Causes and Impacts of Fishery Decline

Overview of the Declining Fishery

The fishery is currently experiencing a persistent decline, as evidenced by several biological indicators. Using cod as a representative species, available data show consistently low body weight and length at age, reduced fecundity, and diminished recruitment rates. These symptoms point toward broader underlying challenges affecting fish populations.

Initial Hypothesis: Food Limitation

Initially, it was believed that insufficient food resources were the primary cause of the fishery’s decline. This assumption was based on the observation that the populations of the three main forage fish species—menhaden, herring, and mackerel—are at or near historic lows.

Forage Fish Index and Energy Density

However, the NOAA State of the Ecosystem Report for New England indicates that the forage fish index has remained constant over several years. The report also includes data on fish energy density, which refers to the lipid content (essentially fat) present in forage fish.

Nutritional Benefits of High-Lipid Forage Fish

1. A diet rich in high-lipid forage fish significantly enhances female fish fecundity, both in terms of the total number of eggs produced and the number of eggs per gram of body weight.
2. Broodstock fed diets based on high-lipid forage fish, or fish-oil equivalents high in EPA and DHA, produce larger eggs with greater diameter and weight.
3. Research consistently demonstrates higher fertilization rates, improved hatching success, and better larval survival in fish fed fish-oil or forage-based diets compared to those fed plant oils or low-lipid feeds.
4. In wild marine food webs, a shift toward energy-rich fish prey (as opposed to squid or invertebrates) improves the condition of predators.

Evidence from Aquaculture and Nutritional Principles

While direct comparisons among wild populations are limited due to varying predator diets, nutritional principles and aquaculture data strongly support the conclusion that dependence on high-lipid forage fish maximizes female fecundity and offspring viability more effectively than diets based on invertebrates or crustaceans. The depletion of forage fish stocks threatens the reproductive output of fish predators, highlighting the critical importance of these prey species in sustaining healthy fisheries.

Conclusion

In conclusion, the root cause of the ongoing decline in fishery populations is a lack of nutritious food.