A new study detected clear negative impacts of salmon lice in terms of reduced growth of salmon during the first months at sea, but more so in years when the general sea survival was low.
Another important result from this study was that the growth was better in wild than in hatchery-reared salmon of the same stock during the first months of the ocean migration.
GROWTH OF FISH CAN BE READ FROM THEIR SCALES, AND CAN BE USED AS AN INDICATION OF SURVIVAL
Recording ocean survival of salmon is challenging, and such data have been largely missing. However, there are collections of scales sampled from salmon caught over several years in many rivers. From these scales, it is possible to analyse the growth of the fish during the marine migration.
This study confirms fish growth based on scale reading can be used as an indication of the marine mortality of salmon in the same time-period. But in the period when the ocean mortality was high and there was an additional mortality from salmon lice, post-smolt growth was not such a good measure of mortality.
FIRST SeaSalar PUBLICATION
This study was performed by Knut Wiik Vollset and colleagues, in an area with intensive salmon farming in western Norway. Salmon were tagged in the River Vosso, detailed scale patterns from returning salmon were analysed, and spawners were counted in rivers in the area over a twelve-year period (2003–2014). A marked shift from low to higher ocean survival occurred in 2009, which could not be explained by effects of salmon lice alone.
This was the first publication from the SeaSalar-project. Congratulations with an interesting and important publication Knut and co-authors!
The study was done in collaboration with other projects.
The new publication is available here.