Cod: Cod has long been the iconic fish of the Atlantic fishery, with the Norwegians fishing them as the glaciers retreated and they settled Norway. In the early 1500’s the Portuguese fishermen began fishing for them in North America. Heavy fishing continued after WW2 with new technology until the fishery finally collapsed in the early 1990’s
Question 5: Studies of cod show they have low length at age, weigh at age, fecundity, and recruitment. In addition, their forage base has a lot of low lipid components like invertebrates and crustaceans. Could they be related and would more high lipid forage fish like herring, mackerel and menhaden help?
Grok: Yes, the low length-at-age, weight-at-age, fecundity, and recruitment in many Atlantic cod (Gadus morhua) stocks (especially in the Gulf of Maine/Northwest Atlantic) are plausibly linked to a forage base dominated by low-lipid, lower-energy-density prey like invertebrates and crustaceans, rather than high-lipid pelagic fish. This connection is supported by bioenergetics, diet studies, and condition metrics. Increasing high-lipid forage fish (herring, mackerel, menhaden) could help, though it is not a standalone fix due to other factors like warming temperatures.
Why the connection makes sense (and is supported by evidence)
Cod are generalist predators whose diet shifts ontogenetically: juveniles rely more on crustaceans/invertebrates, while larger cod prefer fish when available. Fish prey (especially clupeids and scombrids) have higher energy density (typically 5–7+ kJ/g wet weight, driven by lipids) than most benthic invertebrates/crustaceans (often <4–5 kJ/g, more protein-heavy and chitinous).
-Energy intake and growth/condition: Low-energy prey requires cod to consume more volume (or forage more) for equivalent calories, reducing net energy for somatic growth. This leads to slower length/weight-at-age and poorer condition (e.g., lower Fulton’s K or liver index). Bioenergetic models show diet composition directly affects growth rates—e.g., crustacean-heavy diets yield lower consumption and growth than fish- or flatfish-heavy ones.
– Fecundity: Cod store lipids primarily in the liver (up to 40–80% fat content). Pelagic fish diets boost liver index and somatic condition, increasing spawning potential and reducing atresia (egg resorption). Benthic/invertebrate-heavy diets do the opposite.
– Recruitment: Better adult condition improves egg quality/quantity (maternal effects). Juveniles also benefit from higher-quality prey for survival and growth to recruitment size. Poor forage quality contributes to the observed weak recruitment in stressed stocks.
Direct evidence from diet shifts in the Gulf of Maine/northern areas:
– Historic (late 1800s–mid-1900s) cod diets included more fish prey; modern (2000s) cod in areas like Passamaquoddy Bay show a stronger focus on lipid-poor invertebrates, with fish being sporadic.
– Analogous findings from Newfoundland/Labrador stocks: cod on shrimp-dominated (benthic/low-lipid) diets had lower condition, liver lipids, and spawning potential than those with more pelagic fish (e.g., capelin, a high-lipid forage similar to herring). Rebuilding was hypothesized to require a return to pelagic feeding.
– In the NW Atlantic, forage fish like herring and mackerel have high lipid/energy content (driven by zooplankton like Calanus), while declines in their energy density force predators to eat more biomass for the same energy—directly impacting growth and recruitment.
Gulf of Maine cod have shown persistently poor condition and truncated size/age structure in recent assessments, consistent with these diet/quality issues (exacerbated by historical overfishing and ecosystem changes).
Would more high-lipid forage fish (herring, mackerel, menhaden) help?
Yes, in principle—they are higher-energy-density prey that cod preferentially consume when abundant, directly addressing the energy deficit. Herring and mackerel are already key prey in the region; menhaden (pogie/bunker) are lipid-rich and have expanded northward with warming, potentially filling a similar role.
– Potential benefits: Higher prey quality → improved cod growth rates, weight-at-age, condition, liver energy stores → higher fecundity and better egg/larval quality → stronger recruitment (all else equal). Forage abundance also influences cod aggregation and catchability.
– Real-world context: Ecosystem models and predator studies (e.g., tuna shifting to menhaden as herring varies) show these species support energy transfer up the food web. Managing forage fisheries (herring/mackerel quotas) with predator needs in mind is already discussed in the Northeast.
Caveats and limitations (it’s not the only factor):
– Multifactorial problem: Warming in the Gulf of Maine increases cod metabolic demands while potentially reducing suitable habitat and zooplankton quality for their prey. Overfishing history, predation (seals, dogfish), and competition also play roles.
– Ecosystem tradeoffs: More forage fish could benefit cod but requires balanced management (e.g., avoiding overharvest of herring, which many predators rely on).
– Not guaranteed: If temperature or other stressors dominate, diet improvements alone may not reverse declines. However, forage quality is a modifiable lever via ecosystem-based management.
In summary, the evidence strongly supports a nutritional link: a low-lipid invertebrate-heavy forage base contributes to the observed poor cod metrics, and bolstering high-lipid pelagic forage (herring, mackerel, menhaden) would likely improve outcomes as part of broader recovery efforts. This aligns with bioenergetic principles and field studies across cod stocks. For the latest stock-specific details, NEFSC assessments often discuss condition and prey trends.
References:
Here is a list of the key scientific references and sources that informed my previous response on Atlantic cod growth, condition, fecundity, recruitment, and the role of forage quality (high-lipid pelagic fish vs. low-lipid invertebrates/crustaceans). These draw from peer-reviewed studies, NOAA/NEFSC reports, and related ecosystem analyses focusing on the Northwest Atlantic (particularly Gulf of Maine, Newfoundland/Labrador, and broader regions).
Core studies on diet, prey quality, and cod performance
– Sherwood, G.D. et al. (2007). Influence of diet on growth, condition and reproductive capacity in Newfoundland and Labrador cod (Gadus morhua): Insights from stable carbon isotopes (δ¹³C). Deep Sea Research Part II. This shows benefits of more pelagic diets (e.g., capelin) for somatic condition, liver index (lipid stores), and spawning potential (reduced atresia) compared to shrimp-dominated benthic diets.
– Wuenschel, M.J. et al. (2024). Variation in energy density of northwest Atlantic forage species: Ontogenetic, seasonal, annual, and spatial patterns. Marine and Coastal Fisheries. Quantifies energy density (ED) of key forage like Atlantic herring, mackerel, alewife, sand lance, butterfish (>6 kJ/g, high-quality) vs. others; notes recent declines in herring ED (nearly half historical values), requiring predators to consume more biomass for equivalent energy.
– Anthony, J.A. et al. (2000). Lipid content and energy density of forage fishes from the northern Gulf of Alaska. Journal of Experimental Marine Biology and Ecology. Demonstrates lipid as the primary driver of energy density variation (up to 5-fold differences); pelagic schooling fish often higher quality than demersal/low-lipid species.
Diet composition and historical shifts in cod
– Studies on trophic ecology of Atlantic cod on the Northeast US Continental Shelf (e.g., ontogenetic shifts from pelagic/benthic invertebrates in juveniles to more fish in larger cod).
– Tracking cod diet preference over a century in the northern Gulf of Maine (historic vs. modern data showing shifts toward invertebrates).
– Broader reviews of cod as a trophic generalist, with pelagic fish (capelin, herring) improving condition/growth when available vs. benthic prey.
Forage fish energy and ecosystem context
– NEFSC-related work on forage energy density trends and implications for predators (including declines in herring quality linked to broader predator condition issues, e.g., bluefin tuna).
– Bioenergetics and maternal effects in cod: Links between adult condition/liver lipids (influenced by diet) and fecundity, egg quality, and recruitment.
Stock assessment and condition context
– NEFSC Atlantic Cod Research Track Assessments (e.g., 2023 reports) discuss poor condition, truncated size/age structure, low weight-at-age, and ecosystem influences (temperature, prey) in Gulf of Maine and Georges Bank stocks.
– Life history and habitat documents for Atlantic cod in the Gulf of Maine region.
Additional supporting context comes from ecosystem models, stable isotope diet studies, and comparisons showing that high-lipid clupeids/scombrids (herring, mackerel, menhaden/capelin analogs) provide superior energy transfer compared to crustacean-heavy diets, consistent with bioenergetic principles.
These references collectively support the plausible nutritional link while noting multifactorial stressors (e.g., warming, historical overfishing). For full texts or the most recent NEFSC assessments, check NOAA repositories or academic databases like ResearchGate/Google Scholar. If you’d like me to expand on any specific paper, provide PDFs where available, or focus on a subset (e.g., Gulf of Maine only), just let me know!