The tasty migratory fish are typically born in rivers, swim to the ocean where they reach maturity and can remain for several years, before returning to their native rivers to spawn and die.

But drought-hit rivers with too little flow or unusually warm water can fatally disrupt that cycle, causing California's fish and wildlife department to take "the proactive measure of trucking millions of hatchery-raised" juvenile salmon to the sea.

"Trucking young salmon to downstream release sites has proven to be one of the best ways to increase survival to the ocean during dry conditions," said northern California hatchery chief Jason Julienne in a recent statement.

While road transportation of salmon dates back to the 1980s, early drought conditions this summer mean authorities plan to boost the size of the operation by 20 percent.

Trucks bearing nearly 17 million young salmon will travel more than 30,000 miles (48,000 kilometers) between April and June, helping the fry avoid streams where many have perished in the past. 

One beneficiary will be the Feather River Hatchery, just below a dam north of Sacramento, which produces around eight million salmon fry annually from fish that naturally return to the rivers in which they were born.

The hatchery was built in 1967 to compensate for the destruction of natural spawning sites above the Oroville Dam, with a "fish ladder" -- a narrow channel around a mile long with man-made steps -- simulating the rapids of a mountain stream.