Question: Studies of cod show they have low length at age, weigh at age, fecundity, and recruitment. In addition, the 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?

Cod Condition and Forage Base: Nutritional Links and Management Implications

Overview of Cod Metrics and Forage Base

Studies of cod reveal several concerning biological trends: low length-at-age, weight-at-age, fecundity, and recruitment. These issues are particularly notable in Atlantic cod stocks, such as those found in the Gulf of Maine and the Northwest Atlantic. One key factor appears to be the composition of their forage base, which is dominated by low-lipid, lower-energy-density prey—including invertebrates and crustaceans—rather than high-lipid pelagic fish. The relationship between cod condition and forage quality is supported by bioenergetics research, diet studies, and assessments of fish health. While increasing the availability of high-lipid forage fish (such as herring, mackerel, and menhaden) could benefit cod populations, this strategy is not a standalone solution due to additional environmental and ecological challenges.

Why the Connection Makes Sense

Cod are generalist predators whose diet changes as they grow. Juvenile cod mainly consume crustaceans and invertebrates, while larger cod prefer fish prey when available. Fish such as clupeids and scombrids, which include herring and mackerel, have a higher energy density (typically 5–7+ kJ/g wet weight, mainly from lipids) compared to benthic invertebrates and crustaceans (often less than 4–5 kJ/g, with more protein and chitin). This difference in energy content is critical to cod growth and condition.

Energy Intake, Growth, and Condition

Consuming low-energy prey requires cod to eat greater volumes or expend more effort for equivalent calories, which reduces net energy available for somatic growth. This results in slower growth rates and poorer physical condition, as reflected by metrics such as Fulton’s K or liver index. Bioenergetic models demonstrate that diet composition directly influences growth rates: crustacean-heavy diets lead to lower consumption and growth than diets dominated by fish or flatfish.

Fecundity

Cod store lipids primarily in their liver, with fat content reaching up to 40–80%. Diets rich in pelagic fish increase liver index and overall somatic condition, thereby boosting spawning potential and reducing atresia (egg resorption). Conversely, diets heavy in benthic invertebrates have the opposite effect, diminishing reproductive capacity.

Recruitment

Improved adult condition results in higher egg quality and quantity—a phenomenon known as maternal effects. Juvenile cod also benefit from higher-quality prey, which enhances survival and growth to recruitment size. Poor forage quality has contributed to the weak recruitment observed in stressed cod stocks.

Direct Evidence from Diet Shifts

Historical records show that cod diets in the late 1800s to mid-1900s included a greater proportion of fish prey. In recent decades, cod in areas such as Passamaquoddy Bay have focused more on lipid-poor invertebrates, with fish prey becoming sporadic. Similar findings have been reported for Newfoundland and Labrador stocks, where cod feeding primarily on shrimp—a low-lipid, benthic prey—exhibited lower condition, liver lipids, and spawning potential compared to those consuming more pelagic fish like capelin. Recovery efforts for these stocks have emphasized the need to return to pelagic feeding.

In the Northwest Atlantic, forage fish such as herring and mackerel offer high lipid and energy content, largely derived from zooplankton. Declines in the energy density of these forage fish force predators to consume more biomass to meet their energy needs, thereby impacting growth and recruitment.

Recent assessments of Gulf of Maine cod confirm persistently poor condition and a truncated size and age structure, consistent with dietary and forage quality issues, which are compounded by historical overfishing and broader ecosystem changes.

Would More High-Lipid Forage Fish Help?

In principle, increasing the abundance of high-lipid forage fish—such as herring, mackerel, and menhaden—would improve cod condition. These prey species are higher in energy density and are preferentially consumed by cod when available, directly addressing the nutritional deficit. Herring and mackerel are already important prey in the region, and the northward expansion of menhaden with warming temperatures suggests they could play a similar beneficial role.

Potential benefits include improved growth rates, weight-at-age, condition, and liver energy stores, which translate into higher fecundity and better egg and larval quality, supporting stronger recruitment. Forage abundance also influences cod aggregation and catchability. Ecosystem models and studies of other predators (such as tuna) show that these species support energy transfer up the food web. Managing forage fisheries—like setting herring and mackerel quotas with predator needs in mind—is an ongoing topic in Northeast fisheries management.

Caveats and Limitations

The challenges facing cod are multifactorial. Warming temperatures in the Gulf of Maine increase metabolic demands for cod while reducing suitable habitat and the quality of zooplankton (food for their prey). The history of overfishing, predation by other species (such as seals and dogfish), and competition further complicate recovery efforts.

Ecosystem tradeoffs must be considered: increasing forage fish could benefit cod but requires balanced management to avoid overharvesting species like herring, which are vital to multiple predators. Additionally, improvements in diet alone may not be sufficient if other stressors, such as temperature or predation, dominate. Nonetheless, forage quality remains a modifiable factor through ecosystem-based management.

Summary

Evidence strongly supports a nutritional link: a low-lipid, invertebrate-heavy forage base contributes to poor cod growth, condition, fecundity, and recruitment. Enhancing populations of high-lipid pelagic forage fish—such as herring, mackerel, and menhaden—would likely support improved outcomes for cod as part of broader recovery efforts. This conclusion aligns with bioenergetic principles and field studies across multiple cod stocks. For up-to-date information on stock-specific trends, Northeast Fisheries Science Center (NEFSC) assessments often detail cod condition.