Development of Baltic cod eggs at different levels of temperature and oxygen content

The influence of ambient temperature (2-7°C) and oxygen level (1.0-8.3 mi i i) on the development of Baltic cod eggs was investigated in laboratory experimenrs. The incubation period, i.e. the time from fer tilization to 50% hatching, decreased from 27.5 days at 2°C to 13.0 days at 7°C. Reduced oxygen levels did flot significantly affect the time of hatching. Throughout the incubation period highest mortality rates were found during gastrulation and immediately prior to hatching at ali tested oxs’gen levels. Egg survival decreased from around 30% at an oxygen level of 8 ml .11 to less than 10% at 2 m1 l oxygen content. At oxygen concentrations below 2 ml 021 the development ceased at a very early stage. Field observations revealed that in the past years Baltic cod eggs were most abundant below the halo dine, depth with unfavourable oxygen condition. Besides the effect on egg survival, low environmental oxygen may also affect the initial viability of larvae and consequently their ability to approach the feed ing areas close to the sea surface. Thus, the effective reproduction volume of water for cod m the central Baltic may have been smaller than expected and it is suggested that oxygen depletion was the limiting factor determining the reproductive success of cod in this area during the last decade. Keytvords: cod, egg development, mortality, oxygen.