Genetic population structure of European sprat Sprattus sprattus: differentiation across a steep environmental gradient in a small pelagic fish

Abstract

Factors such as oceanographic retention, isolation by distance and secondary contact zones have, among others, been suggested to explain the low, but statistically significant, neutral pop- ulation structure observed in many marine fishes. European sprat Sprattus sprattus L. is not known to display philopatric spawning behaviour or to exhibit local retention of eggs and larvae. It thus consti- tutes a good model for studying population structure in a characteristic small pelagic fish with high dis- persal potential and an opportunistic life history. We analysed 931 specimens of sprat from 9 spawning locations in and around the North Sea and Baltic Sea area and from a geographically distant popula- tion from the Adriatic Sea. Analyses of 9 microsatellite loci revealed a sharp genetic division separat- ing samples from the northeastern Atlantic Ocean and the Baltic Sea (pairwise θ = 0.019 to 0.035), con- curring with a steep salinity gradient. We found, at most, weak structure among samples within the northeastern Atlantic region and within the Baltic Sea (pairwise θ = 0.001 to 0.009). The Adriatic Sea population was highly differentiated from all northern samples (pairwise θ = 0.071 to 0.092). Overall, the observed population structure resembles that of most other marine fishes studied in the North and Baltic Sea areas. Nevertheless, spatially explicit differences are observed among species, probably reflecting specific life histories. Such fine-scale population structures should be taken into account when considering complex ecosystem functions, e.g. in multispecies stock management.