Hybridization in natural populations of salmonids in South-West Europe and in an experimental channel.

Interspecific hybridization between Atlantic salmon and brown trout has been found in different European countries and in Canada. A study was initiated to determine the incidence of hybrids in salmonid populations of some rivers in Asturias (North Spain) and Southwestern France. It was complemented by experiments in a controlled environment to examine the conditions of the breaking down of reproductive isolation. Working hypotheses were that hybridization was favoured by the sneaking behaviour of precocious male salmon parrs and the stocking of juveniles of the two species. Results confirmed the widespread occurrence of this phenomenon. Significant proportions of salmonid populations were locally affected by hybridization (9.4 % in the Narcea river, Asturias). Within the native range of the two species, the direction of crossing was female salmon x male trout. Under normal sympatric conditions, pre-mating behavioural isolating barriers seemed to be strong because of the aggressive behaviour of the dominant conspecific males who successfully chased away competing heterospecific males. In the absence of conspecific males hybridization did occur. However, females modified their spawning behaviour and their reproductive success decreased, particularly in trout. Working hypotheses were not confirmed. To avoid hybridization, managers should make sure that sufficient numbers of spawning zones are available, that spawner populations are well balanced, and that the stocking of exotic juveniles is limited.