GENETIC IMPACTS OF TROUT (SALMO-TRUTTA-FA RIO) STOCKING PRACTICES ON IN-SITU POPULATIONS - CASE OF THE ORB BASIN (HERAULT)

The aim of this work is the evaluation of the genetic impact of the st ocking practices, in the Orb basin, south of France. The effectiveness of the stocking policy, and its impact on the genetic pool of the nat ural populations, was followed in the Orb basin, where the number of t routs stocked each year between 1966 and 1989 was equivalent to that o f the natural populations. Trouts collected by electro-fishing from th is river and two of its tributaries, the Tes and the Mare, were charac terized by starch gel electrophoresis at 25 loci. The three localities were stocked at least for twenty years, but the intensity of stocking and the stages used differ. The LDH-5 locus, which codes for the eye -specific lactate deshydrogenase in brown trout, distinguishes between Atlantic and Mediterranean French populations, with two allelic terms respectively LDH-5100 and LDH-5*105 (KRIEG and GUYOMARD, 1985). The hatchery reared trouts are Atlantic strains, that is why the LDHd-5 r epresents a genetic detectable tag for the four hatchery strains, curr ently stocked in the Mediterranean populations of the Orb basin. The h atchery strains show high frequencies of LDH-5100 (87.5 to 100%). The three populations collected in the Orb basin show frequencies of LDH- 5105 between 86 to 97%. This high percentage of LDH-5*105 shows the p oor interbreeding between natural and hatchery-reared trouts. It may i ndicate that the stocked trouts do not reach the age of maturity.