Mortality caused by cold stress has been identified as a factor regulating the abundance of gizzard shad populations, particularly in waters near the northern edge of the species' distribution where prolonged cold winters might almost eliminate them. Sudden cold spells causing water temperatures to drop several degrees in already cold water can cause massive kills.
In a study by Cornell University researchers, young-of-the-year gizzard shad were held in cages in Oneida Lake, New York, during the winter prior to ice-up.* Shad had low mortality in water above 46°F and mortality was high in water less than 39°F. Researchers also tested survival in cold rooms under controlled temperature treatments of 34°F, 36°F, and 39°F to simulate mid-winter conditions. First, shad were allowed to acclimate to 46°F, then the tank temperatures were reduced by about a half of a degree per day until the final test temperatures were reached.
Mortality was low during the acclimation period. A larger proportion of shad survived for longer periods in the 39°F tanks and mortality was highest in the coldest tanks.
Within each temperature treatment, small fish died faster. The researchers also observed that the average size of shad in field collections increased through winter, indicating higher cold tolerance of larger individuals. They conclude that cold stress and the inability to acclimate to decreasing temperatures, rather than starvation, are key factors in winter mortality.
These findings suggest that you can expect large shad winterkills when severe cold-fronts decrease water temperatures quickly in early winter before ice-up. Once lakes are covered in ice and buffered more from sudden changes in air temperatures, the severity of overwinter mortality should coincide with the duration of the ice-cover period.
*Fetzer, W. W., Brooking, T. E., J. R. Jackson, and L. G. Rudstam. 2011. Overwinter mortality of gizzard shad: Evaluation of starvation and cold temperature stress. Trans. Am. Fish. Soc. 140:1460-1471.