Estimates of larval survival of cod by releases of genetically marked yolk‐sac larvae

In the spring of 1995, 18 million genetically marked yolk‐sac cod larvae Gadus morhua were released into a 2.9‐km 2 , nearly land‐locked fjord 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 was 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 1996), the number genetically marked I‐group cod in the fjord was estimated to be <120. The effect of the historical programme of large‐scale releases of yolk‐sac larvae on recruitment were evaluated and found to be small.