ESTIMATES OF LARVAL SURVIVAL OF COD BY RELEASES OF GENETICALLY MARKEDYOLK-SAC LARVAE

In the spring of 1995, 18 million generically marked yolk-sac cod larv ae Gadus morhua were released into a 2.9-km(2), nearly land-locked fjo rd in western Norway. Four quantitative surveys were conducted, 11, 33 , and 63 days, and 1 year after the first release. Almost 100% of the collected cod larvae were successfully identified to GPI-1 genotype. The marked cod larvae constituted 18% of all sampled cod larvae in the first survey and 9% in the two next. The average rate of mortality wa s estimated to be 23% day(-1) for the first 10 days after release and 12% day(-1) during the next month after release. After 1 year (April 1 996), the number genetically marked I-group cod in the fjord was estim ated to be <120. The effect of the historical programme of large-scale releases of yolk-sac larvae on recruitment were evaluated and found t o be small. (C) 1997 The Fisheries Society of the British Isles.