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Why are the alewives dying?

 

There are several factors involved in the recent alewife die-off and the apparently large numbers that are washing up on the beaches. These factors are origin and life history of the fish, population abundance, and weather.

Origin

The alewife (Alosa pseudoharengus) is native to the Atlantic Coast; alewives 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. 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.

Life History

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 55-60^o 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 upwelling, can cause the fish to die.

So, we see there are two underlying factors that relate to alewife mortality in the spring: their fragile condition due to poor osmotic balance and being exposed to environmental changes when they enter near shore waters to spawn. This year, two other factors are involved: age and abundance.

Abundance

The spawning run of 1995 produced a strong year class of alewives. In addition to being abundant, these fish were robust, larger than fish produced in other years. These fish are now four years old, getting towards the end of their life. The spawn of 1998 produced a strong year class as well, however, these fish, though numerous, were not as robust. Though these were relatively strong year classes, the population is not considered to be 'over abundant' and the numbers of alewife in the lake are much lower than they were in the 1960's.

Weather

As these two strong year classes, and other smaller year classes moved from the deeper waters to near shore areas this spring, they were exposed to temperature fluctuations. These fluctuations probably contributed to the die off and the large numbers of dead fish that subsequently washed upon the beach. The graphs below (see end of story) illustrate the daily high and low Lake Michigan surface water temperatures from areas near Port Washington and Sturgeon Bay.

Notice that the water temperature generally increased through June, but that on at least two occasions, there was a sharp drop in temperature with a 24-hour period, probably related to upwelling events. The effect of this temperature change on the alewives would likely have been most profound in the latter part of the month, around the 19^th or 20^th. By this time, the water temperature had reached about 55^oF and the fish had probably begun to spawn. As indicated by the mid-lake buoy, wind direction in the two days preceding the temperature drop, was predominantly south, southwest. For about two days after the upwelling event, the wind was from as easterly direction. Fish that became weak or died during the rapid temperature change would have been blown into windrows close to shore or washed onto the beaches.

Thus, in addition to the normal, die-off of alewives, this year we had large two relatively abundant year classes, one of older fish and one with small, weaker fish near shore during an upwelling event. The upwelling of cold water occurring during the spawning season probably weakened or killed many of these fragile, saltwater-adapted fish. East winds following the upwelling event contributed to the large numbers of fish accumulating on the shoreline. The number of fish washing up on the beaches should begin to diminish as water temperatures rise, spawning ends, and the fish move out to deeper water.

--Philip Moy, UW Sea Grant Fisheries Specialist