Morphological divergence of three-spined stickleback in the Baltic Sea-Implications for stock identification

Morphometrics is a beneficial, straightforward and relatively simple tool for fish stock delineation and identification. Analysis of spatial divergence, and morphological traits in relation to other factors, are of high interest both for management and in ecological studies. Morphology can help to delineate stocks, inform about local adaptations as well as ecological role the species play in the ecosystem. Morphological variation and differentiation of three-spined stickleback (Gasterosteus aculeatus) has been thoroughly studied in small freshwater systems, but less is known about differentiation of the species in an open marine system. The Baltic Sea inherits substantial environmental gradients in salinity, temperature, as well as in the nature and intensity of predation, gradients that all can foster local morphological adaptation and potential population differentiation. Previous studies have revealed some morphological differentiation of the species along the coast, but studies from the offshore are lacking. In this study we used traditional as well as geometric morphometrics to analyse divergence in morphological traits like body plate numbers, body- and otolith shape among sticklebacks from two major genetic clusters inhabiting different parts of the Baltic Sea. Our results show that fish from one area (Curonian Lagoon) representing one of the clusters had significantly higher number of body plates, potentially indicating a response to higher predation pressure compared to the other areas studied. Body shape also significantly differed among locations with deeper bodied fish in the Curonian Lagoon. The most conservative and least plastic trait studied, otolith shape, did, however, not show any divergence among the areas studied. Our results suggest morphological divergence in plastic traits like body plates and body shape in response to local environmental conditions. The lack of divergence in otolith shape further suggests that the degree of population differentiation is weak or rather recent, as also highlighted in earlier studies using molecular markers or synchrony in population abundances. In all, our study shows that at least the number of body plates can serve as an effective stock delineator among genetic clusters of three-spined sticklebacks in the Baltic Sea.