The alewife (Alosa pseudoharengus) is one of the most important planktivorous fishes in the Great Lakes, providing forage for the valuable stocks of introduced salmonines in Lake Michigan and Lake Ontario.  The Lake Michigan alewife population has been in decline for a number of years, however, and alewife in Lake Ontario are also showing signs of stress.  Fisheries managers are trying to maintain high stocking rates of pacific salmon while also attempting to prevent a catastrophic collapse of the alewife populations.  Alewife winter mortality is still one of the key destabilizing forces in the interactions between salmonines and alewife in the Great Lakes, and the basic question of why Great Lakes alewives survive some winters and die in others has never been satisfactorily explained. While alewives have been present in the Great Lakes since the 1800's, blueback herring (Alosa aestivalis) are just beginning to move into the Great Lakes watershed from their native range in the Hudson and Mohawk Rivers.  In 1994, more than 9,000 blueback herring were collected in the Oswego River near Syracuse, New York and in fall 1995 two juveniles were captured in eastern Lake Ontario. Despite the general similarities between alewife and blueback herring, they appear to have different temperature, food, and depth preferences.  If established in Lake Ontario, blueback herring could alter forage base dynamics and potentially influence the ability of the ecosystem to support the valuable salmonine fisheries. My research, in collaboration with Dr. Todd Hennessey at SUNY Buffalo, examines physiological factors associated with winter mortalities of alewife and blueback herring, specifically the influence of dietary lipids and fatty acids on their lower lethal temperatures.  The basic approach is to manipulate the diet of these fish in the laboratory, and then to challenge them with cold temperatures.  Lipid analysis, including percent lipid estimation and fatty acid analysis using gas chromatography, is then carried out to correlate low temperature tolerance with lipid composition.  This information will be used to develop mechanistic models which will predict the severity of winter mortalities of these fish, and to assess the likelihood that blueback herring will successfully survive winters in Lake Ontario and the other Great Lakes.