There are all sorts of drinking traditions. Some people sing songs as they down their alcohol. Others dance to thumping music. Somewhere in the vicinity of Rotterdam, in the Netherlands, a group of young men, apparently inspired by the American television show "Jackass," got in the habit of capping off their boozing by swallowing live fish.

This, it turns out, is a bad idea. Especially in the event that the fish have evolved to fight back.

According to a recent case report published on Jan. 17 in the journal Acta Oto-Laryngologica Case Reports, the young men typically swallowed live goldfish out of their home aquarium — small, squishy creatures that don't put up much of a fight. The fun stopped on April 3, 2016, when one of the men tried to take their tradition a bit further by swallowing a bronze catfish (Corydoras aeneus), a popular aquarium fish with some powerful natural defenses. [27 Oddest Medical Case Reports]

"Most animals know better"

Most animals know better than to eat bronze catfish, said Kees Moeliker, a director at the Rotterdam Natural History Museum who reviewed the catfish remains after doctors removed them from the man's throat. That's for a good reason: Their cute 2- to 3-inch bodies (5 to 8 centimeters) are defended with spines, mounted on their pectoral fins. When the fish get stressed out — say, for example, when they're being swallowed by a predator — those spines become erect and can pump venom into the mouths of their attackers.

Because of this, bronze catfish "don't have predators like birds and other fishes," Moeliker told Live Science. "Those who give it a try die, and natural selection does its work."

Indeed, in the man's case, it appears that he quickly realized that he had made a grave mistake, according to a video of the incident that was described by the case report authors. Unfortunately, the video was not available for Live Science to review or share. But the report includes a vivid description of what it showed. In the video, a crowd of men stood around drinking and shouting "grote vis, grote vis!" (Dutch for "Big fish! Big fish!") One guy, holding a glass of clear water with the live catfish in it, tips it back, attempting to swallow it whole. Four seconds later, he spit the water and the fish out into his hands, and threw it on the table, where it floundered, appearing "distressed" and "agonized," according to the report.

That might have been the end of it, if someone hadn't plucked the flopping catfish off the table and handed it to a third man, a 28-year-old whose trauma would become the subject of the case report. This unfortunate fellow swallowed some beer, and then dropped the still-living creature into his throat. [11 Weird Things People Have Swallowed]

Immediately, it was obvious something had gone wrong. The man tried to swallow more beer but couldn't. Ten seconds later, he was "gagging vigorously" and vomiting liquid. "In extreme distress," he shoved two fingers down his throat, trying to make himself gag but no luck. Someone administered what the doctors described as a "wrongly-applied Heimlich maneuver," which again failed to produce the fish. The man spewed some blood into a bucket, and then the camera switched off.

Even so, the man apparently waited "several hours" before going to the hospital, after trying but failing to dislodge the fish with "more beer, honey and ice cream."

A "fish-like" structure

When the man finally made it to the emergency room, doctors looked down his throat using a tool called a laryngoscope, and spotted what they described as "a fish-like structure," according to the report. [Photos: The Freakiest-Looking Fish]

"This is definitely in the top three of weirdest medical cases I’ve encountered," said case report co-author Dr. Linda Benoist, a medical resident at Rotterdam’s University Medical Center who treated the patient. Benoist told Live Science that she had been aware that the fish-swallowing game was a "bizarre" tradition among some young people in the area.

The catfish, she said, was already dead when the patient arrived, pressed up against the entrance to the man's esophagus, at the bottom of his throat. (The fish had probably suffocated, Moeliker said, noting that a few swallows of beer do not contain enough water for a fish to breathe in).

The man needed surgery to remove the fish, with the surgeons paying very close attention to carefully remove the fish's spines from the delicate tissue in the throat. .

Fortunately, the procedure was a success. Though not much is known about the effects of bronze catfish venom on humans, it didn't appear to complicate the situation. As of the man's most recent follow-up with doctors, in March 2017, he is doing well.

The fish, meanwhile, ended up preserved at the Rotterdam Natural History Museum, which is right next door to the hospital. It joined an exhibit called "Dead Animal Tales" on dramatic collisions between animals and humans, Moeliker said.

Asked whether the drinking game is dangerous when only goldfish are involved, Benoist said, "I'm not an expert in goldfish swallowers, but I can imagine that fish species without [spikes] would slide easier into the stomach."

Still, the researchers did highlight some other case studies where people choked on live fish, including one instance where a fisherman attempted to kiss a fish but it slid into his throat. Live Science recommends eating fish dead and in bite-size pieces.