How do you restore a population of fish when every time you release them into the wild, they suffer a quick and almost complete die-off because, having been born in a hatchery, they have no idea how to fend for themselves? Enroll them in a wilderness survival class.
The species in question is the Baltic sturgeon, a fish that can grow to the size of a Volkswagen Beetle. Historically, the gigantic fish was found in rivers throughout the northern hemisphere, but fishing, habitat destruction, and pollution have driven it to extinction in many places. In Germany, no sturgeon have been caught in the wild in about two decades.
Since 2006, a German reintroduction program has released three million tank-reared sturgeon into the Elbe and Oder Rivers. It’s a vast number of fish. But studies elsewhere have shown that 97 to 99 percent of released sturgeon die, usually within a few days.
Recently, Joern Gessner, coordinator of the German program, got an inkling as to why. With colleagues from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin, Gessner investigated how well hatchery-raised sturgeon forage for food in conditions similar to those in the wild. Badly, it turned out. They went about it “sweetly stupid,” Gessner says.
Sturgeon suck up worms, shrimp, and other invertebrates from the sand in rivers and lakes. They smell their prey and use receptors along their mouths to sense electrical impulses from the twitching muscles of nearby organisms. In tests, however, hatchery-raised juveniles didn’t seem to know that. Juveniles often took six hours on their first day to track down food. Some never managed at all.
So, in an experiment, Gessner and his team tried teaching sturgeon how to hunt. They hid mosquito larvae—one of the fish’s favorite prey—inside piles of sand in an otherwise empty fish tank. Since the larvae were so simple to find, the strategy seemed to give the fish a better understanding of how to find their food.
After two weeks, the sturgeon were tested in a tank with a completely sand-covered bottom. On average, the fish that had undergone training took only 58 minutes to hunt down their first meal. Over the course of the next few days, both trained and untrained sturgeon got better at foraging, but the untrained fish never caught up.
Similar strategies have been used to help some mammals and birds get ready to be released into the wild. In Mexico, macaws are taught to pick fruit from branches, and wildcats in the United Kingdom are taught to pounce on their prey.
Little is known about how fish learn, but when Gessner’s team compared the brains of the trained and untrained sturgeon, they saw differences in their structures. Those of the trained fish seemed to be producing more neurons, suggesting that learning to search for food was making the animals’ brains more active.
“Given the results, it seems likely that deficient foraging behavior may be part of why the stocked fish don’t survive very well,” says Joacim Näslund, a biologist at Stockholm University in Sweden who was not involved in the study.
Other factors might matter, too. Gessner has been experimenting with exposing young sturgeon to the smell of predators and fluctuating water temperatures to mimic natural variations.
He’ll have to wait a long time before he knows if the training actually helps, however. Once sturgeon that have gone through training are released into the wild, they will spend a decade at sea before returning to their home rivers. But even if they make it, that will open new questions. “Will they reproduce?” says Gessner.