Is the cod food supply low in lipids?

Is the cod food supply low in lipids?

**No, the primary food supply (forage/prey) for Atlantic cod in the Gulf of Maine (GOM) and Georges Bank (GB) is generally not considered low in lipids overall—key prey species like northern sand lance and Atlantic herring are lipid-rich and provide high energy density—but there is significant seasonal, interannual, and spatial variation, with some evidence of reduced quality or availability in recent decades contributing to cod’s poor condition. (LINK)

### Main Prey and Lipid/Energy Content

Cod are generalist predators feeding on small pelagic fish (e.g., sand lance, herring, mackerel), juvenile gadids, invertebrates (e.g., shrimp, crabs), and squid. The most energetically important forage species in the region are:

– **Northern sand lance (*Ammodytes dubius*)**: A high-quality, lipid-rich prey (often classified as high-energy density, >6 kJ/g wet weight). They accumulate lipids heavily from late winter through summer (peaking around August, with whole-body lipid content often 20-25% or higher in peak months), feeding primarily on lipid-rich copepods like *Calanus finmarchicus*. They then use these stores for overwintering and spawning in fall. (LINK)

– **Atlantic herring**: Traditionally high-lipid (clupeids are energy-dense), though recent studies show energy density (ED) values roughly half of some historical reports in the northwest Atlantic, still qualifying as high-quality prey. (LINK)

– Other prey: Atlantic mackerel and butterfish are also high-ED; silver hake and squids are moderate (4-6 kJ/g). Gadids like juvenile pollock or cod themselves tend to be lower-lipid. (LINK)

Forage fish ED varies strongly by season (higher in fall after summer feeding than spring post-spawning/winter), size/age (ontogenetic shifts), depth, location (e.g., GOM vs. GB), and year. High-lipid pelagic forage generally outperforms low-lipid demersal species in energy transfer to predators like cod. (LINK)

### Factors Affecting Prey Quality and Cod Condition

– **Positive aspects**: Sand lance and herring provide critical high-lipid pulses that support cod growth, energy reserves (e.g., liver condition via hepatosomatic index), and reproduction. Historical and recent data confirm these as key drivers of predator condition in the region. (LINK)

– **Challenges and declines**:

  – Shifts in forage abundance (e.g., sand lance declines in some periods, oscillations with herring) and availability affect cod aggregation and feeding success.

  – Warming waters, changing zooplankton (e.g., *Calanus* declines or shifts), and stratification can reduce prey productivity or lipid accumulation.

  – Cod condition (weight-at-length, energy reserves) has shown long-term variability and periods of decline linked to prey quality/quantity, compounded by overfishing history and climate stress. Poor condition reduces growth, survival, and recruitment. (LINK)

  – In analogous systems (e.g., Baltic cod), diet shifts away from lipid-rich clupeids have led to energy reserve declines.

Overall, while individual prey items are not inherently “low-lipid,” the effective food supply for cod can be suboptimal due to lower availability of the best high-lipid options, seasonal gaps, or environmental pressures reducing total energy transfer. This contributes to the stocks’ ongoing poor status, slow rebuilding, and truncated age structures. (LINK)

For the latest data, refer to NOAA’s Northeast Fisheries Science Center reports on forage fish energy density, ecosystem status reports for GOM/GB, or sand lance/herring assessments. Climate-driven changes continue to influence this dynamic.