Use of egg size differences in anadromous (sockeye salmon) and non-anadromous (kokanee) forms of Oncorhynchus nerka to infer ancestral origins of a landlocked population

Life history traits reflect interactions between evolutionary lineage and environmental conditions. Translocations of populations to new environments, and changes in their natal environment, provide insights into the factors controlling life history. For example, the trade-off between egg size and egg number is a well-studied adaptation in fishes, and especially salmon and trout. We used existing and new data on this tradeoff in anadromous sockeye salmon, Oncorhynchus nerka , and the non-anadromous form of the species (kokanee), to investigate the likely origin of a population of uncertain ancestry, land-locked for a century above an impassable dam. Native kokanee have smaller eggs than do the larger-bodied anadromous sockeye salmon. However, the land-locked population in Lake Sutherland, in the Elwha River system, Washington, USA had much larger eggs for their body size than any other kokanee population, similar only to the land-locked descendants of anadromous sockeye salmon in New Zealand. After evaluating and rejecting a series of competing explanations for the unusually large eggs, we infer that the population was mostly likely of anadromous origin, retaining the ancestral tendency to produce large eggs, despite the sacrifice in fecundity that is necessitated by the limited female energy resources. This study revealed the utility of life history traits for studying the ancestral origins of a population for which molecular genetic tools were not informative. Worldwide, many populations have been transplanted or exposed to new conditions, affording similar opportunities to investigate phenotypic plasticity and evolutionary adaptations.