The effects of shell collecting on the abundance of gastropods on Tanzanian shores

Shores at Dar es Salaam and Zanzibar are subject to severe shell collection, reflected in the scarcity of commercially valuable gastropods. Several species, but particularly the large cowries, were scarce at these locations but common on shores at Mafia Island where there is little shell collecting. In particular Cypraea tigris, C. historio and C. lynx were respectively at least 18, 10 and 5 times less abundant on shores at Dar es Salaam and Zanzibar than on the shores of Mafia Island. Overall, commercially valuable species were less abundant (p < 0.·05) on shores at Dar es Salaam and Zanzibar than at Mafia Island. However, this was not generally the case for non-commercial species. Differences in the abundance of commercially valuable gastropods between shores at Zanzibar and Mafia Island are probably attributable to the over-collection of these species at Zanzibar. Over-collection may also be an important factor reducing their abundance at Dar es Salaam though its influence is difficult to separate from other environmental problems also occurring there.