Menhaden’s impact on the Ecosystem

Researchers from the National Oceanic and Atmospheric Association, the University of Miami, and the University of Florida recently completed a study which modeled the ecosystem of the Gulf of Mexico with a focus on Gulf menhaden. The model was developed to evaluate the varying effects of different harvest policies on marine species to support ecosystem-based fisheries management in the Gulf. The study can be used to inform stakeholders and policymakers of the trade-offs between different management actions, while considering predator-prey interactions, fishing pressures, and environmental variables. The authors were primarily interested in evaluating the effects of menhaden harvest on predators like speckled trout, redfish, mackerel, and many other fish that anglers like to catch.Hundreds of diet studies and dozens of coastal species were included in the study’s Gulf-wide food web, which modeled predator-prey interactions from 1980 to 2016. Menhaden, called pogies in the Gulf, were found to support the diets of 32 predator groups. When categorized by age class, the main predators of juvenile menhaden were redfish, speckled trout, and seabirds. Menhaden over one year of age were mainly preyed upon by reds, trout, and king and Spanish mackerel, as well as apex predators like blacktip sharks, gag grouper, dolphins, and coastal piscivores—including jacks, tuna, and mahi mahi. It is interesting to note that menhaden accounted for about 85 percent of all forage fish biomass in this model, but that number is probably smaller in reality. Because other forage fish in the Gulf—like sardines, anchovies, and mullet—are not as well-studied as the menhaden, more data is needed to determine the role of all forage species in the ecosystem. Certainly, other forage species contribute to the diets of predators like snapper and grouper, so more data would be useful for future assessments of those species. Commercial and recreational fishery statistics were added to the model to evaluate historical fishing pressure and compare potential scenarios with different harvest strategies. Notably, the Gulf menhaden reduction fishery was responsible for 48.1 percent of all commercial catch (in weight) from 1980 to 2016. When looking at scenarios where menhaden fishing pressure differed, the data showed the current amount of pressure in the Gulf cuts king mackerel biomass in half compared to where there is zero menhaden fishing. Other species are reduced, as well, including the extremely popular speckled trout and redfish. Based on this study, Gulf menhaden support about 40 percent of the diets of both Spanish and king mackerel and about 20 percent of the diet for speckled trout. This information could be integral to the development of an ecosystem-based management plan for the Gulf menhaden fishery, considering the needs of such predators. During the entire period, there was a 30-percent increase in fishery landings (commercial and recreational) across the Gulf. Although certain gamefish—like yellowfin tuna and swordfish—showed increased biomass estimates, large oceanic sharks, other tunas, Spanish and king mackerel, and reef piscivores decreased in biomass. While the authors of the study noted there are still data gaps regarding predator diets throughout the Gulf, this is the most up-to-date and comprehensive diet model available. Based on the latest stock assessment published in 2019, king mackerel are not considered overfished, nor are they experiencing overfishing, but it is clear from this study that Gulf menhaden and king mackerel are connected. The next stock assessment for Spanish mackerel is scheduled to be published in 2023, which will hopefully close some of the data gaps. With further study, it may be found that the health of king or Spanish mackerel stocks can serve as indicators of ecosystem impacts in the Gulf, like striped bass are in the Atlantic. The TRCP and its partners will be working diligently to gain more information on the role of menhaden in the Gulf of Mexico ecosystem and how redfish, speckled trout, tarpon, jacks, and grouper are affected by the industrial menhaden fishery that currently has no catch limit and very little state and federal oversight. Learn more about our efforts to conserve menhaden in the Gulf and Atlantic.