Two studies from San Francisco state oceanographer Richard Dungdale blame ammonia from a nearby sewage treatment plant.
The Sacramento Regional County Sanitation district releases about 146 million gallons of treated wastewater into the Sacramento River each day.
Sewage officials estimate an upgrade to filter out the ammonia would cost $740 million, and they say doing so would force them to triple sewage rates for customers.
The study claims the ammonia does not make the fish unsafe to eat, but it does disrupt the Sacramento River's eco system.
