Effects of freshwater and marine overwintering environments on life histories of threespine sticklebacks: evidence for adaptive variation between anadromous and resident freshwater populations.

Life history theory predicts that migratory fishes should delay reproduction, be larger at first reproduction, and have higher fecundities than nonmigrants. We tested this hypothesis by comparing life histories of anadromous ("estuary") and resident freshwater ("upstream") threespine sticklebacks (Gasterosteus aculeatus L.) from the Navarro River, California, USA. Using a split-brood, two-environment breeding design, families from cach population were divided and reared in both freshwater and seawater overwintering environments. In both treatments, the more migratory estuary sticklebacks were larger at first reproduction and had large initial clutch sizes; in the freshwater treatment, the estuary sticklebacks matured later than the upstream fish. Population means varied little across treatments, indicating that the average effects of the different overwintering conditions were slight. The responses of individual families to a given overwintering treatment were highly variable in both populations, as reflected in significant family x treatment effects for all traits. Phenotypic correlations among life history traits were significant and positive for most traits, and were similar in magnitude in both populations. Differences in the relative degree of specialization for migration may in part explain variation in life history between these populations.