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

Abstract

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.