Abundance,breeding and growth of the crab Scylla serrata in two South African estuaries

Populations of the crab Scylla serrata (Forskal) were studied in two South African estuaries from 1971 until 1974. One estuary was open to the sea, the other was closed by a sandbar. Population density in the closed estuary was estimated by means of mark-release-recapture data from tagged crabs. The population of S. serrata was estimated as 1 crab/124m² and the production at 3.4 g/m²/annum. Catch per unit effort data from the closed estuary indicated a natural mortality of 41% in the crabs' second year and 60% in the third. Growth was studied in both estuaries by means of size-frequency analysis and from tagging returns. Growth was rapid in the first 12 to 15 months, when the crabs attained a carapace width of 80 to 160 mm. Thereafter growth slowed, and after 3 years crabs were between 140 and 180 mm in carapace width. Females mated in summer at a carapace width of 103 to 148 mm, males at 141 to 166 mm. After mating, females migrated out of both estuaries into the sea.