Use of estuarine environments by migratory salmonids

Many salmonid fish species follow a migratory life history strategy, where they spawn in freshwater and migrate to the sea as juveniles to feed and mature, returning to freshwater to breed. The transition through the estuary is a difficult phase, physiologically (moving from fresh to salt water), behaviourally (navigating novel directional cues) and ecologically (encountering a new set of resources, pressures and predators). There are suggestions that mortality of juveniles is high as they pass through this difficult phase in their migration, yet we know little about how they use estuaries and the risks they face as they transition from fresh to salt water. This project will build on long-term monitoring of salmonid populations and will include environmental fieldwork and fish sampling. The project will assess the spatial distribution, resource use and pressures of salmonids (Salmo salar and S. trutta) and their potential predators in estuaries of contrasting morphology using techniques such as eDNA, tracking, physical sampling and gut contents analysis, potentially leading to the development of simulation models of movement. Outputs of the project will have direct relevance for the management of fish populations of commercial and conservation importance.

The project aims to elucidate the spatial distribution, resource use and pressures on juvenile salmonids (Salmo salar and S. trutta) in estuaries. The plan will involve four key objectives, with the student engaged in all objectives with input from both supervisors and the CASE partner. The first objective will be to collect and process samples of eDNA from the study locations to understand the distribution of salmonids and potential predators during their migration through the estuaries. The second objective will be to track individuals through the estuaries. The third will be to use gut contents and stable isotopes to assess changes in predator diet during the salmonid migration. Finally, the student will model the behaviour and survival of juvenile salmonids through the estuary and incorporate this information into existing life history models for the two target species. Training will be given in electrofishing, experimental use of animals, and stable isotope analysis.