The Threat to the Baltic Salmon - a Combination of Persistent Pollutants, Parasite and Oxidative Stress

Abstract: Atlantic salmon (Salmo salar) from the Baltic Sea suffer from a reproduction disease known as the M74-syndrome. Newly hatched fry develop nerve disorders and die between 3-5 days after the first symptoms are seen. This is a maternally transmitted disease that is casued by a thiamine (vitamin B1) deficiency. The factor(s) that casue(s) this thiamine deficiency are largely unknown. However, several theories have been put forward including persistent pollutant exposure, parasite infestation, fungal or viral infections, thiminase-containing prey or genetic variation in salmon. In the second paper I have measured persistent pollutant concentrations in a Baltic salmon population and correlated that with stress levels in the liver (CYP1A1 and EROD) as well as genetic variation in the expression of the AH receptor (which aids in breaking down pollutants in the body). However, I suspected that there was a large within-fish variation in lipids (and thereby in lipophilic pollutant concentration) in salmon. Therefore, I measured lipid- and pollutant- variation within salmon in paper II, and discuss how such within-fish variation may have consequences both methodologically during sampling, but also for PCB intake in humans after consumption of fish. To further look into stress factors that may affect Atlantic salmon and give rise to a thiamine deficiency, I wanted to investigate nutrient uptake in intestinal parasites of salmon. In paper III, I measured stable isotopes of carbon and nitrogen and found that the cestode Eubothrium crassum and Atlantic salmon differ in nutrient uptake. It seems like parasites rely mainly on passive uptake, and can (when there is an excess of nutrients in the host) benefit from lipid rich hosts by accumulating excess lipids. In paper IV, I wanted to see if E. crassum can accumulate persistent pollutants from the diet. I collected salmon and parasites from salmon intestines as well as sprat from salmon stomachs (sprat is prey both for salmon and for parasites). It was found that salmon accumulate persistent pollutants from the prey, whereas parasites did not. In addition, salmon biomagnified all biphenyl congeners from the diet, whereas parasites seemed unable to biomagnify pollutants. In paper III and IV, the literature on nutrient uptake and biomagnification are discussed. In paper V, PCB and DDT concentrations were measured in three populations of salmon from the Baltic Sea and the two largest lakes in Sweden. The Baltic Sea salmon were the most contaminted on a wet weight basis. Pollutant concentrations in Swedish salmon were compared to concentrations in the rest of the world by an examination of the literature, and it was found that European salmon were more contaminated than salmon from North- or South America