Indirect Effects of Fishery Exploitation and Pest Control in a Riverine Food Web

Abstract

We used an energy-based food web model to evaluate indirect effects of fishery exploitation and aquatic insect pest control on food web structure and recreational fisheries in the New River, West Virginia. Key groups represented in the model were aquatic insects, age-1 and age-2 crayfish (Cambaridae), age-1 and age-2 larvae of the dobsonfly Corydalus cornutus (hellgrammites), prey fishes, and age-0 to adult smallmouth bass Micropterus dolomieu, rock bass Ambloplites rupestris, and flathead catfish Pylodictis olivaris. We focused on exploitation of crayfish and adult smallmouth bass because of their strong prey–predator interaction and their importance as fishery resources. We first simulated a range of harvest rates for adult smallmouth bass, while keeping crayfish harvest rate at its field-estimated value. Outcomes suggested that adult smallmouth bass were being harvested near the maximum sustainable yield (MSY), and that more restrictive harvest regulations would not necessarily increase production and harvest (by weight) but would increase biomass. An a priori expectation that production would increase was not met because adult smallmouth bass in the New River were already near maximum production potential. Also, more restrictive harvest regulations for smallmouth bass would promote decreases in rock bass and flathead catfish stocks that compete with smallmouth bass for a diminishing crayfish resource. We then simulated a range of harvest rates for crayfish, while leaving adult smallmouth bass harvest rate at its field-estimated value. Outcomes suggested that management of the crayfish assemblage for MSY would probably result in substantial declines in biomass, production, and harvest of smallmouth bass and other crayfish predators. Harvest predictions for 40 combinations of smallmouth bass and crayfish harvest rates were consistent with the tenet that predators and their prey cannot be harvested for MSY concurrently. Evaluation of scenarios for control of black fly Simulium jenningsi as a pest suggested strong “bottom-up” effects on species and functional groups that prey on aquatic insects or that feed on insectivorous prey. Our work suggested that fishery management outcomes are strongly dependent on key prey–predator interactions and that distinct harvest regimes produce unique food web structures with varying potentials for sustainable yields. Resource managers who monitor abundances and food habits of prey and predators and who apply the principles of bioenergetics can identify and manipulate key trophic linkages in adaptively managed riverine food webs.