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Summary
The St Lucia system is a South African estuarine lagoon which experiences wide salinity fluctuations; recent records show a range of 2%0 to 89%0. The population of the crab Scylla serrata in the St Lucia estuary system was surveyed in January 1977. An estimate based upon catch per unit effort indicated a population of about 180 000 adult crabs. Population density declined with increasing distance from the mouth of the estuary. S. serrata was more abundant in muddy than in sandy areas. The crabs survived a four-month period of low salinity (2%0) in 1976, but their hypersalinity tolerance (60%0) is too low to permit their existing in most of the system during periods of high salinity. S. serrata feeds chiefly on benthic molluscs and crustacea; the death of these food organisms during periods of extreme salinity may reduce the carrying capacity of the system for crabs.
