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Advancing understanding of Atlantic Salmon at Sea: Merging Genetics and Ecology to resolve Stock-specific Migration and Distribution patterns

Final Report Summary - SALSEA-MERGE (Advancing understanding of Atlantic salmon at sea: Merging genetics and ecology to resolve stock-specific migration and distribution patterns)

Over the past two decades, an increasing proportion of North Atlantic salmon are dying at sea during their oceanic feeding migration. Arguably the greatest challenge in salmon conservation is to gain insight into the spatial and ecological use of the marine environment by different regional and river stocks. The SALSEA-MERGE programme was designed to advance our understanding of oceanic scale, ecological and ecosystem processes.

Through a partnership of nine European nations the programme delivered innovation in the areas of genetic stock identification techniques, new genetic marker development, fine scale estimates of growth, the use of novel high seas pelagic trawling technology and estimates of food and feeding patterns. In addition it provided for the first time fine scale migration and distribution models merging hydrographic, oceanographic, genetic and ecological data.

Research cruises to designated areas in the northeast Atlantic took place in 2008 and 2009. In total 1 728 post- smolts and 53 adult salmon were captured in 233 trawl tows. The tissues from these fish and associated environmental data collected at sea were combined with a subset of 1 800 tissue samples selected from archival material. A unique, comprehensive biological and environmental database, the so-called 'Salsea Pgnapes' was developed to facilitate any future analyses.

The SALSEA-MERGE programme facilitated the development of a unique molecular assignment protocol, the genetically-based regional assignment of Atlantic salmon protocol (GRAASP), based on a suite of 14 microsatellite loci. The database comprised 26 813 individuals from 467 locations in 284 rivers, representing approximately 85 % of non-Baltic European salmon production. A total of 3 871 of the 4 151 fish were assigned on a regional basis. Significant temporal and spatial heterogeneity in the distribution of the regional stock groups was found and fish of farm origin were identified, demonstrating the value and power of the tool.

Over 23 000 scales of Atlantic salmon from seven rivers, located in six countries, and smolt age and fine scale growth of 2 242 sea caught post-smolts were analysed. The average rate of circuli formation in the marine zone of scales was estimated to be 6.3 days per circulus. Both the age structure and the number of marine circuli in the scales suggested that the majority of the post-smolts originated in rivers in southern Europe. Marine growth rates varied among years, with highest growth rates in 2002, followed by 2003 and 2009. The lowest growth rates were in 2008. Growth rates during the first period at sea were lowest for salmon of southernmost origin. Growth indices were linked to prevailing environmental and biological conditions. There was clear evidence found that growth was linked to oceanic conditions for all rivers. The diet of salmon, herring and mackerel was studied for four years, namely 2002, 2003, 2008 and 2009. Although the fish examined fed in close proximity in the ocean, the diet differed between the three species. Salmon showed clear differences in diet among years from 2002 to 2009. The condition factor of salmon decreased from 2002 to 2009.

The distribution pattern of several specific populations of salmon were spatially mapped at different genetic assignment levels. Likely migration routes were assembled for some individual river stocks such as the Loire Allier in France, the Namsen in Norway and the Bann River in Northern Ireland. The distribution of post-smolts was clearly linked to ocean currents. South of 61.5 degrees north the post-smolts were not randomly distributed within the migration path, but were located in areas where the currents were stronger than average. A migration drift model for specific stocks of post-smolts was developed. When temperature and salinity preferences were included with active swimming behaviour this proved to be an important mechanism for changing the migration routes and the post-smolt distribution pattern. Also, inter-annual variation in wind fields, and thus the surface currents, altered the migration pathways. Several key areas in the migration routes, where shifts in the migration direction might occur due to climate change, were also identified. A conceptual ecological model was developed, where the main factors relating to the survival of salmon at sea were identified and described. Important factors that were described included predators, growth and fisheries, which in turn depended on post-smolt distribution, climate variability and change, specific stocks and synergetic effects and lifehistory changes.

SALSEA-MERGE provided for the first time a comprehensive overview of the migration and distribution pattern of regional and river specific populations of Atlantic salmon post-smolts in the northeast Atlantic. The study clearly indicated a need to move away from a paradigm centred on the study of adult survivors to an all-embracing vision of salmon populations which encompassed survival corridors.