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African Journal of Marine Science

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Long-term trends in catch and effort of commercial Linefish off South Africa’s Cape Province: Snapshots of the 20th century

MH Griffiths

Abstract


The Cape commercial linefishery, established during the first half of the 19th century, currently consists of about 2 500 vessels (±20 000 crew), which land some 15 500 tons of fish each year. In spite of a long history, a
lack of a long-term catch and effort data series has severely hindered the management of the fishery. This paper provides commercial catch and effort data for three periods during the 20th century: 1897–1906, 1927–1931 and 1986–1998. Trends in catch per unit effort (cpue) were verified with additional data from the inshore trawlfishery, from fishery-independent surveys and, where possible, stock assessment. According to a Linefish Management Protocol developed for the linefishery, any stock demonstrating a historical reduction in cpue or catch contribution of more than 75% is to be regarded as overexploited. Evaluations based on present datasets support this arbitrarily determined reference point for cpue, but reveal that catch composition is a poor indicator of stock status and should be used with caution. In spite of technological advances such as the advent of combustion engines, nylon lines, echo-sounders, electronic navigational aids, onboard freezer facilities and larger vessels,
declines in catch rate indicative of severe overexploitation (i.e. 75–99%) were observed for many important linefish species during the 20th century. Most of these were higher-trophic-level species from the warm/temperate
East Coast, several of which are also endemic. Life-history characteristics of the vulnerable species (i.e those demonstrating declines in cpue of >75%) include predictable location in time and space (either coastal migrant
or resident), longevity (>15 years) and late maturity (relative to maximum age). Apart frorreduced productivity associated with stock depletion, other setbacks, such as ecosystem alteration, loss of genetic diversity and shortterm commercial extinction, are also anticipated. In order to rebuild depleted linefish stocks it is deemed essential to create additional marine reserves, dramatically improve enforcement, develop extensive public awareness programmes and substantially reduce commercial effort.



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