The palates of endangered southern resident killer whales are refined to the point that they choose to eat Fraser River chinook salmon, say researchers, who hope new information about orcas' menu choices will help the population recover.A joint Canadian-U.S. study that used highly refined molecular genetic techniques to look at the DNA of fish scales and whale feces found that, during spring and summer, up to 90 per cent of chinook eaten by the whales come from the Fraser River.The study appears in this month's edition of the journal Endangered Species Research."When you compare chinook DNA, it can tell you which particular watershed and which river it came from. It's a very powerful tool," said lead author Brad Hanson, a marine mammal scientist with the National Marine Fisheries Service in Seattle.Although the science is high-tech, collecting whale feces is distinctly low tech."You look for the brown stuff floating in the water and scoop it up with a mesh net," Hanson said.A study by Department of Fisheries and Oceans whale expert John Ford -- also one of the authors of this report -- previously showed that southern resident killer whales prefer chinook, but it's the first time there has been information on the origin of the fish.While 80 to 90 per cent of the fish come from the Fraser, only six to 14 per cent come from Puget Sound, the scientists found."When you figure out which stocks are important, it's information that fisheries managers might want to take into consideration when making decisions," said Hanson, adding that it is puzzling the orcas choose Fraser River chinook over the slightly smaller Puget Sound chinook and more abundant sockeye."Do the whales pass by on the smaller Puget Sound fish? Or maybe they can't detect them," Hanson said."Also, the Columbia River produces more chinook salmon than the Fraser, so why aren't they hanging out at the mouth of the Columbia?"After a baby boom over the winter, there are now 89 members of the three endangered southern resident orca pods.Some of the Fraser River chinook stocks they feast on are in trouble, however, with "medium to high conservation concerns," the study says.A partial solution could be increasing the number of hatchery fish in those areas and paying particular attention to those fisheries because of their importance to the orcas' diet, the study says.According to the Canadian and U.S killer whale recovery strategies, shrinking runs of salmon are a factor in the slow recovery of the southern resident population.It's believed poor nutrition can also lead to higher blood contaminant levels, another risk factor.