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The reproductive characteristics of five populations of American shad (Alosa sapidissima) on the Atlantic coast 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 that are 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 …