Cod and Alewives: QUESTION 6: I have heard that there might be a correlation between cod and alewives, that cod might gather at the mouths of rivers for juvenile alewives, and they fatten up on them prior to spawning? is there anything to that story?
GROK: Yes, there is solid scientific and historical evidence supporting this story. Atlantic cod (and other gadids like haddock, pollock, and white hake) in the Gulf of Maine historically aggregated at the mouths of rivers to feed on alewives—both adult alewives migrating in during spring and, especially, juvenile (young-of-the-year or YOY) alewives emigrating in the fall. This prey abundance helped the cod fatten up and maintain condition, which would support energy reserves needed before their winter/early spring spawning period.
The key source is a 2013 study by fisheries scientists Edward P. Ames and John Lichter, titled “Gadids and Alewives: Structure within complexity in the Gulf of Maine.” Using historical fishing grounds data from the 1920s, groundfish surveys, tagging studies, and fishermen’s ecological knowledge, they showed that coastal subpopulations of cod and other gadids were centered near rivers with strong alewife spawning runs. These groups disappeared in the 1950s after local alewife populations collapsed (largely due to dams blocking spawning habitat), contributing to the broader decline of inshore cod stocks.
Seasonal Timing and Behavior
Spring: As adult alewives migrated upstream to spawn (typically May–June in New England rivers), gadids moved inshore toward warmer coastal waters and river mouths. The fish were drawn by the concentrated prey.
Fall: Coastal cod groups specifically shifted to grounds right in the path of YOY/juvenile alewives leaving the rivers and heading to coastal nurseries (out-migration peaks mid-July to early November). In places like Muscongus Bay (fed by the Damariscotta and St. George Rivers), cod and other gadids stayed inshore through fall and into winter to gorge on these schools. Large, stable biomass of YOY alewives (estimated at over 1,100 metric tons per year in some bays historically) was a major attractant.
Direct quote from the paper: “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: “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 build energy reserves (condition/“fatness”) in the fall and winter ahead of spawning (which peaks in late winter to early spring for many stocks). The fall pulse of lipid-rich juvenile alewives provided exactly the high-energy forage needed for this—similar to the herring/mackerel/menhaden we discussed earlier. Historical accounts and diet studies confirm cod (and other groundfish) consumed alewives heavily when available, helping sustain growth, condition, and reproductive output. The paper notes that these concentrated prey sources supported resident coastal groups and even guided some cod back to natal spawning areas.
When alewife runs dropped sharply (from tens of millions to a tiny fraction by the late 1800s/early 1900s, and further in the mid-20th century), the inshore cod aggregations vanished. Many scientists and historians now link this loss of diadromous forage (alewives + blueback herring) to the collapse of localized cod subpopulations—compounding overfishing and other stressors.
This fits with broader NEFSC findings on forage quality affecting cod condition, growth, fecundity, and recruitment: alewives are another high-lipid clupeid (like herring) that used to boost the prey base right in coastal/estuarine areas where cod historically fed and prepared for spawning.
Modern river herring restoration efforts (dam removals, fishways) are partly motivated by this—hoping to bring back not just alewives but the cod and other predators that once relied on them. So yes, the story you heard is grounded in real ecology and history, not just folklore. It’s a great example of how tightly linked coastal rivers and the ocean food web used to be in New England.
Rederencess:
Here is a list of the primary sources that informed my response to your question about the potential correlation between Atlantic cod and alewives (including cod aggregating at river mouths to feed on juvenile alewives and fattening up prior to spawning):
– Ames, E.P., and Lichter, J. (2013). “Gadids and Alewives: Structure within complexity in the Gulf of Maine.” Fisheries Research, 141: 70–78.
This is the central scientific paper directly addressing the topic. It uses historical fishing grounds data from the 1920s, groundfish surveys, tagging studies, and ecological knowledge to document how coastal subpopulations of cod (and other gadids like haddock, pollock, and white hake) were historically centered near rivers with strong alewife runs. It details seasonal movements: gadids shifting to river mouths in spring for adult alewives and in fall for large concentrations of young-of-the-year (YOY/juvenile) alewives emigrating from rivers. The paper explicitly links the loss of these alewife concentrations (due to dams and other factors) to the disappearance of inshore cod aggregations, noting that without alewives, cod and other predators “had no reason to return inshore.” It also discusses how these high-energy prey pulses supported condition and population structure. Full text available via University of Maine Digital Commons or ScienceDirect.
– Ames, E.P. (2004). “Atlantic cod stock structure in the Gulf of Maine.” Fisheries*, 29(1): 10–28.
Earlier work by the same lead author on cod population structure in the Gulf of Maine, including links to inshore habitats and riverine influences. It provides foundational context for the 2013 paper.
– NEFSC State of the Ecosystem reports (New England, 2024 and 2025 editions).
These reports discuss forage fish energy density (including alewife as a listed species in energy density figures), the role of high-lipid clupeids (like alewives and herring) in predator condition and productivity, and broader ecosystem links between prey quality and groundfish performance (growth, fecundity, recruitment). They support the nutritional importance of lipid-rich forage like alewives for species such as cod.
– Supporting studies and reviews citing the Ames & Lichter work:
– Willis et al. (various years) and other diet studies confirming cod predation on alosines (river herring including alewives) in nearshore habitats, with greater contributions in areas near river mouths.
– Broader literature on river herring restoration (e.g., Maine Sea Grant, ASMFC reports) noting historical importance of alewives as high-lipid prey for cod and other groundfish, and how dam removals aim to restore these cross-ecosystem subsidies.
These sources confirm the story is well-grounded in historical ecology, not just anecdotal. The Ames & Lichter (2013) paper is the most direct and frequently cited reference for the specific behavioral and spatial linkage you described.
If you’d like direct links, PDFs (where openly available), or more details/excerpts from any of these, or sources for other parts of our conversation, just let me know!