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16 Settlement of the mid - continental and coastal prairies and mere human presence, as opposed to alteration of the habitat, may have interfered with the continued use of prairie and wetlands by breeding whooping cranes. Man's conversion of pothole and prairie to hay and grain production made much of the original habitat unsuitable for whooping cranes. Disruptive practices included draining, fencing, sowing, and all of the human activity associated with these operations. The advent of rural electrification brought power lines, and collisions are known to have accounted for the death or serious injury of at least 19 whooping cranes since 1956. Whooping cranes adhere to ancestral breeding areas, migration routes, and wintering grounds. There is little likelihood of pioneering new habitat, except locally. The only self- sustaining wild population remains vulnerable to destruction through a hurricane or contaminant spill, due primarily to its limited wintering distribution along the GIWW of the Texas coast. The GIWW experiences some of the heaviest barge traffic of any waterway in the world. Much of the tonnage is in petrochemical products. An accident resulting in a spill could potentially destroy whooping cranes and /or their food resources. Transport of petroleum products and other chemicals by barge along the GIWW has for many years been considered a potential danger to whooping cranes and other wildlife at Aransas NWR. During summer, 1974, 25 to 50 barrels of crude petroleum leaked from a barge. The high viscosity of the oil, and the prompt action by clean -up crews, limited the spill to an area averaging about 1.6 m wide and extending 16 km along the canal. This spill, and other more recent ones, emphasize the hazards which accompany the shipping of dangerous cargoes on the GIWW. A consultant to the Army Corps of Engineers (Gulf Engineers and Consultants, Inc. 1992) assessed threats to the whooping crane and its habitat from spills of vessel fuels and cargoes. Each concluded that the hazard of spill exists, but the probability of their occurrence is low. Catastrophic events, such as a large spill are infrequent, and therefore, difficult to predict. There is a great potential for acceleration of traffic, and an increase in accidents as traffic increases from Mexico associated with ratification of the North American Free Trade Agreement. Thus, the probability of occurrence of the most likely spill (1 per 1,075 years) and worst case spill (1 per 7,982 years) are very likely conservative (Gulf Engineers and Consultants, Inc. 1992). The worst case spill estimated by the Environmental Protection Agency 0 992) would be approximately 33,000 barrels of liquids. The U.S. Coast Guard has the lead responsibility for spill response and containment. The U.S. Fish and Wildlife Service has response plans for the Gulf of Mexico (U.S. Fish and Wildlife Service 1979) and Aransas National Wildlife Refuge specifically (Robertson et al. 1993). The latter Plan (Robertson et al. 1993) "...is inadequate in providing full protection for the whooping cranes. Such a plan is not possible since chemicals are transported right through the center of the whooping crane winter range. Spills of hazardous materials may threaten human health so that approach could only be done by personnel wearing special protective suits and breathing apparatus. Spill of gaseous materials could directly kill all whoopers downwind of the disaster. Response time at best is 1 -2 hours by the refuge staff, and 3 -4 hours by spill control specialists. An event occurring at night or during bad weather (the