Using Management Strategy Evaluation to address problems arising as a result of competing users of the South African horse mackerel resource

Includes bibliographical references.The Cape horse mackerel (Trachurus trachurus capensis) has traditionally made an important contribution to the South African fishing industry and is a key component of the Benguela ecosystem. This thesis concerns the assessment and management of the South African horse mackerel resource. It starts with a brief review of the biology of the Cape horse mackerel and the history of the fishery, as well as of the Management Strategy Evaluation approach, which was applied in this work. Assessments of the horse mackerel resource are currently undertaken through the combined efforts of the Demersal and Pelagic Scientific Working Groups (SWGs) of the South African Department of Agriculture, Forestry and Fisheries (DAFF). A joint effort is required because the resource is available to multiple fisheries: as directed catch to the midwater trawl fishery and as bycatch to the demersal trawl and pelagic purse-seine fisheries. Management of the resources complicated by differences in the age-structures of the horse mackerel caught in each of these three fisheries. The data available for the assessments are described, including the details of their collection and processing. Four age-structured production models (each reflecting different assumptions about the horse mackerel resource) are fitted to those data using the maximum-likelihood estimation method, and are used to provide assessments. Estimates of the current status of the stock indicate that it is healthy, putting it well above its Maximum Sustainable Yield (MSY) level. For the directed midwater fishery, MSY is estimated to be in the region of 50 000-100 000 tonnes per annum. However, the results of constant catch projections suggest that there is a pronounced yield-per-recruit effect, with even small bycatches of juvenile horse mackerel in the pelagic fishery having a pronounced negative effect on the level of a catches in the midwater fishery that can be sustained