Acoustic measures of the relationship between winter severity and overwinter mortality of alewives

Freshwater alewives have been shown to be quite sensitive to cold water, and evidence exists to suggest that entire year classes have died in severe winters. Although a number of factors influence alewife survival rates, it is likely that overwinter mortality is substantial in most years. In spite of the apparent importance of overwinter mortality, estimates of overwinter mortality rates for this species do not exist. In this study, acoustic methods were used to estimate overwinter mortality of alewives in a lake dominated by this species by comparing abundances in fall and the subsequent spring in five years. Overwinter mortality rates were quite variable, with daily instantaneous rates between 0.0022-0.0079, corresponding to percent mortality between 38-83%. These mortality estimates were similar in magnitude to annual estimates for adult alewives in Lake Michigan during the 1960s. Daily instantaneous mortality rates presented here were also similar to daily instantaneous rates of juvenile Pacific herring. Daily instantaneous mortality during winter was positively correlated with an index of winter severity (cumulative freezing degree days), but was not related to mean length in fall, nor to summer water temperature. Because abundance data from fall and subsequent spring acoustic surveys presented here were separated by nearly half a year, we suggest that overwinter mortality is a significant portion of annual natural mortality even under mild winter conditions.