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

The stocking of hatchery-produced whitefish 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 ontlle target population are rarely considered, even though supportive stocking might exelt a series of negative effects. The present study discusses Ulese potentiiLl impacts, taking Upper Lake Constance whitefish as an example. In 2003 , a propOltion of the larvae stocked into the lake were labeI1ed willl Alizal'inRed 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 fish and those incubated in hatcheries in 2003 were· compared with those estimated from records pertaining to the cohorts of 19251939. Survival rates for lake-hatched larvae in 2003 fell between 1 and 10% of historic values. Several factors that may have conu'ibuted to this decline are discussed, of which the most potent seems to .be the massive stocking of the lake with delayed hatchery larvae. Continne(l successful stocking will .increase t.he contribution to the populat.ion of fish with partial or complete hatchery ancestry, and it may compromise t.he stock's ability to adapt to a changing env.ironment through natU.1'al se.lection.