Massive stocking with hatchery larvae may constrain natural recruitment of whitefish stocks and induce unwanted evolutionary changes

The stocking of hatchery-produced whitefi sh is a common practice in most countries of the species’ natural range. Rigorous monitoring of the effects of stocking is less common, however, and possible negative impacts of stocking on the target population are rarely considered, even though supportive stocking might exert a series of negative effects. The present study discusses these potential impacts, taking Upper Lake Constance whitefi sh as an example. In 2003, a proportion of the larvae stocked into the lake were labelled with Alizarin Red S. The contribution made by stocking to virtual cohort size was estimated at 83%. Survival rates from egg or larval stages to adulthood for naturally produced fi sh and those incubated in hatcheries in 2003 were compared with those estimated from records pertaining to the cohorts of 1925–1939. Survival rates for lake-hatched larvae in 2003 fell between 1 and 10% of historic values. Several factors that may have contributed to this decline are discussed, of which the most potent seems to be the massive stocking of the lake with delayed hatchery larvae. Continued successful stocking will increase the contribution to the population of fi sh with partial or complete hatchery ancestry, and it may compromise the stock’s ability to adapt to a changing environment through natural selection.