Evaluating the success of large-scale whitefish stocking at Lake Constance

Abstract

Artificial incubation of whitefish eggs at Lake Constance (L.C.) started in 1887/88. Today there are six hatcheries at Upper L.C. which all use refrigerated water to delay hatching. They stock around 350 million recently hatched and 3-5 million prefed whitefish larvae annually. Despite this long history of artificial incubation at L.C., it remains unknown whether stocking indeed fulfils its aims, to dampen yearly yield fluctuations and/or to increase yields. We used a newly developed method to label the otoliths of whitefish embryos during incubation with Alizarin Red S (ARS) to evaluate the contribution of unfed hatchery larvae to cohort size. In winter 2003, 600 L of whitefish eggs were treated with ARS and by early April around 40 million larvae were released. In autumn 2003, the otoliths of age-0 whitefish sampled from the lake were examined. We found that 18 out of a total of 290 fish originated from the marking experiment. Since only 10% of stocked larvae had been marked, the contribution of hatchery-reared fish to the cohort 2003 was estimated as 62% with binomial confidence intervals of 42 and 79%. It should, however, not be concluded that stocking is necessary to maintain the current level of commercial yields, since it still remains unknown whether hatchery fish are added to the naturally recruited stock leading to stronger cohorts, or whether stocking only increases intraspecific competition without enhancing cohort size.