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.