Long-term survival of de-oiled Cape gannets Morus capensis after the Castillo de Bellver oil spill of 1983

After oil spills, oiled wildlife are regularly cleaned at considerable cost. Yet the conservation value of this intervention has been questioned, mostly because cleaned animals appear to have poor post-release survival. However, reliable long-term data are needed to judge the success of such programs. We used 16 ring recoveries to estimate survival of 932 Cape gannets (Morus capensis) that were oiled, cleaned and released in 1983. For the period 1989–2006, we further compared survival of 162 surviving cleaned gannets to that of 10,558 non-oiled gannets using capture–recapture data. We used modern statistical tools that account for recapture probabilities, recovery rates, transience, and temporary absence from the breeding colonies. Mean annual survival rates of de-oiled gannets ranged from 0.84 (se = 0.05) to 0.88 (se = 0.02), depending on analysis and colony. Between 1989 and 2006, rehabilitated gannets survived slightly less well than unoiled birds, but the difference was similar to the difference in survival between the two colonies where the birds were studied. Our results show subtle long-term effects of oiling and subsequent treatment. However, they also show that cleaned gannets can survive at almost the same rate as unoiled birds, at least if they survived the initial years after release. Rehabilitation of these birds may thus be a valuable conservation intervention for this localised species where a single spill can threaten a large proportion of the world population.