The salmon swam out of the rice fields Wednesday, and they came out fatter than ever.

The experiment to raise juvenile salmon in flooded rice fields began in February when scientists put 50,000 pinky-sized fish into flooded test fields on 18 acres in the Yolo Bypass north of Woodland.

The goal of the experiment, now in its second year, is to verify whether flooded rice fields are better for young salmon than life in the river, and to find the best floodplain conditions to help the fish thrive. The idea is to mimic historical conditions, when flooding gave salmon access to virtually the entire Sacramento Valley.

On Wednesday, when researchers retrieved and weighed their test subjects, they found the fish packed on weight at an impressive rate: 0.17 grams per day on average.

In plain terms, that means pinky-sized baby salmon grew into chubby, palm-sized adolescents. In six weeks, they grew nearly 1.5 inches longer.

Their kin in the river? Salmon that started at the same size and were released into the river at the same time grew only about half as fast.

"For the second year, these fish have grown so fast that we're calling them our 'floodplain fatties,' " said Carson Jeffres, a researcher at UC Davis, which partnered in the experiment with California Trout and the California Department of Water Resources.

The salmon were able to swim back to the Sacramento River via canals that link the test fields to drains in the Yolo Bypass. Along the way, they were netted for counting and measuring.

The results are significant because research has shown that bigger juvenile salmon survive better when they reach the ocean, and are more likely to return as spawning adults three years later.

The fish grow fat because floodplains produce a bonanza of food: insects and zooplankton that hatch from the soil during floods.

To help struggling fish populations, water management and wildlife officials are looking for ways to create these conditions more often. More frequent flooding in the Yolo Bypass is one answer.

Another is to fit salmon rearing into the rice growing cycle: Rice grows in shallow flooding conditions and is already the dominant crop in the bypass.

This year's fish grew about 30 percent bigger than those in last year's test. This is due, ironically, to drought conditions this year, said Jacob Katz, a biologist at Cal Trout and the project manager.

Reduced runoff volume in the water supply serving the test plots caused the food supply in the water to become even more concentrated, Katz said. This made an even richer buffet for the salmon.

It also offers a good illustration of how floodplains are supposed to work, even in years with erratic rainfall.

"A floodplain starts out, during a flood, with a big surface area and then rapidly shrinks down," Katz said. "It produces a bunch of food that becomes denser as the floodplain shrinks. So there was just a lot more food in less water going directly into the fields."

Contact The Bee's Matt Weiser at (916) 321-1264. Follow him on Twitter @matt_weiser.

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