(Background/Question/Methods )
Commercially exploited fishes are part of complex interaction webs, but most studies leading to fishery management focus on small ecosystems. Moreover, traditional resource economics—and intuition about the effects of overexploitation—is almost entirely built on the concept of logistically growing resources. This view ignores the fact that fish standing stock size feeds back to its own carrying capacity through density-mediated indirect interactions. To study how human exploitation impacts ecosystem function, we model marine food webs using a three-tier Allometric Trophic Network (ATN) model, which includes consumer-resource body-size ratios and nonlinearities in feeding behavior. We use a model reduction method to construct the one dimensional logistic equivalent of the ATN and address the question, what conclusions about exploitation change when moving from logistic models of species growth to multi-species ATN equivalents?
(Results/Conclusions )
Our results highlight a major difference between expectations based on logistically growing populations and the less simplistic and more realistic, ATN model. The optimal exploitation rate predicted by the logistic model results in the extirpation of the top species in the ATN; however, this result depends on parameter estimations from the model reduction. Our results suggest that the frequent effect of reduced fish body-size due to economic exploitation shifts systems from those optimized by more profitable fishing with less effort to those optimized by less profitable fishing with more effort; this result is reproducible in the logistic model. We also found subtler results solely within the ATN including "hydra" effects whereby fish standing stock and profit are counter-intuitively optimized simultaneously with greater exploitation.