The number of juvenile endangered fish recovered in the Colorado River declined dramatically after officials flooded the Grand Canyon in an effort to aid them and their fragile ecosystem. But scientists aren’t sure what the fish decline means or why it happened.

"We’re trying to get the same answer," said Jeff Lovich, chief of the Grand Canyon Monitoring and Research Center in Flagstaff. "We don’t know what their fate is." Following a 90-hour experimental water release from Glen Canyon Dam into the canyon in November, 63 percent fewer endangered humpback chub juveniles were trapped than before flooding.

Lovich said there are three possible explanations: The fish were washed downstream, they died, or they’re still in the river and scientists couldn’t accurately sample them after the simulated flood.During the flooding, the Bureau of Reclamation released as much as 41,000 cubic feet of water a second from four of the dam’s giant steel tubes. Scientists hoped to redistribute 800,000 metric tons of sediment to create beaches, substrate used by plants and backwaters and pools to help the fish breed.

The dam, built 40 years ago to assure the West’s water supply, has altered the landscape dramatically. The chub is the last of four endangered fish species surviving in the canyon and one of only four native species of fish left among eight that once swam through it. Scientists set out hoopnets centered around the mouth of the Little Colorado River — a spring-fed tributary into the Colorado — and at two other locations downstream to catch the juvenile humpback chubs to count, tag and release before the flooding.

Scientists are looking at installing temperature control devices in the future to release warmer water closer to the surface of the reservoir. They also note that sampling conditions were different during the flood because of an unexpected natural flood in the Little Colorado. "We have to admit that all of our results have to be interpreted with that caveat — that we had different conditions," Coggins said.