Quantifying the density of mangrove crabs: Ocypodidae and Grapsidae

Numerous studies have highlighted the importance of crabs within mangrove ecosystems, but tested methods of enumeration are virtually non-existent. The aim of the present study was to assess the accuracy of quick, non-invasive methods for estimating the population density of three species of mangrove crabs in East Africa: Uca annulipes, Sesarma guttatum and Neosarmatium meinerti. Methods employed depended on the behaviour and habitat of each species. Counts of U. annulipes and N. meinerti individuals that were active on the surface (visual counts) and burrow counts (of U. annulipes) were compared to actual densities. Actual densities were determined by excavation of U. annulipes, and by multiplying counts of N. meinerti burrows by the mean number of crabs per burrow. Visual counts of marked individuals of S. guttatum were compared to known densities of marked crabs. For N. meinerti two visual counting methods were tested: (1) instant counts - crabs were counted every 5 min during 0.5 h and the highest count taken; and (2) continuous counts - all burrows were tagged, and those from which a crab emerged during 0.5 h were totalled. For U. annulipes on Inhaca Island (Mozambique) visual counts underestimated excavated numbers by 27%, whereas burrow counts overestimated numbers by 46%. These values were similar for both sexes, and on both spring and neap tides. Burrow counts and visual counts estimated absolute numbers of U. annulipes with equal consistency. At Mida Creek (Kenya) visual counts underestimated U. annulipes numbers by 37%, again with no differences between spring and neap tides. In Zanzibar 81% of N. meinerti burrows contained a crab. The instant visual count underestimated N. meinerti numbers by 41%, the continuous count, by 12%. Visual counts of S. guttatum in Zanzibar (Tanzania) underestimated crab numbers by 49%.