After the fish was full of water -- so full that the water ran back out its mouth -- Thomas hoisted the Great Lakes sturgeon from the tub and gently released it back into the St. Clair River.

And then the fun began as Bouse and Thomas began sifting through the water in the tub in the dark of night, looking for anything that might have come out.

The pair -- Bouse works for the U.S. Fish and Wildlife Service, Thomas for the Department of Natural Resources -- is trying to unlock still another secret to one of the Great Lakes' most celebrated fish: Just what are they eating?

"We don't really have a good idea what these fish consume when they're in the river system," said Bouse, who is assigned to the Service's Alpena station but spends his time in southeast Michigan. "And we've got a lot of them in the river, from adults to juveniles."

Sturgeon, members of one of the oldest families of fishes in the world, are the rock stars of Great Lakes fish these days. They have been revered, despised, exploited, then ignored for much of the time since Europeans settled North America.

But now, they have captured the imagination of both the fish heads and the public. There has been more research on them in the last decade than in the previous 10.

The DNR has done some research on the food habits of the freshwater leviathans, but has found its typical collection methods -- catching the big fish on set lines -- typically yields fish with empty stomachs.

"Potentially, (with set lines) they're on there for 24 hours, so you have to deal with digestion and issues like that," Bouse said. Netting surveys are difficult to run because of the fast-flowing current in the river.

So, Thomas and Bouse hit on an unusual way to survey fish: get together with recreational anglers and survey the fish as soon as they're caught? With cell phone technology, the biologists can get to an angler's boat within minutes after a fish has been brought aboard.

The men haven't found much, mostly tiny creatures -- blood worms, caddis, black flies, gnats and midge larvae.

"There's the potential that these fish are regurgitating when they're being captured," Bouse said. "The fish are coming up from the deep portion of the river and you're not going to get the absolute best example of their diet. But, we are getting some representation of what they're consuming."

One of the problems with the study, Bouse says, is that the sturgeon tend to hold in sandy bottom areas where there is little aquatic insect life. But, it's obvious that sturgeon don't get to be 100 pounds without eating something.

One theory is that sturgeon, which feed by swimming over the bottom and suctioning in food items, have taken to eating zebra mussels. Bouse, who studied sturgeon in graduate school at the University of Michigan, said he hasn't found that yet.

One of the difficulties in studying sturgeon is that some of the fish are world-class travelers. One fish that was tagged in southern Lake Huron has been recaptured in Saginaw Bay, in Lake Michigan's Green Bay and then back in southern Lake Huron again. But, others are homebodies.

"We've seen adults in the St. Clair River in the three years I've followed them (with radio telemetry) that have never left the North Channel," Bouse said. "There's no rhyme nor reason in any of this."

Sturgeon, which have been exploited for caviar as well as their flesh, are well protected by regulations. Anglers in the St. Clair system can fish for them from July 16 to Sept. 30. There's an unusual limit -- anglers can keep one per year (they must get a tag from the DNR and register creeled fish) and they must measure between 42 and 50 inches. Larger or smaller specimens must be returned immediately.

But the fishery seems to be highly catch-and-release oriented these days anyway. Thomas says the DNR gives out about 80 or 90 tags a year -- not enough to damage the population -- and sees very few fish registered.

"Certainly, there are thousands out there," Thomas said. "The question is, is it 10,000 or 20,000 or more.

"Eventually, we hope to make a reasonable population estimate."

In the meantime, however, Bouse and Thomas will go out in the boat on warm summer evenings, hoping to get a phone call from a successful sturgeon angler so they can motor over and pump a sturgeon's stomach. And perhaps unlock one more secret of the Great Lakes' most mysterious denizen.