Contribution of ecosystem analysis to investigating the effects of changes in fishing strategies in the South Brazil Bight coastal ecosystem [Review]

摘要

Ecosystem effects of recent changes in fishing strategies in the South Brazil Bight (SBB) area, including increasing squid catches by shrimp bottom trawlers and fishing for young sardines as bait, for the skipjack tuna pole-and-line fishery were investigated by modelling the SBB coastal ecosystem for the 1998-1999 fisheries period, using the mass-balance modelling software, Ecopath with Ecosim. Based on fisheries information and on previous models, 25 species groups were defined. The mean trophic level of all fisheries was 2.99 with small trawlers and the live baitfish fleet occupying the lowest trophic levels. The change in squid fishing rate and the most important squid predators fishing rate were simulated by increasing fishing mortality (F) from 0 to 1 per year, while maintaining F constant for other exploited groups. Also, the impact of fishing pressure by trawlers on squids and by the live baitfish fleet on young sardines were simulated. Different hypotheses of flow control were tested by setting the maximum instantaneous mortality rate that consumers could exert on food resources by (a) "top-down" control by predators (predator control); (b) all interactions of mixed control type; and (c) "bottom-up" control of predators by their prey (donor control). For squid, predicted biomass changes were more pronounced under top-down control that under bottom-up. Biomass of the weakfish, which are important squid predators, were the most affected under all hypotheses. A similar pattern of decrease in other squid predators was seen. The ecosystem consequence of overexploiting weakfish and large pelagic fish biomass was speculated regarding the increasing squid catches. Simulations of increasing live-baitfish fleet did not show prominent impact in the inner shelf, where present F values are low. Given young sardines concentrate in shallow waters, where the conflicts converge, a complementary model of the adjacent coastal system and artisanal fisheries could better address fleet's impact. Nevertheless, it was noted a decrease of sharks and rays reflecting their vulnerability in the system. A precautionary measure for fisheries management in the region would be to assume that top-down control is a dominant force in energy flows, differing from the custom emphasis placed only in environmental forces and productivity. The ecosystem approach will require the integration of information from a wide range of disciplines, levels of ecological organization and temporal and spatial scales, as well as concrete management measures. (C) 2003 Elsevier B.V. All rights reserved. [References: 144]