The origins of Atlantic salmon re-colonizing the river Mersey

This study genotyped 138 adult salmon sampled between 2001 – 2011 from the river Mersey; we used assignment analysis, based on 14 microsatellite loci and the recently compiled pan-European SALSEA-Merge baseline, to identify the most probable region of origin of adult fish entering the river – to date, no smolt run has been observed in the river and only three juvenile fish have been sampled. Assignment analysis suggests that the adult fish originate from multiple sources, with the greatest proportion assigning to rivers in the region just north of the Mersey, which includes rivers in Northwest England and the Solway Firth. Surprisingly, however, the number of fish originating from proximal rivers to the west of the Mersey, e.g. the river Dee, was much lower than might be expected. Our results suggest the majority of salmon sampled in the Mersey are straying in a clockwise north to south direction, in accordance with the predominant clockwise gyre present in the eastern Irish Sea.  This study highlights the complementary roles of prevailing marine currents, improving water quality and in-river navigability in restoring salmon to a river and underlinines further the potential benefits of restoration over stocking as a long-term solution to declining fish stocks. 

Authors: Charles Ikediashi (Exeter), Sam Billington (Environment Agency, NW Region) & Jamie Stevens (Exeter)

This work is published in the Open Access journal Ecology and Evolution and is available online at: www.ecolevol.org