What is the optimum energy density of forage for fish like cod and striped bass?
**There is no single universal “optimum” energy density value for forage (prey) of predatory fish such as Atlantic cod (*Gadus morhua*) or striped bass (*Morone saxatilis*), as it depends on context (bioenergetics efficiency, growth, digestibility, prey availability, season, and size). However, scientific literature consistently classifies and values prey quality by wet-weight energy density (ED), with higher values generally preferred for efficient growth and energy transfer.**(LINK)
Note, Conversion rate from kJ/gram to Cal/lb=106.3387, or 106.34
### Prey Quality Classification (Common Thresholds)
Studies of northwest Atlantic and other forage species (relevant to both cod and striped bass) use these approximate categories on a wet-weight basis: (LINK)
– **High-quality prey**: >6 kJ/g (638 Cal/lb)
– **Moderate-quality prey**: 4–6 kJ/g (425-638 Cal/lb)
– **Low-quality prey**: <4 kJ/g (<425 Cal/lb)
High-quality forage maximizes energy return relative to handling/digestion time and supports better predator growth and condition. Values vary substantially by species, size, season, location, and year (often higher in fall after summer feeding; lower in spring after spawning or overwintering). (LINK)
### Typical Values for Relevant Forage
– **For cod and similar northwest Atlantic predators**: Preferred or high-value prey such as Atlantic herring, capelin, and certain squid historically reached ~9–10+ kJ/g (or higher in some older datasets); more recent measurements for h(LINK)erring are often lower (sometimes ~5–7+ kJ/g). Other common prey (sand lance, various gadids, shrimp, plaice) are frequently in the ~4–5.5 kJ/g range. Capelin is noted as especially high in digestive quality/energy return in some gut-limited foraging models for cod. (LINK)
– **For striped bass (and hybrids)**: Pelagic forage such as threadfin shad, gizzard shad, alewife, blueback herring, and menhaden commonly range ~3–6+ kJ/g (size- and season-dependent; larger individuals and summer/fall values tend higher). Bioenergetics models for striped bass routinely incorporate these measured values; higher-ED prey (e.g., certain clupeids) improve predicted growth potential. (LINK)
Across broader marine/freshwater forage fishes, individual ED can range from ~2 to >10 kJ/g wet weight (driven primarily by lipid content), with means often near 4–5 kJ/g. Dry-weight values are substantially higher (commonly 16–27+ kJ/g). (LINK)
### Additional Context
– In wild systems, predators benefit from selecting higher-ED prey when available (all else equal), but real diets also reflect abundance, size, capture probability, and digestibility. Gut limitation can shift optimal diet composition away from pure energy-maximizing predictions. (LINK)
– In aquaculture (hybrid striped bass), “nutrient-dense” formulated diets are preferred (e.g., higher protein/lipid combinations such as ~40% digestible protein / 18% lipid, or energy: protein ratios around 7–9 kcal available energy per g protein), but these are not direct equivalents to natural forage ED. (LINK)
In short, **forage exceeding ~6 kJ/g wet weight is generally considered high-quality/optimum for energy transfer to piscivores like cod and striped bass**, while values in the 4–6 kJ/g range are moderate but still commonly utilized. Always use species-, size-, and season-specific measurements when possible for accurate bioenergetics or ecological modeling, as averages can mislead.