Maternal effects on egg size and egg buoyancy of Baltic cod, Gadus morhua - Implications for stock structure effects on recruitment

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 va rying between 11 and 20 psu. Due to oxygen-depletion commonly prevailing in these areas. neutral egg buoyancy above oxygen-critical levels is of subst antial importance for egg and larval survival in the Baltic. In the present study, large females were found to produce larger eggs with neutral egg bu oyancy at a lower salinity, implying egg development in more favourable oxy gen conditions. An age- and batch-specific model of egg production, egg siz e and egg buoyancy showed a substantial increase in the probability of egg survival with female age. The relationship between egg production by old fe males and subsequent recruitment was tested in two separated periods with d ifferent 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 p roduced by old females (greater than or equal to5 years). That recruitment was positively related to the proportion of eggs spawned by old fish reveal s that stock age structure should be seriously considered in stock-recruitm ent relationships and in the management of the cod stock in the Baltic. (C) 2000 Elsevier Science B.V. All rights reserved.