The origins of A tlantic salmon ( Salmo salar L.) recolonizing the R iver M ersey in northwest E ngland

Abstract By the 1950s, pollution had extirpated A tlantic salmon in the river M ersey in northwest E ngland. During the 1970s, an extensive restoration program began and in 2001, an adult salmon was caught ascending the river. Subsequently, a fish trap was installed and additional adults are now routinely sampled. In this study, we have genotyped 138 adults and one juvenile salmon at 14 microsatellite loci from across this time period (2001–2011). We have used assignment analysis with a recently compiled pan‐ E uropean microsatellite baseline to identify their most probable region of origin. Fish entering the M ersey appear to originate from multiple sources, with the greatest proportion (45–60%, dependent on methodology) assigning to rivers in the geographical region just north of the M ersey, which includes N orthwest E ngland and the Solway Firth. Substantial numbers also appear to originate from rivers in western S cotland, and from rivers in W ales and S outhwest E ngland; nonetheless, the number of fish originating from proximal rivers to the west of the M ersey was lower than expected. Our results suggest that the majority of salmon sampled in the M ersey are straying in a southerly direction, in accordance with the predominantly clockwise gyre present in the eastern Irish Sea. Our findings highlight the complementary roles of improving water quality and in‐river navigability in restoring salmon to a river and underlines further the potential benefits of restoration over stocking as a long‐term solution to declining fish stocks.