The effect of downstream translocation on Atlantic salmon Salmo salar smolt outmigration success.

Trap and transport, the capture and subsequent translocation of fish during the freshwater phase of their migration, is becoming more common as a management intervention. Although the technique can be successful, it is costly and can have unintended effects on the fish being transported. This study investigates whether trap and transport can be used to increase the migration success of Atlantic salmon, Salmo salar, smolts in naturally flowing rivers. Seaward-migrating S. salar (n = 294) from two UK rivers were tracked using acoustic telemetric techniques. Outmigration success and timing were compared between non-transported (released at the original in-river capture site) and transported (released ca. 23 km downstream of the capture site) individuals. Downstream translocation increased the proportion of fish that successfully migrated to marine waters, and there was no indication that transport reduced post-release survival. The post-release migration speed of transported fish was slower than expected but this was likely a function of their advanced migration timing rather than an inhibition of their capacity to migrate. These results suggest that trap and transport can increase the outmigration success of S. salar smolts, but the earlier river exit dates of transported fish could negatively affect their survival at sea.

Resource instability undermines predictable plasticity-mediated morphological responses to diet in a postglacial fish.

Phenotypic plasticity has been presented as a potential rapid-response mechanism with which organisms may confront swift environmental change and increasing instability. Among the many difficulties potentially facing freshwater fishes in recently glaciated ecosystems is that of invertebrate prey communities becoming significantly altered in species composition and relative abundance. To test how the rapidity of diet resource change may affect phenotypic responses during development, we subjected juvenile brown trout to pelagic-type or littoral-type diets that alternated either daily, sub-seasonally, or not at all over a single growth season. The proportional intake of each diet was traced with stable isotopes of carbon and nitrogen and modelled with morphometric data on head and jaw shape. While those trout exposed to a single diet type developed predictable morphologies associated with pelagic or littoral foragers, those raised on alternating diets expressed more unpredictable morphologies. With extreme (daily) or even sub-seasonal (monthly) resource instability, the association of diet type with the phenotype was overwhelmed, calling into question the efficacy of plasticity as a means of adaptation to environments with rapidly fluctuating prey resources.

Differences in brain morphology of brown trout across stream, lake, and hatchery environments.

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.