The volcanic eruption led to a massive bloom of special phytoplankton called diatoms — an unusually rich source of food for the growing salmon. (Canadian Press)

A volcanic eruption might have helped produce B.C.'s largest sockeye salmon run since 1913.

The 34 million salmon that returned to B.C.'s Fraser River this year were "adolescents" in the Gulf of Alaska when the Kasatochi volcano erupted there in 2008, said Tim Parsons, a research scientist at the Institute of Ocean Sciences in Sidney, B.C.

The ash from that eruption fertilized the ocean, leading to a massive bloom of special phytoplankton called diatoms — an unusually rich source of food for the growing salmon.

"When you have an adolescent of any kind [and] you give them lots of food, they have lots of energy, and they build strong bodies," Parsons said.

"So, we get back, in my hypothesis, 34 million salmon — which was totally unpredicted — instead of the 1.5 million salmon of the previous year, which fed on a diet — which was the normal diet of the Gulf of Alaska — composed of very small plankton."

Parsons said he based his hypothesis on the recent research results reported by Roberta Hamme, an assistant professor at the School of Earth and Ocean Sciences at the University of Victoria.

Hamme, who observed the plankton bloom using satellite imaging, said in a recent paper in Geophysical Research Letters that it was one of the largest such blooms observed in the subarctic North Pacific.

Parsons said the 2009 sockeye run was small because the fish in that run were older, closer to adulthood, and were starting to move out of the Gulf of Alaska at the time the eruption happened.

The link between the plankton bloom and the huge sockeye run of 2010 is consistent with Parsons's own research. In one 1970s experiment, the sockeye run increased seven fold after he fertilized a lake on Vancouver Island. In other studies, he found salmon populations in the Gulf of Alaska depend on the density of phytoplankton.

Parsons suggests that if his hypothesis proves true, it could help fisheries managers make better predictions about salmon populations.

A federal inquiry into the state of B.C.'s wild salmon stocks opened in Vancouver on Monday.