|Stock assessment and some morphometric and biological characteristics of the rock lobster Jasus lalandii on Marshall rocks, its main commercial fishing area off South West Africa, 1971-1974|
De B. Beyers, C.J. (1979). Stock assessment and some morphometric and biological characteristics of the rock lobster Jasus lalandii on Marshall rocks, its main commercial fishing area off South West Africa, 1971-1974. Sea Fisheries Branch Investigational Report, 1147. Dept. Of Industries, RSA: Cape Town. 26 pp.
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Although the rock lobster Jasus lalandii occurs from the Orange River mouth northwards to Walvis Bay along the South West African coast, Marshall Rocks (locally known as the Reef) was chosen as the study area owing to its high yield of rock lobster. Morphometric characteristics such as carapace length relations to total body mass, tail mass, second tail segment length and carapace depth were calculated. Population characteristics were monitored by commercial and research sampling from 1971 to 1974. Tagging with FT2 dart tags enabled the stock size on the Reef to be estimated as 19,2 million rock lobsters larger than 6,4 cm carapace length. Tagging results further showed that males grow at 3 mm per yearly moult and sexually mature females at 1 mm. A fishing mortality rate of 0,26 was estimated, indicating that maximal exploitation has not been reached. Although commercial catch per unit of effort increased for both hoopnets and rectangular traps from 1971 to 1973, effort decreased and the unrealistically high production quota could not be filled. Adverse environmental conditions may also have reduced the catchability of animals. The biomass of Aulacomya ater, the staple diet of Jasus lalandii was higher on the inshore side of the Reef than offshore and the biomass increased with depth up to 20 m inshore while mussel biomass was inversely related to depth off shore.