The influence of ambient temperature (2-7 degree C) and oxygen )eve( (1.0-8.3 ml cntdot l-1) on the development of Baltic cod eggs was investigated in laboratory experiments. The incubation period, i.e. the time from fertilization to 50% hatching, decreased from 27.5 days at 2 degree C to 13.0 days at 7 degree C. Reduced oxygen levels did not significantly affect the time of hatching. Throughout the incubation period highest mortality rates were found during gastrulation and immediately prior to hatching at all tested oxygen levels. Egg survival decreased from around 30% at an oxygen level of 8 ml cntdot l-1 to less than 10% at 2 ml cntdot l-1 oxygen content. At oxygen concentrations below 2 ml O-2 cntdot l-1 the development ceased at a very early stage. Field observations revealed that in the past years Baltic cod eggs were most abundant below the halocline, 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 feeding areas close to the sea surface. Thus, the effective reproduction volume of water for cod in 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.
|Publication status||Published - 1994|