The abundance of Scylla serrata in two closed estuaries, the Mhlanga and the Msimbazi, in Natal, South Africa, was estimated by means of the mark-recapture models of Chao, Burnham and Overton, and Petersen. In view of the size-related variability in capture probability between individuals and the low capture probability of most individuals, the Chao model was considered to yield the most reliable estimates of population size. Crab densities were estimated to be 225 m2·crab−1 or 0,444 crabs·100 m−1 at Mhlanga and 188 m2·crab−1 or 0,532 crabs·100 m−2 at Msimbazi. Probable sources of bias are an increase in the capture probability of crabs during the four-day experiment at Mhlanga and inadequate overlap between the areas of influence of adjacent traps. Asymptotic curves of the form C t = C ∞(1−e−βt ) were fitted to catch data from the two estuaries. Asymptotic catch C ∞ was higher and the rate at which it was reached (β) was lower in the more densely populated estuary. A possible relationship between these parameters and crab density is discussed.
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Robertson et al. (1991) studied this question.