Juvenile Atlantic salmon - mapping the early running component: report

Background

While Atlantic salmon (Salmo salar L.) spawn in late autumn/early winter, their return migration to freshwater from the sea can occur at any time during the year (Juanes et al. 2004; Quinn et al. 2006; Shearer 1990). Individuals, therefore, arrive in rivers earlier than is necessary for them to reach their breeding ground. The evolutionary advantages of this strategy are still unclear (Quinn et al. 2016) and, although the mechanisms behind such a variation in return timing are unknown, the consequences for the management and exploitation of stocks are better understood.

The diversity in the run timing of Atlantic salmon (defined as the time of return of adults to rivers from the sea) is of great significance to fisheries, as it results in fresh-run fish being spread out through the year, increasing the temporal extent of the catching period. In order to maintain the wide diversity observed in salmon fisheries, it is important to maintain the diversity of the populations that underpin them (Hilborn et al. 2003; Potter et al. 2003; Schindler et al. 2010). However, achieving this management aim is only possible if the biological structure of the resource is known.

Recently, a number of single nucleotide polymorphic (SNP) markers were identified to be associated with run timing, most prominently found on chromosome Ssa09 and linked to the Six6 gene (Cauwelier et al. 2018). From that, Cauwelier et al. (2024) developed and tested a panel of twelve SNPs that could be used as a cost-effective tool to determine run timing distributions within and between rivers. Here, we applied this tool to characterise Atlantic salmon populations and produce run timing maps both within and between rivers. For that, genetic samples were taken from juveniles electrofished at 231 sites across eight rivers draining along the East coast of Scotland and 9 sites from the river Ullapool, which drains along the West coast. Samples were screened for the SNP panel and the proportion of early running fish calculated for each site. These site proportions were then mapped out to examine distribution of early and later running stock components within and between rivers.