Genotypic and Phenotypic Divergence of Sockeye Salmon in New Zealand from Their Ancestral British Columbia Populations

Abstract

Embryos of sockeye salmon Oncorhynchus nerka were transplanted from Shuswap Lake, British Columbia (BC), to tributaries of Lake Ohau in the South Island of New Zealand (NZ) in 1901. This single shipment gave rise to a self-sustaining freshwater resident population that spawns primarily in Larch Stream north of Lake Ohau. We compared the genotypic (using protein electrophoresis) and phenotypic traits of the NZ O. nerka with anadromous (sockeye salmon) and nonanadromous (kokanee) populations of the species in the Shuswap Lake system. Genetic analysis confirmed that the transplanted fish had been sockeye salmon, not kokanee, and allele frequencies in the current NZ population were not significantly different from those in anadromous sockeye salmon from Scotch Creek, BC. The NZ O. nerka developed two life history patterns: some spent their entire rearing period in Lake Ohau, whereas others migrated downstream from Lake Ohau after 1 year to mature in Lake Benmore. These latter “migrants” grew faster and matured at a younger age than the residents. Both NZ forms were younger and smaller at maturity than their anadromous ancestors and matured at an earlier age than Shuswap kokanee. Despite their much smaller size, the NZ O. nerka had eggs that were only slightly smaller than their anadromous ancestors' eggs (96.4 mg for NZ versus 101.4 and 98.4 mg for Lower Shuswap River and Adams River, BC, sockeye salmon, respectively). In contrast, their eggs were much larger than those of comparably sized kokanee (74.4 and 68.2 mg for kokanee from the Lower Shuswap River and Sinmax Creek, BC, respectively). Fecundity of NZ O. nerka was less than that of both Shuswap sockeye salmon (because they were smaller) and kokanee (because their eggs were larger). The coloration of NZ O. nerka was consistent with descriptions of “residuals” (nonanadromous offspring of anadromous sockeye salmon parents). Taken together, the suite of traits indicated considerable phenotypic plasticity in the adaptation of sockeye salmon to the NZ environments.