Evaluating ecosystem-based reference points for Atlantic menhaden (Brevoortia tyrannus)
Buchheister, A., Miller, T. J., Houde, E. D. (2017). Evaluating ecosystem-based reference points for Atlantic menhaden (Brevoortia tyrannus). Marine and Coastal Fisheries. doi: 10.1080/19425120.2017.1360420.
- Bottom line: This paper presents a new ecosystem model to explore trade-offs in the management of Atlantic menhaden, the largest fishery on the U.S. East Coast.
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Background: The current fishery management plan for Atlantic menhaden envisions a shift to a new set of guidelines. These include ecological reference points (ERPs, benchmark levels of key metrics (e.g. fishing pressure or species abundance), set with the purpose of maintaining critical ecological functions. Managers have established a working group that is developing ERPs using several multispecies models to synthesize data on menhaden and a handful of major predators.
Ecosystem models, in contrast to most multispecies models, can include a much larger range of species. This allows them to simulate the effects of different management actions on the whole system and to account for complex predator-prey feedbacks. These capabilities are useful for quantifying trade-offs, such as the trade-off between harvesting menhaden and leaving it in the water as food for predators.
- Methods: This paper describes a new ecosystem model called NWACS (for northwest Atlantic continental shelf). It incorporates eight fishing fleets and 61 trophic groups (i.e., species or groups of species), including all major predators of menhaden. To create the model, the researchers first built a food web—a diagram showing what each trophic group eats. They then adjusted the model’s parameters—things like growth rates, feeding rates, and diet compositions—to get the best fit with real-world data. Taken as a whole, the model gives a picture of how biomass flows through the system, from plankton at the base of the food web up through top predators.
- Findings:
- The model produced quantitative estimates of several trade-offs. One of the strongest was between the catch of menhaden and the catch of striped bass. As the simulated catch of menhaden increased from zero to maximum sustainable yield, the catch of striped bass declined by 60 percent.
- Tunas and nearshore piscivorous birds, such as ospreys, were among the other groups showing strong trade-offs. But the trade-offs between menhaden and two other key predators—bluefish and weakfish—were much less pronounced.
- Menhaden proved to be an important prey species: 22 predators consumed it, and it made up 30 percent of the diet of large striped bass (age 7 years and older) and 33 percent of the diet of nearshore piscivorous birds (such as cormorants and ospreys).
- The authors argue that such quantitative estimates of trade-offs are valuable tools for managers considering how to balance conflicting goals for the system.
- Another key benefit of the model is that it incorporates the full system, which means it can be used to compare results across models that include different subsets of the system. This can help in evaluating uncertainty and testing the robustness of conclusions.
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