Intra-Specific Life History Variation of Great Lakes Fishes: Environmental and Fisheries-Induced Selection.

Abstract

Life history traits are important determinants of fish population dynamics and may provide insight as to how populations will respond to current and future environmental conditions and stressors. However, given that most life history traits respond to both genetic and environmental processes, it is difficult to elucidate processes underlying life history trait variation. Through a combination of methodologies, I considered intra-specific variation of important life history traits, i.e., maturation, growth and egg size, for two fish in the Great Lakes: lake whitefish (Coregonus clupeaformis) and walleye (Sander vitreus). Abundance of both lake whitefish and walleye stocks have fluctuated across spatial and temporal scales, due to harvest, habitat degradation, and ecosystem changes. Using several analytic methods (age and length at 50% maturity, age-specific maturity ogives, and probabilistic maturation reaction norms), I demonstrated that spatial and temporal variation of maturation schedules for lake whitefish and walleye could be partitioned into plastic (changes in maturation schedules due to different growth rates) and potential adaptive variation (selection-induced changes in maturation schedules). For example, after accounting for different growth rates, lake whitefish in three Upper Great Lakes (lakes Michigan, Huron, and Superior) expressed intrinsically different maturation schedules, apparently corresponding to relative levels of natural and fishing mortality. Further, to investigate potential fishing-induced selection on maturation schedules and growth, I developed a generalized eco-genetic individual-based model (IBM). By running simulations with various harvest scenarios, I demonstrated that selectively harvesting large fish might impose selection that favored slow growth rates and early maturation schedules, leading to unsustainable fisheries. Conversely, harvesting small fish might achieve sustainability by promoting fast growing and delayed maturity. Moreover, I showed significant variation in egg size among five walleye stocks in the Great Lakes region, and that such inter-stock egg size variation appeared to correspond to quality (e.g., primary production) of spawning habitats. Collectively, this research demonstrates that life history variation is attributable to both genetic and plastic factors and that both environmental and anthropogenic processes can influence life history expression. Thus, effective fisheries management must account for both ecological and evolutionary consequences on life history variation.