Epsilon Archive for Student Projects

Abstract

The Atlantic salmon (Salmo salar L.) is an anadromous species and is due to its complex life cycle highly susceptible to anthropogenic disturbances. Historically the Atlantic salmon of the Baltic sea has faced the M74 outbreak, but in 2014 observations of an unknown sickness causing mortality were made across several Swedish rivers.

In this study the purpose was to investigate whether these health issues have had an impact on where the returning adult salmon choose to spawn since a poorer health could be hypothesised to result in a larger proportion of the salmons spawning earlier in the river. Reaching a higher altitude through the running water takes energy and might cause salmon in poorer condition to not reach the upper spawning areas. In order to investigate this the average electro fished densities of 0+ before (2010-2014) and after (2015-2019) the first reports of health issues were compared, as this density is a good indication of where spawning takes place.

The results however did not show any clear spatial trends, other than in most cases a positive development in densities between the two periods with Vindelälven as a clear exception. Of the 11 Swedish rivers of the Gulf of Bothnia that were included in this study the most severe reports of issues have come from Vindelälven, Kalixälven and Torneälven, and this study indicates that the health issues are not present in the other rivers. The densities of 0+ salmon remained fairly stable between the periods in Torneälven and Kalixälven, indicating that the issues have not had an effect on neither spawning success nor ability to reach the uppermost spawning areas. In Vindelälven on the other hand, densities had decreased dramatically across almost all sites, independently of distance to the sea. This underlines the fact that little is known about the behavioural response of a poorer body condition, which in combination with the poor understanding of the mechanisms behind spawning site choice.