Identification of important forage fish and implications of increased predator demand through Ecopath modeling

Abstract

Fisheries on lower trophic levels act as and compete with predators in aquatic ecosystems for potentially limited prey resources. Multiple predators in a system are often managed to achieve a Bmsy that could result in a total predator consumption level the ecosystem cannot support. We developed an Ecopath model for Pamlico Sound, NC and its tributaries to address this issue. The model is comprised of 43 compartments with an emphasis on piscivores and their prey and represents the 2012–13 time period. We estimated total predator demand for fish prey and identified important species of forage fish and top piscivorous predators in Pamlico Sound. We compared local fishing fleet removals and predator consumption in forage fish, penaeid shrimp, and blue crabs. Finally, we evaluated the impact of increased predator demand on the system, by increasing biomass levels of five predators to equal their management thresholds and using mixed trophic impact analysis. Atlantic menhaden (Brevoortia tyrannus), anchovy (Anchoa hepsetus and A. mitchilli), spot (Leiostomus xanthurus), and Atlantic croaker (Micropogonias undulatus) were the most important forage fish in the system, representing over 80% of the fish consumed (by weight). Lizardfish (Synodus foetens) and bottlenose dolphins (Tursiops truncates) were top predators based on trophic level, while bluefish and longnose gar (Lepisosteus osseus) were top predators in terms of total fish consumption. The highest overlap in forage fish usage exists between fisheries and piscivores for Atlantic croaker and spot. Total forage fish removals through predation and harvest was greater than 65% of the production for each of the four-forage fish and, under the higher predator biomass model, total removals exceeded production for spot; total removals in either scenario did not exceed the production of penaeid shrimp and blue crab. Thus, the tradeoff between fisheries for forage fish and piscivores should be considered when assessing and managing fisheries in Pamlico Sound.