It could be a combination of two things: predation, or changes in prey abundance. These young fish are feeding actively. They need to put on weight so they can survive the huge migratory pathway to the north Pacific Ocean. It's possible the prey they normally consume wasn't there in the amounts they required, or alternatively, predators were there in numbers that were different from normal. Probably a combination of those two might explain what happened.
On October 31st, 2006. See this statement in context.