Walking the bridge from single- to multi-species approaches in Southern African fisheries management

Plaganyi-Lloyd, Eva Elizabeth

Walking the bridge from single- to multi-species approaches in Southern African fisheries management

Doctoral Thesis

2004

Abstract

Fisheries management worldwide is in flux with calls for an EAF (Ecosystem Approach to Fisheries) needing to be balanced with the ongoing requirements to provide timeous and realistic assessment-based advice for management ( often with major economic and social consequences), that is typically based on single-species stock assessment models. This thesis is an attempt to walk the bridge from single- to multi-species approaches to fisheries management by developing a ''traditional" single-species stock assessment model that is used for management purposes, assessing possibilities for extending the model to incorporate multi-species effects and evaluating the potential of a range of multi-species approaches to contribute to the fumishment of practical management advice. The South African abalone Haliotis midae fishery is an example of a commercially valuable resource that is currently experiencing a downturn due to a complicated mix of biological, social, political, economic and environmental factors. Core problems include illegal fishing and recent ecosystem change in the form of a movement of rock lobsters Jasus lalandii into a major part of the range of the abalone. It seems that the lobsters have dramatically reduced sea urchin Parechinus angulosus populations, thereby indirectly negatively impacting juvenile abalone, which rely on the urchins for shelter. A spatial and age-structured production model (ASPM) developed as part of this study has provided the basis for management advice for this resource over recent years by projecting abundance trends under alternative future catch levels. The focus is on the main abalone fishery Zones A-D. The model estimates the reduction in juvenile abalone survival due to the ecosystem change extent and estimates the illegal take using a novel fisheries index - the confiscations per unit of policing effort (CPUPE). As a consequence of the recent explosion of poaching activities, the combined Zones A-D model-predicted 2003 poaching estimate of 933 MT ( corresponding to the assumption that, on average, 36% of all poached abalone are confiscated) is more than seven times the legal 2003 commercial TAC for these Zones. Given the complexity of ecosystem processes, there is a need to critically evaluate the tools used to steer this thinking. The focus here is on both the most widely-employed multi-species/ecosystem approach (ECOPATH with ECOSIM or EwE) as well as a scenario in which there is an urgent need (from management) for scientific evaluations to quantify indirect interactions between marine mammals and fisheries. A critical review of EwE highlights some weaknesses related to, for example, the handling of some life history responses such as compensatory changes in natural mortality rates of marine mammals, overcompensatory stock-recruit relationships, inadequate representation of uncertainty, possible problems in extrapolating from the micro-scale to the macro-scale as well as some (not too far-reaching) mathematical inconsistencies in the underlying equations. Strengths include the structured parameterisation :framework, the inclusion of a well-balanced level of conceptual realism, a novel representation of predator-prey interaction terms and the inclusion of a Bayes-like approach (ECORANGER) to take account of the uncertainty associated with values for model inputs. The potential of EwE to contribute to five important multi-species management quandaries in the marine environments off southern Africa and Antarctica is assessed, leading to the conclusion that EwE has limited predictive capability in these contexts. 3 Aspects of the potential application of other multi-species/ecosystem modelling approaches to advise the management of South African fisheries are discussed. In general, reliable predictive ability from such models is likely to be achieved sooner for top predators because relatively fewer links need to be modelled. Accordingly discussion concentrates on the problems of modelling marine mammal-fisheries interactions. Competition is a primary concern, but existing evidence is inconclusive because of the difficulties of substantiating claims that predation by marine mammals is adversely affecting a fishery or vice versa. Numerous species have been implicated in such conflicts, and long-term studies are essential to evaluate relationships between rates of predation and types and densities of available prey, i.e., functional responses. More realistic modelling studies are needed to address operational or management issues. Such models should reflect uncertainty in data and model structure, describe the influence of model assumptions, focus on systems where there is the greatest chance of success, incorporate a sufficient array of ecological links, and include appropriate spatial and temporal scaling for data collection and modelling exercises. In general, GADGET (Globally Applicable Area-Disaggregated Generic Ecosystem Evaluation Tool) and Minimum Realistic Models (MRM) are seen to show the most promise for use as tools to assess indirect effects between marine mammals and fisheries. The hake-seal-fishery interactions off the west coast of southern Africa are discussed as an example and the initiatives being pursued to further this modelling work are summarized. An important message derived from this study concerns the need to couple multi-species/ecosystem models with a simulation framework to take explicit account of uncertainty and management issues.

Keywords

Mathematics

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PhD / Doctoral

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