VANCOUVER — The death toll from the Gulf of Mexico oil spill goes far beyond the animal corpses washing ashore, says a report that warns that whale and dolphin deaths may be 50 times higher than believed.

The report, by an international team of marine mammal specialists, estimates that for every corpse that washes ashore another 50 may never be found.

"When people present the raw carcass counts without any caveats, without any qualifiers, they are implying we have a 100 per cent carcass recovery rate when we don't have anything like that," says Rob Williams of the University of B.C. He is lead author of the report to be released Wednesday as the one-year anniversary of the Deepwater Horizon disaster approaches.

He and his colleagues take issue with recent reports that point to a carcass count of just over 100 whales, dolphins and porpoises as evidence of the modest environmental impact of the spill.

"The true death toll could be 50 times the number of carcasses recovered," they report in the journal Conservation Letters.

An explosion ripped through BP's Deepwater Horizon drill rig last April, and the damaged well gushed oil onto the sea floor for months as crews scrambled to plug the well and clean up and disperse the oil.

"The Deepwater oil spill was the largest in U.S. history, however, the recorded impact on wildlife was relatively low, leading to suggestions that the environmental damage of the disaster was actually modest," Williams says in summary of the findings. "This is because reports have implied that the number of carcasses recovered, 101, equals the number of animals killed by the spill."

In an interview he said the recovered bodies "represent just the tip of the iceberg."

"It will be a long time before the science is in and we can evaluate the true total environmental impact of the Gulf spill," he said.

He and his colleagues from the United Kingdom, the U.S. and Australia looked at 14 species of marine mammals that frequent the Gulf of Mexico, from sperm whales to spinner dolphins.

Their analysis found that, historically, only two per cent of carcasses have been recovered after they die in the region. They caution in their paper that the recovery rate after the Gulf spill could have been above the historical two per cent because there was such a concerted search for carcasses.

But applying that two per cent recovery rate to the Deepwater Horizon disaster, and assuming all deaths recorded were caused by the oil, they suggest the spill's toll on whales, dolphins and porpoises "would translate to 5,050 carcasses." They also break it down by species and say, for example, that "it is plausible" 29 sperm whales, an endangered species, died because of the Gulf spill.

The scientists ran their calculations late last year when the official carcass count for the marine mammals along the northern coast of the Gulf of Mexico stood at 101. Dozens of young and stillborn baby dolphins have washed ashore since then, and necropsies are underway to find out if the oil spill is responsible.

Bottlenose dolphins appear to have been hit hardest by the spill, and the biologists say "the potential is high for the spill to have caused catastrophic impacts on small, localized populations of bottlenose dolphins in the Gulf."

They call for more study to assess impacts on marine mammals, pointing to the profound and lingering effects of the Exxon Valdez spill.

In the first year after the 1989 Exxon Valdez spill one group of "transient" killer whales experienced a 41 per cent loss and there has been "no reproduction" in the group since the spill, they say.

"Although the cause of the apparent sterility is unknown, the lesson serves as an important reminder that immediate death is not the only factor that can lead to long-term loss of population viability," the scientists say.