Sea trout are the anadromous migratory form of brown trout (Salmo trutta) going to sea to feed and mature, then returning to freshwater to spawn, usually in their natal rivers. As a fish that primarily occupies coastal zones during its marine phase, the sea trout bridges environmental and fishery concerns across freshwater, transitional, and marine habitats. This unique characteristic brings challenges for science, assessment, and management.
Sea trout are a highly valued species in both recreational and commercial fisheries. They frequently coexist with salmon and are caught in the same fisheries but often take second place in management; in some countries, little attention has been paid to monitoring or assesments of sea trout stocks and fisheries. As a result, the quality of catch and fishing effort statistics is variable. Juvenile monitoring is conducted in all countries, although sampling programmes are not always well structured, systematic, or consistent over time. Some countries have developed extensive networks of counters, usually targeted at monitoring salmon, and in some cases, these also provide good data on runs of sea trout.
Stock declines, for example in areas where marine mixed stock fisheries prevail (e.g. the Baltic) and where there is salmon farming, have raised concerns about our lack of knowledge of the complex and variable life cycle of this species. But few sea trout stocks have been studied for sufficient time to allow the development of population models.
WGTRUTTA members come from across the native range of sea trout. We are developing and evaluating different ways to model sea trout populations. We are using stock-recruitment relationships derived from monitoring data on abundance and/or fisheries data (catch and CPUE-data) from a number of rivers across Europe. We are evaluating models with different levels of complexity (taking into account e.g. habitat variation within rivers and between catchments, occurrence of lakes, migration obstacles and resident trout etc), as well as the representativeness of index rivers for larger areas with sparse information.
There is a need to develop assessment methods for sea trout populations and establishing biological reference points (BRPs) is a prerequisite to be able to assess the status of populations. We are evaluating different ways of estimating BRPs from population models developed, based on e.g. stock-recruitment relationships or estimated pristine abundance levels.