Trophic Niches of Two Congeneric Pipefishes in the Aegean Sea (Bargilya Wetland, Turkey), Based on Stable Isotope Analyses

Pipefishes (Family Syngnathidae) are representative fishes in many European coastal areas asso-ciated with vegetal assemblages and other shallow habitats. The polyandrous pipefishes Syngnathus abaster and S. acus co-occur in Metruk Tuzla (Bargilya Wetland, Aegean Sea, Turkey), a protected but threatened wetland due to anthropogenic disturbances. Both species were sam-pled during the breeding season (April-2018 and June-2019) for the assessment of growth and trophic features revealed from stable isotope profiles (δ13C and δ15N) and gut content. Both species showed positive allometric growth, with similar length-weight relationships. The isotope approach indicated that both species occupy similar trophic niches, with mean δ13C and δ15N values of -17.0 ± 1.2 ‰ for δ13C and 13.2 ± 0.6 ‰ for δ15N in S. abaster, and -17.3 ± 1.2 ‰ for δ13C and 13.3 ± 0.7 ‰ for δ15N in S. acus. The niche area for all sex types (undifferentiated specimens, males and females) in S. acus was slightly broader than in S. abaster. The simulation analysis of the trophic position (TP) in both species agreed with the isotopic assessment (TP = 3.67 ± 0.61 in S. abaster and 3.46 ± 0.66 in S. acus). Despite the high trophic resemblance between S. abaster and S. acus, the gut content assessment revealed the large contribution of copepods (especially Calanipeda aquaedulcis) to the dietary regime in both pipefishes. However, while the diet in S. abaster seemed to be almost monospecific, cladocerans were also an important resource in S. acus. Hence, we suggest that potential inter-specific competition for dietary resources would be minimized likely due to the occupancy of different microhabitats, a hypothesis that needs to be confirmed by further research.