|Trends in the size composition, availability, egg-bearing and sex ratio of the rock lobster Jasus lalandiiin its main fishing area off south west Africa, 1958-1969|
Matthews, J.; Smit, N.L. (1979). Trends in the size composition, availability, egg-bearing and sex ratio of the rock lobster Jasus lalandiiin its main fishing area off south west Africa, 1958-1969. Sea Fisheries Branch Investigational Report, 103(103). Dept. Of Industries, RSA: Cape Town. 38 pp.
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Experimental and commercial catches of rock lobster Jasus lalandii in its main commercial fishing area in South West Africa, the Reef area north of Lüderitz, and experimental catches in the rock-lobster sanctuary in Lüderitz Bay were analysed against the background of an increase in the export production quota and a reduction of the minimum legal size and its eventual abolishment. In every instance relaxation of control was followed by increases in the commercial catch and in the number of rock lobsters used as frozen tails, followed in turn by declines, indicative of over-fishing. Annual length compositions for the period 1958-1969 derived from both packing figures of frozen tails and experimental catches in the Reef area show a steep decline in rock lobster of 3 inches and longer after 1965 and 1966 respectively. The modal length of experimental catches had still been 2,2 to 3 inches between 1959 and 1967, but then decreased to between 2 and 2 1/2 inches in 1969. The effect of exploitation is further evidenced by the annual length compositions derived from experimental catches in the rock-lobster sanctuary and the Reef area: from 1965 until 1969 the rock lobsters in the sanctuary were consistently larger than those in the Reef area. The commercial catch per unit effort, based on weight, was approximately one-third lower for the periods 1966-1967 and 1968-1969 than that of 1964-1965. The experimental catch per unit effort in the Reef area, based on the number of rock lobsters, dropped by one quarter between 1962-1963 and 1968-1969 for rock lobsters of 3 inches and longer. However, for rock lobster of all sizes there was no drop in the experimental catch per unit effort, which indicates a relative and absolute increase in the number of small rock lobsters in the Reef area. The data were also analysed on a monthly basis in an effort to establish a relation with reproductive and moulting cycles. In the Reef area the normal ratio of males and females, i.e. 53:47 , is found during February-March, but the percentage of males reaches a climax in May-July as this is the female moulting period. This percentage subsequently drops to the lowest level in November after moulting and during egg-bearing in females and owing to the onset of moulting in males, and is followed by an increase in December and January after moulting in males. The modal length of experimental catches inthe:Reef area diminished from 3 inches (2 1/2 inches in 1968-1969) to a minimum after March, to reach 3 inches again only towards the end of the year. Although the effect of exploitation on the population is clear, biological cycles must also affect availability, for commercial fishing commences in January, whereas the commercial and the experimental catch per unit effort increase until February or March, and the Reef area is closed at the end of April or May, whereas the experimental catch per net per hour decreases until September-October. The average monthly size of rock lobster from the reserve reveals a similar pattern for both sexes: it reaches a low during May, a high in July-August and a somewhat higher low in October-December. Possibly the variations are linked with the biological cycles, but why the sexes react in identical fashion and different sizes of rock lobster differently can at this stage be explained only by postulating the migration of specific size classes of rock lobster .