UBC Theses and Dissertations
Ecosystem impacts of fishing forage fishes : an analysis of harvest strategies for the brazillian sardine Vasconcellos, Marcelo
The ecosystem impacts of fisheries have become an important concern in fisheries management and one of the most important issues in fisheries science. The objectives of this thesis were i) to evaluate the use of trophic models in the analysis of the ecosystem responses to fishing small pelagic forage fish; ii) to forecast the 'fishing down marine food web' phenomenon in Brazil; and iii) to conduct an in-depth analysis of the sardine, Sardinella brasiliensis, fishery off southeastern Brazil to examine hypotheses of ecosystem changes following stock collapse, and to evaluate how uncertainties of ecological processes influence the choice of harvest strategies and controls in this fishery. Dynamic simulations of mass-balance trophic models were used to compare the ecosystem responses to fishing forage fishes across different types of marine ecosystems, and to examine the stability characteristics of ecosystems when impacted by fisheries. The comparative analysis among trophic models indicated that as 'wasp-waist' species in upwelling ecosystems, small pelagics will only sustain much more conservative exploitation rates than the levels that have brought about historical stock-collapses. In the upwelling ecosystem off southeastern Brazil, sardine has been historically the main target of commercial fisheries, although 'fishing down marine food webs' for small pelagic fish species is not yet an overall observed phenomenon in Brazil. The collapse of the sardine fishery in the late 1980s has apparently favored a competing small pelagic fish (anchovy, Engraulis anchoitd) in the ecosystem. The combined effect of fishing and environmental effects makes it difficult, however, to characterize the changes in the sardine population and the ecosystem, and to predict the results of rehabilitation measures for the stock. On the other hand, the explicit recognition of these ecological uncertainties allows a sound choice of precautionary strategies for the fishery, and a better scrutiny of research programs to improve management. In this context, trophic models will have a complementary role to single-species models in the analysis of the broad consequences of fishing policies, and in the test and formulation of hypotheses about the causes of observed changes in marine ecosystems.
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