Quantifying environmental heterogeneity:habitat size necessary for successful development of cod Gadus morhua eggs in the Baltic Sea

Spatial and temporal variability in environmental factors can exert major influences on survival and growth of livlng organisms. However, in many key areas of fisheries sclence (e.g. growth, survival and recruitment determination), environmental heterogeneity is usually ignored because of insufficient environmental or fisheries data or lack of evidence that such heterogene~ty impacts response variables. For the eastern Baltic Sea (ICES Subdivisions 25 to 321, we evaluated spatial and temporal differences in conditions affecting the sunrival of cod Gadus morhua L. eggs at survival on four distinct spawning sites within the assessment area. We intercalibrated ways of quantifying the volume of water ('reproductive volume') at each site where salinity, oxygen and temperature conditions permitted successful egg development. We have developed and compared a time series (1952 to 1996) of reproductive volumes among the areas to identlfy spatial differences. The results of 2 independent volume-estimation methods are comparable, indicating that highly significant differences exist among the sites, and that the westernmost spawning ground, Bornholm Basin, has on average the highest reproductive volume and the lowest variability among the 4 sites. These findings may be useful in evaluating how spatial and temporal variability in environmental conditions affect egg hatchng success and possibly recruitment in the Baltic stock.