Alewife, Alosa pseudoharengus, have been recognized for several decades as one of the most important forage fish in the Laurentian Great Lakes. Although massive alewife die-offs have regularly been observed throughout the Great Lakes and other inland lakes, little substantive information is available regarding physiological mechanisms associated with adult alewife mortality. Long-term field surveys have shown a correlation between cold winter temperatures, poor condition and adult alewife mortality. In this study, adult alewife were raised in replicate pond systems and subjected to contrasting cold temperatures (4 and < 2°C) representing mild and severe winter conditions. We evaluated alewife mortality, condition and immune response to these temperatures. In contrast to our expectations, alewife exposed to mild and severe winter temperatures showed no difference in mortality or condition (measured as the ratio of dry to wet weight). Survival of alewife held in ponds with mild winter conditions (~ 4°C) was similar to that of alewife exposed to prolonged periods (more than six weeks) of temperatures < 2°C. This result contrasts with previous observations indicating that alewife cannot tolerate temperatures < 3°C. Circulating lymphocytes from alewife exposed to severe winter temperatures were significantly lower in number (~40%) compared to fish experiencing milder winter conditions, suggesting sub-lethal immunosuppression in response to the colder winter temperatures. Although colder winter temperatures did not directly induce alewife mortality, these results suggest that winter conditions that result in colder water temperatures can produce immunosuppression, thereby increasing alewife susceptibility to disease and mortality.
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