The Relation between Capacity for Growth and Length of Growing Season: Evidence for and Implications of Countergradient Variation

Evidence suggests that the capacity for growth (i.e., maximum growth potential) within a species may vary inversely with the length of the growing season across a latitudinal gradient. I evaluated this hypothesis with data on three species—American shad Alosa sapidissima, striped bass Morone saxatilis, and mummichog Fundulus heteroclitus—having wide latitudinal ranges (≈29–46°N) along the east coast of North America. For each of these species, the length of the first growing season decreases by a factor of about 2.5 with increasing latitude within the speciesˈ range, yet body size at the end of the first growing season is independent of latitude. Northern fish must, therefore, grow substantially faster within the growing season than do southern fish. This “countergradient variation” in growth rate may be more widespread than has been recognized. A similar latitudinal pattern in growth rate has a genetic basis in the Atlantic silverside Menidia menidia, and data on Atlantic salmon Salmo salar, largemouth bass Micropterus salmoides, and several marine invertebrates also support the hypothesis of countergradient variation. Moreover, the capacity to grow rapidly within the growing season in high-latitude environments may be an adaptive response to size-selective winter mortality (i.e., the tendency of small fish to die more readily than large fish), a phenomenon known to occur in numerous fish species. The countergradient variation model has important implications for choosing genotypes with high growth rates for aquaculture, transplantation of natural stocks across latitudes, and elucidation of the trade-offs between growth rate and other physiological or life history traits.