Effects of temperature on egg and larval survival of cod (Gadus morhua) and sprat (Sprattus sprattus) in the Baltic Sea -: implications for stock development

Stock development of cod and sprat, two major fish species in the Baltic Sea, is linked by trophic interactions. Depending on recruitment success the Baltic may be pushed towards either a cod- or a clupeid dominated system. Both cod and sprat spawn in the Baltic deep basins at strongly varying hydrographical conditions with survival during the egg and early larval stages regarded as a major bottleneck. Due to differences in egg specific gravity, cod and sprat eggs occur at different depths and are thus subject to different hydrographical conditions. For sprat, weak year-classes have been associated with low water temperatures during peak spawning. For cod the shift in peak spawning from spring to summer during the 1990s has been discussed as a reason for the poor recruitment at present as delayed spawning may involve egg development at too high temperatures. In the present study cod and sprat eggs and yolk sac larvae were incubated at different temperatures, 1-11 degreesC for cod and 1-13 degreesC for sprat. No difference in viable hatch occurred in the range 3-9 degreesC for cod and in the range 5-13 degreesC for sprat. Larval viability decreased at 11degreesC for cod and at less than or equal to5 degreesC for sprat. Comparing the results with vertical egg distribution and temperature profiles from field studies suggested no major influence of temperature on cod reproduction, but a considerable effect on sprat. The results imply that different environmental conditions; frequency of major saline water inflows into the Baltic Sea for cod, and water temperature in the upper layers, e.g. following severe/mild winters, for sprat, involve different opportunities for egg and larval survival and may thus cause a displacement in the balance between cod and sprat.