How has the energy content in the diet of cod in the gulf of maine and georges bank changed with the collapse of the herring, mackerel and menhaden populations?
**The energy content (energy density) of Atlantic cod (*Gadus morhua*) diets in the Gulf of Maine (GoM), including northern areas like Passamaquoddy Bay, has decreased in association with major declines in key high-energy pelagic forage fish such as Atlantic herring (*Clupea harengus*), with similar historical patterns for mackerel and menhaden.** (LINK)
Cod are generalist predators, but they preferentially consume lipid-rich pelagic fish (e.g., herring) when available because these provide substantially higher energy density than benthic invertebrates (e.g., crabs, amphipods, echinoderms, bivalves). Pelagic forage fish like herring typically have energy densities of ~6–10+ kJ/g wet weight (higher in historical data), while many invertebrates and lower-quality prey are lower in lipids and thus less energetically advantageous for cod growth, condition, gonad development, and recovery after spawning. (LINK) (LINK)
### Key Evidence from Diet Studies (Primarily GOM)
A century-scale comparison in northern GoM (Passamaquoddy Bay) shows clear shifts tied to changes in forage fish availability:
– In summer 1965 (when motorized trawling/dredging had altered the benthos but forage fish like herring were still relatively available post-early 20th-century exploitation), large cod diets were dominated by fish (~69% by weight), with >70% of identifiable fish prey being Atlantic herring. Fish occurred in 38–55% of stomachs overall. (LINK)
– By 2005–2008 (after collapses in herring stocks and reduced overall forage fish abundance), fish comprised only ~29% of cod diets by weight. Diets shifted toward greater diversity and higher proportions of benthic invertebrates (e.g., crabs, echinoderms, amphipods), resembling the invertebrate-heavy diets of 1896 cod when small finfish were heavily exploited. Modern cod relied more on lipid-poor invertebrates because high-energy pelagic fish (especially herring) were at low abundances and largely absent from stomachs. (LINK) (LINK)
This dietary shift is explicitly described as producing a **less energetically rich diet** overall. Herring and similar clupeids (including historical menhaden) were noted as key high-value prey that became scarce; menhaden had already largely disappeared from the Bay of Fundy/GoM inshore areas by the 1860s due to fisheries, while herring schools were heavily reduced by the late 19th/20th centuries and collapsed further in the 1970s. (LINK)
**Georges Bank (GB) context**: Direct century-scale stomach-content comparisons like those in the GoM are not as detailed for GB in the available literature. However, GB herring stocks also collapsed in the mid-1970s due to distant-water fleet overfishing, with similar broader declines in mackerel and menhaden influencing the regional prey field. Cod diets on the Northeast US shelf (including GB) have long included these forage fish as primary prey, and ecosystem shifts (e.g., increased reliance on sand lance or invertebrates) have been noted regionally. Bioenergetics models comparing GoM vs. GB cod indicate diet energy content and temperature drive differences in growth efficiency, but do not show a post-collapse temporal analysis specific to GB. Regional patterns suggest analogous reductions in high-energy prey availability for GB cod. (LINK) (LINK)
### Additional Context on Forage Fish Energy Density
Recent monitoring shows that energy density of key forage species (including herring) has also declined or remained variable/low compared to historical values (e.g., herring now often <7 kJ/g wet weight vs. 9–10.6 kJ/g in 1980s/1990s data). This compounds the effect of reduced abundance. (LINK) (LINK)
These changes are linked to overfishing of forage fish, ecosystem shifts, and environmental factors (e.g., warming), contributing to poorer cod condition and challenges in stock recovery. Ecosystem reports for the GoM/GB note ongoing work on forage energy as a driver of predator performance. (LINK)