Potential Factors Influencing Reproductive Success of Baltic Cod, Gadus morhua: A Review

摘要

In the present paper we review some factorsconsidered to explain the dramatic decline in theabundance of cod in the eastern Baltic. Environmentalconditions, e.g. salinity and oxygen content, art knownto have a considerable impact on recruitment. Weconclude that, at present, low recruitment due todeteriorated spawning conditions accompanied by highmortality due to commercial fishing and, thus, adecrease in spawning stock biomass are majordeterminants 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 andlarvae may be considerable under certain circumstances.The temporal shift in spawning time towards summermonths has likely resulted in spawning in moredeteriorated oxygen conditions. The change in spawningstock structure towards younger fish probably affectsthe survival of eggs and larvae negatively. Since thischange is closely related to high fishing pressure, lowrecruitment might, to a large extent,be a managementissue. The M74 syndrome, a thiamine deficiency known toaffect the Baltic salmon, has also been discussed inrelation to poor cod recruitment. We conclude that itis unlikely that the Baltic cod suffers from M74 sincethiamine levels in gonads from Baltic cod have been foundto be, on average, almost 10-fold those found in healthysalmon. Moreover, in contrast to salmon, the survivalof cod larvae has been found to be unaffected by thiamine treatment.