Abstract
Experimental survey of the hybridization between the Brown trout (Salmo trutta) and the threatened Marble trout (Salmo marmoratus) : Preliminary results. Populations of the marble trout (Salmo marmoratus), an endemic species of the Adriatic Basin, are critically declining due to introgression by brown trout (Salmo trutta) stocked strains. Hybrids between these two forms are fertile and presently predominant in most rivers of the species' range. A rehabilitation project was initiated in 1993 in the Soca and Idrijca river basins in Slovenia. The overall final objective of the project is to decrease the introgression rate in the upstream Soca basin. Our present study concerns a limited aspect of the overall project as it focuses on the hybridization mechanisms involved. Knowledge of these mechanisms is of the utmost importance if one wants to facilitate rehabilitation of the Marble trout. The mechanisms have been studied through experimentation in nature. Four experimental small rivers were selected in 1997 in Slovenia and emptied by electro fishing the following years: the Driselpoh, Stopnikarca, Prodarjeva and Baca streams. In 1999, we released 50% of marmoratus originating from the Soca basin and 50% of S. trutta from Switzerland. To facilitate later genetic analyses two pure populations of marble trout were crossed in order to increase the level of genetic variation at neutral marker level. Both strains were bred in a local fish farm. The fish were released as 1 + and marked with carlin tags. the main aim was to follow the demographic evolution of the populations, the reproduction features, to identify a possible interspecific competition and to define the genealogy of the offspring born in the experimental streams. The genetic analysis included seven variable microsatellite loci and we used the obtained multilocus genotypes to allocate offspring born in the river to a set of possible parents using a novel approach. A coalescence based method was used to estimate the effective population size (Ne) of each species as well as for hybrids. In the present study, we present the results obtained from the first cohort born in the Driselpoh stream. The main results are (i) a highly significant deficit of hybrids in the cohort born in the experimental stream, (ii) a low effective size during the first breeding and (iii) an apparent high heterosis effect in terms of size and weight of hybrids when compared to pure marble and brown trout offspring born in 2002.