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

In the present paper we review some factors considered to explain the dramatic decline in the abundance of cod in the eastern Baltic, Environmental conditions, e.g. salinity and oxygen content, are known to have a considerable impact on recruitment. We conclude that, at present, low recruitment due to deteriorated spawning conditions accompanied by high mortality due to commercial fishing and, thus, a decrease in spawning stock biomass are major determinants for distribution of cod in the Baltic. Still, a number of additional factors may be important. The impact by sprat and herring predation on eggs and larvae may be considerable under certain circumstances. The temporal shift in spawning time towards summer months has likely resulted in spawning in more deteriorated oxygen conditions. The change in spawning stock structure towards younger fish probably affects the survival of eggs and larvae negatively. Since this change is closely related to high fishing pressure, 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 in 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 survival of cod larvae has been found to be unaffected by thiamine treatment.