Frederic Santoul had been researching Wels catfish for nearly a decade when he received a call in 2011 from a fisherman fishing the Tarn River in southern France. As an assistant professor of biology and ecology at Paul Sabatier University in Toulouse, Santoul often works with fishermen, who help collect stomach contents and additional data that Santoul uses to study the ecological impact of large catfish species. But this time, the fisherman’s field report bordered on the unbelievable.
“He said to me, ‘Something strange is happening in the town of Albi,'” Santoul says. “It looked like some catfish were foraging on the pigeons.”
Albi is home to many wild pigeons that take advantage of the shallow gravel bars in the River Tarn. Catfish Wels lined up at sunrise to grab pigeons from these small islands, sometimes beaching themselves in the process.
Wels catfish are an invasive species in Western Europe
Wels catfish are native to Eastern Europe. The species is thought to have been introduced into the waterways of Western Europe by fishermen in the 1970s and 1980s. They are now found throughout France, Italy and Spain. Chickens can grow up to 10 feet long and weigh more than 300 pounds. Females lay up to 25,000 eggs when they spawn. Members of the species can live for decades, and some scientists believe that the warmer waters of western European rivers may allow them to spawn more often than in their native range. Understanding the impact of catfish on anadromous fish such as salmon, lamprey and shad had long been the focus of Santoul’s research—until he discovered that they also prey on pigeons.
After the fisherman’s report, Santoul spent a summer in Albi filming the wells as they practiced the “amazing adaptation” of feeding pigeons. He noticed fish hiding in the shallows with their dorsal fins and backs sticking out of the clear water, waiting for the doves to come. As the birds scrambled for position, the catfish swooped in, opening their mouths wide to swallow their prey, sometimes thrusting their bodies far out of the water to snatch the birds from the gravel bars.
“The first time we saw this, we said it was exactly the same beaching behavior that is well known in killer whales,” says Santoul. “It’s unbelievable.” One element makes the maneuver even more dangerous for the catfish than for the orcas, which snatch seal pups off the beach on Patagonia’s Valdes Peninsula before catching the waves back out into the ocean. “Killer whales breathe oxygen from the air,” he notes. “The stack needs oxygen from the water. So it is more dangerous for catfish because if they stay [on land] too long, it can be fatal.”
Santoul co-authored and published a paper in the scientific journal PLOS-One in 2012 called “Freshwater Killer Whales: Beaching an Alien Fish to Hunt Land Birds.” In 24 surveys, the researchers observed one to nine wells floating near a gravel pit where the pigeons congregated. They filmed 54 attacks on the beach, noting that in 40 percent of those attacks, the fish brought more than half of their bodies ashore. On one occasion, a well catfish covered his entire body. That attack was unsuccessful, but Santoul found that 28 percent of attacks were successful—a kill rate similar to that of terrestrial predators.
Closer examination of the footage suggests that catfish use their barbels — whisker-like appendages of muscle and nerves near their mouths — to detect their prey. Pigeons that did not move were not attacked, even when the catfish were only inches away. For this reason, larger flocks of pigeons appear to increase the likelihood of well success.
“When there’s only one or two chicks, it’s really hard for the catfish,” he says. “When there are 20 or 30, it can be really dangerous for the pigeons because there is competition between them to get to the water.” While pigeons flock to each other to reach the best drinking and bathing spots, fish are more tactical.
“In some places, you can observe up to five or six fish in the same place, and there is some kind of strategic interaction,” says Santoul. “The base position is for the biggest catfish of the group, while the smaller fish are close by waiting. When the older fish catches a dove, a younger fish takes its place. They certainly seem to be learning these behaviors step by step.”
The novel fishing technique seems to have paid off for catfish
The researchers collected further data on the well catfish by taking fin clips from fish caught by fishermen and muscle samples collected by scuba divers carrying spear guns equipped with biopsy-tipped spears. Using a technique called stable isotope analysis, the researchers were able to determine how much of each fish’s diet consisted of pigeons. They found that for some birds, 50 to 60 percent of their diet came from birds, while others did not feed on them at all.
“You have to imagine that sometimes they stand in one place, waiting, for 10 hours, and there are no pigeons,” explains Santoul. The fact that they use the tactic, despite the energy required to constantly fight the current and maintain their prime hunting grounds, says something about the benefit. “I remember one morning, the same catfish caught three pigeons.”
Santoul noticed that only catfish in a certain size range — roughly 3 to 6 feet long — seemed to use the beaching tactic. No fish longer than 6 feet have been observed following pigeons on land. “When and why do they decide to stop using this behavior?” wonders Santoul. “We have never seen a dead catfish on the beach. They seem to learn and understand that, perhaps, [when they get larger,] they are no longer successful or it is too dangerous and they stop.”
Santoul and his team were the first scientists to document this behavior. Beaches have since been spotted in Italy, Spain and other parts of France – but never in their native Wales in Eastern Europe. Santoul initially thought that the invasive fish were feeding on the pigeons because they were struggling to compete with the larger catfish for dwindling native fish populations, but he later abandoned the hypothesis. Instead, he says it’s simply a matter of opportunistic feeding. “They are able to forage for any type of prey,” says Santoul. “In this case it’s pigeon, but it could also be lobster, moray eel or salmon. The plasticity of this species is one of the reasons for their success in conquering Western Europe.
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He adds that pigeons are particularly vulnerable targets. The birds have evolved to be wary of predatory predators, but seem oblivious so far to the dangers lurking at the water’s edge. “They care about the birds, but they don’t care about what happens in the water,” says Santoul. “That makes them a good target for catfish – for now.”