| Summary: | Disturbances in Baltic cod reproduction, additional to the basic problems related to low salinity and oxygen content, are discussed. Abnormal cod embryos appeared first in 1979 and have been observed since. Still recruitment has been relatively well correlated to the "reproduction volume" (sufficient saiinity and oxygen content) until year-class 1991. Since that year, year-class strengths have been lower than expected considering available reproduction volurne. Many experimental investigations on Baltic cod reproduction have shown a lower hatching success compared to other cod stocks. Simultaneously with cod, other fish species in the Baltic Sea have experienced reproduction problems. Several hypotheses regarding the causes of the reproductive disturbances are reviewed, including a mis-match of hatching and environmental factors, including food availability and predator abundances. Also the impact of xenobiotic substances on peroxidation of unsaturated lipids (polyunsaturated fatty acids and cholesterol) as well as reactive intermediates, as DNA adducts, and their effects on the quality of spawning products are discussed. Also other fatty acid xenobiotic interactions are highlighted. Insufficient hormone level for induction of maturation and spawning, causing a delay in spawning activities, is also suggested to be caused by anthropogenic substances. We conclude that the causes of reproductive problems in Baltic Sea cod include factors other than overfishing and salinity - oxygen interactions.
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