In recent decades conservation measures for the rarest freshwater fish in the U.K., the vendace (Coregonus albula), have included attempts to form conservation refuge populations in Scotland. Here we report that at two of these refuge sites where the status of the introduced vendace was previously unknown (Loch Earn and Daer Reservoir) surveys have established that vendace are reproducing successfully in situ, albeit that these populations appear to be relatively numerically small. At a third site, Loch Valley, there was no evidence of vendace in the survey, but it is possible that a small, but as yet undetectable population is in the process of establishing.

The fragmented, heterogeneous and relatively depauperate ecosystems of recently glaciated lakes present contrasting ecological opportunities for resident fish. Across a species, local adaptation may induce diverse and distinct phenotypic responses to various selection pressures. We tested for intraspecific phenotypic structuring by population in a common native lake-dwelling fish species across a medium-scale geographic region with considerable variation in lake types. We investigated potential lake-characteristic drivers of trophic morphology. Using geometric morphometric techniques, we quantified the head shapes of 759 adult brown trout (Salmo trutta L.) from 28 lakes and reservoirs across Scotland. Multivariate statistical analyses showed that almost all populations differed from one another. Trout from larger and deeper lakes had deeper, but shorter heads, and smaller eyes. Higher elevation lakes were associated with fish with shorter heads and jaws. Within-population shape variation also differed by population, and was positively correlated with lake surface area and depth. Trout within reservoirs differed subtly from those in natural lakes, having larger eyes, shorter jaws and greater variability. This study documents an extraordinary morphological variation between and within populations of brown trout, and demonstrates the role of the extrinsic environment in driving phenotypic structuring over a medium-scale and varied geographic area.

As a conservation measure to protect European whitefish in Scotland, a translocated population was established in Loch Sloy from Loch Lomond stock between 1988 and 1990. Previous study has assumed that current morphological differences between adults from the donor and refuge lakes have arisen through phenotypic plasticity. The present study compared the morphologies of whitefish at three life stages: alevins and fry raised in a common garden, and wild-caught adults. Alevins were clearly distinguishable by their lake of origin. Loch Sloy alevins were distinguishable also by family, although this was not the case for Loch Lomond. Differential allometric trajectories facilitated the persistence of morphological differences associated with lake of origin through the fry stage into adulthood. Overall, the whitefish from Loch Lomond displayed morphologies associated with pelagic feeders, while the more robust heads and ventrally positioned snouts of the Loch Sloy whitefish conformed to expectations for more benthic feeding habits. That differences between populations were present not only in wild adults, but also in alevins and fry from a common garden setup, strongly suggests that the divergence between populations is due to inheritance mechanisms, rather than differential plastic responses, and questions the effectiveness of translocation as a conservation measure.

It has been suggested that a trade‐off between cognitive capacity and developmental costs may drive brain size and morphology across fish species, but this pattern is less well explored at the intraspecific level. Physical habitat complexity has been proposed as a key selection pressure on cognitive capacity that shapes brain morphology of fishes. In this study, we compared brain morphology of brown trout, Salmo trutta , from stream, lake, and hatchery environments, which generally differ in physical complexity ranging from low habitat complexity in the hatchery to high habitat complexity in streams and intermediate complexity in lakes. We found that brain size, and the size of optic tectum and telencephalon differed across the three habitats, both being largest in lake fish with a tendency to be smaller in the stream compared to hatchery fish. Therefore, our findings do not support the hypothesis that in brown trout the volume of brain and its regions important for navigation and decision‐making increases in physically complex habitats. We suggest that the observed differences in brain size might be associated with diet quality and habitat‐specific behavioral adaptations rather than physical habitat complexity.

The trade-off between cognitive capacity and developmental costs drive brain size and morphology across fish species, but this pattern is less explored at intraspecific level. Physical habitat complexity has been proposed as a selection pressure on cognitive capacity that shapes brain morphology of fishes, but development of brain is also inherently linked to supply of energy and nutrients, particularly of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA). In this study, we compared brain morphology of brown trout Salmo trutta from stream, lake, and hatchery environments, which differ in physical complexity and availably of dietary n-3 LC-PUFA ranging from low habitat complexity and high n-3 LC-PUFA availability in hatchery to high habitat complexity and low n-3 LC-PUFA availability in streams. We found that brain size, and size of optic tectum and telencephalon differed across the three habitats, being largest in lake fish. We suggest that these differences appeared to associate with diet quality and habitat specific behavioural adaptations rather than physical habitat complexity.