Introduction
Atlantic cod and alewives have a well-documented ecological connection, particularly in the Gulf of Maine. Scientific and historical evidence shows that cod, along with other gadids such as haddock, pollock, and white hake, historically gathered at the mouths of rivers to feed on alewives. Both adult alewives migrating upstream in the spring and juvenile alewives emigrating in the fall served as important food sources. This prey abundance allowed cod to fatten up, maintaining energy reserves necessary for their spawning period in winter and early spring.
Historical Evidence and Key Studies
A pivotal study by Edward P. Ames and John Lichter in 2013, titled “Gadids and Alewives: Structure within complexity in the Gulf of Maine,” used historical fishing data, groundfish surveys, tagging studies, and fishermen’s ecological knowledge to demonstrate the importance of alewife runs. Their research revealed that coastal subpopulations of cod and other gadids were centered near rivers with robust alewife spawning runs. However, these subpopulations disappeared in the 1950s following the collapse of alewife populations, primarily due to dams blocking spawning habitat. This contributed to the broader decline of inshore cod stocks.
Seasonal Timing and Behavior
Spring: During May and June, adult alewives migrate upstream to spawn in New England rivers. Cod and other gadids move inshore toward warmer coastal waters and river mouths, attracted by the concentrated prey.
Fall: Coastal cod groups specifically shift to areas where juvenile alewives leave rivers for coastal nurseries. Out-migration peaks from mid-July to early November. In regions like Muscongus Bay, cod and other gadids remain inshore through fall and into winter, feeding on the large schools of juvenile alewives. Historically, the biomass of these alewives was substantial, estimated at over 1,100 metric tons per year in some bays.
Direct quotes from the Ames and Lichter paper emphasize this behavior: “Without alewives to draw them back to the mouths of rivers in spring, or schools of YOY alewives to attract them in fall, cod and other predators had no reason to return inshore.” Another states, “Predators are known to seek areas where their prey will be concentrated, such as when alewives are entering or leaving their natal rivers.”
Fattening Up and Link to Spawning
Cod in the Gulf of Maine accumulate energy reserves in the fall and winter, preparing for spawning which peaks in late winter to early spring. The fall influx of lipid-rich juvenile alewives provides high-energy forage, similar to herring, mackerel, or menhaden. Historical records and diet studies confirm that cod and other groundfish heavily consumed alewives when available, supporting growth, condition, and reproductive success. Concentrated prey sources also guided cod back to their natal spawning areas.
When alewife runs declined dramatically—from tens of millions to a fraction of their historical numbers—the inshore cod aggregations disappeared. Scientists and historians now link the loss of diadromous forage species, such as alewives and blueback herring, to the collapse of localized cod subpopulations, compounding the effects of overfishing and other environmental stressors.
These findings align with broader Northeast Fisheries Science Center (NEFSC) research showing that forage quality influences cod condition, growth, fecundity, and recruitment. Alewives, as high-lipid prey, once boosted the food base in coastal and estuarine areas where cod historically prepared for spawning.
Modern Restoration Efforts
Current river herring restoration initiatives, including dam removals and fishway construction, are motivated by the desire to restore not only alewives but also cod and other predators that once relied on them. This story is firmly rooted in ecology and history, illustrating the close connection between coastal rivers and the ocean food web in New England.