The brevetoxin theory for the dolphin die-off had its flaws. For example, red tides occur frequently in the Gulf of Mexico, yet no similar die-off of dolphins has ever been reported there. In addition, brevetoxin was found in only eight of the 17 animals tested and in widely varying levels. Clinical data on the specific effects of chronic exposure to brevetoxin does not exist.

Public scepticism led the US Congress to hold a hearing and convene a scientific panel on 7 and 8 May 1989, to review the investigation's conclusions. One panelist, Canadian marine mammal scientist Dr Pierre Beland, stated that although 'little hard evidence implicating brevetoxin exists... remarkably little effort was put into investigating other possibilities for the die-off.'

This sentiment was echoed by other members of the same scientific panel. Veterinarian pathologist Daniel Martineau presented a possible scenario in which organochlorine pollutants, particularly PCBs, may have played an important role in the die-off.

Indeed, PCB levels in the beached dolphins were among the highest ever recorded in cetaceans. Levels found in the dolphins' blubber ranged between 13 and 620 parts per million (ppm). There was one exception - a dolphin found to contain an incredible 6,800ppm, the highest level ever recorded in a marine mammal. (Products in the US are required to be labelled as hazardous and handled in toxic waste containers if they contain 5 ppm of PCBs.)

That PCBs cause various lesions, including damage to the liver and to the nervous, reproductive and immune systems in a wide range of mammals and birds is well documented. Many of the dolphins found on the east coast had the same types of lesions as those recorded in laboratory and domestic animals experimentally subjected to PCBs in their diet.

Alternative theories were presented during and after the investigation. For example, the US Department of Agriculture suggested that some event may have caused normally benign bacteria to increase in potency and wreck havoc on the dolphins.

Others posited a combination of events. Dr Gabe Vargo, another member of the scientific panel, questioned whether the mass mortality would have occurred if contaminants such as PCBs and DDT had not been present.

A number of ocean disposal sites used for dumping sewage sludge, acid waste and chemical warfare agents exist within the die-off area. There was no investigation of possible links between these sites and the die-off, nor into the possibilit that there had been an accidental release from one of the military and civilian facilities in the area which manufacture chemical weapons and genetically engineered micro-organisms.

The US government certainly appeared anious to avoid the implication of PCBs, or any other form of pollution, in the dolphins' deaths. In a leaked memo, read out at the Congressional hearing, one government scientist asked another for 'data generated on PCB/pesticides.' He added that no matter what was found in the dead dolphins, 'no special attention will be drawn to these data... [and] a blanket statement will be made that the levels were not out of the ordinary.'

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