Adaptive, genetically based differences in life history between estuary and freshwater threespine sticklebacks (Gasterosteus aculeatus L.)

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Abstract
Life history theory predicts that migratory fishes should delay reproduction, be larger at maturity, and have higher fecundities than nonmigrants. We tested this hypothesis by comparing life histories of laboratory-reared estuary and freshwater threespine sticklebacks (Gasterosteus aculeatus L.) from the Navarro River, California. We also estimated phenotypic correlations and genetic variance (broad-sense heritabilities) for reproductive characteristics of these fish. The more migratory estuary sticklebacks delayed reproduction, were larger at first reproduction, and had higher fecundities than the freshwater fish. We found no significant differences in interclutch interval or average size of eggs. The body size – fecundity relationship did not differ among these populations, unlike the findings in previous reports comparing anadromous and freshwater threespine sticklebacks. We found significant levels of genetic variance for age and size at first reproduction in both populations, and for fecundity in the freshwater population. The estuary and freshwater sticklebacks did not differ in average number of fin rays, gill rakers, or lateral plates, indicating that differentiation in life history has not been accompanied by significant changes in these characteristics. These results provide evidence of genetic divergence between these populations with respect to life histories, and the nature of these differences suggests that adaptation to different migratory life-styles has occurred.
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