Historical Range Contraction and Extent of Harbour Porpoises (Phocoena phocoena) in the Baltic Sea Revealed by Archival Newspapers

Over the past several centuries, the Baltic Proper harbour porpoise (Phocoena phocoena) has undergone significant population declines, resulting in its current IUCN classification as critically endangered. While conservation efforts are extensive and multinational, the population’s abundance has only been estimated once (SAMBAH Project, 2011-2013). Ad hoc historical sources and the sub-fossil archaeological record, dating to 7,000 years BP, suggest that the population once had a wider distribution in the Baltic Sea. However, the historical abundance and distribution of the Baltic Proper harbour porpoise population remain largely unknown, especially before the mid-20th century. This study examines archival Swedish newspaper records from the late 1700s to the early 1900s to assess the presence and distribution of porpoises and human interactions with porpoises. Digitized articles were searched for by keyword in the National Library of Sweden Newspaper Archive. This dataset was combined with HELCOM/ASCOBANS historic data. The records show that harbour porpoises historically occurred regularly along the entire Swedish coast, including the northernmost parts of the Gulf of Bothnia, where the species is virtually absent today, suggesting a notable range retraction. While harbour porpoises appeared less frequently in the Baltic Sea than along the west coast, the number of Baltic records indicates that porpoises occurred more frequently than today. The peak occurrence of porpoises during spring and summer suggests that the Gulf of Bothnia historically constituted an important foraging habitat for migrating animals from the southern Baltic. Bycatch is a common anthropogenic impact, consistent with the historic importance of fishing in the region. By integrating historical records with modern conservation data, this study provides critical insights into the long-term effects of human activity on the Baltic Proper harbour porpoise. Understanding past ranges, population size and the history of negative anthropogenic impacts is vital for guiding more effective conservation strategies.

Baltic Proper harbour porpoises ( Phocoena phocoena ) are listed as critically endangered by the IUCN (Carlström et al. 2023) and the Baltic Marine Environment Protection Commission (HELCOM 2025), with an extant population of ~500 individuals (Amundin et al. 2022). While research and monitoring initiatives are extensive and multinational (Benke et al. 2014; Carlén et al. 2018; Döhring 2022; Koschinski et al. 2024), the population’s abundance has only been estimated once, about a decade ago (SAMBAH Project, 2011-2013; Carlén et al. 2018; Amundin et al. 2022). Although archaeological evidence suggests that the harbour porpoise was once more widespread in the southern and central Baltic region (Sommer et al. 2008; Lõugas and Bērziņš 2023; van den Hurk and Aiken 2025), there is no reliable information regarding historic range and presence. Here, we use a marine historical ecology (MHE) approach, examining records of harbour porpoises from archival Swedish newspapers and Baltic region documents from the late 1700s to the early 1900s to assess the presence and distribution of harbour porpoises in the Baltic region. MHE offers valuable insights into the conservation of modern ecosystems by combining historical perspectives with contemporary ecological practices (Al-Abdulrazzak et al. 2012; Kittinger et al. 2014; Engelhard et al. 2016). This interdisciplinary approach integrates previously disparate fields, including ecology, history, archaeology, economics, and fisheries science, to enhance understanding of long-term human-ocean interactions, informing effective management strategies for marine environments. By placing modern studies into context through a historical perspective, MHE combats the often deleterious effects of shifting baseline theory (Pauly 1995; Keegan 2009; Orton 2016; Atmore et al. 2021), providing evidence regarding the drivers of past change (Lotze and Worm 2009). Through the development of historical baselines and ranges, MHE has aimed to understand the effects of anthropogenic disturbances on marine ecosystems, including their structure and function (Engelhard et al. 2016; Thurstan 2022). Historic newspapers serve as valuable resources for examining past faunal distributions by reporting sightings and observations of animals. These archives have been used to gather data on species presence (Vuorisalo et al. 2001; Brito and Sousa 2011; Herbst et al. 2023), relative abundance (Cochran and Elliott 2012), seasonal behaviours (Brown and Wiedenmann 2024), and human-animal interactions (Sandoval Gallardo et al. 2021), revealing patterns that contribute to our understanding of historical biodiversity. This does not come without difficulties: historic newspapers can present challenges in understanding inconsistencies and variability in how natural phenomena were reported, as they often reflect local, regional, and national interests, without a focus on scientific accuracy. The reports are often disparate, requiring researchers to manually filter through records (van Erp et al. 2018). Moreover, descriptions can be vague or imprecise, making it difficult to definitively identify species. However, the benefits are considerable. In Sweden, historic newspaper accounts offer valuable snapshots of the environment from periods when natural history was becoming a more prominent interest, with growing public fascination with documenting and understanding wildlife (Hodacs 2010; Hodacs 2011). Although the accounts may lack the precision of modern scientific observations, they provide a unique resource to track the presence and distribution of species that would otherwise be inaccessible (Ferretti et al. 2014).

Reports of harbour porpoises in the Baltic and Belt Sea were obtained from Swedish Archival newspapers digitized and held by the National Library of Sweden (https://tidningar.kb.se/). The newspaper database was searched by year, using key terms from the earliest records in the archive, from 1645 to 1924 (newspapers published after 1924 were not publicly available due to copyright law). The terms included: phocoena, phocaena, phocana, tumlare (Swedish for “porpoise”), marsvin (alternative Swedish name for “porpoise”), communis (a binomial nomenclature used “Phocoena communis” for harbour porpoises), delphinus (“Delphinus phocoena”), sjöodjur (Swedish for “sea monster”), pyöriäinen (Finnish for “porpoise”) and val (Swedish for “whale”). Hits from the search terms were then manually assessed to identify mentions of harbour porpoises. This excluded information about harbour porpoises in other bodies of water (i.e., in accounts of travel across the Atlantic), mentions in serialized fiction, and accounts that were not harbour porpoises (based on size or other physical characteristics). From the mentions of harbour porpoises in the Baltic and the Belt Sea waters, specific information was recorded: the newspaper, the search term used, the year, the date of publication, the most exact date (i.e., if the article mentioned the date of observation or catch either as a numerical date or more colloquially, for example: “last Friday”), the toponym associated with the mention, the coordinates associated with that toponym (estimated from the description, for example: “off the coast of Umeå”), and a translation of the report into English. Each mention was given a “Report ID” (RID) to identify the report. The RIDs were collated by Observation ID (OID) (Supplementary Table 1), combining observations that were reported multiple times. This included the most exact date available, toponym, coordinates, and a combined version of the published stories. From the newspaper stories, the following information was collected for each OID: the number of reported harbour porpoises (using two if multiple porpoises were seen, but no number was given), the gear involved (i.e. gun, fishing net, etc.), biological information about the porpoise(s) (length, weight, girth, sex, and information about pregnancy or calving), and the type of observation. The types of observation were categorized into seven distinct outcome types. These include (alphabetically): 1. Bycatch: Porpoises that were caught unintentionally in fishing gear. 2. Capture : Porpoises that were caught, where it is unclear if it was intentional (hunted) or by accident (bycatch). 3. Hunting : Porpoises that were actively hunted. 4. Ship Strike : Porpoises that were struck by vessels. 5. Sighting : Porpoises that were seen alive but not otherwise interacted with. 6. Stranding : Porpoises that were washed ashore or found dead in the water. 7. Unknown : Porpoises where the type of observation was unclear or not specified. Each OID was assigned a single observation type, with the observations ranked (in cases of multiple observation types being applicable) to ensure only one was applied per OID. Additionally, each OID was categorized based on whether the porpoise was killed as a result of human activities. The “human-induced mortality” category can overlap with observation types. For instance, a porpoise that was reported as bycatch or hunting may also be recorded as a human-induced mortality. The archival Swedish data were combined with data from the HELCOM/ASOBANS Harbour porpoise database (HELCOM 2023), filtered to match the same time period (up to 1924) as the Swedish data. The filtered HELCOM/ASOBANS dataset comprised data from the Finnish Ministry of the Environment and Gauja National Park. The HELCOM/ASOBANS dataset was compared to the Swedish archival dataset to ensure that no duplicate observations were included. The filtered HELCOM/ASCOBANS data included 123 new reported observations, reporting a minimum of 189 porpoises. Data Analysis Data analysis was conducted in R version 2024.12.1+563 (R Core Team 2022). To avoid the inclusion of non-porpoise cetaceans that were incorrectly identified as porpoises in the newspapers and not excluded during data collection, a set of exclusion criteria based on size was developed. This included upper limits on weight (100 kg) and length (2 m) based on maximum sizes of harbour porpoises and the lower averages for other species known the visit the Baltic region, including the common dolphin ( Delphinus delphis ), bottlenose dolphin ( Tursiops truncatus ), white-beaked dolphin ( Lagenorhynchus albirostris ), and newborn orcas ( Orcinus orca; Supplementary Table 2) . The exclusion criteria identified seven (~0.6%) animals as non-porpoise (OID: 10, 101, 118, 157, 230, 281, and 292). Additionally, OID 4 was identified as a bottlenose dolphin. These animals were excluded from the downstream analysis. The location and number of porpoises observed were visualized by OID in R using hexbin maps (Wickham 2016) with a bin width of 75 km. To examine any correlation between the number of newspapers published and the number of observations over time, several different tests were carried out. The normality of the OID data and the number of archived newspapers were tested using the Shapiro-Wilk normality test, which showed that the data were not normally distributed. Spearman’s test was used to examine the correlation between the number of OIDs and the number of newspapers published, as well as the number of porpoises observed and the number of newspapers published. Poisson generalized linear models (GLM) were used with a log link function to examine the effects of newspaper counts and time period on the number of porpoises reported by year. The number of OIDs was visualized geographically, using hexbins, as well as temporally in a timeseries plot. The annual number of newspapers archived by the National Library of Sweden over the time period was extracted from the database and included. To examine the seasonality of the number of observed harbour porpoises by region, the monthly number was summarized by four areas: West of the Baltic Sea, the Baltic Sea, the Bothnian Sea, and the Bothnian Bay. The delineation between the West of the Baltic Sea and the Baltic Sea is based roughly on the management border suggested by Carlén et al. (2018); here, longitude 16 ° E is used (Supplementary Figure 1). The delineation between the Baltic and Bothnian Sea is just above the Åland Islands (latitude 60.75 ° N), and the delineation between the Bothnian Sea and Bothnian Bay is at Vasa, Finland (latitude 63.17 ° N). A time series plot was made to visualize the number of harbour porpoises observed over time, and seasonality of harbour porpoise locations was examined by plotting the number of porpoises observed by month and by region, including the Belt Sea, Baltic Sea, Bothnian Sea, and Bothnian Bay. For this purpose, data from all years were combined, enabling a broader view of seasonal trends without year-specific bias. Two calves were identified based on newspaper reports (OID: 109, 214). Animals were assigned to an age class (neonate, calf, juvenile, and adult) based on length (m) following the four-class criteria established by Lockyer (1995) and van Elk (2019), as detailed by Neimanis (2022). Since sex information was unavailable for the historic dataset, sex-specific differences in maturation length required an adaptation of the criteria (Supplementary Table 3). Specifically, the cutoff for the adult class was set at 1.29 m. This length corresponds to the minimum published length for maturity in modern male harbour porpoises and is lower than the same for female harbour porpoises (1.39 m). The 1.29 m length was selected to ensure that all individuals considered mature in the modern population (both male and female) were included in the historic “adult” category. While inclusive of all adults, the 1.29 m cutoff likely results in the inclusion of some larger juvenile females within the historic “adult” class. Information on porpoise length was included in the descriptions for and weight was included for ~20% (n=79). These were standardized to the metric system to allow comparison. Several measurement systems were recorded in the newspapers. In addition to the metric system, traditional Swedish units of measurement, which were standardized in 1665, were also used, including aln (ell; pl. alnar): from 1605-1863 59.38 cm, 1863 onward 59.37 cm; famn (fathom): 3 alnar; fot (foot): ½ aln. Mass measurements included skålpund: 0.42507 kg, and lispund: 8.502 kg. Individuals were excluded if both measurements were included in the newspaper report but only one met the criteria (ex., weight: 17 kg and length: 2 m would not count as a juvenile). The geographic location and the number of juvenile porpoises over time were plotted. The length of the porpoises is plotted over time, separating the five age classes.

The first records of harbour porpoises in Swedish newspapers are from May 1772 and were published by the newspaper “Hvad Nytt?” (What’s New?). Reports of porpoises in Swedish newspapers did not become regular, with yearly mentions, until the 1880s. In total, between 1772 and 1925, harbour porpoises were reported 1,490 times in Swedish archival newspapers, resulting in 318 unique OIDs (Supplementary Table 1). The reports indicate a minimum of 1,266 harbour porpoises observed. Combined with the historic HELCOM/ASCOBANS dataset, there are a total of 441 observations accounting for at least 1,455 porpoises. Distribution & Occurrence The combined dataset shows that from 1772 to 1924, harbour porpoises were observed along the entire Swedish and most of the Finnish coast, as well as in Danish, German, Lithuanian and Latvian waters (Figure 1). The highest historic number is reported from the Little Belt Sea in Denmark, where, during the hunting season in November 1880, 800 harbour porpoises were reported to have been killed. Specifically, from the 27th to 30th November 1880, 349 harbour porpoises were caught over these three days (OID: 31; see Kinze 2008). Other hotspots include Kattegat in southern Sweden, where a 1775 report mentions the presence of over 100 porpoises in the Kattegat “accompanying” the herring (OID: 2), and Northern Gotland in the Baltic Sea, where in addition to the porpoises regularly caught, 100 porpoises are reported as observed and killed by drift ice in March 1924 (OID: 324). The Bothnian Bay region around Sundsvall also has a high number of porpoises reported (n = 120), but these are the results of a large number of individual reports (31 OIDs), rather than a single larger event. Örnsköldsvik has the highest amount of OIDs within a 50 km radius, 36, with a relatively low number of porpoises observed per OID, with a total of 52 porpoises. Luleå is similar, with the sixth highest amount of OIDs, 12, but the number of porpoises is only 20. The seasonality of the harbour porpoises’ location shows that they were less present in the Bothnian Bay and Bothnian Sea in the winter months, with no observations from February through April and few animals in December and January (Figure 3). In the Baltic Sea, a higher number of reports occurs in all seasons but autumn. West of the Baltic Sea, the highest number of animals are reported during the summer months, and during the autumn and winter hunt (Figure 3). Calving, Juveniles, and Size Two porpoises are reported as females with fetuses (OID: 7, 11). The two pregnant porpoises were reported in May and June in Umeå, and Karlshamn, respectively (Figure 4C). Two additional porpoises were identified as potentially being a mother and calf pair (OID: 209, 214) near Hudiksvall and Örnsköldsvik, respectively (Figure 4C). One report from 1886 describes the annual porpoise hunt in Bramsnæs Bay, Denmark, that occurs from March to May, including that “many of the animals have a small cub in their belly but never more than one” (OID 60). Of the harbour porpoises for which there are lengths reported, approximately 43% (n=52) are classified as sub-adults. This includes six neonates (3.4%), 24 calves (13.6%), and 22 juveniles (12.5%) (Figure 4). Additionally, ~57% (n=68) are classified as adults (Figure 4). The first juvenile harbour porpoise was reported in 1854, and they are regularly reported starting in1865 (Figure 4B). The juvenile harbour porpoises are reported along the majority of the Swedish coast and along the Finnish coast, including the northernmost part of the Bothnian Bay (Figure 4C). The average size of porpoises in the “adult” age-class (> 1.29 m length) was 1.57 m (n=68). This is slightly larger than the combined average length of modern adults in the study region. The historic average is greater than the modern male average (1.45 m) but falls slightly below the modern female average (1.6 m). Critically, the historic animals could not be classified by sex. Thus, the 1.29 m length cutoff used to define “adult” for the historic animals likely includes some juvenile females, whose length at physical maturity is generally higher (1.39 m). The inclusion of these smaller, sub-adult females likely results in a slight underestimation of the true historic average adult length, suggesting the difference between historic and modern porpoises may be more pronounced than reported here. While the historical lengths are likely approximate and often reported in rounded figures, as is typical of historical, non-scientific accounts, the relatively wide range of reported length measurements (0.7 m to 2 m) suggests that the data captures real variation in body size. Observation Types The distribution of observation types of harbour porpoises across the entire Baltic region and by sub-region is shown in Figure 5. The most common outcome across the whole Baltic region was sightings, accounting for 39% (n=564) of the total number of porpoises. This is closely followed by hunted porpoises (29%, n=421), then bycatch (12%, n=170), stranded (8%, n=122), unknown (7%, n=101), caught (4%, n=64), and shipstrike (0.2%, n=3). This varies by sub-region, with, for example, 45% of the number of porpoises from West of the Baltic Sea were hunted (n=363). Across the whole dataset, 45% (n=656) of the total number of porpoises recorded are killed by human actions (Figure 5). Regionally, the proportion of killed animals varies greatly, ranging from making up 65% of the animals West of the Baltic Sea (n=518) to 16% of the porpoises in the Baltic Sea (n=51). Harbour porpoises are strongly associated with fish and are mentioned alongside fish in almost 20% of the reports (OID: n=66), accounting for about 16% of the porpoises recorded (n=195). Herring makes up 61% of the fish mentions, followed by salmon (20%, n=41) and the general “fish” category (15%, n=30). 1% (n=12) of the observations reported strandings of porpoises, and 0.2% (n=2) reported ship strike. Impact of Archived Newspapers on Porpoise Observations The number of Swedish newspapers archived decreases after 1907 as the National Library of Sweden Newspaper Archive shifts to documenting the main newspapers, reducing the number of newspaper titles archived from strong influence between the number of newspapers archived and the number of OIDs (rho = 0.528, p = <0.001) and a moderate influence between the number of newspapers and the number of harbour porpoises reported (rho = 0.438, p = <0.001). The linear regression did not find significant linear relationship between the number of newspapers and the number of porpoises (β = 3.26×10⁻ 4, p = 0.412), suggesting a linear model was not appropriate for the data. The GAM revealed a significant nonlinear relationship (χ² = 859.8, p < 0.001), indicating that porpoise counts increased with newspaper availability but in a non-linear fashion. The Mann-Whitney U test revealed a significant difference between the number of porpoises before and after 1900 (W=241.5, p < 0.001). The Poisson GLM, accounting for differences in the number of newspapers archived, showed that while the number of newspapers is highly significant positive predictor of the number of porpoises (β = 1.79×10⁻⁵, p < 0.001), the early 1900s period was associated with fewer porpoise reports after adjusting for the number of newspapers (β = -0.27, p < 0.001). However, regional Poisson GLMs show differences. The Bothian Bay exhibited strong temporal trends, with porpoise counts nearly doubling in the early 1900s (β = 0.684, p < 0.001), largely independent of newspaper activity (β = 4.78×10⁻⁶, p = 0.395). The Bothnian Sea counts increased moderately over the same period (β = 0.429, p = 0.028), but the higher newspaper coverage likely caused the increase (β = 1.70×10⁻⁵, p = 0.004). The number of porpoises West of the Baltic decrease in the early 1900s (β = –1.51, p < 0.001), with little effect from the number of newspapers (β = 2.23×10⁻⁶, p = 0.421). In the Baltic Sea, the number of reported counts remained relatively stable (β = –0.255, p = 0.103), but the number of archived newspapers was negatively associated with porpoise reports (β = –2.09×10⁻⁵, p < 0.001). These results indicate that temporal trends in the number of porpoises differ by sub-region, with some regions showing clear early-1900s declines or increases while others remain stable. This reduction in archived newspapers corresponded to a slight decrease in porpoises reported (Figure 2B), though the larger newspapers occasionally reprint observations from non-archived smaller newspapers.

Presence and Distributions The historic distribution of harbour porpoises in Swedish waters appears to be very distinct from the modern one. In modern times, few animals have been observed north of the Åland Islands (HELCOM/ASCOBANS Harbour Porpoise database), and a northern management border has been proposed between 60.5°N at the Swedish coast and 61°N at the Finnish coast (ICES 2020). The historic range of the harbour porpoise, however, extends to the northernmost parts of the Gulf of Bothnia with higher concentrations (30+ individuals) along the Swedish coast between Hudiksvall and Örnsköldsvik. Their frequency in the Bothnian Bay, however, is not clear. Reports from the 1800s and 1890s suggest that porpoises are rare in the region (OID: 47, 54, 55, 115, 131, 331) and “should rarely be caught” (OID, 115). Conversely, their occurrence was noted as not unusual by 1899 and into the 1900s (337). In 1899, the newspaper Umeåbladet reported: “The presence of porpoises in our waters is far from as rare as one might think due to the information regarding the sea animal visible in the Sundsvall area.” This contrasts sharply with the contemporary spatial distribution of harbour porpoises in the SAMBAH project, which generally reported low predicted detection rates in the waters of 5–80 m depth from north of Gotland to the Åland and Archipelago Seas (Carlén et al. 2018). It is important to note that the SAMBAH acoustic monitoring did not survey deeper waters, which include considerable parts of the Baltic Proper, north and east of Gotland. Areas exceeding a 20% monthly probability of detection were restricted to the south and west of the Baltic Proper, though these highly important areas accounted for only ∼30% of all recorded detections (Carlén et al. 2018). National monitoring programs and the SAMBAH project have consistently identified seasonal movements of harbour porpoises within the Baltic Sea and West of the Baltic Sea (Benke et al. 2014; Carlén et al. 2018; Owen et al. 2021), characterized by an increased presence around the offshore banks east and southeast of Öland and Gotland in summer, and a more coastal distribution in during winter. Specifically, analyses carried out in German waters have linked the winter distribution of the assumed Baltic Proper population to cold air temperatures and air temperatures lower than water surface temperatures (Gallus et al. 2012), which is interpreted as a migration pattern toward the southwest during winter months. Since the archival data primarily comes from Sweden and Finland, the absence of porpoises outside these countries coastlines should not be interpreted as a historical absence, as their presence not captured in the available records. A historical presence of harbour porpoises in the Gulf of Bothnia is a key finding and suggests a significant retraction in distribution, now missing from approximately a third of its historic range in the Baltic region. Range contractions, linked to environmental stressors and anthropogenic impacts, affect species distribution and, eventually, survival. It is especially concerning for marine mammals, which often rely on large, stable environments for migration, foraging, breeding, and social behaviours. The historical presence of harbour porpoises in the Gulf of Bothnia challenges the idea that the current range reflects ecological limits of the harbour porpoise in the Baltic Sea region. Indeed, it could suggest that porpoises may return with warming waters, a trend seen in other marine species (Stafford et al. 2022; Coulon et al. 2024; for exceptions, see Le Luherne et al. 2024) and marine mammals (Hamilton et al. 2019; Lameris et al. 2021; Stafford et al. 2022). Several factors could contribute to the retraction of harbour porpoises to south of the Åland Islands, which can be broadly categorized into three interconnected factors: climatic and oceanographic factors, prey availability, and anthropogenic impact. These factors may be further exacerbated by demographic constraints inherent to a small population, creating a mechanism that actively drives the maintenance of a retracted range. The Gulf of Bothnia is characterized by its low salinity, brackish waters, a state that has existed since the Late Holocene (~3,000 BP; Widerlund and Andersson 2011). This environment results in a sensitive ecosystem and significantly limits biodiversity as few species are capable of colonizing the region (Snoeijs-Leijonmalm 2017). The lower salinity water favours brackish and even freshwater species over marine species, resulting in a shift in community composition and negatively impacting large-bodied marine taxa, potentially affecting marine predators reliant on these species as prey (Rousi et al. 2013; Weigel et al. 2015; Mäkinen et al. 2017; Snoeijs-Leijonmalm 2017). This makes the region a sensitive ecosystem. Harbour porpoises are known to visit areas of low salinity such as estuaries and rivers (Cucknell et al. 2020; Stern et al. 2017; Wenger and Koschinski 2012; Rekdahl et al. 2023; Koschinski 2001; Kesselring et al. 2017), and genomic analyses have suggested an adaptation to the salinity gradient (Celemín et al. 2023), suggesting that low salinity is not a physiological barrier. However, the brackish environment means the few adapted species are sensitive to change, making the Gulf of Bothnia’s species-poor ecosystem acutely vulnerable to disturbance, particularly the loss of prey, impacting the feasibility of the habitat. Eutrophication, driven by nutrient runoff from expanding agriculture and urban centres, has had deleterious effects on benthic communities and has led to reduced biodiversity (Österblom et al. 2007; Rousi et al. 2013; Weigel et al. 2015). The severity of the nutrient over-enrichment in the Gulf of Bothnia is debated (Andersen et al. 2011; Voss et al. 2011). The decline in benthic species has reduced the prey available for demersal fish and other predator species (Rousi et al. 2013; Mustamäki and Mattila 2015). Similarly, there has been a reduction in dissolved oxygen concentrations since the mid-1950s, with a trend of negative 0.1 ml L -1 per decade in the Bothnian Bay (Polyakov et al. 2022). Deoxygenation of the sea has led to expanding areas of oxygen minimum zones and hypoxia, primarily linked to increased nutrient input (Carstensen et al. 2014). In 2023, 35% of the bottoms of the Baltic Sea were anoxic or hypoxic (Hansson and Viktorsson 2024). These hypoxia/anoxic regions are less likely to be habitable for harbour porpoises as prey will be less available (Almroth-Rosell et al. 2021; Krapf et al. 2022; Koschinski et al. 2024). The weakening food web in the Bothnian Sea has been linked to overfishing and eutrophication (Faithfull and Bergström 2025). Beyond these primary stressors, the extremely small population size may itself be a barrier to range expansion. If the remaining population of harbour porpoises is spread throughout its full historical range, the resulting low population density would likely induce the Allee effect. An Allee effect occurs when the positive benefits of groups, such as cooperative defence or improved mate finding, are lost as a population becomes too sparse, resulting in a correlation between population density and individual fitness (Courchamp et al. 1999; Walter et al. 2017). Essentially, the population growth rate declines at low population densities. The Allee effect has been suggested in other marine mammal species, most notably relating to reproductive success due to low-density populations. For example, the difficulty in finding a mate across a large habitat at extremely low population densities is proposed to be a factor limiting the recovery of the critically endangered North Atlantic right whale ( Eubalaena glacialis ; Caswell et al. 1999). A similar concern is driving population modelling for the polar bear ( Ursus maritimus ), where low population densities, unpredictable habitat and harvest-depleted male populations lead to infrequent mating encounters (Molnár et al. 2008; Molnár et al. 2014). Models predict that this mate-finding Allee effect can induce a rapid reproductive collapse if the operational sex ratio is severely skewed, offering a mechanism for the slow recovery of populations like the overharvested Viscount Melville Sound stock (Molnár et al. 2014). Evidence for an Allee effect was also suggested during the early stages of recovery from overexploitation in the Antarctic Fur Seal ( Arctocephalus gazella ; Nagel et al. 2021). In the case of the Baltic proper harbour porpoise, an Allee effect incentivizes individuals to stay in the core, higher-density area for mating, thus perpetuating a range retraction that was caused by an initial stressor. This low-density scenario suggests that the costs of mandatory seasonal migrations south to avoid ice-covered water may no longer be justified. Instead, the need for reproductive assurance, likely concentrated in the core southern part of the Baltic, outweighs the benefits of returning to the now-peripheral, low-density habitat, essentially incentivizing a maintained range retraction. Thus, while not being the initial driver of habitat retraction, a component Allee effect on the much-reduced population size may be a factor keeping the population in a smaller range. Population Size Zooarchaeological analysis suggests that harbour porpoises were once more abundant in the southern Baltic Sea region (Sommer et al. 2008; Lõugas and Bērziņš 2023; van den Hurk and Aiken 2025). Medieval texts further support a sizable harbour porpoise population in the Baltic region (van den Hurk and Aiken 2025). 18th and 19th century historical documents provide evidence of harbour porpoises present in Swedish waters (Linné et al. 1792; Flower et al. 1866), including a report of a “whale and salmon fishery” in northern Gotland (Caddy 1887). Historical data from bounty projects, bycatch records, and observations of dead stranded animals show that harbour porpoises were numerous in the Baltic and Gulf of Bothnia during the first half of the 20th century (Johansen 1929; Lönnberg 1940; Tägström 1940; Psuty 2013; HELCOM 2023). Current estimates, however, suggest a small Baltic Proper harbour porpoise population of around 500 individuals concentrated in the southwestern Baltic (Amundin et al. 2022) and a larger, but declining, Belt Sea population of around 14,000 individuals (Gilles et al. 2023). Harbour porpoises in the Baltic Region are comprised of two morphologically (Börjesson and Berggren 1997; Huggenberger et al. 2002; Galatius et al. 2012) and genetically (Wiemann et al. 2010; Celemín et al. 2023; Autenrieth et al. 2024) distinct populations: the Baltic Proper and the Belt Sea population. While these populations are considered distinct, their overlapping range creates a complex dynamic for conservation and population management. Differentiation of these populations relies on genomic or geometric morphometric analysis of skeletal material (Galatius et al. 2012; Celemín et al. 2023); thus, differentiation based on visual observation is not possible. The harbour porpoises included in the archival newspaper accounts are likely a mix of Belt Sea and Baltic proper populations. While historic documents cannot provide quantitative estimates of population size, the number of observations is greater than what is observed in modern-day, suggesting that the historic population size was larger. Anecdotal newspaper reports indicate an increase in harbour porpoise observations in the Bothnian Sea during the early 1900s. Several newspapers described “a mass immigration” of porpoises into the region, often highlighting potential negative impacts on fisheries. Our data confirm an increase in porpoise observations in the Bothnian Bay and Sea, but not in the Baltic Sea or west of the Baltic. Newspaper records from the 1920s show no evidence of a population decline, suggesting that the reduction occurred later in the 20th century. Although newspaper observations provide some indication of relative abundance, they should be interpreted cautiously as a proxy for true population size. Systematic surveys beginning in the 1950s documented a significant decrease in harbour porpoise numbers (Berggren and Arrhenius 1995), but the absence of consistent data between the 1920s and 1950s means that the exact timing of the onset of the decline remains uncertain. Calving, Juveniles, and Size Female harbour porpoises give birth to a single calf every year (or every other year) once reaching sexual maturity at 3-5 years of age (Lockyer and Kinze 2003; Kesselring et al. 2017). Currently in the Baltic region, mating mainly occurs from July to August, with calves subsequently born from May to August after an approximately 10-month gestation period (Börjesson and Read 2003; Lockyer and Kinze 2003). The three individuals reported as pregnant were reported in May and June, fitting with modern calving timing. OID 7 and 11 had size descriptions (40 cm and 4.6 kg, respectively) of the fetuses. While Belt Sea porpoises are often observed around Kullen (Sweden) in the Kattegat during the reproductive season (Sveegaard et al. 2011; Sveegaard et al. 2011; Teilmann et al. 2022; Stedt et al. 2023), the location(s) of calving is less clear for the Baltic Proper population, though it is thought to occur around Hoburgs bank and Midsjöbankarna (Figure 4C). When considering the porpoises assigned to the age category neonates, as well as the reported mother-calf pairings and pregnant porpoises, there is a distribution as far north as the northernmost part of the Bothian Sea. The range of juvenile harbour porpoises largely mirrors that of the whole distribution, suggesting that juvenile harbour porpoises occur in the same places as adult harbour porpoises. Outcomes Porpoise hunting has been known historically in the Baltic region (Lõugas and Bērziņš 2023), with laws regarding the exploitation of harbour porpoises dating from 1200 CE in Denmark and 1378 CE in Poland (van den Hurk and Aiken 2025), and a guild of harbour porpoise hunters in Middelfart, Denmark, since 1367 (Kinze 2011). Hunting harbour porpoises was banned in Sweden in 1967 with a law that prohibited hunting of any animals unless a specific hunting period was designated by the government. Specific protections for the harbour porpoise in Swedish waters were enacted in 1973. In 1994, the Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) took effect, providing protection for harbour porpoises in the Baltic, Belt, and North Sea. Bycatch is a critical threat to the Baltic proper harbour porpoise population (Skóra and Kuklik 2003; Cervin et al. 2020; Carlström et al. 2023). The newspaper records show a long history of bycatch of harbour porpoises in the Baltic region, with accounts of bycatch porpoises as early as 1858 (OID: 7) in Umeå, caught in a salmon net. Bycatch only became systematically documented after World War II, leaving earlier impacts on harbour porpoises and cetacean populations more generally largely unknown. While bycatch is expected to have occurred in the Baltic, there was previously little concrete evidence beyond anecdotal reports until the 1970s (Berggren 1994). There is limited knowledge about ship strikes in smaller cetaceans (Van Waerebeek et al. 2007; Schoeman et al. 2020), even in contemporary populations. The relatively low number of reports is likely due to reporting bias rather than a lower frequency of interactions (Schoeman et al. 2020). Necropsies conducted on 128 harbour porpoises stranded along the Swedish coast from 2008-2020 do not identify any instances of ship strike (Neimanis et al. 2022). In contrast, necropsies of 612 harbour porpoise from the Dutch coast in the North Sea suggest 14 (~4%) have trauma consistent with propeller or ship-hull collision (IJsseldijk et al. 2022). Harbour porpoises have been reported to have been hit by vessels in British (Deaville et al. 2018) and Dutch (Camphuysen and Siemensma 2011) waters, and possibly in Canadian waters (Fenton et al. 2017). The earliest known account of a fatal ship strike of a cetacean occurred in Rhode Island, USA, in 1877 when the steamship “Munroe” hit an unidentified small whale, possibly a minke whale ( Balaenoptera acutorostrata ) or a small fin whale ( Balaenoptera physalus ; Laist et al. 2001). OID 111, reported in 1895, is described as stranded due to being hit by a ship, an assessment made based on the injury on the animal’s back. This might be one of the earliest examples of a ship strike in an identified species, with the second oldest being a sperm whale ( Physeter macrocephalus ) in 1902 (Laist et al. 2001; Schoeman et al. 2020).

Through the lens of MHE, using data collected from Swedish archival newspapers between the late 18th and early 20th centuries, this study established a crucial time-depth for the conservation of harbour porpoises in the Baltic Sea region. Our findings provide compelling evidence of a substantially different historical population structure and distribution, with four key implications for conservation: 1. Range Retraction : We demonstrate a notable range retraction, suggesting the loss of approximately a third to half of the historic range in the Gulf of Bothnia. Historical sighting patterns confirm the seasonal utility of these areas, with reports of porpoises West of the Baltic Sea (or approximately the Belt Sea assessment unit) primarily from April to November, the Baltic Sea from January to August, and the Bothnian Sea and Bay in summer. This was likely driven by an interconnected set of pressures that collectively led to a significant regional decline in the availability of prey, making the Gulf of Bothnia unsuitable for sustaining a harbour porpoise population. We suggest this range retraction is likely reinforced by the Allee effect, with the extremely small population size and correspondingly low density may promote a maintained retraction toward the core area in the Baltic Sea. Due to ice cover in the winter months, harbour porpoises cannot remain resident in the Bothnian Bay year-round. The harbour porpoises observed in the Bothnian Sea and Bay may be part of the Baltic Proper population or a large migration from the Belt Sea population. However, animals equipped with satellite tags within the Belt Sea assessment unit in modern times are rarely registered to move farther east than 15.0°E (Sveegaard et al. 2015). 2. Larger Historical Population Size: Historical observation patterns indicate a larger historical population size. Harbour porpoises were reported with higher frequency than seen today, and key indicators of reproduction were observed across the historic range: sub-adults were present alongside adults and pregnant females were identified substantially farther north than currently seen. 3. Long-Term Human-Induced Mortality: The archival records document a long history of early anthropogenic impacts that precedes modern conservation concerns. High rates of human-induced mortalities are reported alongside early observations of ship strike and frequent bycatch, hunting, and catching of harbour porpoises. This demonstrates that porpoises in the region have been subject to ongoing human-related threats for centuries. 4. Utility of Historical Sources: Finally, the utility of historical sources in defining the former range and robustness of harbour porpoises in the Baltic region is shown. While historic newspapers are subject to selection and reporting biases, preferentially reporting “interesting” and extraordinary events, the temporal consistency and wide geographic coherence of the observations validate the findings and enable the contextualization of modern population dynamics. Together, these data suggest that the current population distribution is not reflective of the species’ potential range. The historic period around the mid-20th century appears to be a critical time for the modern population. Looking forward, the potential for increased abundance through successful conservation efforts, combined with warming ocean trends due to climate change, suggests porpoises in the Baltic region may attempt to reoccupy their historic northern range, at least during spring-summer-fall when sea ice is absent; however, ecological heterogeneity between the Baltic Proper and the Bothnian Sea and Bay indicates that porpoises are unlikely to occur there year-round, either historically or in the future. Therefore, proactive protection and spatial management in these historic northern regions are essential for future conservation efforts. This necessitates future ecological and genomic analyses to precisely examine past population size and changing environments, which will ultimately inform minimum viable population targets and future distribution scenarios for the region’s harbour porpoise populations.

The authors would like to thank Kylie Owen for participation in the acquisition of funding, Markus Ahola for help translating Finnish newspapers and Morten Tange Olsen for constructive feedback on the draft manuscript. This work was funded by Carl Tryggers Postdoctoral Scholarship and the Swedish Agency for Marine and Water Management, grant number 2024-001531. Data Availability Statement All data used in this study are included within the manuscript and its appendices.

Al-Abdulrazzak, Dalal, Robin Naidoo, Maria Lourdes D. Palomares, and Daniel Pauly. 2012. Gaining perspective on what we’ve lost: the reliability of encoded anecdotes in historical ecology. PloS one 7: e43386. Almroth-Rosell, Elin, Iréne Wåhlström, Martin Hansson, Germo Väli, Kari Eilola, Pia Andersson, Lena Viktorsson, Magnus Hieronymus, and Lars Arneborg. 2021. A regime shift toward a more anoxic environment in a eutrophic sea in northern Europe. Frontiers in marine science 8. Frontiers Media SA. https://doi.org/10.3389/fmars.2021.799936. Amundin, Mats, Julia Carlström, Len Thomas, Ida Carlén, Jonas Teilmann, Jakob Tougaard, Olli Loisa, et al. 2022. Estimating the abundance of the critically endangered Baltic Proper harbour porpoise (Phocoena phocoena) population using passive acoustic monitoring. Ecology and evolution 12: e8554. Andersen, Jesper H., Philip Axe, Hermanni Backer, Jacob Carstensen, Ulrich Claussen, Vivi Fleming-Lehtinen, Marko Järvinen, et al. 2011. Getting the measure of eutrophication in the Baltic Sea: towards improved assessment principles and methods. Biogeochemistry 106. Springer Science and Business Media LLC: 137–156. Atmore, Lane M., Magie Aiken, and Fabricio Furni. 2021. Shifting Baselines to Thresholds: Reframing Exploitation in the Marine Environment. Frontiers in marine science 8. Frontiers Media SA: 1644. Autenrieth, Marijke, Katja Havenstein, Binia De Cahsan, Julia Canitz, Harald Benke, Anna Roos, Christophe Pampoulie, et al. 2024. Genome-wide analysis of the harbour porpoise (Phocoena phocoena) indicates isolation-by-distance across the North Atlantic and potential local adaptation in adjacent waters. Conservation genetics 25: 563–584. Benke, H., S. Bräger, M. Dähne, A. Gallus, S. Hansen, C. G. Honnef, M. Jabbusch, et al. 2014. Baltic Sea harbour porpoise populations: status and conservation needs derived from recent survey results. Marine ecology progress series 495. Inter-Research Science Center: 275–290. Berggren, P. 1994. Bycatches of the harbour porpoise ( Phocoena phocoena ) in the Swedish Skagerrak, Kattegat and Baltic Seas 1973-1993. Report of the International Whaling Commission (Special Issue) . Newcastle University. Börjesson, Patrik, and Per Berggren. 1997. Morphometric comparisons of skulls of harbour porpoises (Phocoena phocoena) from the Baltic, Kattegat, and Skagerrak seas. Canadian journal of zoology 75. Canadian Science Publishing: 280–287. Börjesson, Patrik, and Andrew J. Read. 2003. Variation in Timing of Conception between Populations of the Harbor Porpoise. Journal of Mammalogy 84. Oxford Academic: 948–955. Brito, Cristina, and Andreia Sousa. 2011. The environmental history of cetaceans in Portugal: ten centuries of whale and dolphin records. PloS one 6: e23951. Brown, Danielle M., and John Wiedenmann. 2024. Newspapers describe long-term trends in whale occurrence in the nearshore New York Bight. Ocean & coastal management 255. Elsevier BV: 107224. Caddy, Florence. 1887. Through the fields with Linnæus; a chapter in Swedish history, by Mrs. Florence Caddy . Vol. 2. London,: Longmans, Green, and co.,. Camphuysen, C. J., and M. L. Siemensma. 2011. Conservation plan for the Harbour Porpoise Phocoena phocoena in The Netherlands: towards a favourable conservation status. NIOZ Royal Netherlands Institute for Sea Research. Carlén, Ida, Len Thomas, Julia Carlström, Mats Amundin, Jonas Teilmann, Nick Tregenza, Jakob Tougaard, et al. 2018. Basin-scale distribution of harbour porpoises in the Baltic Sea provides basis for effective conservation actions. Biological conservation 226: 42–53. Carlström, J., I. Carlén, M. Dähne, P. S. Hammond, S. Koschinski, K. Owen, S. Sveegaard, and &. R. Tiedemann. 2023. Phocoena phocoena Baltic Sea subpopulation, Harbour Porpoise . The IUCN Red List of Threatened Species. https://doi.org/10.2305/iucn.uk.2023. Carstensen, Jacob, Jesper H. Andersen, Bo G. Gustafsson, and Daniel J. Conley. 2014. Deoxygenation of the Baltic Sea during the last century. Proceedings of the National Academy of Sciences of the United States of America 111. Proceedings of the National Academy of Sciences: 5628–5633. Caswell, H., M. Fujiwara, and S. Brault. 1999. Declining survival probability threatens the North Atlantic right whale. Proceedings of the National Academy of Sciences of the United States of America 96: 3308–3313. Celemín, Enrique, Marijke Autenrieth, Anna Roos, Iwona Pawliczka, María Quintela, Ulf Lindstrøm, Harald Benke, et al. 2023. Evolutionary history and seascape genomics of Harbour porpoises (Phocoena phocoena) across environmental gradients in the North Atlantic and adjacent waters. Molecular ecology resources . https://doi.org/10.22541/au.167120655.52464008/v2. Cervin, Linnea, Tero Harkonen, and Karin C. Harding. 2020. Multiple stressors and data deficient populations; a comparative life-history approach sheds new light on the extinction risk of the highly vulnerable Baltic harbour porpoises (Phocoena phocoena). Environment international 144: 106076. Cochran, Philip A., and Robert F. Elliott. 2012. Newspapers as sources of historical information about lake sturgeon (Acipenser fulvescens Rafinesque, 1817). Archives of natural history 39. Edinburgh University Press: 136–146. Coulon, Noémie, Sophie Elliott, Nils Teichert, Arnaud Auber, Matthew McLean, Thomas Barreau, Eric Feunteun, and Alexandre Carpentier. 2024. Northeast Atlantic elasmobranch community on the move: Functional reorganization in response to climate change. Global change biology 30. Wiley: e17157. Courchamp, F., T. Clutton-Brock, and B. Grenfell. 1999. Inverse density dependence and the Allee effect. Trends in ecology & evolution 14. Elsevier BV: 405–410. Cucknell, Anna-Christina, Anna Moscrop, Oliver Boisseau, and Richard McLanaghan. 2020. Confirmation of the presence of harbour porpoise (Phocoena phocoena) within the tidal Thames and Thames Estuary. Mammal communications 6. Mammal Society. https://doi.org/10.59922/okac5968. Deaville, R., P. D. Jepson, M. Perkins, A. Brownlow, N. Davidson, and Doeschate M. 2018. Final Contract Report 1st January 2011 to 31st December 2017 . Report MB0111 2011-2017 by the UK Cetacean Strandings Investigation Programme to Defra. Döhring, T. A. 2022. The Politics of Porpoises–A Critical Assessment of the Biodiversity Conservation Efforts by the European Union within the Baltic Sea. diva-portal.org. van Elk, Cornelis E., Marco W. G. van de Bildt, Peter R. W. A. van Run, Paulien Bunskoek, Jolanda Meerbeek, Geoffrey Foster, Albert D. M. E. Osterhaus, and Thijs Kuiken. 2019. Clinical, pathological, and laboratory diagnoses of diseases of harbour porpoises (Phocoena phocoena), live stranded on the Dutch and adjacent coasts from 2003 to 2016. Veterinary research 50. Springer Science and Business Media LLC: 88. Engelhard, Georg H., Ruth H. Thurstan, Brian R. MacKenzie, Heidi K. Alleway, R. Colin A. Bannister, Massimiliano Cardinale, Maurice W. Clarke, et al. 2016. ICES meets marine historical ecology: placing the history of fish and fisheries in current policy context. ICES journal of marine science: journal du conseil 73. Oxford Academic: 1386–1403. van Erp, Marieke, Jesse de Does, Katrien Depuydt, Rob Lenders, and Thomas van Goethem. 2018. Slicing and dicing a newspaper corpus for historical ecology research. In Lecture Notes in Computer Science, 470–484. Lecture Notes in Computer Science. Cham: Springer International Publishing. Faithfull, C. L., and L. Bergström. 2025. Temporal changes in the Bothnian Sea food web reveal a deterioration linked to fishing pressure and recent eutrophication. ICES journal of marine science: journal du conseil 82. Oxford University Press (OUP): fsaf025. Fenton, H., P. Y. Daoust, M. J. Forzán, R. V. Vanderstichel, J. K. Ford, L. Spaven, and Raverty. 2017. Causes of mortality of harbor porpoises Phocoena phocoena along the Atlantic and Pacific coasts of Canada. Diseases of aquatic organisms 122: 171–183. Ferretti, Francesco, Larry B. Crowder, and Fiorenza Micheli. 2014. Using disparate datasets to reconstruct historical baselines of animal populations. In Marine Historical Ecology in Conservation, ed. John N. Kittinger, Loren McClenachan, Keryn B. Gedan, and Louise K. Blight, 63–85. Berkerley: University of California Press. Flower, William Henry, Daniel Friedrich Eschricht, Wilhelm Lilljeborg, and J. Reinhardt. 1866. Recent memoirs on the Cetacea . London: Pub. for the Ray society by R. Hardwicke. Galatius, A., and C. C. Kinze. 2012. Population structure of harbour porpoises in the Baltic region: evidence of separation based on geometric morphometric comparisons. Biological Association of … . search.proquest.com. Gallus, A., M. Dähne, U. K. Verfuß, S. Bräger, S. Adler, U. Siebert, and H. Benke. 2012. Use of static passive acoustic monitoring to assess the status of the “Critically Endangered” Baltic harbour porpoise in German waters. Endangered species research 18. Inter-Research Science Center: 265–278. Gilles, A., Authier, M., Ramirez-Martinez, N.C., Araújo, H., Blanchard, A., Carlström, J., Eira, C., Dorémus, G., Fernández-Maldonad, C., Geelhoed, S.C.V., Kyhn, L., Laran, S., Nachtsheim, D., Panigada, S., Pigeault, R., Sequeira, M., Sveegaard, S., Taylor, N., Owen, K., Saavedra, C., Vázquez-Bonales, J.A., Unger, B., Hammond, P.S., 2023. Estimates of cetacean abundance in European Atlantic waters in summer 2022 from the SCANS-IV aerial and shipboard surveys. University of Veterinary Medicine Hannover. Hamilton, Charmain D., Jade Vacquié-Garcia, Kit M. Kovacs, Rolf A. Ims, Jack Kohler, and Christian Lydersen. 2019. Contrasting changes in space use induced by climate change in two Arctic marine mammal species. Biology letters 15. The Royal Society: 20180834. Hansson, Martin, and Lena Viktorsson. 2024. Oxygen Survey in the Baltic Sea 2023 - Extent of Anoxia and Hypoxia, 1960-2023 . RO 76. SMHI. HELCOM. 2023. Abundance and population trends of harbour porpoises . HELCOM pre-core indicator report. HELCOM. 2025. HELCOM Red List II of the Baltic Sea species in danger of becoming extinct . Baltic Sea Environment Proceedings No. 205. HELCOM Red List Marine Mammal Expert Group. 2013. Species Information Sheet: Phocoena phocoena . HELCOM. Herbst, Dannieli Firme, Jara Rampon, Bruna Baleeiro, Luiz Geraldo Silva, Thiago Fossile, and André Carlo Colonese. 2023. 180 years of marine animal diversity as perceived by public media in southern Brazil. PloS one 18. Public Library of Science: e0284024. Hodacs, Hanna. 2010. In the field: exploring nature with Carolus Linnaeus. Endeavour 34. Elsevier BV: 45–49. Hodacs, Hanna. 2011. Linnaeans outdoors: the transformative role of studying nature “on the move” and outside. British journal for the history of science 44. Cambridge University Press: 183–209. Huggenberger, S., Benke, H., Kinze, C.C., 2002. Geographical variation in harbour porpoise (Phocoena phocoena) skulls: support for a separate non-migratory population in the Baltic proper. Ophelia 56, 1–12. van den Hurk, Y., and M. Aiken. 2025. From Mesolithic Hunters to Medieval Nobility: The Spatiotemporal Exploitation and Changing Role of Harbour Porpoises ( Phocoena phocoena ) in Northern Europe. In Subsistence Whaling, ed. J. M. Savelle, N. Kishigami, and G. G. Monks. Singapore, Singapore: Springer. ICES. 2020. EU request on emergency measures to prevent bycatch of common dolphin (Delphinus delphis) and Baltic Proper harbour porpoise (Phocoena phocoena) in the Northeast Atlantic. ICES. https://doi.org/10.17895/ICES.ADVICE.6023. IJsseldijk, Lonneke L., Mardik F. Leopold, Lineke Begeman, Marja J. L. Kik, Lidewij Wiersma, Maria Morell, Elisa L. Bravo Rebolledo, Thierry Jauniaux, Hans Heesterbeek, and Andrea Gröne. 2022. Pathological findings in stranded harbor porpoises (Phocoena phocoena) with special focus on anthropogenic causes. Frontiers in marine science 9. Frontiers Media SA: 997388. Johannesson, Kerstin. 2024. Climate change and Baltic Sea genetic diversity. Oxford Research Encyclopedia of Climate Science . Oxford University Press. https://doi.org/10.1093/acrefore/9780190228620.013.925. Johansen, A. C. 1929. Om Dødeligheden blandt Marsvin, Fisk og større Krebsdyr i Farvandene omkring Danmark under strenge Vintre. Beretning til ministeriet for søfart og fiskeri. fra den danske Biol. Station 35: 60–89. Kankaanpää, Harri T., Pekka Alenius, Pekka Kotilainen, and Petra Roiha. 2023. Decreased surface and bottom salinity and elevated bottom temperature in the Northern Baltic Sea over the past six decades. The Science of the total environment 859. Elsevier BV: 160241. Keegan, William F. 2009. The Synergism of Biology and Culture. The Journal of Island and Coastal Archaeology 4. Routledge: 240–248. Kesselring, Tina, Sacha Viquerat, Ralph Brehm, and Ursula Siebert. 2017. Coming of age: - Do female harbour porpoises (Phocoena phocoena) from the North Sea and Baltic Sea have sufficient time to reproduce in a human influenced environment? PloS one 12: e0186951. Kinze, C. C. 2008. Marsvinefangsten ved Middelfart—en naturhistorisk nyfortolkning. Vends 8: 55–65. Kinze, Carl Christian. 2009. White-beaked dolphin. In Encyclopedia of Marine Mammals, 1255–1258. Elsevier. Kinze, C. C. 2011. Die Schweinswal-Treibjagd in nördlichen Kleinen Belt 1357 bis 1944. Meer und Museum 23: 83–92. Kittinger, John N., L. Blight, K. Gedan, and Loren McClenachan. 2014. Managing human legacies in a changing sea. In Marine Historical Ecology in Conservation : Applying the Past to Manage for the Future, ed. John N. Kittinger, Loren McClenachan, Keryn B. Gedan, and Louise K. Blight, 1–11. Berkerley: University of California Press. Koschinski, Sven. 2001. Current knowledge on harbour porpoises (Phocoena phocoena) in the Baltic Sea. Ophelia 55. Taylor & Francis: 167–197. Koschinski, Sven, Kylie Owen, Kristina Lehnert, and Katarzyna Kamińska. 2024. Current species protection does not serve its porpoise-Knowledge gaps on the impact of pressures on the Critically Endangered Baltic Proper harbour porpoise population, and future recommendations for its protection. Ecology and evolution 14. Wiley: e70156. Krapf, Karina, Michael Naumann, Cyril Dutheil, and H. E. Markus Meier. 2022. Investigating hypoxic and euxinic area changes based on various datasets from the Baltic Sea. Frontiers in marine science 9. Frontiers Media SA. https://doi.org/10.3389/fmars.2022.823476. Laist, David W., Amy R. Knowlton, James G. Mead, Anne S. Collet, and Michela Podesta. 2001. Collisions between ships and whales. Marine mammal science 17. Wiley: 35–75. Lameris, Thomas K., Jeroen Hoekendijk, Geert Aarts, Aline Aarts, Andrew M. Allen, Louise Bienfait, Allert I. Bijleveld, et al. 2021. Migratory vertebrates shift migration timing and distributions in a warming Arctic. Animal Migration 8. Walter de Gruyter GmbH: 110–131. Le Luherne, Emilie, Lionel Pawlowski, and Marianne Robert. 2024. Northeast Atlantic species distribution shifts over the last two decades. Global change biology 30. Wiley: e17383. Linné, Carl von, Johann Friedrich Gmelin, Robert Kerr, and J. Archer. 1792. The animal kingdom, or zoological system, of the celebrated Sir Charles Linnæus. containing a complete systematic description, arrangement, and nomenclature, of all the known species and varieties of the mammalia, or animals which give suck to their young . Edinburgh: Printed for A. Strahan, and T. Cadell, London, and W. Creech, Edinburgh. Lockyer, C. 1995. Aspects of the biology of the harbour porpoise, Phocoena phocoena, from British waters. Developments in marine biology 4. Elsevier: 443–457. Lockyer, Christina, and Carl Kinze. 2003. Status, ecology and life history of harbour porpoise ( Phocoena phocoena ), in Danish waters. NAMMCO Scientific Publications 5: 143–175. Lönnberg, E. 1940. Massdöd av tumlare, Phocaena communis. Fauna och flora . Lotze, Heike K., and Boris Worm. 2009. Historical baselines for large marine animals. Trends in ecology & evolution 24: 254–262. Lõugas, Lembi, and Valdis Bērziņš. 2023. Natural History and Exploitation of the Harbor Porpoise (Phocoena phocoena Linnaeus, 1758) during the Neolithic (ca. 4000–2000 cal. BC) in the Eastern Baltic Region. Animals 13. Multidisciplinary Digital Publishing Institute: 909. Mäkinen, K., I. Vuorinen, and J. Hänninen. 2017. Climate-induced hydrography change favours small-bodied zooplankton in a coastal ecosystem. Hydrobiologia 792. Springer Science and Business Media LLC: 83–96. Molnár, Péter K., Andrew E. Derocher, Mark A. Lewis, and Mitchell K. Taylor. 2008. Modelling the mating system of polar bears: a mechanistic approach to the Allee effect. Proceedings. Biological sciences 275. The Royal Society: 217–226. Molnár, Péter K., Mark A. Lewis, and Andrew E. Derocher. 2014. Estimating Allee dynamics before they can be observed: polar bears as a case study. PloS one 9. Public Library of Science (PLoS): e85410. Mustamäki, N., and J. Mattila. 2015. Structural changes in three coastal fish assemblages in the northern Baltic Sea archipelago. Estuarine, coastal and shelf science 164. Elsevier BV: 408–417. Nagel, Rebecca, Claire Stainfield, Cameron Fox-Clarke, Camille Toscani, Jaume Forcada, and Joseph I. Hoffman. 2021. Evidence for an Allee effect in a declining fur seal population. Proceedings. Biological sciences 288. The Royal Society: 20202882. Neimanis, Aleksija, Jasmine Stavenow, Erik Olof Ågren, Emil Wikström-Lassa, and Anna Maria Roos. 2022. Causes of death and pathological findings in stranded harbour porpoises (Phocoena phocoena) from Swedish waters. Animals: an open access journal from MDPI 12. MDPI AG: 369. Orton, David C. 2016. Archaeology as a Tool for Understanding Past Marine Resource Use and Its Impact. In Perspectives on Oceans Past, ed. Kathleen Schwerdtner Máñez and Bo Poulsen, 47–69. Dordrecht: Springer Netherlands. Owen, K., Sköld, M., & Carlström, J. (2021). An increase in detection rates of the critically endangered Baltic proper harbor porpoise in Swedish waters in recent years. Conservation Science and Practice, 3, e468. https://doi.org/10.1111/csp2.468 Österblom, Henrik, Sture Hansson, Ulf Larsson, Olle Hjerne, Fredrik Wulff, Ragnar Elmgren, and Carl Folke. 2007. Human-induced trophic cascades and ecological regime shifts in the Baltic sea. Ecosystems (New York, N.Y.) 10. Springer Science and Business Media LLC: 877–889. Pauly, D. 1995. Anecdotes and the shifting baseline syndrome of fisheries. Trends in ecology & evolution 10. cell.com: 430. Perrin, William F. 2014. Killer Whale - Orcinus orca. In Encyclopedia of marine mammals, ed. William F. Perrin, Bernd Wursig, and J. G. M. Thewissen, 2nd ed. Academic Press. Perrin, William F. 2018. Common Dolphin. In Encyclopedia of Marine Mammals, 205–209. Elsevier. Polyakov, Igor V., Kimmo Tikka, Jari Haapala, Matthew B. Alkire, Pekka Alenius, and Harri Kuosa. 2022. Depletion of oxygen in the Bothnian Sea since the mid-1950s. Frontiers in marine science 9. Frontiers Media SA. https://doi.org/10.3389/fmars.2022.917879. Psuty, I. 2013. Records of harbour porpoises (Phocoena phocoena) in fishing nets during the interwar period in Poland: Verification of archival materials. Aquatic Mammals 39. https://doi.org/10.1578/AM.39.3.2013.270. R Core Team. 2022. R: A Language and Environment for Statistical Computing. Vienna, Austria: R Foundation for Statistical Computing. Rekdahl, Melinda L., Sarah G. Trabue, Carissa D. King-Nolan, Samantha Strindberg, and Howard C. Rosenbaum. 2023. Hardly seen, often heard: acoustic presence of harbour porpoises (Phocoena phocoena) in one of the most urbanised estuaries in the world. Frontiers in marine science 10. Frontiers Media SA: 1167945. Rousi, Heta, Ari O. Laine, Heikki Peltonen, Pentti Kangas, Ann-Britt Andersin, Jouko Rissanen, Eva Sandberg-Kilpi, and Erik Bonsdorff. 2013. Long-term changes in coastal zoobenthos in the northern Baltic Sea: the role of abiotic environmental factors. ICES journal of marine science: journal du conseil 70. Oxford University Press (OUP): 440–451. Sandoval Gallardo, Santiago, Thiago Fossile, Dannieli Firme Herbst, Alpina Begossi, Luiz Geraldo Silva, and André Carlo Colonese. 2021. 150 years of anthropogenic impact on coastal and ocean ecosystems in Brazil revealed by historical newspapers. Ocean & coastal management 209. Elsevier BV: 105662. Schoeman, Renée P., Claire Patterson-Abrolat, and Stephanie Plön. 2020. A global review of vessel collisions with marine animals. Frontiers in marine science 7. Frontiers Media SA: 476648. Skóra, Krzysztof E., and Iwona Kuklik. 2003. Bycatch as a potential threat to harbour porpoises ( Phocoena phocoena ) in Polish Baltic waters. NAMMCO scientific publications 5. UiT The Arctic University of Norway: 303. Snoeijs-Leijonmalm, Pauline. 2017. Patterns of Biodiversity. PDF. In Biological oceanography of the Baltic sea, ed. Pauline Snoeijs-Leijonmalm, Hendrik Schubert, and Teresa Radziejewska, 1st ed., 123–192. Dordrecht, Netherlands: Springer. Sommer, Robert S., Juliane Pasold, and Ulrich Schmölcke. 2008. Post‐Glacial immigration of the harbour porpoise ( Phocoena phocoena ) into the Baltic Sea * . Boreas 37. Wiley: 458–464. Stafford, Kathleen, University of Washington, Edward Farley, Megan Ferguson, Kathy Kuletz, and Robert Levine. 2022. Northward range expansion of subarctic upper trophic level animals into the Pacific Arctic Region. Oceanography (Washington, D.C.) 35. The Oceanography Society: 158–166. Stedt, J., M. Wahlberg, J. Carlström, P. A. Nilsson, M. Amundin, N. Oskolkov, and P. Carlsson. 2023. Micro-scale spatial preference and temporal cyclicity linked to foraging in harbour porpoises. Mar Ecol Prog Ser 708. academia.edu: 143–161. Stepien, Emilie Nicoline, Jacob Nabe-Nielsen, Kirstin Anderson Hansen, Jakob Højer Kristensen, Marie-Anne Blanchet, Sabrina Brando, Geneviève Desportes, et al. 2023. Determination of growth, mass, and body mass index of harbour porpoises (Phocoena phocoena): Implications for conservational status assessment of populations. Global ecology and conservation 42. Elsevier BV: e02384. Stern, S. Jonathan, William Keener, Isidore D. Szczepaniak, and Marc A. Webber. 2017. Return of Harbor Porpoises (Phocoena phocoena) to San Francisco Bay. Aquatic mammals 43. Aquatic Mammals Journal: 691–702. Sveegaard, Signe, Anders Galatius, Rune Dietz, Line Kyhn, Jens C. Koblitz, Mats Amundin, Jacob Nabe-Nielsen, Mikkel-Holger S. Sinding, Liselotte W. Andersen, and Jonas Teilmann. 2015. Defining management units for cetaceans by combining genetics, morphology, acoustics and satellite tracking. Global Ecology and Conservation 3: 839–850. Sveegaard, Signe, Jonas Teilmann, Per Berggren, Kim N. Mouritsen, Douglas Gillespie, and Jakob Tougaard. 2011. Acoustic surveys confirm the high-density areas of harbour porpoises found by satellite tracking. ICES journal of marine science: journal du conseil 68. Oxford University Press (OUP): 929–936. Sveegaard, Signe, Jonas Teilmann, Jakob Tougaard, Rune Dietz, Kim N. Mouritsen, Geneviève Desportes, and Ursula Siebert. 2011. High-density areas for harbor porpoises (Phocoena phocoena) identified by satellite tracking. Marine mammal science 27. Wiley: 230–246. Tägström, B. 1940. Massdöd bland tumlarna i södra Östersjön. Svensk Fiskeritidskrift 49: 207–208. Teilmann, Jonas, Rune Dietz, and Signe Sveegaard. 2022. The use of marine waters of Skåne by harbour porpoises in time and space. DCE-Nationalt Center for Miljø og Energi, Aarhus Universitet. Thurstan, Ruth H. 2022. The potential of historical ecology to aid understanding of human-ocean interactions throughout the Anthropocene. Journal of fish biology 101. Wiley: 351–364. Van Waerebeek, K., A. N. Baker, F. Félix, J. Gedamke, M. Iñiguez, G. P. Sanino, E. Secchi, D. Sutaria, A. Van Helden, and Y. Wang. 2007. Vessel collisions with small cetaceans worldwide and with large whales in the Southern Hemisphere, an initial assessment. The Latin American journal of aquatic mammals: (LAJAM) 6. Sociedad Latinoamericana de Especialistas en Mamiferos Acuaticos (SOLAMAC): 43–69. Voss, Maren, Joachim W. Dippner, Christoph Humborg, Jens Hürdler, Frederike Korth, T. Neumann, Gerald Schernewski, and Markus Venohr. 2011. History and scenarios of future development of Baltic Sea eutrophication. Estuarine, coastal and shelf science 92. Elsevier BV: 307–322. Vuorisalo, Timo, Rauno Lahtinen, and Hannu Laaksonen. 2001. Urban biodiversity in local newspapers: a historical perspective. Biodiversity and conservation 10. Springer Nature: 1739–1756. Walter, Jonathan A., Kristine L. Grayson, and Derek M. Johnson. 2017. Variation in Allee effects: evidence, unknowns, and directions forward. Population ecology 59. Wiley: 99–107. Weigel, B., H. C. Andersson, H. M. Meier, T. Blenckner, M. Snickars, and E. Bonsdorff. 2015. Long-term progression and drivers of coastal zoobenthos in a changing system. Marine Ecology Progress Series 528: 141–159. Wellsa, R. S., and M. D. Scott. 2024. Common and Tamanend’s bottlenose dolphins Tursiops truncatus (Montagu, 1821) andT. erebennus (Cope, 1865). In Coastal dolphins and porpoises: Ridgway and Harrison’s handbook of marine mammals, volume 1, ed. Thomas Allen Jefferson, 155–256. San Diego, CA: Academic Press. Wenger, Denise, and Sven Koschinski. 2012. Harbour porpoise ( Phocoena phocoena Linnaeus, 1758) entering the Weser river after decades of absence. Marine biology research 8. Informa UK Limited: 737–745. Widerlund, Anders, and Per S. Andersson. 2011. Late Holocene freshening of the Baltic Sea derived from high-resolution strontium isotope analyses of mollusk shells. Geology 39. GeoScienceWorld: 187–190. Wiemann, Annika, Liselotte W. Andersen, Per Berggren, Ursula Siebert, Harald Benke, Jonas Teilmann, Christina Lockyer, et al. 2010. Mitochondrial Control Region and microsatellite analyses on harbour porpoise (Phocoena phocoena) unravel population differentiation in the Baltic Sea and adjacent waters. Conservation genetics 11. Springer Science and Business Media LLC: 195–211. Appendix Supplementary Table 1. Table with OIDs and RIDs. (see supplementary documents) Supplementary Table 2. Maximum and minimum weights and length for cetacean species used for making exclusion criteria, including harbour porpoise (Pp) (Christina Lockyer and Kinze 2003) and dolphin species known to visit the Baltic region: common dolphin (Dd) (Perrin 2018), bottlenose dolphin (Tt) (Wellsa and Scott 2024), white sided dolphin (La) (Kinze 2009), and neonate orcas (Oo) (Perrin 2014). | Species | Min Weight | Max Weight | Min Length | Max Length | | Pp | 89kg | 1.9m | || | Dd* | 70 kg | 230 kg | 1 m | 2.3m | | Tt | 150 kg | 200 kg | 2 m | 4 m | | La | 180 kg | 360 kg | 2.4 m | 3.1 m | | Oo** | 200 kg | 2 m | 2.5 m | Supplementary Table 3. Age-length classification categories for porpoises used to analyze historic data, adapted from the established criteria (Lockyer 1995; van Elk et al. 2019; Neimanis et al. 2022) to account for sex-specific maturation lengths in the absence of individual sex data; the male adult length (>1.29 m) was used. | Age Classification | Length Range (m) Used for Historic Harbour Porpoises | Sex Based | | | Male (m) | Female (m) | || | Neonate | ≤ 0.91 | 0.91 to ≤ 1.17 | >0.91 to ≤ 1.17 | | | Juvenile | >1.17 to ≤ 1.29 | >1.17 to ≤ 1.29 | >1.17 to ≤ 1.39 | | Adult | >1.29 | >1.29 | >1.39 | Supplementary Material File (supplementary_table_1 copy.xlsx) - Download - 149.93 KB Information & Authors Information Version history Peer review timeline Published Ecology and Evolution Version of Record11 May 2026Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection

Authors Metrics & Citations Metrics Article Usage 575views 164downloads Citations Download citation Magie Aiken, Pia Eriksson, Carl Kinze, et al. Historical Range Contraction and Extent of Harbour Porpoises (Phocoena phocoena) in the Baltic Sea Revealed by Archival Newspapers. Authorea. 10 November 2025. DOI: https://doi.org/10.22541/au.176276384.47843202/v1 DOI: https://doi.org/10.22541/au.176276384.47843202/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.