Does opercular shortening influence seaward migration success of stocked Atlantic salmon (Salmo salar) smolts?

Stocked Atlantic salmon ( Salmo salar ) used to supplement wild populations often exhibit lower survival than their wild counterparts. The causes of their reduced survival in nature are multifaceted and likely include morphological deformities that can arise in the rearing environment. Opercular shortening, characterized by a partially or completely missing gill cover, is a common deformity in hatchery-reared salmon. However, although the opercula are important for respiration and protecting the gills, the causes and consequences of opercular shortening are not well known. Here, we assessed whether shortened opercula influenced the seaward migration success of stocked salmon smolts using passive integrated transponder (PIT) telemetry. One-year-old smolts with intact opercula and varying degrees of opercular shortening were PIT-tagged and released into Gudsø stream in 2017 and River Omme in 2018, Denmark. Their downstream spring migration was subsequently tracked using stationary PIT antenna stations. The overall migration success was higher in Gudsø stream, where 61.9 % of smolts were classified as successful migrants, compared to 31.1 % in River Omme. However, the degree of opercular shortening did not influence the likelihood of successful seaward migration in either study system. Similarly, opercular shortening had no detectable effect on migration timing or bird predation risk of smolts. Despite these findings, it remains important for hatcheries to minimise the occurrence of opercular deformities to promote fish welfare during rearing. As the salmon must still undertake an extensive marine migration to reach their feeding areas, further studies are needed to assess marine survival before robust conclusions can be drawn about the long-term effects of opercular shortening.