Hendry, Andrew Paul. 1995. Sockeye salmon (Onchorhynchus nerka) in Lake Washington: An investigation of ancestral origins, population differentiation and local adaptation. M.S.
Currently sockeye salmon spawn in many locations within the Lake Washington drainage. These populations could have been derived from transplants and/or from indigenous fish. Nevertheless, major anthropogenic disturbances in the form of introductions, transplants and hatchery propagation have largely been absent from the system since 1954 (12 generations). This study analyzed genetic and phenotypic variation among five of the major Lake Washington populations and two of the introduced populations (Baker and Cultus lakes) in order to: (1) provide insights into ancestral origins, (2) determine the level of population differentiation, and (3) investigate the possibility that each population has become adapted to its local environment.
Analysis of allelic variation at protein coding loci revealed two major population groups within Lake Washington. The first group was comprised of three anadromous populations (the Cedar River, Issaquah Creek and the Pleasure Point beach) and the second group was comprised of two other anadromous populations (Bear Creek and Cottage Creek) and of resident Lake Washington kokanee. Populations in the first group appeared to have had their origin in transplants from Baker Lake whereas populations in the second group were postulated to be derived from fish native to the Lake Washington drainage. Within the first grouping, the presence of genetic differences among populations indicated that divergence, both between the ancestral and derived groups and among derived groups, may have occurred following the introductions.
The level of population sub-division within Lake Washington attested to at least the partial reproductive isolation of each population. In fact, the only two population pairings which could not be distinguished using allozymes were Bear Creek vs. Cottage Creek and the Cedar River vs. the beach. Of these two, phenotypic variables such as body size, male body depth and embryo development rates served to delineate the beach population from the Cedar River. Hence, this study provided evidence for at least four distinct populations within Lake Washington: the Cedar River, Issaquah Creek, Bear/Cottage Creek, and the Pleasure Point beach.
Many phenotypic differences were detected among the Lake Washington populations, some of which matched a priori predictions about local adaptation. For example, young fish were common in the small creeks (Bear and Cottage), large females were common in the Cedar River, and deeply-bodied males were common at the beach. Some differences (e.g., spawning time, hatching time, emergence time) conformed well to those which would be expected from local adaptation, while others (egg size, rheotaxis, fry size and condition at emergence) did not match initial intuitive expectations. In summary, this study provided evidence for (1) the persistence of native and introduced fish within a drainage, and (2) the divergence of populations from each other and from a common ancestor, which may be driven by some combination of reproductive isolation and local adaptation.