Assessment of egg and larval viability in cod, Gadus morhua: methods and results from an experimental study

Abstract

Despite improved environmental conditions for successful spawning during the 1990s recruitment of Baltic cod, Gadus morhua, is low. A discussion about egg quality has developed; is recruitment being affected by high levels of toxic substances and/or by changed stock parameters due to high fishing mortality? We performed a series of experiments to find an unbiased method for assessment of cod egg quality and we compared egg and larval viability of two stocks: Baltic and Skagerrak cod. Our incubation method resulted in high egg and larval survival to day 10 post hatch [on average 84.7±11.1% (SD) and 83.4±7.9% (SD), respectively], with low variation between replicates (on average 3.1±2.2% (SD) and 7.2±5.6% (SD), respectively). An incubation procedure (light until onset of hatching; in darkness during hatching) resulted in an average period of 5 h from start to 50% hatching, enabling sampling of larvae of approximately the same age. Egg survival data revealed that consideration should be given to `overripening' and ambient temperature effects. Stripped and spontaneously spawned batches yielded equally high survival rates. No correlation was found between viable hatch and egg size but larval length at hatching and at yolk sac depletion was significantly correlated with egg size, as was survival up to day 10 post hatch. Evaluation of egg and larval survival revealed no difference between the stocks for newly caught fish, but for Baltic cod kept in captivity >1 year some females displayed somewhat lower egg and larval survival. The results suggest that recruitment of Baltic cod is not influenced by toxic substances but, as larval viability was correlated to egg size, recruitment may be influenced by female age/length and condition; this may be a fishery management issue.