DRAKESVILLE — The fish are back at Lake Wapello, even the ones conservation authorities hoped were gone for good.

It has been more than a year since Lake Wapello was up to full strength. The Iowa Department of Natural Resources drained it in 2008 to make repairs and kill off an invasive population of gizzard shad. The work also allowed workers to add new fishing areas and improve fish habitats in hopes that the fishing would come back strong.

The total project cost $400,000.

Everything seemed to work. Crews closed the lake’s dam in April and the water level is back to 75 percent of its normal level.

Now everything is in doubt again. Mark Flammang, a fisheries biologist with the DNR, said checks on the fish the state re-stocked found a new gizzard shad population.

Flammang still sounds dazed by the discovery.

“We can’t have gone through all this work and start with a situation where we have to go back and start all over again,” he said.

Gizzard shad are not native to this part of the country. They can grow to over 22 inches in length, though the ones caught in early July were much smaller. Experts believe they hatched this spring.

The real problem is the shad don’t fit lakes the size of Lake Wapello. Flammang said they can be absorbed in large lakes, but they eat the same food as other hatchling fish. They grow quickly and wind up out-competing the bluegill and sport fish in small lakes.

“It’s kind of a complicated issue when you start talking about these critters,” said Flammang. “In a small lake like this ... they just don’t work very well. Time and time again they result in the collapse of the fishery.”

Some of the fish the shad put pressure on were already becoming re-established in Lake Wapello. Bass were back in the 10-inch range and bluegill were already spawning. Flammang said the survey showed the fish were ahead of projections and encouraged biologists until the shad surfaced.

The upside, if there is one, is that the discovery of gizzard shad came now instead of two or three years from now. Flammang said the effort to restore Lake Wapello was set back a year, but it could have been much worse had the shad stayed hidden.

The state will target the shad again, but it is too soon to know just what form that effort will take.

Just how the shad wound up in Lake Wapello again is a mystery. The effort to wipe them out included killing out feeder streams and checking both public and private ponds in the area. None of those checks uncovered an independent population of the shad.

A DNR press release emphasized that it is against the law to introduce any live fish to any public water in Iowa. They sent the same message Monday afternoon on Twitter to reinforce it. Flammang went farther, saying it is illegal to possess live gizzard shad in Iowa.