Posted by: barn owl | April 20, 2008

Earth Week Environment Posts: 1. Oil Spills and Iguanas
Although maritime oil spills have declined in frequency and volume in the decades since 1960, they remain a major source of ocean pollution, with a devastating impact on many species of marine birds, mammals, fish, and invertebrates. In a historical and geographical survey of major oil spills, designed in part to determine whether the apparent high incidence of oil tanker accidents in the European Atlantic reflected a genuine trend, Vieites and colleagues (2004) reported that spills correlated with all major crude production areas and maritime transport routes. Unfortunately, major oil spills overlapped with productive areas of the oceans (NOAA-defined Large Marine Ecosystems), and with zones containing vulnerable coral reef ecosystems.

The European Atlantic is not a crude oil production area, but the analyses by Vieites and colleagues revealed that it is an accident hotspot, with one-fifth of the global amount of spilled oil (1,097,359 tons, between 1960 and 2002). The English Channel and the Galician coast, site of the devastating Prestige accident in November 2002, were the most affected areas, with an oil spill incident rate of one every 2.3 years. Between 1970 and 2002, most of the world’s oceans experienced a decreasing trend in the number of oil spills, but there were nine tanker accidents in the European Atlantic during the 1990s, and a single accident in 2002 spilled 60,000 tons of oil in this region. Hopefully, the 2003 ban on single-hulled tankers in EU ports will prevent major oil spill incidents like those following the wrecks of the Prestige in 2002, and the Erika in 1999.


Marine Iguana (Amblyrhyncus cristatus).
Photo: Dr. Jim Smith

Even a small amount of oil spilled in a fragile marine ecosystem can have a major impact on local animal species. The tanker Jessica accident, which affected the Galápagos archipelago in January 2001, was considered to be a low-level oil spill. Nevertheless, the impact on one species, the marine iguana (Amblyrhyncus cristatus), was severe, with a 62% mortality rate among animals on the oil-contaminated island. Two biologists, who have studied environmental stressors and glucocorticoid levels in Galápagos marine iguanas, reported (Romero and Wikelski, 2002) that low-level oil contamination can elicit a stress response in these animals, potentially leading to a weakened immune system and declining reproductive rates.

Under natural conditions, the main cause of marine iguana deaths is starvation, due to insufficient amounts of their algae food source. Such a reduction in food supply occurred in the late 1990s, with a strong El Niño event that restricted nutrient upwelling and algal growth. Levels of the glucocorticoid corticosterone were elevated in malnourished animals, as an adaptation that increases protein catabolism to prolong survival. Corticosterone levels proved to be a predictor of marine iguana mortality rates, not only under the El Niño starvation conditions, but in the wake of the Jessica oil spill as well. Prior to the oil spill, corticosterone levels in the marine iguana populations were low, as the La Nina cycle had restored their algae forage; however, after the tanker grounded, even those animals not visibly contaminated with oil exhibited elevated corticosterone levels, indicative of stress. Romero and Wikelski predicted that mortality would be 40% on affected islands, and the actual mortality was higher on Santa Fe (62%). The researchers speculated that ingested oil killed intestinal bacteria necessary for proper digestion of algae consumed by the iguanas, and proposed that wildlife populations exposed to environmental contaminants should be monitored carefully for the stress response.


Romero, L.M., and Wikelski, M. (2004). Severe effects of low-level oil contamination on wildlife predicted by the corticosterone-stress response: preliminary data and a research agenda. Spill Science and Technology Bulletin 7, 309-313.

Vieites, D.R., Nieto-Rom�n, S., Palanca, A., Ferrer, X., Vences, M. (2004). European Atlantic: the hottest oil spill hotspot worldwide. Naturwissenschaften, 91(11), 535-538. DOI: 10.1007/s00114-004-0572-2


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