Aspects of the biology and population dynamics of the deep-water commercially exploited crabs off South West Africa/Namibia

Doctoral Thesis


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University of Cape Town

Two crab species have been exploited on the South West African/ Namibian continental slope since 1973. The deep-sea red crab Geryon maritae forms >99 percent of the catch made by traps, the balance being a by-catch of stone crab Lithodes tropicalis. Historical information dating back to the late 1970's has been examined, but most of the data analysed in this study have been collected since 1980. Most of the research has been conducted on red crab. Four aspects of its biology have been examined in some detail, namely its reproduction, growth, movements and causes of its specific dep t h zonation off Namibia. These (particularly the first two) aspects have been used, together with information from commercial sampling and catch statistics, to examine possible changes in the yield-per-recruit should the age at first capture be increased, or fishing effort reduced. On the basis of these results, some management options are presented. Several techniques (photography, trawling, effective fishing area and tag-recapture) have been used to examine the abundance and density of red crab off Namibia, and are critically compared . Research on the stone crab has been largely restricted to an analysis of commercial catch data, though an unusual symbiotic relationship with a liparid fish has been identified and discussed.

Bibliography: pages 170-172.