For folks who imagine themselves to be extra ethical due to eating fish rather than “meat,” What a Fish Knows: The Inner Lives of Our Underwater Cousins may be unwelcome:
Just how exploited are they? One author, Alison Mood, has estimated, based on analysis of Food and Agriculture Organization fisheries capture statistics for the period 1999–2007, that the number of fishes killed each year by humans is between 1 and 2.7 trillion.* To get a handle on the magnitude of a trillion fishes, if the average length of each caught fish is that of a dollar bill (six inches) and we lined them up end to end, they would stretch to the sun and back—a round-trip of 186 million miles—with a couple hundred billion fishes to spare.
However you slice it, it’s a lot of fishes, and they do not die nicely. The leading causes of death for commercially caught fishes are asphyxiation by removal from the water, decompression from the pressure change of being brought to the surface, crushing beneath the weight of thousands of others hoisted aboard in massive nets, and evisceration once landed.
The bulk of the book is a survey of research regarding what we know about fish intelligence and emotional life. First, let’s talk about what a “fish” is.
What we casually refer to as “fish” is in fact a collection of animals of fabulous diversity. According to FishBase—the largest and most often consulted online database on fishes—33,249 species, in 564 families and 64 orders, had been described as of January 2016. That’s more than the combined total of all mammals, birds, reptiles, and amphibians. When we refer to “fish” we are referring to 60 percent of all the known species on Earth with backbones.
We conveniently classify animals with backbones into five groups: fishes, amphibians, reptiles, birds, and mammals. This is misleading because it fails to represent the profound distinctions among fishes. The bony fishes are at least as evolutionarily distinct from the cartilaginous fishes as mammals are from birds. A tuna is actually more closely related to a human than to a shark, and the coelacanth—a “living fossil” first discovered in 1937—sprouted closer to us than to a tuna on the tree of life.
According to the National Oceanic and Atmospheric Administration, less than 5 percent of the world’s oceans have been explored. The deep sea is the largest habitat on Earth, and most of the animals on this planet live there. A seven-month survey using echo soundings of the mesopelagic zone (between 100 and 1,000 meters—330 to 3,300 feet—below the ocean surface), published in early 2014, concluded that there are between ten and thirty times more fishes living there than was previously thought.
Between 1997 and 2007, 279 new species of fishes were found in Asia’s Mekong River basin alone. The year 2011 saw the discovery of four shark species. Given the current rate, experts predict the total count of all fishes will level off at around 35,000.
The smallest fish—indeed, the smallest vertebrate—is a tiny goby of one of the Philippine lakes of Luzon. Adult Pandaka pygmaea are only a third of an inch in length and weigh about 0.00015 of an ounce. If you were to put 300 of them on a scale they wouldn’t equal the weight of an American penny.
Another fish superlative is their fecundity, which is also unmatched among vertebrates. A single ling, five feet long and weighing fifty-four pounds, had 28,361,000 eggs in her ovaries. Even that pales compared to the 300 million eggs carried by an ocean sunfish, the largest of all bony fishes.
An older organism isn’t necessarily simpler. Evolution does not trend relentlessly toward increased sophistication and size. Not only were the largest dinosaurs much larger than modern reptiles, paleontologists have recently unearthed evidence that they were social creatures with parental care and modes of communication at least as complex as those of modern reptiles.
There is a great chapter on fish vision:
But how do fishes perceive what they themselves see? What is the mental experience of a fish, and how might it compare to our own? One way of probing this question is by considering optical illusions. If an animal is unaffected by a visual image that fools us, then it would seem that that animal perceives visual fields in a mechanical way, as a robot might “perceive” them. If, however, they fall for the illusion as we do, it suggests that they have a similar mental experience of what they are seeing.
Are fishes fooled by optical illusions? Well, in a captive study of redtail splitfins—small fishes that originate from highland Mexican streams—they learned to tap the larger of two disks to get a food reward. Once they had mastered the task, the scientists presented them with the Ebbinghaus illusion, which consists of two disks of the same size, one of which is surrounded by larger disks, making it appear smaller (at least, to human eyes) than the other, which is surrounded by smaller disks (see Figure 1). The splitfins preferred the latter disk.
Similarly, an earlier study found that redtail splitfins also fall for the more familiar Müller-Lyer illusion, in which two identical horizontal lines appear to have different lengths
So fish do see more or less the same way that we do! How about hearing. Here’s one from the Department of the Science is Settled:
And yet, as recently as the 1930s, scientists believed that fishes were deaf. This prejudice probably arose from the fact that fishes lack an external hearing organ. With our human-centric view of the world, such a lack could only mean one thing: no hearing. Now we know better: fishes don’t need ears, thanks to water’s incompressibility, which is why water is an excellent conductor of sounds. It is not until we peer inside a fish that we find structures modified and recruited for producing and processing sounds. Karl von Frisch (1886–1982), the Austrian biologist famous for his discovery of the dance language of honeybees, was also a devoted student of fish behavior and perceptions. Decades before he became the corecipient of the Nobel Prize in 1973 for his contributions to the emergence of ethology (the science of animal behavior), von Frisch was the first to demonstrate hearing in fishes. In the mid-1930s, he devised a simple but ingenious study in his lab with a blind catfish named Xaverl. He did this by lowering a piece of meat on the end of a stick into the water near the clay shelter in which Xaverl spent most of his days. Having an excellent sense of smell, Xaverl would soon emerge from his hiding place to retrieve food. After a few days of this routine, von Frisch began to whistle just before delivering the food. Six days later, he was able to lure Xaverl from his lair just by whistling, thereby proving the fish could hear him.
Not only can they hear, but they can learn about music:
Ava Chase, a research scientist at Harvard University, was interested to see if fishes could learn to categorize sounds as complex as music. She conducted an experiment using three pet store–bought koi named Beauty, Oro, and Pepi. Chase set up a sophisticated apparatus in the fishes’ tank that included a speaker at the side for presenting sounds, a response button on the bottom that fishes could push with their bodies, a light that signaled to the fish that his response had been recorded, and a nipple near the surface that dispensed a food pellet when the fish swam up and sucked it after a “correct” response. She then trained the fishes by rewarding them (with a food pellet) when they responded to a certain genre of music and not rewarding them for responding while another genre emanated from the speaker. She found that the koi were not only able to discriminate blues recordings (John Lee Hooker guitar and vocals) from classical recordings (Bach oboe concertos), but that they could generalize these distinctions when presented with new artists and composers for each genre.
They have taste and smell:
The sophistication of the smelling organs of fishes varies greatly, but the basic design is shared among all the bony fishes (the 30,000 or so fish species that are separate from the sharks and rays group). Unlike those of other vertebrates, fishes’ nostrils do not do double duty as organs of smell and openings for breathing; they are used exclusively for smell.
A sockeye salmon can sense shrimp extract at concentrations of one part to a hundred million parts water, which translates in human terms to five teaspoons in an Olympic-size swimming pool. Other salmon can detect the smell of a seal or sea lion diluted to one eighty billionth of water volume, which is about two-thirds of a drop in the same pool. A shark’s sense of smell is about 10,000 times better than ours. But the champion sniffer among all fishes (as far as we know) is the American eel, which can detect the equivalent of less than one ten millionth of a drop of their home water in the Olympic pool. Like salmons, eels make long migrations back to specific spawning sites, and they follow a subtle gradient of scent to get there.
game. Female sheepshead swordtails from Mexico can discriminate the smell of well-fed males from hungry males of their species—two- to three-inch denizens of tropical rapids—and you can probably guess which they prefer: all else being equal, a well-nourished fish is a more resourceful one, which makes him the better sperm donor. Female swordtails do not discriminate the odor of well-fed females from hungry females, suggesting they are responding to male sex pheromones and not merely to food-based excretions.
Taste buds are also more numerous in fishes than in any other animal. For instance, a fifteen-inch channel catfish had approximately 680,000 taste buds on his entire body, including fins—nearly 100 times the human quota.
There are weakly electric fishes that can sense using electrical pulses.. They also may enjoy the sensation of touch. But can they feel pain?
There are some good reasons to expect that fishes are sentient. As vertebrates, they have the same basic body plan as mammals, including a backbone, a suite of senses, and a peripheral nervous system governed by a brain. Being able to detect and learn to avoid harmful events is also useful to a fish. Pain alerts animals to potential damage that may lead to impairment or loss of life. Injury or death reduces or eliminates an individual’s reproductive potential, which is why natural selection favors the avoidance of these dire outcomes. Pain teaches and motivates animals to avoid a noxious past event.
Fish love opioids enough to qualify as true Americans:
The trouts’ negative reactions to the insults were dramatically reduced by the use of a painkiller, morphine. Morphine belongs to a family of drugs called opioids, and fishes are known to have an opioid-responsive system. Their behavior in response to it here is consistent with their experience of relief of pain by the drug.
Lynne Sneddon used what I consider to be a most convincing way to examine pain in zebrafishes: she asked if they were willing to pay a cost to get pain relief. Like most captive animals, fishes like stimulation. For instance, zebrafishes prefer to swim in an enriched chamber with vegetation and objects to explore rather than in a barren chamber in the same tank. When Sneddon injected zebrafishes with acetic acid, this preference didn’t change; nor did it change for other zebrafishes injected with saline water (which causes only brief pain). However, if a painkiller was dissolved in the barren, unpreferred chamber of the tank, the fishes injected with the acid chose to swim in the unfavorable, barren chamber. The saline-injected fishes remained in the enriched side of the tank. Thus, zebrafishes will pay a cost in return for gaining some relief from their pain.
The author describes that fish consciousness and
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