What causes alewives to die off in great numbers at certain times of the year?

Credit: Phil Moy, Wisconsin Sea Grant


The alewife (Alosa pseudoharengus), native to the Atlantic Coast, entered the Great Lakes through the Welland Canal and made their way to Lake Michigan by 1949. Alewives are not well adapted to the osmotic stress associated with life in fresh water. In freshwater, the salt concentration in a fish’s body is higher than the surrounding water. For this reason water tends to leak into the cells of the fish, a process called osmosis. Freshwater fish must constantly 'pump' water out of their bodies; fish that are well adapted to a freshwater environment have larger kidneys than their saltwater counterparts. Because of this physiological stress, alewives are rather sensitive to disturbances in their Great Lakes environment.

Alewives spend most of the year in the deeper waters of the open lake, but come into near shore waters in the summer when they are ready to spawn. Alewives begin to spawn when the water temperatures reach about 55-60o F. In their native habitat alewives are anadromous, swimming upstream to spawn in the spring. In the Great Lakes, the fish congregate near the outlets of rivers or streams or near harbors that occur at the outlet of a river. Generally, alewives begin reproducing at about two years of age. Alewives do not necessarily die after they spawn, but when the fish move from the deeper water to near shore areas they are exposed to fluctuating temperatures. A severe change in water temperature, such as can occur with

Credit: Phil Moy, Wisconsin Sea Grant
upwelling, can cause the fish to die.

Underlying factors that relate to alewife mortality in the spring include: (1) their fragile condition due to the physiological stress of being in fresh water (2) a weakened condition due to lack of forage in the winter (3) stress related to spawning and (4) being exposed to rapid temperature changes when they enter nearshore waters to spawn. Whether we will see them die off in large numbers depends on population abundance, age of the fish, general physical condition, and the weather.