Subsistence utilization of the crab Neosarmatium meinerti in the Kosi Lakes ecosystem, KwaZulu-Natal, South Africa

Abstract

Members of the rural communities living around the edges of the Kosi Lake system in northern KwaZulu Natal have been subsistence users of the resources of the lakes, estuary and surrounding swamp forest for many generations.Women and children dig up a variety of crabs in the mangrove ecosystem, and in the late 1980s collected as many as 638 000 crabs a year. The red mangrove crab Neosarmatium meinerti de Man, 1884 (Crustacea: Decapoda: Grapsidae: Sesarminae) constituted the major proportion of these catches, while Scylla serrata and Cardisoma carnifex made up the rest. However, subsistence fishing effort and catches have been reduced steadily, declining from about 5000 outings p.a. and between 2 and4×105 crabs p.a. in the 1980s, to about 1000 outings and 1.3 × 105 crabs in the late 1990s. CPUE has increased from 66 to 145 crabs/person/outing during this period. Crab densities ranged between 0.35/m2 on the high shore and 11.4/m2 on a densely vegetated island, but it was not clear what factors most affected crab density. Densities inside and outside a ‘sanctuary area’ were not significantly different, although there were significant differences in densities among sites within the ‘sanctuary area’. Stock size within the entire mangrove area of Kosi was estimated as 728 514 N. meinerti, but this is almost certainly an underestimate of the total stock size. Sex ratios in the N. meinerti population were not significantly different from 1:1 but above 38 mm carapace width (CW) male crabs considerably outnumbered females. Females matured at 24.9 mm CW and mean sizes of crabs captured by subsistence harvesters were 33.4mmand 31.3mmCWfor males and females, respectively. Instantaneous natural mortality was estimated at M = 2.27 and fishing mortality F = 0.83. The N. meinerti population in the Kosi mangrove system appears to be sustainably exploited.

Key words: subsistence fishery, sex ratios, crab density, Brachyura, Crustacea.