The effects of shore‐based diamond‐diving on intertidal and subtidal biological communities and rock lobsters in southern Namibia

Abstract

Abstract Divers mine shallow‐water diamonds on the west coasts of South Africa and Namibia. In the process they cut kelp, suck up gravel that is sorted on the shore and then deposited intertidally, and uncover and overturn subtidal boulders. The impacts of these divers on intertidal and subtidal reefs, and on the population structure of the commercially important rock lobster Jasus lalandii, were monitored for 5 years near Lüderitz in southern Namibia. Sampling was undertaken at unmined reference (control) sites and at sites mined at various times in the past. Percentage cover and densities of benthic organisms were recorded, and the abundance, length‐frequencies and sex ratios of rock lobsters determined. High natural variability in benthic community structure made it difficult to distinguish mining impacts even a short time after mining had ceased. Nevertheless, mining reduced the species diversity and abundance of both intertidal and subtidal communities. Recovery did, however, occur within 2 years. During, or immediately after mining, the intertidal community became characterized by the near disappearance of grazers, proliferation of fast‐growing, opportunistic foliose algae and decreased cover of filter‐feeders. Subsequently, grazers increased, curtailing the algae. In contrast, mining reduced subtidal algal cover. Kelp was cut to facilitate mining, foliose algae were removed and smothered as rocks were overturned, and reef‐building filter‐feeders diminished. The abundance and population structure of rock lobsters appeared to be largely determined by environmental conditions and habitat suitability. Their abundance and catch per unit effort were more often higher at reference sites and recently mined sites than at sites mined > 5 years previously. Sex ratios and mean sizes of rock lobsters were not statistically affected by mining. Mining does alter the biological and physical characteristics of the habitat, and in the process, it may temporarily increase the availability of suitable rock‐lobster habitats. No negative impacts of shore‐based diamond‐diving on rock lobsters were detected. Copyright © 2002 John Wiley & Sons, Ltd.