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Catch-and-release fishing is supposed to be the best of both worlds: The anglers get the joy of a day spent on the water, and the fish gets to live another day. But sometimes, a hook will damage a fish's gills or esophagus and cause severe or even fatal bleeding.

For years, catch-and-release anglers have claimed that there's a straightforward solution to this: Just pour some soda on the fish to stop the bleeding and save its life. The idea was that the carbon dioxide in sodas constricted the blood vessels in the gills, thus stopping the bleeding. But some biologists thought the idea sounded a little bit fishy.

In a 2020 study, they decided to test this claim and figure things out for real. In their study, the researchers got to do what I'm sure they spent all of grad school dreaming about:. They poured soda on bleeding fish.

They tested Coca-Cola, Mountain Dew, and carbonated lake water and compared that to no treatment. Alas, none of the drinks reduced the duration or intensity of bleeding. Technically, this study is a preprint, so it hasn't gone through the peer-review process yet, and corrections could turn up.

But at this point, it does seem to effectively debunk this idea. That said! In this process, the researchers did find clues about why so many people think this strategy works.

They noticed that while bleeding wasn't reduced overall, it would slow down for about 30 seconds. Which might be as much time as an angler spends with their catch. Researchers think this happens because the carbon dioxide in the drinks causes bradycardia, or a very slow heart rate.

And if the heart slows down, blood is pumping more slowly through a fish's body, so it won't bleed as much. At least, until its heart rate goes back up again. Now, heart rate wasn't specifically looked at in this study, but several previous papers have shown that when fish were briefly exposed to water with high CO2 levels, they had a substantial drop in heart rate.

So there is support for this idea. Researchers think this drop is triggered by sensitive chemical receptors in the fish's gills. That's based on things like how, when you sever the nerve that allows the gills to communicate with the rest of the body, fish no longer have this response to elevated CO2.

But even though the how is clear, the why is another story. Despite decades of research, scientists still haven't conclusively demonstrated why this happens. One idea is that high carbon dioxide levels in water generally happen at the same time as low oxygen levels.

And since the heart needs lots of oxygen to work, the bradycardia may help protect the heart muscle when there isn't as much oxygen available. Sadly, though, this still means there's no good evidence that pouring carbonated drinks on fish does them any good. So if you've got a bleeding fish on your line, it's just going to be a rough day for that fish.

But I guess you can console yourself with a nice, cold soda. If you have a science question for us, we would love to hear it. You can leave it in the comments below, or you can submit them through our Patron QQ inbox over at patreon.com/scishow.

And who knows: Your question might inspire our next episode. But in the meantime, thank you for watching this episode, too. [♪OUTRO].