Maternal effects on egg size and egg buoyancy of Baltic cod, Gadusmorhua: Implications for stock structure effects on recruitment

Abstract

Successful spawning of cod in the Baltic is restricted to the deep basins: the Bornholm basin, the Gdansk Deep and the Gotland basin, at salinities varying between 11 and 20 psu. Due to oxygen-depletion commonly prevailing in these areas, neutral egg buoyancy above oxygen-critical levels is of substantial importance for egg and larval survival in the Baltic. In the present study, large females were found to produce larger eggs with neutral egg buoyancy at a lower salinity, implying egg development in more favourable oxygen conditions. An age- and batch-specific model of egg production, egg size and egg buoyancy showed a substantial increase in the probability of egg survival with female age. The relationship between egg production by old females and subsequent recruitment was tested in two separated periods with different conditions: 1967–1980 and 1981–1994. The number of recruited cod (age 2), in the Baltic Sea, was positively related to the fraction of eggs produced by old females (≥5 years). That recruitment was positively related to the proportion of eggs spawned by old fish reveals that stock age structure should be seriously considered in stock–recruitment relationships and in the management of the cod stock in the Baltic.