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. The fish, which belong to the minnow family and spawn primarily in the warmer-water Little Colorado, were counted again after flooding.
The fish caught after the flooding tended to be somewhat larger, up to a bit over 7 inches in length. That suggested that the smallest fish declined most significantly, said Lew Coggins, a fisheries biologist.
Change in temperature
Water releases from the Glen Canyon Dam are much colder, generally between 46 and 54 degrees Fahrenheit. The humpback chub's eggs have a much higher survival rate in the Little Colorado's temperatures of about 61 degrees. 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.
The change in conditions could have affected fish behavior. "People want to know right now what did the fish do, and sampling showed this reduction of about 63 percent," Lovich said. "But we don't have the answer right now."
Officials estimate there may be about 3,000 fish in the Grand Canyon today, down from about 10,000 in the last decade.
Environmental critics such as Living Rivers, based in Moab, Utah, said the flooding likely pushed the dwindling chub population closer to extirpation. It contends that the dam, which creates Lake Powell along the Arizona-Utah line, is exacerbating the collapse of the Grand Canyon's ecosystem and should be decommissioned.
"It makes sense to get rid of Glen Canyon Dam, because it's the one changing the ecosystem of the Grand Canyon," said spokesman John Weisheit. "It's dying."