RESCUE MISSION by Marilyn Davis
The fish are two native species of river sturgeon. The pallid sturgeon, found in the Mississippi River and also in the Missouri River and some of its tributaries, is federally endangered. The shovelnose sturgeon, which is much more common and widespread, can be fished commercially. Both are increasingly valuable. Hard times and lax environmental regulation in the former Soviet states have led to overfishing and the collapse of the caviar fisheries there. The result? Eggs from the shovelnose—hardly a prime caviar species—now sell for $160 per pound. "We think that harvest pressure on shovelnose sturgeon has been going up," says Robert Sheehan, a professor of zoology with SIUC’s Fisheries and Illinois Aquaculture Center. Sheehan is a member of the national Pallid Sturgeon Recovery Team. He and other scientists and students at the Fisheries Center are studying these fish in the middle Mississippi River—from the mouth of the Missouri, just north of St. Louis, to the mouth of the Ohio, at Cairo, Ill.—in an effort to save the species. Some have argued that the pallid sturgeon is just a strain of shovelnose. If that were so, there’d be no worry about potential harvest of an endangered species. A full-grown pallid sturgeon can be 6 feet long and 60 pounds; the biggest shovelnose will reach only 15 pounds. And mature pallids look almost white, whereas shovelnose are dark brown. But identification can be tricky with juveniles. Further muddying the picture are apparent hybrids, which, like many hybrid fish, can produce offspring. Commercial fishermen usually are experienced enough to tell the sturgeon apart, Sheehan says—especially out west, where pallids are larger. But he thinks that pallids are being taken, deliberately or accidentally. When there’s any doubt about a catch, he says, it’s unrealistic to think that most fisherman would cut loose such a profitable haul. Whereas a female shovelnose can be worth up to $500 for its eggs, a mature pallid sturgeon can net $2,000 or more. Currently there’s no way to distinguish the eggs of the two species—meaning there’s no easy way to clamp down on illegal catches. The SIUC team has developed methods to distinguish pallid, shovelnose, and hybrid sturgeon by taking certain measurements and counting fin rays. But genetic evidence is the gold standard of proof. So Sheehan, doctoral student Brian Sloss, and colleague Bernie May at the University of California—Davis developed genetic markers to settle the species issue. Because the two sturgeon lineages have diverged recently, their genes differ very little. The team focused instead on microsatellites—repeating "letters" in parts of the DNA alphabet that don’t seem to carry any genetic instructions. They found that the lengths of the repeats differ between pallid and shovelnose sturgeon. Their results indicate that pallid and shovelnose are separate species, with hybrids a third distinct group. "These genetic markers are going to be useful for sturgeon work worldwide," Sheehan adds. The microsatellite work distinguishes between populations, not individuals. So the research team is now refining genetic techniques to identify pallid sturgeon flesh or eggs. The aim: enabling enforcement agencies to prove their case if a market is dealing in the endangered fish. The genetics work also will let the researchers determine if propagation programs at hatcheries are using pure pallid sturgeon rather than hybrids as their broodfish. If hatcheries are unwittingly producing hybrids, those fish will swamp the natural stock—contributing to the pallid’s extinction rather than its survival. For several years, Sheehan, recently retired Fisheries Center director Roy Heidinger, master’s student Keith Hurley, and postdoc Paul Wills have tracked pallid sturgeon in the middle Mississippi to study their habitat use and seasonal movements. For example, while pallid sturgeon are most often found in the main river channel, they choose the downstream tips of islands when they can. But such habitat is now rare in the middle Mississippi thanks to levees and channelization. "A lot of federal and state agencies, as well as the Army Corps of Engineers, agree that restoration of islands is a high priority," says Sheehan, who will help the Corps draft a river management plan to protect the sturgeon. This offers other benefits, he says. "Habitat diversity in our large rivers has greatly declined. But different species at different periods of their lives rely on vastly different habitats. So it just makes sense that [increasing] habitat diversity will lead to species diversity." The SIUC team has not found large pallid sturgeon in the middle Mississippi. That’s a classic sign of exploitation, says Sheehan: "The larger individuals are taken first." More ominously, only three young have been found in the past five years. "We may not have reproductively active females out there," he says. Females must be much larger and older (15-20 years) than males before they can reproduce. Hybrids are probably the result of male pallid sturgeon fertilizing shovelnose eggs. Sheehan advocates temporarily closing the burgeoning shovelnose fishery so scientists can gather data to set acceptable harvest limits. That would also keep the pallid sturgeon safe while scientists refine the genetic testing methods needed to protect it permanently. "It would be win/win for both species," he says. Why save the pallid sturgeon, since it’s genetically similar to the shovelnose? "It’s an impressive species," Sheehan says, "and once they’re gone, they’re gone. If we let the pallid sturgeon become extinct, we’re making that decision for everyone from here on out—and I don’t think we have the right to do that." For more information, contact the Fisheries and Illinois Aquaculture Center at (618) 536-7761. This research has been funded by the U.S. Fish and Wildlife Service, the St. Louis District of the U.S. Army Corps of Engineers, SIUC, and UC-Davis. Fall 2000 Contents | Perspectives Home | SIUC Home
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Politics
can affect conservation in bizarre ways. So it is that the breakup of the
Soviet Union has affected the harvest of one Midwestern fish species—and
may hasten the extinction of another.