Appraisal of the selective properties of gill nets and implications for yield and value of the fisheries at the Itaipu Reservoir, Brazil-Paraguay

Abstract

We studied the selective properties of gill net mesh-sizes used in the fisheries at the Itaipu Reservoir to assess the likelihood of growth and recruitment overfishing, and to explore modifications of current harvesting approaches and their effect on the fish stock and the value of the fishery. We used empirical growth estimates and gill net selectivity to model yield per recruit, size distributions, value per recruit, and spawner biomass per recruit under different natural mortalities, fishing mortalities, and gill net mesh-size restrictions. Our results suggest growth overfishing may be limiting fishery yield and commercial value. Increasing mesh-size (in most cases) and increasing fishing effort (in some cases) would lead to higher yields and commercial values. Nevertheless, such increases would further reduce the biomass of the spawning stock, and we could not predict whether such reductions would lead to recruitment overfishing, nor their effect on the interactions between the fishery species and the associated fish assemblage. However, we identify how fishing effort and mesh-size may be juggled to boost yield and commercial value without reducing current biomass of the spawning stock.