Seasonal and Diel Distribution of Alewife in Lake Ontario

Disruptions to the forage base in Lakes Huron and Michigan have had drastic effects on salmon fisheries.  Despite similarities between these systems and Lake Ontario, patterns in population metrics observed in these lakes for alewife Alosa pseudoharengus, the dominant forage species, are not apparent for Lake Ontario.  Alewife age- and size-structure remain constant, and fish condition is high despite concerns about epilimnetic zooplankton declines.  We used hydroacoustics to evaluate offshore seasonal (May-November) and diel summer (July-September) distributions of alewife in 2013.  We describe the biotic and abiotic factors most strongly influencing their vertical distribution (temperature, zooplankton distribution), and evaluate seasonal (June-November) patterns in night distribution to better understand drivers of alewife population dynamics at a whole-lake level.  Alewife night time vertical distribution from vertical gillnet catches were bimodal with one peak near the surface and the second in the lower epilimnion.  Acoustic sampling confirmed this distribution pattern.  During the day, alewife schools were found deeper in the water than during the night and partly merged with the deep chlorophyll layer (DCL), indicating that alewife may be feeding on zooplankton in the DCL.  Better understanding alewife spatial ecology will aid in fisheries management and balancing predators and prey in Lake Ontario.