Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—The reproductive success of cod <em>Gadus morhua </em>from the Baltic Sea and the Barents Sea was compared. The offspring of 17 family pairs from the Baltic Sea and 12 family pairs from the Barents Sea were investigated during the embryonic and larval development stages. Frequencies of mortality over time and frequencies of different disorders at hatch were analyzed. The results indicated that the reproductive success of cod from the Baltic Sea was seriously impaired. The Baltic cod showed high mortality before hatch. In newly hatched larvae, different kinds of disorders were seen, such as vertebrae deformity, disrupted yolk sac or subcutaneous edema in the yolk sac, and precipitate in the yolk. To compare mortality and early developmental abnormalities in Baltic cod and Baltic salmon <em>Salmo salar</em>, the offspring of 20 salmon family pairs, caught in the River Dalälven in Sweden, were investigated analogically. The results showed that the majority of the salmon offspring experienced a thiamine deficiency-dependent mortality at different stages of larval development and that five family pairs experienced high mortality before hatch. In salmon, different kinds of disorders were also seen at hatch, such as vertebrae deformity, blood disorders, subcutaneous edema in the yolk sac, and precipitate in the yolk. The disorders at hatch were not correlated to later thiamine deficiency-dependent mortality. Aliquots of newly fertilized salmon eggs were injected with thiamine by the nanoinjection method. This treatment had only a minor effect on the frequency of disorders at hatch, but it protected the salmon larvae almost completely from later thiamine deficiency-dependent mortality. This indicates that factors other than thiamine deficiency are involved in the developmental disorders. In both salmon and cod from the Baltic Sea, the mortality and disorders among the offspring were mainly correlated to the female, and in both species some females produced offspring that experienced high mortality before hatch. Both salmon and cod also showed disorders that might have similar biochemical mechanisms, because the formation of precipitates and edema in the yolk sac occurs in both species.