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.
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.
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 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.
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
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 19th
or 20th. By this time, the water temperature had reached about 55oF
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