Split-tail spawn in the Yolo Bypass and live in the Delta. Dan Castleberry heads the Fish and Wildlife Service's Bay-Delta office. He says populations of the fish fluctuate naturally…depending on what kind of water year we have.

"Split-tail do well in high-water years when flood-plains inundate. Recently we've had a series of dry years and the numbers are lower."

But environmentalists say the fluctuations have more to do with changes to the fish's habitat. Jeff Miller is with the Center for Biological Diversity. He says too much Delta water is being exported.

"We're creating conditions where we're diverting water and altering habitats to the point where every year is a bad year for split-tail and for all our other native fish species."

Miller says environmental groups plan to file a legal challenge over the Fish and Wildlife Service decision.