Ecology: Alewife Explosion

From the Chicago waterfront to the Mackinaw Bridge, the shores of Lake Michigan were taken over last month by dead alewives. The fish,*members of the herring family, washed ashore on every incoming wave, piling up on the beaches faster than bulldozers and tractors could clear them away. They filled the air with the odor of decay and drew swarms of mosquitoes and flies.

Chicago's municipal water-supply inlets and those of industries that draw water directly from the lake became clogged time and again with the little (two-to-seven-inch) alewives. Off Benton Harbor, Mich., an aerial photographer reported a ribbon of dead fish 50 ft. wide and 40 miles long floating on the surface of the lake.

Scientists have investigated a number of less spectacular alewife "die-offs" in recent years, but they still have conflicting theories about the cause of the phenomenon. Some believe that alewives head for shallow coastal waters in such great numbers every spring that they exhaust the oxygen supply in their immediate vicinity and suffocate. Others suggest that plankton—tiny water plants and animals on which alewives feed—suddenly begin dying just as the fish are crowding into coastal waters in the spring. U.S. Fish and Wildlife Biologist Melvin Greenwood theorizes that the alewives are killed by sudden temperature drops caused by violent spring storms that drive colder waters from the center of Lake Michigan into the shore areas.

Steelhead Hopes. Originally an ocean fish, the alewife could not penetrate very far into the Great Lakes until the 1930s, when rebuilding of the Welland Canal provided it with a convenient bypass around Niagara Falls. Even so, their numbers remained relatively small until the 1950s, when the sea lamprey—also an oceanic interloper—wiped the Great Lakes clean of the trout and burbot that were feeding on alewives. Too small a target for the lamprey (which is now being eliminated by chemical controls), and left with no natural enemies, the alewives promptly began a population explosion.

Commercial fishermen now take about 50 million Ibs. of the plentiful alewives from Lake Michigan each year, for processing into fish meal, fish oil, and cat and chicken food. Worried fed eral and state agents have stocked the lake with 2,000,000 steelhead trout and 300,000 coho salmon, hoping that they will take to an alewife diet and proliferate, thus bringing the ecology of the lake back into balance.

The trout, salmon and fishermen have their jobs cut out for them. Despite the deaths of hundreds of millions of ale wives in the current die-off, there are still an estimated 175 billion in Lake Michigan alone.