EarthTalk, the environmental section of online magazine Blast was asked: Why did 34 million wild sockeye salmon return to the Fraser River in British Columbia this year? The run had been declining for 20 years before now.

The miraculous sockeye salmon run in western Canada’s Fraser River watershed in the summer and fall of 2010—indeed the biggest run in 97 years—still has fishers, researchers and fishery managers baffled. Just a year earlier only one million fish returned to spawn. No one seems to be able to say for sure what caused the massive 2010 run, but most agree that it probably had to do with the very favorable water conditions that were present in 2008 when the sockeyes were juveniles. “They’re very vulnerable at that stage of their life,” reports John Reynolds, a salmon conservation expert at Canada’s Simon Fraser University.

Read the full answer here.