The potential for advective exchange of the early life stages between the western and eastern Baltic cod (Gadus morhua L) stocks

In order to clarify mechanisms influencing the reproductive success of Balt ic cod (Gadus morhua L.), a modelling exercise was performed to examine the effects of the wind-driven circulation on the transport of early life stag es between the western and eastern Baltic. Because the different stocks spa wn in different areas and environments at different times of the year, the occurrence of variable age/length distributions of juveniles within the dif ferent potential nursery areas can be explained by the circulation pattern. A three-dimensional circulation model of the Baltic was utilized to invest igate the temporal evolution of egg and larval distributions of the western Baltic cod stock, which spawns preferentially in the Danish Straits, in Ki el Bay as well as in Mecklenburg Bay. For different scenarios (1988 and 199 3), within, and between-year variability of egg and larval transport showed large differences, primarily due to variations in wind forcing. In 1988, r elatively low and variable wind forcing prevailed, whereas, due to sustaine d strong, mainly westerly, winds, in January 1993, the recent major Baltic inflow to the Baltic Sea occurred. Differences in contributions of early li fe stages from the western to the eastern cod stocks, depending on the phys ical forcing conditions, suggest that this process can be controlled by var iations of atmospheric forcing conditions. The potential for early life sta ges from the western Baltic cod stock to drift into the Arkona Basin and th e Bornholm Basin, and to contribute there to the juvenile population, has b een recognized as being mainly due to strong westerly winds. During cold wi nters, retention of eggs, larvae and juveniles within their original spawni ng grounds may predominate. Transport of cod early life stages from the Ore sund, as well as from the Great Belt, can occur only during periods of stro ng westerly winds, but significant east, wards orientated drift from Kiel B ay and Mecklenburg Bay was also evident during periods of minor westerly wi nd influence.