Our knowledge about stenohaline freshwater fish living on the edge of their maximum salinity
tolerance in estuaries and coastal areas is currently limited. The present PhD research project
therefore aimed to describe the ecology and experimentally determine physiological responses to
salinity of European perch (Perca fluviatilis) in brackish water. A field survey was conducted to
elucidate the physical and chemical environment, migratory behavior, and determine population
dynamics of a European perch population in the western Baltic Sea. Laboratory experiments
determined the maximum salinity tolerance and the effect of salinity on swimming performance
and metabolism, in relation to origin habitat salinity. The field survey showed that European
perch in the western Baltic Sea live at salinities from 0 to 22. Despite this, the population
exhibited a high growth rate and a large maximum size. The maximum salinity tolerance was
substantially higher for fish originating from brackish water (17.5) compared to fish from fresh
water (10-12.5). Brackish water European perch possessed the ability to both hyper- and hypoosmoregulate,
in contrast to the fresh water European perch, which could only hyperosmoregulate.
Furthermore, brackish water reduced the critical swimming speed in fresh water
European perch, while it remained unaffected in brackish water European perch. Salinity had no
measurable effect on metabolism in any of the fish. The results showed that European perch
thrive in the western Baltic Sea, despite living on the edge of their salinity tolerance. They also
revealed intraspecific plasticity in the maximum salinity tolerance as well as swimming capacity
in relation to salinity, indicating an emerging speciation. This raises conservation issues on
brackish water European perch. It is concluded that the high growth rates of brackish water
European perch probably is due to low inter-specific competition and high food availability in
estuaries and coastal areas, and not the energetic cost of osmoregulation. Brackish water
European perch in the western Baltic Sea ought to be managed carefully to avoid local
extinction.