Alewife Invasion: Ecological Disruption in the Great Lakes

The alewife (Alosa pseudoharengus), a small anadromous herring native to the Atlantic, invaded the Great Lakes via the Welland Canal (bypassing Niagara Falls) in the 1930s–1950s. It exploded into a dominant biomass, triggering cascading ecological changes.

Mechanisms of Impact

1. Zooplankton Depletion (Bottom-Up Control)

• Alewives are efficient filter-feeders on large zooplankton (Daphnia, Limnocalanus, Leptodora).
• They decimated large-bodied zooplankton, shifting communities to small, less nutritious species (copepods, rotifers).
• Consequence: Reduced food for native larval fish (lake trout, whitefish, cisco, perch) and competitors (bloater, smelt).

2. Predation on Native Larvae (Top-Down Control)

• Alewives consume eggs and larvae of lake trout, whitefish, walleye, yellow perch, cisco.
• This recruitment failure prevented natural reproduction of key native predators for decades.

3. Competition with Native Forage Fish

• Outcompeted bloater, kiyi, deepwater ciscoes, emerald shiner for zooplankton.
• Contributed to extirpation/extinction of several deepwater cisco species.

4. Thiamine Deficiency Syndrome (M74/Cayuga Syndrome)

• Alewives are rich in thiaminase, an enzyme that destroys thiamine (Vitamin B1).
• Predators eating alewives (lake trout, salmon, walleye) develop thiamine deficiency → reproductive failure, neurological damage, early mortality in offspring.
• This biochemical blockade hindered lake trout rehabilitation for decades.

5. Population Cycles & Die-Offs

• Alewives exhibit boom-bust cycles. Massive die-offs (thermal stress, starvation) foul beaches, clog water intakes, cause botulism outbreaks killing waterbirds.

Management Response: Top-Down Control

Intentional stocking of Pacific salmon (Chinook, Coho) and lake trout rehabilitation created a predator gauntlet that suppressed alewife biomass by >90% in Lakes Michigan, Huron, Ontario. This allowed partial zooplankton and native prey fish recovery—but created a predator-prey imbalance (salmon now food-limited).

Current Status

Alewives remain abundant in Lakes Michigan and Ontario; collapsed in Huron (2003); low in Erie, Superior. They are now a managed forage base for the multi-billion dollar salmonid sport fishery—a novel, human-maintained ecosystem state.