THE DEAD OF WINTER - SIZE-DEPENDENT VARIATION AND GENETIC-DIFFERENCESIN SEASONAL MORTALITY AMONG ATLANTIC SILVERSIDE (ATHERINIDAE, MENIDIA-MENIDIA) FROM DIFFERENT LATITUDES

Genetic differences among populations of Atlantic silverside (Menidia menidia) are hypothesized to be evolutionary responses to intense, siz e-selective winter mortality at high latitudes. Three experiments were conducted to test features of winter mortality. In the first experime nt, we varied size and whether food was provided or withheld; temperat ures were permitted to follow ambient (New York) wintertime fluctuatio ns. Mortality and depletion of energy reserves were more rapid in the units receiving no food. Small fish died before larger fish in these u nits, but not in the units receiving food. Energy depletion of fish in the no-food treatment resembled that of fish in the wild. In the seco nd experiment, we varied size and population of origin, representing h igh-latitude (Nova Scotia), midlatitude (New York), and low-latitude ( South Carolina) populations. These fish were provided food and showed minimal depletion of energy reserves, but mortality rates were high wh en water temperatures were low. Mortality did not vary with size in Ne w York and South Carolina fish, but was highest in intermediate-size f ish from Nova Scotia. There was a pronounced population difference in survival rate (Nova Scotia > New York > South Carolina). In the third experiment, food was withheld and extreme low temperatures were modera ted. Energy depletion was rapid and small fish died before large fish. We conclude that populations in seasonal environments are likely to b e subject to size-selective winter mortality when energy reserves are depleted and that juvenile growth rates have evolved in response to th is selection pressure. In addition, high-latitude populations have evo lved greater tolerance to other winter stresses associated with low te mperatures.