Juvenile Atlantic salmon - mapping the early running component: report

Executive summary

While Atlantic salmon spawn in late autumn/early winter, their return migration to freshwater from the sea can occur at any time during the year. 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 the past decades, the number of the highly prized early running (spring) fish has declined and continues to decline, most markedly along the East coast of Scotland. As such, identifying areas dominated by early running adults would aid with the development and execution of management plans.

Recently, a number of genetic markers associated with run timing were identified and developed into a tool that could be used to screen juvenile salmon and characterise stocks according to their run timing composition.

Here, we describe the application of this tool to juvenile Atlantic salmon caught at 231 locations across nine Scottish rivers. The early running component was mapped out for each location.

In general, the proportion of the early running component increased with increasing distance to the river mouth. This pattern was found across the nine rivers and corresponded well with previous findings based on tracking studies.

This would suggest that this genetic tool can be used to identify spawning areas containing high numbers of early running fish and assess whether local interventions might aid with conservation and management of the species.