Salinity tolerance of rudd (Scardinius erythropthalmus) and risk for range expansion via brackish water

Abstract

Rudd (Scardinius erythropthalmus) is a naturally occurring species in Norway which over the last four decades has expanded its distribution far beyond its natural range. This invasion represents a threat to local ecosystems and biodiversity. In order to find the most efficient measures to prevent further spreading, it is crucial to find the spreading mechanism for this species. It is assumed that anglers are responsible for introducing rudd to new water bodies, but in this study I explore whether along-coast migration via brackish water may serve as an additional route. Rudd from Storelva water system in southern Norway regularly performs seawards migrations entering brackish water. In periods of high input of freshwater to the fjord system, the brackish surface layer expands outwards, and salinities along the coast decrease. To test whether brackish water represents a possible route for range expansion, the salinity tolerance of rudd was tested in a laboratory experiment, and was found to be between 12 and 15 ppt. These results were applied in a risk model to determine the potential distribution of rudd in relation to observed and simulated salinities in the Storelva- Sandnesfjorden system. According to this model, there is high chance (given salinity tolerance at 24 hrs) that rudd may spread all the way through the Storelva-Sandnesfjorden system at medium (8 m3/sec) to high (100 m3/sec) Storelva discharges. Similar assessments can be made for other water systems to predict the risk of range expansion along the Skagerak coast of Norway.