Latitudinal Variation in Reproductive Characteristics of American Shad (Alosa sapidissima): Evidence for Population Specific Life History Strategies in Fish

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Abstract
The reproductive characteristics of 5 populations of American shad (A. sapidissima) on the Atlantic coast [St. Johns River, Florida; York River, Virginia; Connecticut River, Connecticut [USA]; and St John River and Miramichi River, New Brunswick [Canada].] were studied. The proportion of repeat spawners increased with the latitude of the home river. Relative and absolute fecundities decreased as the proportion of repeat spawners increased. These reciprocal trends in reproductive characteristics are independent of growth parameters. The principal factor influencing reproductive strategies in shad appears to be variability in the thermal regime of the home river which influences egg and larval survival. Northern populations, spawning in environments thermally harsh and variable, allocate a greater proportion of their energy reserves to migration, thereby ensuring higher postspawning survival. This is accomplished by reducing the energy allocated to gonads. The pattern of reproductive responses of shad to the thermal environment of the natal river is consistent with existing ecological theory concerning the evolution of reproductive strategies in response to differing environmental conditions. Available literature for several other fishes suggests that fine tuning of reproductive strategies to local environmental conditions may be widespread among fish and may be the ultimate basis for the evolution of homing.

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