Impact of the Iron Baron Oil Spill on Subtidal Reef Assemblages in Tasmania

Abstract

The biological impact of the grounding of the bulk carrier Iron Baron on Hebe Reef in northern Tasmania, with release of approximately 350 tonnes of Bunker C fuel oil, was assessed using quantitative underwater censuses at numerous reef sites before and after the spill. Physical abrasion from the ship's hull during grounding caused the complete destruction of the subtidal reef community within a localized area of ≈170 m by ≈20 m on Hebe Reef. However, the release of fuel oil did not appear to have substantially affected populations of subtidal reef-associated organisms in the near vicinity. Analyses of changes over time outside the hull impact area at oiled sites before and after the spill, and comparisons with undisturbed reference sites, indicated no significant change in number of species on reefs or in densities of the most abundant animal and plant species. Post-impact monitoring of the grounding zone and adjacent reference sites on Hebe Reef indicated that the fish assemblage associated with the hull scar recovered rapidly in terms of species composition and species richness within one year, whereas plant and invertebrate assemblages had not reached inferred pre-disturbance levels after two years. Wave disturbance appeared to be hindering re-establishment of large macroalgae over part of the abrasion zone where the reef substrata had been converted to unstable gravels.