What Caused the Big Alewife Die-Off of 1999?

An Old Alewife's Tale

By Philip Moy and Elizabeth Seaman, edited by Stephen Wittman

MANITOWOC, Wis. (07/02/99) —  A couple of sharp drops in water temperature that caught abundant numbers of elderly and young alewives in shallow water is the likely cause of last month's larger-than-usual die-off of the fish along Wisconsin's Lake Michigan shore, according to University of Wisconsin Sea Grant Fisheries Specialist Philip B. Moy.

Thousands of dead alewives continued to wash up on the state's Lake Michigan beaches last week following one of the larger alewife die-offs in recent years, posing a pungent threat to the plans of Fourth of July beach-goers.

"The spawning run of 1995 produced a large number of alewives," Moy said. "These fish are now four years old and getting towards the end of their life. The spawn of 1998 also produced a large number of alewives, but these fish were less robust."

These two large age-groups — one near the end of its lifespan, the other young and weak — had both moved into shallow nearshore waters to spawn, Moy said, when on at least two occasions in June, there was a sharp drop in water temperature. The effect of this temperature change would have been most devastating around June 19-20, he said, when the lake's water was warm enough (55°F) to induce spawning by the fish.

One other factor played an important part, according to Moy. "For the two days before the temperature drop, the wind direction was predominantly south-southwest. Then, for about two days after that, the wind was from an easterly direction," he said. "Fish that became weak or died during the rapid temperature change would have been blown in close to shore or washed up in windrows on the lake's beaches."

Alewife die-offs occur almost every spring and early summer, when the small silvery fish come into nearshore waters to spawn. These waters are much more subject to temperature fluctuations, Moy said, and a sudden severe change in water temperature can be fatal to the fish.

While several factors figure into the annual alewife die-off in Lake Michigan, the most fundamental one is a flaw in the fish's design resulting from its oceanic origins.

"The alewife is native to the Atlantic Coast and not well adapted to life in fresh water," Moy said. "Freshwater fish must constantly 'pump' water out of their bodies, so they need larger kidneys than their saltwater counterparts. Alewives don't have this adaptation, which means that they are already fragile. Any disturbance in their Great Lakes environment can be too much for them."

He said the good news is that 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.

For a more in-depth explanation of the die-off, see "Why Are the Alewives Dying?"
or contact Philip Moy, UW Sea Grant's Advisory Services Fisheries Specialist at UW-Manitowoc, phone (920) 683-4697

Some additional background from the pages of the Milwaukee Journal-Sentinel:
Alewives a bigger stink than usual (06/28/99)

Created in 1966, Sea Grant is a national network of 30 university-based programs of research, outreach and education dedicated to the protection and sustainable use of the United States' coastal, ocean and Great Lakes resources. The National Sea Grant Network is a partnership of participating coastal states, private industry and the National Sea Grant College Program , National Oceanic & Atmospheric Administration , U.S. Department of Commerce . The University of Wisconsin Sea Grant College Program is administered by the Sea Grant Institute on the UW-Madison campus in Madison, Wisconsin.