Impact of the Treasure oil spill on African penguins Spheniscus demersus at Dassen Island: case study of a rescue operation

Abstract

The bulk ore carrier MV Treasure sank between Robben and Dassen islands off western South Africa in June 2000. Although releasing less oil than the Apollo Sea spill six years earlier, the number of African penguins Spheniscus demersus oiled (approximately 19 000) in the Treasure spill was almost double the number oiled in the Apollo Sea incident. The majority of oiled penguins was collected from Robben Island. The capture, stabilisation and transport of oiled penguins following the Treasure spill were substantially more successful than during the Apollo Sea rescue operation and resulted in an increase in the percentage of oiled penguins that were released back to the wild. In addition to the 19 000 oiled penguins that were caught, 19 500 un-oiled birds (12 345 from Dassen Island) were evacuated to Cape Recife, 800 km to the east, to prevent them from becoming contaminated. The evacuated birds returned rapidly to Dassen Island and many resumed breeding soon after their return. The restoration success (proportion of birds observed at Dassen Island that were recorded breeding) was substantially greater for evacuated birds than for de-oiled birds. Lightly oiled birds that were released without being cleaned had lower survival rates and were less successfully restored into the breeding population than de-oiled birds, especially if more than 5% of their body was covered in oil. Reproduction exerted a greater cost to de-oiled birds than to evacuated birds. A total of 1 787 orphaned chicks was hand-reared at two separate facilities on the mainland and were released at Robben and Dassen islands. The chicks did not necessarily return to the colony at which they were released. The survival rates and restoration success of chicks reared at the two facilities differed, probably as a result of one group being overfed. Despite the death of about 2 000 African penguins in the Treasure spill, and the substantial disturbance that resulted from the rescue operation, the penguin colonies at Robben and Dassen islands increased by 18% and 26% respectively in the year following the spill. This increase was attributed to a significant increase in the biomass of anchovy Engraulis encrasicolus and sardine Sardinops sagax in 2001. These findings highlight the importance of preventing oil spills in the first place, and in the event of a large oil spill, the conservation value of evacuating un-oiled penguins to minimise their risk of becoming contaminated.