The Biggest Loser: Shark Edition

Ocean acidification could be the ultimate weight-loss plan for fish.
A smooth dogfish swims in a flume at the Woods Hole Oceanographic Institution in Massachusetts. Arrows point to the source of the food odor.
Credit: Photo: Danielle Dixson

Are you an overweight shark having trouble saying no to second, third, and fourth helpings? Do you stay up late at night slamming squid juice after squid juice? Are you consumed by the constant desire to find, hunt, and devour prey in a way that has haters calling you an “eating machine” behind your back?

Well, do I have the thing for you—it’s called the ocean acidification diet!

This amazing diet has already been proven to work on oysters, clams, coral, and mollusks, as they feel the burn (the burning of their calcium shells away in more acidic seas, that is.) Now new research has found that sharks that used our “secret cleanse formula” for just four days appeared to totally lose their drive to sense and attack prey! (Fine print: Active ingredients are just carbon pollution and seawater. Patent pending.)

But don’t take our word for it! Here to provide scientific testimony is Dr. Danielle Dixson, a marine biologist at the Georgia Institute of Technology. Dixson studies how fish sniff out chemical clues in seawater in order to feed and reproduce. Ocean acidification, she’s found, could profoundly affect those behaviors.

And now a word from our scientists: Fish (including sharks) “breathe” when water passes through their gills. As the ocean absorbs more carbon dioxide from atmospheric pollution, chemical reactions are causing seawater all over the world to become more acidic. As a fish sucks that more-acidic water into its gills, its respiratory and circulatory systems also become more acidic. Its body then tries to counterbalance the lower pH by upping its levels of bicarbonate, a base. And that’s where things get funky with the fish’s central nervous system (more on that later).

To see how this might play out with sharks, Dixson and a team of researchers from the Woods Hole Oceanographic Institution, James Cook University, and Boston University scientists put three groups of smooth dogfish (a species of houndshark) into a custom-made flume of seawater. Basically it was like a treadmill tank for fish, with a current flowing from front to back. Tiny nozzles up front would release different scents into the left or right side of the pool. The scents in this case were concentrated squid rinse, a.k.a. delicious squid juice, to test the animals’ ability to detect prey in the water. (Concentrated squid rinse sold separately.)

The tank for Shark Group 1 contained water like you would find in the oceans today. Those sharks spent more than 60 percent of their time on the side that smelled like chopped up squid. (Those fatties!) The same was true for Group 2. Those sharks took a dip in medium-range CO2 levels, similar to what scientists expect the oceans will be like in the next 50 years.

Group 3, on the other hand, got the full ocean-acidification treatment. Those sharks swam through high concentrations of CO2 that coincide with projections for the year 2100. Instead of preferring the blended-squid side of the pool as Groups 1 and 2 did, the sharks in Group 3 actively avoided it, spending less than 15 percent of their time on the squid-scented side.

What’s more, when the sharks were allowed to interact with the nozzles squirting out the squiddy smell, the sharks in the high-carbon pools bumped or tried to attack the source significantly less frequently than the sharks from Group 1 (what a bunch of gluttons!).

These results are preliminary, and Dixson says the experiment doesn’t define how the more acidic water affects sharks specifically—only that it does. But her previous research on clownfish could offer some clues about what’s going on.

A clownfish uses its sense of smell to distinguish between predators and friendlies. But when you treat Nemo with CO2-heavy seawater, he loses the ability to tell the difference between friend and foe. It gets worse: In experiments where clownfish had the option between swimming toward the odor of a predator or heading toward nothing at all, the clownfish on acid swam headlong toward the existential threat 100 percent of the time.

What’s happening here? Dixson found that the bicarbonate produced by a clownfish’s body in response to higher acidities disrupts communication between the fish’s sniffer and its brain by deactivating something called GABAA receptors. Those neural receptors, found in all vertebrates, play a crucial role in decision-making—as in, don’t swim toward that barracuda!

“It’s kind of a catch-22,” says Dixson. “The way they’re coping with the acidity is the reason why they’re affected.”

So there you have it, fish—like all fad diets, this one comes with risks. But look on the bright side: Nothing helps you get in your cardio time like having to constantly escape from the jaws of death. And wait, there’s more! The ocean acidification diet allows you to eat all the scrumptious seafood you love, provided you can find it. (Don’t worry—ha ha—you won’t!)

Heard enough? Then call now for your introductory offer. Or don’t! Supplies are not limited! If humans don’t get it together and do something to control their own carbon binges, every fish you know will be on the ocean acidification diet by 2100.

This article was originally published on onEarth, which is no longer in publication. onEarth was founded in 1979 as the Amicus Journal, an independent magazine of thought and opinion on the environment. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. This article is available for online republication by news media outlets or nonprofits under these conditions: The writer(s) must be credited with a byline; you must note prominently that the article was originally published by and link to the original; the article cannot be edited (beyond simple things such grammar); you can’t resell the article in any form or grant republishing rights to other outlets; you can’t republish our material wholesale or automatically—you need to select articles individually; you can’t republish the photos or graphics on our site without specific permission; you should drop us a note to let us know when you’ve used one of our articles.