Why Do Humans Eat So Many Sea Bass?

“Wild fish did not come into this world just to be our food,” Paul Greenberg writes toward the end of his book Four Fish.  “They came into this world to pursue their own individual destinies.”

Yet the way in which different fish species pursue those destinies – or, if you don’t like that quasi-mystical word, behave as their genes dictate – goes a long way in determining how likely a fish is to wind up pan-seared in butter and lemon juice.

Accordingly, one of Four Fish’s big questions is: Why do we eat certain species?  Well, you might answer, because they’re delicious, or plentiful, or good for you.  But what, specifically, about a fish’s anatomy makes it tasty?  What specific behaviors make it easy to catch?

Take, for example, the European sea bass.  Bass, according to Greenberg, owe their culinary popularity to one organ: their highly evolved swim bladder.  (In Thailand, I discovered, bladder is often served in a dubious dish called “fish maw soup,” which even Thais don’t even seem to like very much.) Greenberg doesn’t go in-depth about what traits make bass’ swim bladders so evolutionarily excellent, so I’ll try to fill in what he leaves out.


A fish’s swim bladder has a basic function: it keeps the fish from sinking.  Without the bladder, the fish would be heavier than its watery milieu, and would have to work very hard to stay afloat.  (Some fish, including sharks and coelacanths, manage this trick.  That’s why sharks have tails with longer upper lobes than bottom – heterocercal tails provide better lift).

A long upper tail lobe provides lift to help compensate for the shark's lack of swim bladder. Picture courtesy of hookedondestin.com

By filling their bladders with air, fish remain neutrally buoyant – ie, they weigh as much as the water around them – and expend far less energy than they would otherwise.  As fish move up and down, and the water pressure around them changes, they inflate and deflate their swim bladders correspondingly. Anybody who has ever dived has used a Buoyancy Control Device, and understands this intuitively.

Would you eat a soup made of that organ? Ummm... Courtesy of http://www.earthlife.net.

Now for the cool stuff – how do fish inflate their swim bladders?

There are two kinds of swim bladder: physostomous, or open, bladders; and physoclistous, or closed, bladders.

Human lungs and swim bladders share an ancestral organ, and that homology is very apparent when you look at an open bladder: a duct, like our windpipe, connects the fish’s mouth to its bladder.  The fish simply sticks its head out of the water and gulps some air when it wants to inflate and gain buoyancy; and burps when it wants to deflate.  (Think about Charlie and his grandpa in the original Charlie and the Chocolate Factory.)  Salmon, pike, goldfish, and many other species use this primitive system.

A Yellowstone cutthroat trout with its swim bladder fully inflated. Always nice to be able to use a photo that I took.

But sea bass, like all perciforms (perch-shaped fish), evolved the second, advanced kind of bladder: the closed bladder.  Closed bladders are so-called because they have no duct connecting to the mouth, or to any outside sources of air*.  Instead, perciforms regulate buoyancy by absorbing gas, usually oxygen, from the bloodstream into the bladder; and releasing that gas, when necessary, back into the blood.  To accomplish this, closed-bladder fish have an organ called the gas gland which secretes lactic acid, causing the eventual diffusion of oxygen from the blood into the bladder.  If you’re interested in exactly how this complicated process works, I suggest you consult either this Wikipedia page or a nephrologist.

*Closed-bladder fish have temporarily open bladders during their larval stage (approximately the first two weeks of their lives). A common cause of mortality among farmed fish is closed, or oil-covered, tanks, which prevent the larvae from taking gulps of air. The swim bladder forms around the first bubble of air that a fish swallows; without this bubble, larvae die, sink, or grow up deformed.

The upshot is that fish like sea bass have two big advantages over fish with open bladders: they don’t have to swim to the surface to freshen up their air bladders; and their buoyancy system is simply more efficient – they’re better at staying neutral.  This means they expend less energy than other fish, and, says Greenberg, is why they’re so delicious:

“Without a need to fight gravity all the time, perciforms became more efficient swimmers and were able to trade in their heavy, energy-demanding “red muscle” tissue for lighter, more delicate flesh.  Hence the white, light meat of many perciforms.  Perciforms also evolved an efficient muscle structure that is principally attached only to the central spine column.  The result: a smooth, mostly boneless fillet, very pleasant to eat.”

This unfortunate but delectable sea bass will never again diffuse internal gases across a chemical gradient. Courtesy of SFgate.com

Not only is smooth, delicate, boneless bass flesh tasty to eat, it was once easy to obtain.  Although a closed swim bladder allows its owner to remain below the surface, it also limits the depth to which the fish can dive – go too deep and the bladder may implode.  In general, the fish that can go the deepest, and can change depths most rapidly, are the fish that have no swim bladders at all, like mackerel, sharks, and flounder.  Because coastal sea bass are confined to shallow water by their large bladders, they’re much easier to spear and catch on a line than fish that lack bladders, or have small ones*.

*There are some perciforms, like the Patagonian toothfish (rebranded the Chilean sea bass, in a stroke of marketing genius) which live at great depths. The toothfish’s bladder has been replaced by glands which secrete lighter-than-water fat into the fish’s skin and allow it to maintain buoyancy effortlessly. The toothfish’s high fat content, incidentally, is what makes it so delicious, and thus so over-fished.

The author's brother gets his hands on a sea robin, a bottom-dwelling fish that uses its swim bladder to make drumming noises.

Europeans’ taste for sea bass, then, is the product of fish morphology and centuries of consequent exploitation.  Humans developed and maintained a taste for sea bass centuries ago; began to farm them intensively in the 1980’s; and eat them ravenously today.  All thanks to their remarkable closed-bladder buoyancy system that improves their energy efficiency and, in turn, makes them tastier, less bony, and comparatively easy to catch.  

I’m sure I’ll return to both swim bladders and Paul Greenberg’s great book in future posts, but enough for now. Big hat tip to Davidson Biology for a great site on this subject.

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6 Responses to Why Do Humans Eat So Many Sea Bass?

  1. Monica says:

    Idea: You should write a book about animals on the verge of extinction that people don’t care about because they have some unfortunate name like “grey-tailed marsh rat” and they don’t look like panda bears… just a thought, because my only real interest in fish is in eating them and maintaining healthy ecosystem balances so we can keep eating them, but this post is really awesome and interesting, even though it’s about fish bladders!

  2. bgoldfarb09 says:

    Thanks a lot, Monica! Just wait until the next update: sea slugs. I’ll do some grey-tailed marsh rat research and get back to you…

  3. According to some history sources, the sole reason that many people in East Asia are delighted in having “Fish Maw/Fish Swim Bladder” in festive meals is due to the fact that it is a favourite among the ancient emperors. Hence, the “social status” gained from ordering the “fish-maw soup/dishes”.

    The same can be said of abalones, shark fins, sea cucumber, etc.. But what’s really sad is that the emerging East Asian economies have generated a large well-off population and this increased the affordability to the “status-symbol food”, fuelling overharvesting in every seafood/exotic animals industry. (without the consumers knowing it; or worst, consumers ignoring the warnings)

    Although many NGOs in East Asia are working hard to change the people’s thinking about these severely depleted food resources, the lack of social and political will impedes their efforts. Only time will tell if the next generation will be more environmentally-conscious…

    • bgoldfarb09 says:

      Hey JK, that’s really interesting… I had no idea that fish maw was considered a prestigious food. But, come to think of it, that explains why I only saw it served at banquets and parties. The sad thing is that nobody even seems to like these “status” foods: although I would never try shark fin soup, I’ve heard that it’s pretty much flavorless. Apparently the Japanese don’t like whale very much – a few years ago the government tried to introduce it in school lunches, and the students rebelled. And whenever I went to a party in Thailand where fish maw was served, nobody touched it. I never saw somebody order it off a menu, either. Ridiculous. How to convince people that fish is a means of sustenance, not status?

  4. That may be a sign of winds of change, Ben. Some young couples in Asia have also joined the “environmental bandwagon” by dropping out all exotic meats/dishes (spelled- dishes from endangered species) in their wedding dinner/parties too. Hope this trend will spread like wildfire soon!

  5. Monica says:

    I have to concur with all the above. Exotic foods are a status symbol, but people also (wrongly) believe that these foods have medicinal qualities. What’s more, in certain social and business contexts you might be seen as a bad host if you don’t shell out for the expensive items on the menu, invariably including some endangered species. (Might be worth noting that, at least in China, people almost never split the bill; somebody is always the host.) This, I think, is probably the root of the problem and the most difficult issue to fix. I’ve never heard anybody praise a restaurant’s shark’s fin soup, here or in Asia, because it tastes like… nothing, and yet it ends up on so many tables just because ordering it is a way of fulfilling a social expectation.

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