Working Group to Develop and Test Assessment Methods for Sea Trout Populations (anadromous Salmo trutta) (Outputs from 2020-2023 Meetings) (WGTRUTTA)

In its second term, WGTRUTTA focused on increasing the understanding of life history drivers of trout across Europe, further developing methods for modelling sea trout populations, developing approaches to quantify external pressures acting on populations in different regions, and approaches to facilitate sustainable governance of sea trout. Members’ research on life history drivers and sea trout distributions were supported by the expansion of the environmental and biological databases, benefiting from regional and national programmes collecting age and growth data, juvenile abundance and distributions, and habitat availability. Genetic tools were applied to investigate gender-specific performances over different year classes, and genetic structuring, identity and diversity of trout/sea trout populations in major rivers along the south coast of Britain and northern coast of France, the relationship between migrating spawners and subsequent juvenile recruitment in Ireland, and gene flow between populations inhabiting adjacent river basins in Portugal. Research to quantify external pressures on trout populations has included mapping anthropogenic impacts at regional and national scales, tracking studies to evaluate impacts, potential risks from bycatch and the river stocks at threat, and studies of the effectiveness of solutions such as dam removal and habitat restoration. The working group continued to develop and test the application of the Trout Habitat Scores (THS) method outside the Baltic region, and to advance approaches to estimate smolt productivity. A paper was written examining methods to characterize juvenile density and habitat and proposing strategies and guidelines to improve data comparability. This study concluded that opportunities exist to harmonize data through selective use of fish surveys, integration of range-wide macro-scale spatial and habitat data, and using appropriate modelling approaches, and highlighted the importance of detailed and complementary population data from index rivers to validate and scale juvenile assessments. New solutions for sustainable governance are being pursued across many countries, e.g. the development of a sea trout effect indicator for the Norwegian salmon ‘traffic light’ system, and research to examine national exploitation and ecosystem services provided by trout in Portugal. New data have contributed to the revision of the Portuguese Red List for freshwater and migratory fish.