Every year, wildlife agencies around the world pour millions of young fish into lakes, streams and rivers. The work is usually done with the twin goals of propping up fish populations and giving anglers something to catch.
While the actions might seem like a straightforward equation (add more fish, get more fish), an elaborate study suggests that the math doesn’t work that way. Just adding more fish has little long-term effect. It’s the habitat that matters.
“Restoring central ecological processes and habitats—ecosystem-based management—is likely to have stronger long-term effects for rebuilding fish species and populations than narrow, species-focused conservation actions,” said Johannes Radinger, a scientist at Germany’s Leibniz Institute of Freshwater Ecology and Inland Fisheries, who helped lead the research.
The new findings raise questions not just about the benefits of artificially supplementing fish populations, but about the potential pitfalls of any initiative aimed at boosting wildlife numbers by raising and releasing more animals.
It can be tricky to tease out the effects of programs such as putting more animals into the wild. While lab studies are useful, it’s nearly impossible to replicate the dizzying number of forces that shape an ecosystem. Meanwhile, natural habitats are so complex—and wild animals so, well, wild—that it’s hard to track what’s happening to their numbers and why.
Radinger and his colleagues tried to address these difficulties with a hybrid approach, turning a network of 20 German ponds into outdoor laboratories. The ponds, which formed in the bottoms of defunct gravel mines—have many of the characteristics of natural lakes, such as tree and grass-lined shores. But they are small enough that they lend themselves to being manipulated and monitored.
The scientists wanted to test two basic approaches to improving fish populations: enhancing habitat or adding more fish. In some lakes they used heavy machinery to build shallow underwater habitat. In some of those same lakes, they also dumped bundles of sticks that could serve as hiding places for fish. Other ponds received no habitat treatments, just bucket-loads of fish commonly put there by local fishing clubs, including northern pike, tench, roach, bream and pikeperch.
Over the span of 6 years, including two years before the alterations to the lakes and four years after, the scientists visited each of the lakes and caught fish using nets and electrical currents that stun the animals. When they counted their haul, several key patterns emerged. Fish numbers grew 2.7 times more in lakes with new shallow habitat, compared to lakes that were left untouched and unstocked. Adding more wood had little effect, with a few exceptions. Fish stocking was a dud. Numbers in stocked lakes either didn’t change or, in some cases, even declined over the course of the study, the scientists reported March 2 in the journal Science.
The benefits of building the shallow-water zones likely stem from their attractiveness as spawning grounds and as homes to baby fish, which can live there while avoiding predators such as pike that lurk in deeper waters. The populations of juvenile fish smaller than 10 centimeters was more than 5 times larger in lakes with new shallow habitat, based on the fish caught during electrofishing surveys. Given the small amount of new habitat involved, “the positive outcome of this measure is noteworthy,” the scientists wrote.
The added wood was a more complicated story. Overall, it had little effect. Predatory perch numbers increased in lakes with woody bundles, though it wasn’t statistically significant. At the same time, there was some evidence that the numbers of roach—a smaller, silvery fish that perch eat—had decreased in the same lakes. That might be because roach avoided wood-filled areas that harbored perch, or because such spots made the prey fish more vulnerable to being gobbled up.
The experience with wood adds a cautionary note about habitat improvements as the key to increasing wildlife numbers. It’s not simply enough to meddle with the habitat. Some kinds of changes work better than others.
Why did adding more fish not make a dent in fish numbers? In lakes where habitat and food sources imposed a limit on how many fish could live there, adding more fish just created more competition, the researchers surmised. Stocking can leave survivors malnourished as they fight over the scraps. It could also introduce hatchery-raised animals less genetically suited to surviving in the wild.
That’s not to say stocking will have no effect in all places. The researchers note that the results can hinge on the condition of the lakes and the species that live there. For instance, it seems possible that in a place with healthy habitat but severe fishing pressure, restocking could help fish numbers rebound. Terrestrial examples include species such as wolves and beaver that have been wiped out by hunting, then bounced back with the help of reintroductions.
Nevertheless, the new results add a sobering, and potentially expensive lesson. Habitat improvement generally is more costly than simply raising more of a species and setting them loose. Then again, it might be the only thing that works.
Radinger, et. al. “Ecosystem-based management outperforms species-focused stocking for enhancing fish populations.” Science. March 2, 2023.