Stop Pouring Soda on Fish Gills

A couple years ago, I saw a blog post that encouraged bass anglers to keep a bottle of Coke with them at all times. And no, it wasn’t for the caffeine high.

Instead, fishermen were supposed to use this magical elixir as a healing agent for bleeding fish. The thought went that when a fish is gill hooked or gut hooked, you could pour Coke on the wound to stop the bleeding. The soda provided a combination of carbonation and acid to clean and cauterize the sore, saving the fish from bleeding out.

This seemed too good to be true, but the fish-saving wisdom spread like wildfire. Soon, publications like Field & Stream and Outdoor Canada were promoting the theory, making it common knowledge that a Coke could bring a fish back to life.

Most recently, though, a handful of videos on the subject have garnered over 1.6 million views on Facebook. One of them is titled “Saving a Giant Musky with Diet Coke” where a fishing guide confidently releases a bleeding musky.

“The effervescence from soda pop swells the blood vessels on a bleeding fish and stops the bleeding. It’s successful for me probably 75 or 80 percent of the time,” the guide said. “That sealed it and that’s not bleeding. That’s going to make this fish survive.”

Another viral video features a shoreline angler holding a bass with blood-filled gills. He spoke of the healing powers of Mountain Dew, then poured the citrus soda into the fish’s gullet. The blood disappeared, and he lowered the bass into the water and watched it swim away.

“And voila! Instant cauterization. Save more fish, drink Mountain Dew.”

In short order, 8,000 people had liked the video in approval, with another 12,000 sharing it with friends. This was the last straw for me, as I needed to set the record straight on this. Sure, the fish in those videos swam away looking strong and healthy, but did the soda have anything to do with that?

I reached out to Dr. Solomon David, aquatic ecologist at Nicholls State University, to see what his expertise had to say on the subject.

“Coke on fish gills is as dumb as it sounds,” Dr. David explained. “My initial take is that the acidic nature of soda is bad for sensitive gill tissues. The best thing one could do is get the fish back in water that’s the appropriate temp.”

The theory is about to be tested in a laboratory setting. Dr. Steven J. Cooke, fish ecologist at Carleton University and editor of Conservation Physiology, will be the first person to research the effects of soda on fish gills. He expects their study to wrap up in September of this year.

“There’s no science on the topic, but enough anecdotes that it is worth asking the question in the context of a formal research project, which is what we intend to do,” Dr. Cooke said. “I remain the skeptical academic as always—but also smart enough to realize that if anglers are doing it there must be some merit. The videos are interesting, but far from conclusive.”

While Dr. Cooke wants to let his research play out before making any assumptions, the majority of scientists I spoke to have said that not only is this soda theory wrong, but it’s also potentially bad for fish. Introducing a weakly acidic substance that a fish would never otherwise encounter could throw off its body chemistry. And, even if the fish’s gills were cauterized, that’d mean there’s no more gas exchange, which is crucial for a fish to, you know, breathe.

Something that everyone should have an easy time accepting is that any time a fish is out of water, it can’t get oxygen. These guys who keep an already struggling fish out of water to pour soda on them are “doing more harm than good,” one biologist noted. In the case of the musky angler from the video referenced above, the fish was kept with its head out of water for an extra 20 seconds as the fisherman did the Diet Coke trick. It also resulted in about 30 additional seconds of unnecessary handling.

What’s more is that fish quickly lose their “slime” the longer they’re held out of water. This slime—which is crucial to their immune systems—dries when exposed to air. It also gets removed from a fish’s body through handling or having a foreign liquid like Mountain Dew poured all over it . If a fish really is wounded, it needs to retain this slime to help keep out pathogens and bacteria, especially when it’s already vulnerable from a wound.

Northern pike and musky specifically need that slime to fight off infections, as these species typically inhabit some of the shallower areas in lakes that are more likely to house harmful microorganisms. A fish without slime is more prone to infections, and Mountain Dew isn’t helping out there, either.

As for what’s really happening, it’s likely that anglers are witnessing blood clotting when releasing fish, but it has nothing to do with Coke. Fish blood naturally clots in water. That’s why they can survive small dings and scrapes. Unlike mammals, who have platelets to clot blood, fish have thrombocytes. Although they have the same function, platelets don’t work under water, while thrombocytes do.

As Dr. David said earlier, the best thing you can do for an ill-hooked fish is to get it back into the water.

Still, it’s likely that many anglers will continue to embrace this pseudoscience, as fishing personalities push this bass-saving technique. It’s an important reminder that the biologists and physiologists who study these creatures have a better idea of what’s going on than Joe Bassfisherman.

You can’t deny the good intentions of these catch-and-release fishermen, though. While their science is off, their hearts are clearly in the right place. I would just prefer that they swiftly dispatch of mortally wounded fish, put them in a cooler next to their soda and take them home for supper, rather than giving them a Coke baptism and sending them off into a watery grave.

Update from June 21, 2020
Dr. Steven J. Cooke’s results from the study are now available. In the study, northern pike with damaged gills were exposed to Mountain Dew, Coca Cola, or carbonated lake water. Scientists examined the amount of blood loss, bleeding intensity, and gill color. They found that it had no positive affect on the fish.

“As such, there is no scientific evidence to support the use of carbonated beverages for reducing or stopping blood loss for fish that have had their gills injured during recreational angling based on the context studied here. This study reinforced the need to scientifically test angler anecdotes and theories when it comes to best practices for catch-and-release fishing.”

So, in case you need to hear it again: Stop pouring soda on fish gills.