Overwinter Mortality of Gizzard Shad: Evaluation of Starvation and Cold Temperature Stress

Overwinter mortality of gizzard shad Dorosoma cepedianum has been attributed to starvation, cold stress, and predation; however, the interactions among these factors are not well understood and can change across winter. We evaluated possible causes for overwinter mortality of age-0 gizzard shad through a combination of experiments and field sampling during the winters of 2005-2006 and 2006-2007. In the first experiment, gizzard shad were placed into cages in Oneida Lake, New York, during three time periods prior to ice formation. The fish exhibited low mortality at temperatures above 8 degrees C but high mortality (>75%) in all cages as temperature dropped below 8 degrees C. We observed no consistent patterns of length-dependent mortality or changes in total percent dry weight (DW). However, the viscerosomatic index (VSI) decreased during all time periods, indicating disproportionate use of visceral tissues. In the second experiment, gizzard shad were exposed to temperature treatments of 1, 2, and 4 degrees C in experimental cold rooms. Little mortality occurred as temperature dropped from 8 degrees C to 4 degrees C, but mortality increased after temperature reached 4 degrees C and was highest in the two coldest treatments. Within a temperature treatment, small fish died faster, and mortality exhibited a weak negative correlation with VSI and percent DW. Average size of fish in field collections increased through the winter, indicating higher mortality of smaller individuals and proportional changes in somatic DW and visceral DW. Results from both experiments and the field collections suggest that cold stress and an inability to acclimate-rather than starvation-are driving gizzard shad mortality at low temperatures.