Where are you from? Finding the origin of the recently observed sprat in Iceland using a panel of SNPs

Where are you from? 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Finding the origin of the recently observed sprat in Iceland using a panel of SNPs María Quintela, Roger Lille-Langøy, Christophe Pampoulie, Jón Sólmundsson, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7416962/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The European sprat is a small pelagic fish characterised by genetically distinct populations including the Norwegian fjords, the Baltic Sea, the oceanic component ranging from the North Sea, Kattegat–Skagerrak, Celtic Sea and Bay of Biscay, as well as the southern groups such as the Mediterranean (Adriatic) and Black Seas. Additionally, a self-recruiting population evolved in Landvikvannet, a lake on the Norwegian coast of Skagerrak that turned brackish following artificial connection to the sea. Sprat was first reported in Icelandic waters in 2017, and in subsequent years it has become increasingly frequent and has spread along the south and west cost of the country. As the population of origin of this introduction was unknown, we used a panel of 91 SNP loci that display high genetic resolution in this species to characterize the genetic background of 64 sprat individuals collected in Icelandic waters in 2021. Analysis of Icelandic sprat, compared with existing reference data, clearly identified the oceanic component as the likely source of the introduction. While this aligns with expected colonisation routes along the Greenland-Scotland Ridge associated with range-expansion, it does not preclude an anthropogenically-driven vector, such as transport via ballast water. Population Genetics Sprattus sprattus European sprat colonization range-expansion genetic clustering Greenland-Scotland Ridge Figures Figure 1 Figure 2 Figure 3 INTRODUCTION The Arctic Ocean and adjacent seas are unique and vulnerable marine ecosystems facing anthropogenic challenges such as accelerated warming (IPCC 2007 ) – as much as four times the rate of the rest of the world oceans (Rantanen et al. 2022 )– and loss of ice cover (Stroeve et al. 2007 ; Årthun et al. 2019 ), which promote northward range-shifts of non-native sub-Artic species, and are foreseen to increase the liability of these habitats to invasive alien species. Limited functional redundancy in Arctic ecosystems involves that the loss of a single species could have dramatic and cascading effects on the polar and subpolar ecosystems’ state and function (Post et al. 2009 ). Icelandic waters, despite more moderate warming than the Arctic Ocean, have increasingly experienced the introduction and/or colonisation of non-native marine taxa, numbering 22 in 2021 (ICES 2022 ), and including phytoplankton, macroalgae, crustaceans, molluscs, tunicates and fish. Introduction vectors have been either anthropogenically-mediated transport or passive transport of plankton or planktonic stages via oceanic currents (see Hoad 2022 for review). Species such as the European flounder Platichthys flesus , the brown shrimp Crangon crangon and the Atlantic rock crab Cancer irroratus , first recorded in 1999, 2003 and 2006 respectively, have rapidly spread and can be already considered invasive (Gíslason et al. 2021 ; Henke et al. 2025 ; Thorarinsdóttir et al. 2014 ). Likewise, European sprat Sprattus sprattus (Linnaeus, 1758) – hereafter referred to as sprat– was first reported in Iceland in 2017 (Fig. 1 ), and subsequent records have been increasingly documented in research trawls with numbers escalating since 2020 (Suppl. Table S1). In addition, the presence of mature/spent individuals suggests that sprat now spawns in Icelandic waters (Hoad 2022 ; Pálsson et al. 2022 ). The sprat is a fast-growing, small, short-lived pelagic shoaling fish (Moore et al. 2019 ; Peck et al. 2012 ) that plays a crucial ecological role as prey for different piscivorous fishes, marine mammals and seabirds (ICES 2013 , 2018 ). As a batch spawner, it releases pelagic eggs near the surface over a long time period (de Silva 1973 ); eggs, that together with larvae, are passively advected by horizontal currents thus resulting in high gene flow (Glover et al. 2011 ; McKeown et al. 2020 ; Quintela et al. 2020 ). The species ranges from Morocco to northern Norway, the Baltic Sea, the northern Mediterranean basins (Adriatic Sea) and the Black Sea (Debes et al. 2008 ). Throughout most of its natural distribution, sprat sustains multiple fisheries, for many of which the International Council for the Exploration of the Sea (ICES, www.ices.dk ) provides management advice. Stock boundaries have been defined based upon Quintela et al. ( 2020 ) using a panel of 91 SNPs revealing patterns of differentiation later confirmed by full genome sequencing (Pettersson et al. 2024 ). Sprat is divided into several highly distinct and relatively homogenous genetic groups: the Norwegian fjords, the Baltic Sea, a Northeast Atlantic oceanic component from ranging from the North Sea, Kattegat–Skagerrak, Celtic Sea and the Bay of Biscay, and the southern components composed of the Adriatic and the Black Seas, respectively (Pettersson et al. 2024 ; Quintela et al. 2020 ), in addition to Landvikvannet, a once freshwater lake on the Norwegian coast of Skagerrak that became brackish following artificial connection to the sea in 1880 (Quintela et al. 2021 ). Evidence of genetic admixture, and possibly physical mixing, was also detected in the transition zone between the North Sea and the Baltic Sea, but otherwise, the aforementioned genetic groups are highly genetically distinct. Here, we aimed to elucidate the genetic origin of the recent invasion of sprat in Icelandic waters by genotyping 64 individuals collected in two different locations in 2021 and comparing them to appropriate reference data sourced from Quintela et al. ( 2020 , 2021 ). MATERIALS AND METHODS Sampling and genotyping Sprat was sampled in NW Iceland in October 2021 during an annual shrimp survey using a shrimp trawl of 40 mm mesh size in the codend. A total of 64 individuals were collected at two sampling stations located at coordinates 65°57.87N– 22°32.47W (N = 25 individuals) and 65°48.53N – 22°30.34W (N = 39 individuals), respectively. DNA was extracted from fin clips stored in ethanol using the Qiagen DNeasy 96 Blood & Tissue Kit in 96-well plates. Individuals were genotyped using the 91 SNP loci published by Quintela et al. ( 2020 ). SNP amplification and genotype calling was performed using the Sequenom MassARRAY iPLEX Platform as described by Gabriel et al. ( 2009 ). To assess the provenance of the sprat collected in Iceland, the two Icelandic samples were analysed in combination with a set of 43 reference samples of sprat (total of 2,694 individuals) from a range of locations in the Atlantic Ocean as well as the Baltic, Adriatic and Black Seas, all characterised in Quintela et al. ( 2020 ) (Fig. 2 ). The reference dataset was completed with individuals from Landvikvannet (Quintela et al. 2021 ). Genetic identification Statistical analyses were aimed at identifying the genetic provenance of the Icelandic sprat by analysing these two samples against the existing reference. Genetic structure was assessed in an unsupervised manner using Principal Component Analysis (PCA) using the function dudi.pca in ade4 (Dray, Dufour 2007 ) after replacing missing data with the mean allele frequencies, and using none scaled allele frequencies (scale = FALSE). In addition, the Bayesian clustering approach implemented in STRUCTURE v.2.3.4 (Pritchard et al. 2000 ) and conducted using the software ParallelStructure (Besnier & Glover, 2013) was used to identify genetic groups under a model assuming admixture and correlated allele frequencies without using LOCPRIORS, a burn-in period of 100,000 replications, and a run length of 1,000,000 MCMC iterations. Supervised genetic structure using geographically explicit samples was assessed through the pairwise F ST (Weir, Cockerham 1984 ) computed with Arlequin v.3.5.1.2 (Excoffier et al. 2005 ). The False Discovery Rate (FDR) correction of Benjamini, Hochberg ( 1995 ) was applied to p -values to control for Type I errors. Furthermore, the relationship among samples was examined using the Discriminant Analysis of Principal Components (DAPC) (Jombart et al. 2010 ) implemented in the R (Team 2025 ) package adegenet (Jombart 2008 ) in which groups were defined using geographically explicit locations. To avoid overfitting, both the optimal number of principal components and discriminant functions to be retained were determined using the cross-validation function (Jombart, Collins 2015 ; Miller et al. 2020 ). The individual assignment of Icelandic individuals to their potential source was conducted with the program GeneClass 2 (Piry et al. 2004 ) using the Rannala, Mountain ( 1997 ) method. The reference baseline was built by merging sampling locations into genetic units, i.e. , Atlantic Ocean, Norwegian fjords and Baltic Sea (Suppl. Table S2). RESULTS The PCA biplot aiming to frame the two Icelandic samples within the 43 reference ones revealed that Landvikvannet was singled out by axis 1, whereas axis 2 separated the Adriatic and Black Seas (Suppl. Fig. S1). The absolute lack of overlap between Icelandic samples and these three geographic locations ruled them out as putative sources of the Icelandic sprat, also supported by the outcome of DAPC, pairwise F ST and STRUCTURE (Suppl. Fig. S2–S4 and Table S2). Therefore, Landvikvannet, as well as the Adriatic and Black Seas were discarded from all analyses henceforth to gain clarity in the graphic representations. The dendrogram based upon pairwise F ST in the trimmed dataset revealed that the Icelandic sprat closely aligned within the oceanic cluster (Suppl. Fig. S5) but still formed its own branch within the oceanic samples. The DAPC aligned to this result (Suppl. Fig. S6a) as the third axis of the DAPC, accounting for 4.4% of the variation, slightly discriminated the Icelandic samples from the bulk of the oceanic ones (Suppl. Fig. S6a). Likewise, STRUCTURE barplot ratified the oceanic origin of the Icelandic sprat (Fig. 3 ). Geneclass assignment analyses were conducted using three genetic groups as baseline, i.e. Atlantic Ocean, Norwegian fjords and Baltic Sea. A total of 58 out of the 64 Icelandic sprat individuals ( i.e. 90.6%) were assigned to the Atlantic Ocean component with an assignment score ranging from 80–100% in most of them. The six remaining individuals were assigned to the Baltic Sea (N = 5) and Norwegian fjords (N = 1) with assignment scores of 45–71% and 53%, respectively. The number of individuals assigned to the oceanic component did not change when using a trimmed baseline (i.e. when retaining in the baseline only the individuals with correct self-assignment), whereas the six ones remaining were assigned to the Baltic Sea (N = 4) and Norwegian fjords (N = 2), respectively. The lack of genetic differentiation between the samples integrating the oceanic component (Suppl. Table S2) hindered any precise geographic delimitation of the putative source of the Icelandic sprat. DISCUSSION Sprat was detected in Iceland for the first time in 2017, but its putative source remained elusive. The genotyping and subsequent analysis of sprat collected in Icelandic waters alongside published data covering its natural distribution range (Quintela et al. 2020 ; Quintela et al. 2021 ), demonstrated a close alignment between Icelandic samples and the oceanic genetic component, i.e. the genetic profile displayed across the North Sea, Kattegat–Skagerrak, Celtic Sea and the Bay of Biscay. This finding largely rules out the Norwegian fjords, the Baltic Sea, Landvikvannet and the southern groups such as the Adriatic and Black Sea as potential sources of the introduction/colonisation. One of the working hypotheses about the introduction vector of the sprat in Icelandic waters is that eggs or larvae could have drifted with ocean currents from spawning grounds such as the Faroe Islands or North Sea (Pálsson et al. 2022 ). This idea gets support from the fact that the appearance of “natural” invasive species in this region, such as the Atlantic mackerel Scomber scombrus (Astthorsson et al. 2012 ) or the pink salmon Oncorhynchus gorbuscha (Eliasen, Johannesen 2021 ), mainly occurred along the Greenland-Scotland Ridge, with arrival through the Scotland–Faroe Islands mount with subsequent waves of colonization. Similarly, the European flounder, currently classified as invasive (Thorarinsdóttir et al. 2014 ), was first documented in Iceland in 1999 (Jónsson et al. 2001 ) and microsatellite analyses indicated that the Faroese population was its most likely source, thus displacing the hypothesis of introduction via ballast water from the coasts of northwestern Europe (Henke et al. 2025 ). Sprat occurs in the Faroes ecoregion (ICES 2023 ) but it has not been yet genetically characterized. In spite of this lack of genetic information, it would not be adventurous to speculate that it could match the oceanic pattern as this has been shown to cover a broad geographic range (McKeown et al. 2020 ; Quintela et al. 2020 ). Patterns of strong differentiation between Norwegian fjords, open ocean and Mediterranean Sea coupled with little or no differentiation in the oceanic component across long distances have also been observed in other taxa such as mesopelagic fish (Quintela et al. 2024 ; Quintela et al. 2025 ). However, an introduction driven by an anthropogenic vector cannot be dismissed. In the last three decades, translocation events through released ballast water have been suggested to be the origin of the introduction of several new species that seem to thrive along the Greenland–Scotland Ridge; some of which arrived at larval stage (see Pampoulie et al. 2024 for review). Icelandic waters are now home to species such as the brown shrimp Crangon crangon (Jónsdóttir et al. 2016 ), the Atlantic rock crab Cancer irroratus (Jónsdóttir et al. 2016 ; Magnússon et al. 2024 ), the Newfoundland's razor clam Ensis terranovensis (Gunnarsson et al. 2023 ) and the tunicate Ciona intestinalis , first observed in 2007 and thought to have been originally transported as fouling organisms from the hull of vessels (Thorarinsdóttir et al. 2014 ). The known 10 days’ survival of sprat in ballast water further supports the viability of the hypothesis of an anthropogenically-driven introduction (Wonham et al. 2000 and references therein). Whereas there is no evidence to reject that Icelandic sprat belongs to the oceanic cluster, the limited genetic differentiation observed among samples within it, both using genetic and genomic tools (McKeown et al. 2020 ; Pettersson et al. 2024 ; Quintela et al. 2020 ), hinders a more precise geographical demarcation of the origin of Icelandic sprat. However, a significant differentiation was detected between the recently genotyped Icelandic samples and the remaining oceanic ones (Suppl. Table S2). The temporal aspect does not seem to be a plausible explanation since reference samples span between 2006 and 2018 whereas the Icelandic samples were collected in 2021. In addition, the combined effect of founder effect and drift in the Icelandic samples might have led to this differentiation. Whereas the sparse set of SNPs used here might not be the best tool to investigate this issue, it must be mentioned that F ST per locus in the oceanic cluster became significant for 8% of the markers when including the Icelandic samples in the analysis, which might provide some indication of genetic drift acting on the smaller (in size) Icelandic population. Likewise, it has been shown that this species was able to colonize and develop a genetically highly distinct population in a brackish lake within few decades (Quintela et al. 2021 ). While a draft-genome assembly is available for sprat and revealed the potential presence of inversions (Pettersson et al. 2024 ) distinguishing oceanic, coastal and brackish populations, the current genomic knowledge did not permit the identification of the “Oceanic” population from which sprat in Iceland originates. Further analyses of potential outliers loci and inversions might bring more light into the structure of sprat in the North Atlantic Ocean, and about the origin of Icelandic sprat. In addition, the genetic characterization of additional populations such as sprat from Faroe region could confirm the hypothesis of Faroese sprat belonging to the Atlantic Ocean cluster and, eventually, further support its arrival to Iceland via the Greenland-Scotland Ridge. Furthermore, it is highly recommended to obtain a full reference genome for each putative population to fully fathom genomic structure of marine species when differentiation is low (Thorburn et al. 2023 ). Declarations Sprat is a commercial species that sustains an important fishery in European waters. The individuals from Iceland have been sampled in a scientific survey conducted by an official research centre and thus following all the national regulations that apply. AUTHORS’ STATEMENTS The study was primarily funded by the Norwegian Department of Trade and Fisheries. This manuscript has not been submitted elsewhere before. Authors contributed to the text, agreed with its content and approved it for submission. No competing interest exists and there is no financial support or relationships that may pose any kind of conflict. All research met the ethical guidelines of the study countries. AUTHORS’ CONTRIBUTIONS The study conception and design were performed by MQ, CP, JS, KAG, FB and CK. Funding acquisition was performed by CK. Data collection was performed by CP and JS. Laboratory work was performed by RL-L and FA. Data analyses were performed by MQ and all authors contributed to the interpretation of the results. Fig. 1 was produced by FB, and Fig. 2 was produced by JS. The first draft of the manuscript was written by MQ and all authors commented on previous versions. All authors read and approved the final document. DATA AVAILABILITY The genotype raw data of the Icelandic sprat used in this study can be publicly accessed from the electronic archive of the Institute of Marine Research at https://hdl.handle.net/11250/3212171. ACKNOWLEDGEMENTS This study was primarily funded by the Norwegian Department of Trade and Fisheries. Special thanks are addressed to Ingibjörg Jónsdóttir and Nick Hoad References Astthorsson OS, Valdimarsson H, Gudmundsdottir A, et al. (2012) Climate-Related Variations in the Occurrence and Distribution of Mackerel ( Scomber scombrus ) in Icelandic Waters. ICES Journal of Marine Science 69:1289–1297 Benjamini Y, Hochberg Y (1995) Controlling the False Discovery Rate: A practical and powerful approach to multiple testing. Journal of the Royal Statistical Society: Series B (Methodological) 57:289–300 de Silva SS (1973) Aspects of the reproductive biology of the sprat, Sprattus sprattus (L.) in inshore waters of the west coast of Scotland. 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(2000) Fish and ships: Relating dispersal frequency to success in biological invasions. Marine Biology 136:1111-1121 Årthun M, Eldevik T, Smedsrud LH (2019) The Role of Atlantic Heat Transport in Future Arctic Winter Sea Ice Loss. Journal of Climate 32:3327–3341 Additional Declarations The authors declare no competing interests. Supplementary Files IcelandspratSupplement.docx IcelandspratSuppl.TableS2.xlsx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7416962","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Short Report","associatedPublications":[],"authors":[{"id":503091679,"identity":"c9c2a2e2-3d42-4e8d-b7b2-2dacfcbbc992","order_by":0,"name":"María Quintela","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0003-4762-2192","institution":"Institute of Marine Research Bergen","correspondingAuthor":true,"prefix":"","firstName":"María","middleName":"","lastName":"Quintela","suffix":""},{"id":503091680,"identity":"94780c21-bbb8-4d33-b076-f4f37c1661e1","order_by":1,"name":"Roger Lille-Langøy","email":"","orcid":"https://orcid.org/0000-0002-8010-8542","institution":"Institute of Marine Research Bergen","correspondingAuthor":false,"prefix":"","firstName":"Roger","middleName":"","lastName":"Lille-Langøy","suffix":""},{"id":503091681,"identity":"81566f2b-f5f5-4bc2-991a-3c30c740372f","order_by":2,"name":"Christophe Pampoulie","email":"","orcid":"https://orcid.org/0000-0001-6425-9060","institution":"Marine and Freshwater Research Institute, Hafnarfjörður, Iceland","correspondingAuthor":false,"prefix":"","firstName":"Christophe","middleName":"","lastName":"Pampoulie","suffix":""},{"id":503091682,"identity":"b1eca424-038c-49d2-8d62-4cdc3430c17c","order_by":3,"name":"Jón Sólmundsson","email":"","orcid":"https://orcid.org/0000-0002-2685-777X","institution":"Marine and Freshwater Research Institute, Hafnarfjörður, Iceland","correspondingAuthor":false,"prefix":"","firstName":"Jón","middleName":"","lastName":"Sólmundsson","suffix":""},{"id":503091683,"identity":"31665ac3-54f0-4078-8a98-ccbf363a380b","order_by":4,"name":"Fernando Ayllon","email":"","orcid":"https://orcid.org/0009-0005-6051-7348","institution":"Institute of Marine Research Bergen","correspondingAuthor":false,"prefix":"","firstName":"Fernando","middleName":"","lastName":"Ayllon","suffix":""},{"id":503091684,"identity":"f27267ea-dd13-4fca-b95d-334b01bbb697","order_by":5,"name":"Kevin A. Glover","email":"","orcid":"https://orcid.org/0000-0002-7541-1299","institution":"Institute of Marine Research Bergen","correspondingAuthor":false,"prefix":"","firstName":"Kevin","middleName":"A.","lastName":"Glover","suffix":""},{"id":503091685,"identity":"08b1314d-62a3-448e-b412-86dbfb88ecbe","order_by":6,"name":"Florian Berg","email":"","orcid":"https://orcid.org/0000-0003-1543-8112","institution":"Institute of Marine Research Bergen","correspondingAuthor":false,"prefix":"","firstName":"Florian","middleName":"","lastName":"Berg","suffix":""},{"id":503091686,"identity":"e305b0b1-0750-47c9-b24d-2d3eb396dcb5","order_by":7,"name":"Cecilie Kvamme","email":"","orcid":"","institution":"Institute of Marine Research Bergen","correspondingAuthor":false,"prefix":"","firstName":"Cecilie","middleName":"","lastName":"Kvamme","suffix":""}],"badges":[],"createdAt":"2025-08-20 11:30:43","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-7416962/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7416962/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":89894641,"identity":"c63d9407-7e5e-438d-8ee7-64ef8cef2c5c","added_by":"auto","created_at":"2025-08-26 08:17:44","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":177885,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal and spatial distribution of sprat in Iceland. The data mostly originate from annual standardized trawl surveys for groundfish and shrimp, conducted in March and October. The groundfish surveys cover the whole continental shelf, but sprat has not yet been found in the colder waters north and east of Iceland.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7416962/v1/8ccf6f715257e1dc10439374.png"},{"id":89894649,"identity":"9a998ad0-05c6-456c-a249-eb3d36417050","added_by":"auto","created_at":"2025-08-26 08:17:44","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":258575,"visible":true,"origin":"","legend":"\u003cp\u003eSampling locations of sprat including two sites in Iceland alongside the reference samples published in Quintela et al. (2020, 2021). Colours depict the genetic clusters known today.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7416962/v1/8643383358b57468700a6dc1.png"},{"id":89894642,"identity":"e6a66a5a-1d85-4100-896d-6fd4e5220107","added_by":"auto","created_at":"2025-08-26 08:17:44","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":131731,"visible":true,"origin":"","legend":"\u003cp\u003eProportion of individuals’ ancestry to clusters at K =3 after Bayesian cluster analyses in STRUCTURE assessed from the set of 91 SNP loci to put Icelandic sprat (ICE1, ICE2) in context with the reference samples published in Quintela et al. (2020). Colours depict the three main clusters of sprat in the NE Atlantic: orange represents the Oceanic cluster, green the Norwegian fjords and blue the Baltic Sea. Samples from UV, GB and OES are located in the transition zone between the North and Baltic Seas.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7416962/v1/1861114af7148673edc33194.png"},{"id":89895797,"identity":"3b274d51-8e73-405f-bb02-b0b776593175","added_by":"auto","created_at":"2025-08-26 08:25:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":909062,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7416962/v1/b64df6ba-3988-4eba-b34b-6f47db533431.pdf"},{"id":89895793,"identity":"df8b8754-83e8-4ffb-b2dd-f495dae5ad7b","added_by":"auto","created_at":"2025-08-26 08:25:44","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2837857,"visible":true,"origin":"","legend":"","description":"","filename":"IcelandspratSupplement.docx","url":"https://assets-eu.researchsquare.com/files/rs-7416962/v1/413764540e5d3bc2bc890043.docx"},{"id":89894644,"identity":"c2c03719-11b1-4763-a304-38b4707a2fc8","added_by":"auto","created_at":"2025-08-26 08:17:44","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":26581,"visible":true,"origin":"","legend":"","description":"","filename":"IcelandspratSuppl.TableS2.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7416962/v1/ed2f91f07127d2a435cc6437.xlsx"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eWhere are you from? Finding the origin of the recently observed sprat in Iceland using a panel of SNPs\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eThe Arctic Ocean and adjacent seas are unique and vulnerable marine ecosystems facing anthropogenic challenges such as accelerated warming (IPCC \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) \u0026ndash; as much as four times the rate of the rest of the world oceans (Rantanen et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u0026ndash; and loss of ice cover (Stroeve et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; \u0026Aring;rthun et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), which promote northward range-shifts of non-native sub-Artic species, and are foreseen to increase the liability of these habitats to invasive alien species. Limited functional redundancy in Arctic ecosystems involves that the loss of a single species could have dramatic and cascading effects on the polar and subpolar ecosystems\u0026rsquo; state and function (Post et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIcelandic waters, despite more moderate warming than the Arctic Ocean, have increasingly experienced the introduction and/or colonisation of non-native marine taxa, numbering 22 in 2021 (ICES \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), and including phytoplankton, macroalgae, crustaceans, molluscs, tunicates and fish. Introduction vectors have been either anthropogenically-mediated transport or passive transport of plankton or planktonic stages via oceanic currents (see Hoad \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2022\u003c/span\u003e for review). Species such as the European flounder \u003cem\u003ePlatichthys flesus\u003c/em\u003e, the brown shrimp \u003cem\u003eCrangon crangon\u003c/em\u003e and the Atlantic rock crab \u003cem\u003eCancer irroratus\u003c/em\u003e, first recorded in 1999, 2003 and 2006 respectively, have rapidly spread and can be already considered invasive (G\u0026iacute;slason et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Henke et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Thorarinsd\u0026oacute;ttir et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Likewise, European sprat \u003cem\u003eSprattus sprattus\u003c/em\u003e (Linnaeus, 1758) \u0026ndash; hereafter referred to as sprat\u0026ndash; was first reported in Iceland in 2017 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), and subsequent records have been increasingly documented in research trawls with numbers escalating since 2020 (Suppl. Table S1). In addition, the presence of mature/spent individuals suggests that sprat now spawns in Icelandic waters (Hoad \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; P\u0026aacute;lsson et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe sprat is a fast-growing, small, short-lived pelagic shoaling fish (Moore et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Peck et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) that plays a crucial ecological role as prey for different piscivorous fishes, marine mammals and seabirds (ICES \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2013\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). As a batch spawner, it releases pelagic eggs near the surface over a long time period (de Silva \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1973\u003c/span\u003e); eggs, that together with larvae, are passively advected by horizontal currents thus resulting in high gene flow (Glover et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; McKeown et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Quintela et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The species ranges from Morocco to northern Norway, the Baltic Sea, the northern Mediterranean basins (Adriatic Sea) and the Black Sea (Debes et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Throughout most of its natural distribution, sprat sustains multiple fisheries, for many of which the International Council for the Exploration of the Sea (ICES, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u003ca href=\"https://orcid.org/0000-0003-4762-2192\" target=\"_blank\"\u003ewww.ices.dk\u003c/a\u003e\u003c/span\u003e\u003cspan address=\"http://www.ices.dk\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) provides management advice. Stock boundaries have been defined based upon Quintela et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) using a panel of 91 SNPs revealing patterns of differentiation later confirmed by full genome sequencing (Pettersson et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Sprat is divided into several highly distinct and relatively homogenous genetic groups: the Norwegian fjords, the Baltic Sea, a Northeast Atlantic oceanic component from ranging from the North Sea, Kattegat\u0026ndash;Skagerrak, Celtic Sea and the Bay of Biscay, and the southern components composed of the Adriatic and the Black Seas, respectively (Pettersson et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Quintela et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), in addition to Landvikvannet, a once freshwater lake on the Norwegian coast of Skagerrak that became brackish following artificial connection to the sea in 1880 (Quintela et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Evidence of genetic admixture, and possibly physical mixing, was also detected in the transition zone between the North Sea and the Baltic Sea, but otherwise, the aforementioned genetic groups are highly genetically distinct.\u003c/p\u003e\u003cp\u003eHere, we aimed to elucidate the genetic origin of the recent invasion of sprat in Icelandic waters by genotyping 64 individuals collected in two different locations in 2021 and comparing them to appropriate reference data sourced from Quintela et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eSampling and genotyping\u003c/h2\u003e\u003cp\u003eSprat was sampled in NW Iceland in October 2021 during an annual shrimp survey using a shrimp trawl of 40 mm mesh size in the codend. A total of 64 individuals were collected at two sampling stations located at coordinates 65\u0026deg;57.87N\u0026ndash; 22\u0026deg;32.47W (N\u0026thinsp;=\u0026thinsp;25 individuals) and 65\u0026deg;48.53N \u0026ndash; 22\u0026deg;30.34W (N\u0026thinsp;=\u0026thinsp;39 individuals), respectively. DNA was extracted from fin clips stored in ethanol using the Qiagen DNeasy 96 Blood \u0026amp; Tissue Kit in 96-well plates. Individuals were genotyped using the 91 SNP loci published by Quintela et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). SNP amplification and genotype calling was performed using the Sequenom MassARRAY iPLEX Platform as described by Gabriel et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTo assess the provenance of the sprat collected in Iceland, the two Icelandic samples were analysed in combination with a set of 43 reference samples of sprat (total of 2,694 individuals) from a range of locations in the Atlantic Ocean as well as the Baltic, Adriatic and Black Seas, all characterised in Quintela et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The reference dataset was completed with individuals from Landvikvannet (Quintela et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eGenetic identification\u003c/h3\u003e\n\u003cp\u003eStatistical analyses were aimed at identifying the genetic provenance of the Icelandic sprat by analysing these two samples against the existing reference. Genetic structure was assessed in an unsupervised manner using Principal Component Analysis (PCA) using the function \u003cem\u003edudi.pca\u003c/em\u003e in \u003cem\u003eade4\u003c/em\u003e (Dray, Dufour \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) after replacing missing data with the mean allele frequencies, and using none scaled allele frequencies (scale\u0026thinsp;=\u0026thinsp;FALSE). In addition, the Bayesian clustering approach implemented in STRUCTURE v.2.3.4 (Pritchard et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) and conducted using the software ParallelStructure (Besnier \u0026amp; Glover, 2013) was used to identify genetic groups under a model assuming admixture and correlated allele frequencies without using LOCPRIORS, a burn-in period of 100,000 replications, and a run length of 1,000,000 MCMC iterations.\u003c/p\u003e\u003cp\u003eSupervised genetic structure using geographically explicit samples was assessed through the pairwise \u003cem\u003eF\u003c/em\u003e\u003csub\u003eST\u003c/sub\u003e (Weir, Cockerham \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e1984\u003c/span\u003e) computed with Arlequin v.3.5.1.2 (Excoffier et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The False Discovery Rate (FDR) correction of Benjamini, Hochberg (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1995\u003c/span\u003e) was applied to \u003cem\u003ep\u003c/em\u003e-values to control for Type I errors. Furthermore, the relationship among samples was examined using the Discriminant Analysis of Principal Components (DAPC) (Jombart et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) implemented in the R (Team \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) package \u003cem\u003eadegenet\u003c/em\u003e (Jombart \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) in which groups were defined using geographically explicit locations. To avoid overfitting, both the optimal number of principal components and discriminant functions to be retained were determined using the cross-validation function (Jombart, Collins \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Miller et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe individual assignment of Icelandic individuals to their potential source was conducted with the program GeneClass 2 (Piry et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) using the Rannala, Mountain (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) method. The reference baseline was built by merging sampling locations into genetic units, \u003cem\u003ei.e.\u003c/em\u003e, Atlantic Ocean, Norwegian fjords and Baltic Sea (Suppl. Table S2).\u003c/p\u003e"},{"header":"RESULTS","content":"\u003cp\u003eThe PCA biplot aiming to frame the two Icelandic samples within the 43 reference ones revealed that Landvikvannet was singled out by axis 1, whereas axis 2 separated the Adriatic and Black Seas (Suppl. Fig. S1). The absolute lack of overlap between Icelandic samples and these three geographic locations ruled them out as putative sources of the Icelandic sprat, also supported by the outcome of DAPC, pairwise \u003cem\u003eF\u003c/em\u003e\u003csub\u003eST\u003c/sub\u003e and STRUCTURE (Suppl. Fig. S2\u0026ndash;S4 and Table S2). Therefore, Landvikvannet, as well as the Adriatic and Black Seas were discarded from all analyses henceforth to gain clarity in the graphic representations.\u003c/p\u003e\u003cp\u003eThe dendrogram based upon pairwise \u003cem\u003eF\u003c/em\u003e\u003csub\u003eST\u003c/sub\u003e in the trimmed dataset revealed that the Icelandic sprat closely aligned within the oceanic cluster (Suppl. Fig. S5) but still formed its own branch within the oceanic samples. The DAPC aligned to this result (Suppl. Fig. S6a) as the third axis of the DAPC, accounting for 4.4% of the variation, slightly discriminated the Icelandic samples from the bulk of the oceanic ones (Suppl. Fig. S6a). Likewise, STRUCTURE barplot ratified the oceanic origin of the Icelandic sprat (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eGeneclass assignment analyses were conducted using three genetic groups as baseline, \u003cem\u003ei.e.\u003c/em\u003e Atlantic Ocean, Norwegian fjords and Baltic Sea. A total of 58 out of the 64 Icelandic sprat individuals (\u003cem\u003ei.e.\u003c/em\u003e 90.6%) were assigned to the Atlantic Ocean component with an assignment score ranging from 80\u0026ndash;100% in most of them. The six remaining individuals were assigned to the Baltic Sea (N\u0026thinsp;=\u0026thinsp;5) and Norwegian fjords (N\u0026thinsp;=\u0026thinsp;1) with assignment scores of 45\u0026ndash;71% and 53%, respectively. The number of individuals assigned to the oceanic component did not change when using a trimmed baseline \u003cem\u003e(i.e.\u003c/em\u003e when retaining in the baseline only the individuals with correct self-assignment), whereas the six ones remaining were assigned to the Baltic Sea (N\u0026thinsp;=\u0026thinsp;4) and Norwegian fjords (N\u0026thinsp;=\u0026thinsp;2), respectively. The lack of genetic differentiation between the samples integrating the oceanic component (Suppl. Table S2) hindered any precise geographic delimitation of the putative source of the Icelandic sprat.\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eSprat was detected in Iceland for the first time in 2017, but its putative source remained elusive. The genotyping and subsequent analysis of sprat collected in Icelandic waters alongside published data covering its natural distribution range (Quintela et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Quintela et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), demonstrated a close alignment between Icelandic samples and the oceanic genetic component, \u003cem\u003ei.e.\u003c/em\u003e the genetic profile displayed across the North Sea, Kattegat\u0026ndash;Skagerrak, Celtic Sea and the Bay of Biscay. This finding largely rules out the Norwegian fjords, the Baltic Sea, Landvikvannet and the southern groups such as the Adriatic and Black Sea as potential sources of the introduction/colonisation.\u003c/p\u003e\u003cp\u003eOne of the working hypotheses about the introduction vector of the sprat in Icelandic waters is that eggs or larvae could have drifted with ocean currents from spawning grounds such as the Faroe Islands or North Sea (P\u0026aacute;lsson et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). This idea gets support from the fact that the appearance of \u0026ldquo;natural\u0026rdquo; invasive species in this region, such as the Atlantic mackerel \u003cem\u003eScomber scombrus\u003c/em\u003e (Astthorsson et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) or the pink salmon \u003cem\u003eOncorhynchus gorbuscha\u003c/em\u003e (Eliasen, Johannesen \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), mainly occurred along the Greenland-Scotland Ridge, with arrival through the Scotland\u0026ndash;Faroe Islands mount with subsequent waves of colonization. Similarly, the European flounder, currently classified as invasive (Thorarinsd\u0026oacute;ttir et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), was first documented in Iceland in 1999 (J\u0026oacute;nsson et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) and microsatellite analyses indicated that the Faroese population was its most likely source, thus displacing the hypothesis of introduction via ballast water from the coasts of northwestern Europe (Henke et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Sprat occurs in the Faroes ecoregion (ICES \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) but it has not been yet genetically characterized. In spite of this lack of genetic information, it would not be adventurous to speculate that it could match the oceanic pattern as this has been shown to cover a broad geographic range (McKeown et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Quintela et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Patterns of strong differentiation between Norwegian fjords, open ocean and Mediterranean Sea coupled with little or no differentiation in the oceanic component across long distances have also been observed in other taxa such as mesopelagic fish (Quintela et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Quintela et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eHowever, an introduction driven by an anthropogenic vector cannot be dismissed. In the last three decades, translocation events through released ballast water have been suggested to be the origin of the introduction of several new species that seem to thrive along the Greenland\u0026ndash;Scotland Ridge; some of which arrived at larval stage (see Pampoulie et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2024\u003c/span\u003e for review). Icelandic waters are now home to species such as the brown shrimp \u003cem\u003eCrangon crangon\u003c/em\u003e (J\u0026oacute;nsd\u0026oacute;ttir et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), the Atlantic rock crab \u003cem\u003eCancer irroratus\u003c/em\u003e (J\u0026oacute;nsd\u0026oacute;ttir et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Magn\u0026uacute;sson et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), the Newfoundland's razor clam \u003cem\u003eEnsis terranovensis\u003c/em\u003e (Gunnarsson et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) and the tunicate \u003cem\u003eCiona intestinalis\u003c/em\u003e, first observed in 2007 and thought to have been originally transported as fouling organisms from the hull of vessels (Thorarinsd\u0026oacute;ttir et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The known 10 days\u0026rsquo; survival of sprat in ballast water further supports the viability of the hypothesis of an anthropogenically-driven introduction (Wonham et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2000\u003c/span\u003e and references therein).\u003c/p\u003e\u003cp\u003eWhereas there is no evidence to reject that Icelandic sprat belongs to the oceanic cluster, the limited genetic differentiation observed among samples within it, both using genetic and genomic tools (McKeown et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Pettersson et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Quintela et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), hinders a more precise geographical demarcation of the origin of Icelandic sprat. However, a significant differentiation was detected between the recently genotyped Icelandic samples and the remaining oceanic ones (Suppl. Table S2). The temporal aspect does not seem to be a plausible explanation since reference samples span between 2006 and 2018 whereas the Icelandic samples were collected in 2021. In addition, the combined effect of founder effect and drift in the Icelandic samples might have led to this differentiation. Whereas the sparse set of SNPs used here might not be the best tool to investigate this issue, it must be mentioned that \u003cem\u003eF\u003c/em\u003e\u003csub\u003eST\u003c/sub\u003e per locus in the oceanic cluster became significant for 8% of the markers when including the Icelandic samples in the analysis, which might provide some indication of genetic drift acting on the smaller (in size) Icelandic population. Likewise, it has been shown that this species was able to colonize and develop a genetically highly distinct population in a brackish lake within few decades (Quintela et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eWhile a draft-genome assembly is available for sprat and revealed the potential presence of inversions (Pettersson et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) distinguishing oceanic, coastal and brackish populations, the current genomic knowledge did not permit the identification of the \u0026ldquo;Oceanic\u0026rdquo; population from which sprat in Iceland originates. Further analyses of potential outliers loci and inversions might bring more light into the structure of sprat in the North Atlantic Ocean, and about the origin of Icelandic sprat. In addition, the genetic characterization of additional populations such as sprat from Faroe region could confirm the hypothesis of Faroese sprat belonging to the Atlantic Ocean cluster and, eventually, further support its arrival to Iceland via the Greenland-Scotland Ridge. Furthermore, it is highly recommended to obtain a full reference genome for each putative population to fully fathom genomic structure of marine species when differentiation is low (Thorburn et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eSprat is a commercial species that sustains an important fishery in European waters. The individuals from Iceland have been sampled in a scientific survey conducted by an official research centre and thus following all the national regulations that apply.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHORS\u0026rsquo; STATEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study was primarily funded by the Norwegian Department of Trade and Fisheries. This manuscript has not been submitted elsewhere before. Authors contributed to the text, agreed with its content and approved it for submission. No competing interest exists and there is no financial support or relationships that may pose any kind of conflict. All research met the ethical guidelines of the study countries.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHORS\u0026rsquo; CONTRIBUTIONS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study conception and design were performed by MQ, CP, JS, KAG, FB and CK. Funding acquisition was performed by CK. Data collection was performed by CP and JS. Laboratory work was performed by RL-L and FA. Data analyses were performed by MQ and all authors contributed to the interpretation of the results. Fig. 1 was produced by FB, and Fig. 2 was produced by JS. The first draft of the manuscript was written by MQ and all authors commented on previous versions. All authors read and approved the final document.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDATA AVAILABILITY\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe genotype raw data of the Icelandic sprat used in this study can be publicly accessed from the electronic archive of the Institute of Marine Research at https://hdl.handle.net/11250/3212171.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was primarily funded by the Norwegian Department of Trade and Fisheries. Special thanks are addressed to Ingibj\u0026ouml;rg J\u0026oacute;nsd\u0026oacute;ttir and Nick Hoad\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAstthorsson OS, Valdimarsson H, Gudmundsdottir A, et al. (2012) Climate-Related Variations in the Occurrence and Distribution of Mackerel (\u003cem\u003eScomber scombrus\u003c/em\u003e) in Icelandic Waters. ICES Journal of Marine Science 69:1289\u0026ndash;1297\u003c/li\u003e\n\u003cli\u003eBenjamini Y, Hochberg Y (1995) Controlling the False Discovery Rate: A practical and powerful approach to multiple testing. Journal of the Royal Statistical Society: Series B (Methodological) 57:289\u0026ndash;300\u003c/li\u003e\n\u003cli\u003ede Silva SS (1973) Aspects of the reproductive biology of the sprat, \u003cem\u003eSprattus sprattus\u003c/em\u003e (L.) in inshore waters of the west coast of Scotland. 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Journal of Climate 32:3327\u0026ndash;3341\u003cstrong\u003e\u003cbr\u003e \u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Sprattus sprattus, European sprat, colonization, range-expansion, genetic clustering, Greenland-Scotland Ridge","lastPublishedDoi":"10.21203/rs.3.rs-7416962/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7416962/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe European sprat is a small pelagic fish characterised by genetically distinct populations including the Norwegian fjords, the Baltic Sea, the oceanic component ranging from the North Sea, Kattegat\u0026ndash;Skagerrak, Celtic Sea and Bay of Biscay, as well as the southern groups such as the Mediterranean (Adriatic) and Black Seas. Additionally, a self-recruiting population evolved in Landvikvannet, a lake on the Norwegian coast of Skagerrak that turned brackish following artificial connection to the sea. Sprat was first reported in Icelandic waters in 2017, and in subsequent years it has become increasingly frequent and has spread along the south and west cost of the country. As the population of origin of this introduction was unknown, we used a panel of 91 SNP loci that display high genetic resolution in this species to characterize the genetic background of 64 sprat individuals collected in Icelandic waters in 2021. Analysis of Icelandic sprat, compared with existing reference data, clearly identified the oceanic component as the likely source of the introduction. While this aligns with expected colonisation routes along the Greenland-Scotland Ridge associated with range-expansion, it does not preclude an anthropogenically-driven vector, such as transport via ballast water.\u003c/p\u003e","manuscriptTitle":"Where are you from? Finding the origin of the recently observed sprat in Iceland using a panel of SNPs","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-26 08:17:39","doi":"10.21203/rs.3.rs-7416962/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6f8a81a2-f956-43ba-9074-061416f8a82f","owner":[],"postedDate":"August 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":53444009,"name":"Population Genetics"}],"tags":[],"updatedAt":"2025-08-26T08:17:39+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-26 08:17:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7416962","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7416962","identity":"rs-7416962","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}