Heterosigma akashiwo

High resolution image of Heterosigma akashiwo. 2001 Photo courtesy of Brian Bill, Northwest Fisheries Science Center.

Heterosigma is a microscopic alga called a raphidophyte. It has two flagella and swims in the water, but doesn't have a rigid cell wall so it can change its shape. Most often it looks like a swimming cornflake. When high concentrations (blooms) of Heterosigma are carried by wind or water currents into a salmon net pen, the fish are killed. It is not known exactly how Heterosigma kills fish, but it may be from irritation of the fish gills so the fish cannot take up oxygen from the water. Heterosigma blooms can kill large numbers of fish causing the aquaculture industry to lose millions of dollars as occurred in Washington in 1989 and 1990.