He decided to test the theory. In 1997, Estes and Tinker packed up a dead otter on Amchitka Island and flew it to California, where a colleague ground it up in a giant blender, calculated its calorie load, and compared it with how many calories a killer whale consumes. It turned out that fewer than four whales, 3.7 to be exact, could have eaten 40,000 otters in five years. But orcas had lived in harmony with otters for thousands of years on the Aleutians. To find the answer, biologists simply had to follow the food chain.
Orcas customarily feed on sea lions and seals, which are packed with high-calorie blubber.
But the population of Steller sea lions, the world's biggest sea lions, took a sharp dive in the late 1980s. Harbor seals also declined at a similar rate. By 1992, otters were the only plentiful marine mammals left in Aleutian waters. The orcas, in their hunt for calories, apparently had been forced to switch prey. The effects cascaded rapidly down the food chain.