Preservation of Genetic Variation in the Green Lake Strain Lake Trout Derived from Remnant Domestic and Feral Populations

—The Green Lake, Wisconsin, strain of lake trout Salvelinus namaycush was discontinued as a hatchery brood stock in 1976 after Lake Michigan was stocked with the 1975 yearclass. In 1982, a decision was made to restore the Green Lake strain as a production brood stock. Five groups were produced by spawning marked fish that were survivors from the 1976 stocking of southern Lake Michigan. A sixth group was produced from a remnant of the Green Lake brood stock held at the Genoa (Wisconsin) National Fish Hatchery. Hatchery accidents reduced both the number of groups and the effective population number of each group. We studied a procedure for reconstructing a composite Green Lake brood stock from individuals of these six groups. Genetic variability was evaluated by allozyme electrophoresis in five of these groups. Twelve of 18 loci were polymorphic. Allelic frequencies were similar in all five groups; however, significant differences occurred in nine systems. Heterozygosity level (mean ± SE) was lowest in the domestic group (0.119 ± 0.043) and ranged from 0.139 ± 0.043 to 0.166 ± 0.052 in the feral groups. Cluster analysis of genetic distances grouped the four feral groups together, but separate from the domestic group. Progeny from fish captured on Black Can Reef, Lake Michigan, in 1986 and 1988 were the most similar. A modified diallel mating design was developed to produce a composite brood stock from remnant feral and domestic fish. Pooled families from 1991 and 1992 diallel matings will be reared to maturity, then reciprocal crosses of the two year-classes will be made to form the new composite Green Lake brood stock. Attempts to expand populations of lake trout mission initiated development of a hatchery brood Salvelinus namaycush throughout the Great Lakes stock of Green Lake strain ftsh (Krueger et al. 1983). have emphasized adapted strains that could surBetween 1985 and 1988, feral fish were captured vive and reproduce on the historic spawning reefs from spawning reefs in southern Lake Michigan (Schneider et al. 1983). The Green Lake strain of and used as parents for a series of progeny lots lake trout was used extensively for the stocking of that were transferred to hatcheries. Fish from these southern Lake Michigan; more than 4.9 million lots were to be the parental stock for the new Green yearlings were planted from 1966 to 1976 (Brown Lake brood stock. The present study served to et al. 1981). In 1976, the Green Lake strain was examine the genetic variability within and among discontinued as a hatchery brood stock, except the six Green Lake strain progeny lots, one lot 10,900 eyed eggs were transferred from the Jordan from the 1975 domestic brood stock and five lots River (Michigan) National Fish Hatchery (NFH) from feral brood fish collected in southern Lake to the Crystal Spring (Wisconsin) State Fish Michigan. The objectives were to determine the Hatchery (SFH). When the Green Lake strain was level of genetic differentiation among the six lots discovered to be the last representative of the and to design a mating plan to produce a new southern Lake Michigan deep-water spawning brood stock from the remnant Green Lake gene strains, managers began to examine techniques to pool, recover the strain. Fish from the 1975 and earlier year-classes were present in sport, commercial, and Methods assessment catches from Lake Michigan during the Background of Green Lake strain. — The Green 1980s, but their numbers decreased with each Lake lake trout strain was developed from repassing year. peated stockings of Green Lake, Wisconsin, from In 1983, the Lake Michigan Lake Trout Brood 1886 to 1943 with fish from southern Lake MichStock Committee of the Great Lakes Fishery Comigan. The source of these fish is believed to have