Potential factors influencing reproductive success of Baltic cod, Gadus morhua: A review

In the present paper we review some factors considered to explain the drama tic decline in the abundance of cod in the eastern Baltic, Environmental co nditions, e.g. salinity and oxygen content, are known to have a considerabl e impact on recruitment. We conclude that, at present, low recruitment due to deteriorated spawning conditions accompanied by high mortality due to co mmercial fishing and, thus, a decrease in spawning stock biomass are major determinants for distribution of cod in the Baltic. Still, a number of addi tional factors may be important. The impact by sprat and herring predation on eggs and larvae may be considerable under certain circumstances. The tem poral shift in spawning time towards summer months has likely resulted in s pawning in more deteriorated oxygen conditions. The change in spawning stoc k structure towards younger fish probably affects the survival of eggs and larvae negatively. Since this change is closely related to high fishing pre ssure, low recruitment might, to a large extent, be a management issue. The M74 syndrome, a thiamine deficiency known to affect the Baltic salmon, has also been discussed in relation to poor cod recruitment. We conclude that it is unlikely that the Baltic cod suffers from M74 since thiamine levels i n gonads from Baltic cod have been found to be, on average, almost 10-fold those found in healthy salmon. Moreover, in contrast to salmon, the surviva l of cod larvae has been found to be unaffected by thiamine treatment.