A comparison of genetic variability in artificial and natural populations of brown trout in a regulated river system

Brown trout (Salmo trutta) were sampled from tributaries of Glomma, the lar gest river system in Norway. Brown trout were formerly known to migrate lon g distances, but several dams and river regulations have made migration dif ficult, as fishways constructed at the dams are not efficient. To compensat e for the resultant reduction in brown trout, the river system has been sto cked with hatchery fish reared from native brown trout. Genetic analysis by enzyme electrophoresis was conducted to monitor possibl e genetic effects on native fish. Brown trout were obtained from a fishway at Lopet in the South Rena River, and from a section at Deset, 16 km upstre am of the fishway. One sample was taken from a cohort of first generation h atchery fish, based on only six spawning fish collected in the fishway, and one sample was taken from the second hatchery generation, bred from a mixt ure of two cohorts of first generation hatchery fish. The pooled broodstock of these two first generation cohorts numbered five females and five males . Eight samples were taken from second-, third- and fourth-order streams co ntaining populations differing in size and degree of isolation. Tissue samples taken from eye, liver and muscle were analyzed using starch gel electrophoresis for protein polymorphism to determine genetic populatio n structures. Allele frequencies, heterozygosity and polymorphism were comp ared. The fraction of heterozygosity ranged from 3.3 to 13.5% in the wild p opulations, and the lowest fraction was found in the most isolated populati on. Heterozygosity was 8.0% in the first generation of hatchery reared fish and 7.3% in the second generation. The number of detected polymorphic loci ranged from one to seven, with a mean of 4.5, in wild populations, but was three in the first generation and four in the second generation of hatcher y fish. Polymorphism seemed to be lost at three loci in the first generatio n, but one locus was restored in the second generation, probably due to bre eding with another hatchery cohort. Copyright (C) 1999 John Wiley & Sons, L td.