Declining coastal avifauna at a diamond-mining site in Namibia: comparisons and causes

Abstract

A review of Namibian shorebird densities over two decades and two additional visits to the coastal diamond-mining areas at Elizabeth Bay, southern Namibia, were undertaken to assess the long-term influence of mining activity on density of shorebirds (Charadrii) and particularly threatened African Black Oystercatchers Haematopus moquini and Damara Terns Sterna baleanarum. Oystercatcher numbers remained relatively stable over 25 years, with some recent declines relative to previous surveys along the beach, while that of other shorebirds showed dramatic declines. Density of shorebirds fell 4.3-fold from 220 birds/km to 41 birds/km, relative to similar (pre-mining) surveys 25 years previously. Over the last nine years birds have declined from 12 to nine species. Control sites on nearby sandy beaches over the same (nine-year) period showed no such declines. From the mid-1970s (pre-mining), Damara Terns also declined from 20 breeding pairs to 2–7 breeding pairs between 1996 and 2002; they have remained stable elsewhere in Namibia. Mining activity does not directly impact the known tern nesting sites, but reduced tern numbers have occurred since 1.5 million m³ of fine sediments have been deposited annually into the bay. Observed foraging success of terns at Elizabeth Bay in 2002 was lower than recorded elsewhere in southern Africa, and independent data indicates that the abundance and biomass of their surf zone fish prey is lower in Elizabeth Bay than the nearest control site. Fish availability may also be reduced because most fish occurred within the sediment plume. The decline of the mollusc-eating component of shorebirds can also be explained by the disappearance of the White Mussel and other shellfish favoured by oystercatchers. I conclude that releasing sediment into the bay is detrimental to coastal avifauna and simple conservation measures are given that could reverse and allow further (experimental) study of the reasons for the trends found.

Ostrich 2005, 76(3&4): 97–103