Depending on the type, a fish can be a low-maintenance pet. A tank or a bowl filled with water, maybe some rocks in it, and you're pretty much set, so long as you feed it. The fish doesn't need to search for food, and you have an animal companion that doesn't require too much effort. Everyone's happy, right?

Not so fast.

Numerous studies have taken a look at the cognitive world of fish, both those in the wild and those we regularly keep as pets. The studies include considerations of whether or not they can experience an equivalent to human depression or feel pain. More simply, researchers are attempting to determine the degree to which fish have sentience.

This may seem like a silly thing to investigate, but fish, as a category of animal, are among the most recognizable and used vertebrates on the planet. We raise them as livestock to be consumed. We keep them as pets. We go to visit exotic species of them at aquariums. We use them routinely in scientific research. However, we don't often afford them the same consideration we do other creatures, like dogs, cats, chimpanzees or even mice and rats.

Perhaps we should. And it's possible that by doing so, we could learn not only more about their internal world but our own as well.

How do you know if a fish is depressed?

A zebrafish swimming in a tank Zebrafish can't tell us if they're depressed, but they do demonstrate behavior that we associate with depression in humans. (Photo: topimages/Shutterstock)

Take depression, for instance. About 15 million Americans are affected by depression each year, and while our understanding and treatment for it has grown, there's still plenty we don't know. Enter the zebrafish.

In a chapter from the study Developing Zebrafish Depression-Related Models, Julian Pittman and Angelo Piato discuss zebrafish as the "model organism for experimental studies of affective disorders," such as depression. Their review of previous studies highlights various testing methods for which zebrafish are used, including the "open field test," exposure to stress and dosing with drugs to create responses.

During the open field test, for instance, organisms such as rat, mice and fish are exposed to a new (and thus potentially dangerous) environment. The animals must decide if they will explore and how aggressively. They could be rewarded with resources and shelter if they seek things out, but they may also run into danger.

To test how a zebrafish might behave in such a situation, Pittman kept a zebrafish drunk on ethanol for two weeks and then cut off the supply to induce a sense of withdrawal. The fish was then placed in a new tank, and Pittman monitored how long the fish stayed at the bottom half of a tank versus the top half.

The assumption in this experiment is that languishing on the bottom is representative of a kind of anxious or depressed state for the fish, with the withdrawal effects standing in for a depressive state. When Pittman supplied the fish with antidepressants for a couple of weeks, it began to explore the tank in a more aggressive manner.

Pittman and Piato's survey of zebrafish and depression-related experiments seem to indicate that these tiny fish are indeed the ideal candidates for studying depression. Speaking to The New York Times, Pittman said "the neurochemistry [between humans and zebrafish] is so similar that it's scary."

But it's not exactly the same, either. Pittman induced these symptoms in the zebrafish, after all, and as Pittman and Piato point out in their survey, the idea of depression in fish cannot encapsulate the whole of the human experience of depression, including things like low self-esteem and suicidal thoughts. That being said, there's still value to the studies that Pittman and Piato review because if they can induce depression-like symptoms, it likely means that the fish can experience them on their own.

The brain and the heart

A betta fish at the bottom of its jar Even if they're unable to actually say, 'I'm depressed,' a fish can convey unhappiness through some of its actions. (Photo: thanyathornS/Shutterstock)

So if fish have the capacity to feel depression, does that mean they're capable of feeling other things, have cognition and possibly even some degree of sentience?

Culum Brown, a behavioral biologist at Macquarie University in Sydney, has made the case for fish intelligence and sentience for a number of years. Brown conducted a review of scientific papers in an effort to support his position, one that's far-ranging and worth a look. Some of the highlights regarding the issue of fish cognition include how different types of fish respond to optical illusions, which eye fish use to look at friends and potential predators and how that corresponds to brain processing, tool use and the ability of fish to determine the location of things using geometry.

Brown lays out these particularly studies without much fanfare; the intelligence of fish almost seemed to be taken for granted in his review. When it comes time to map out the discussion around emotions and the sensation of pain, Brown becomes more heated and vocal in his review. The debate largely centers around whether pain is an emotional experience that is separate from the physical sensations, or if, as Brown argues, it's part of an "integrated system" of experiences.

Brown cites various studies that suggest fish do indeed experience pain in a way similar to humans. Pain causes fish to suffer from attention deficits, possibly because pain overtakes other cognitive functions. Basically, it's hard to focus on anything but the pain we're experiencing, and the same is apparently true for fish. On top of that, other studies have demonstrated that fish can even show fear towards pain, a valuable evolutionary tool. Critics argue that since fish lack a neocortex, the presumed area of human consciousness in the brain (though this itself is debated), they can't have an emotional response. Brown's response centers on the differences between human and fish brains, but also that fish have been shown to have similar brain activity to humans when they experience pain.

Adding fish to the moral circle

A little girl watches a gold fish in its bowl Even if it's just giving them more to do in their tanks, we should probably give fish more respect. (Photo: Morrowind/Shutterstock)

With the science in mind, what do we owe fish, especially considering how much we use them for?

On a large scale, the answer is clear to Brown, who concludes his survey of academic literature with this: "This body of evidence strongly suggests that they are sentient and the evidence that they are capably of feeling pain in a manner similar to humans is gradually mounting. I submit that there are compelling reasons to include fish in our 'moral circle' and afford them the protection they deserve."

The moral circle is a concept used to explain increasing the scope of who and what we care about. The center is you and presumably those closest to you, but like a tree ring, you add more circles to include groups that you care about or "deem worthy of consideration." You add other humans, pets, the environment and other nonhuman animals. On a societal scale, we've certainly taken the steps to do the last one. Consider personhood cases regarding elephants and chimpanzees as an expansion of our culture's moral circle.

Brown and other academics want fish included in the moral circle we afford to other vertebrates. Brown's perspective is that if the public better understood the cognitive and emotional world of fish, we'd be more likely to expand our circle to include them. This would mean more legal protections for them in the wild and more humane lab conditions — or if they would even still be used in scientific research.

Like so many things, expanding this moral circle starts at home. If you have a fish as a pet, a bowl without much else in it is the last place you want to keep it.

"One of the things we're finding that fish are naturally curious and seek novel things out," Victoria Braithwaite, a professor Penn State University, who has studied fish intelligence and fish preferences for years, told The Times. She recommends that you change the contents of the tank every once in a while, or at the very least, change the location.

Brown, in the same Times article, concurs with Braithwaite's recommendation. He points to a study he conducted that demonstrated that fish experience brain growth and reduced stress in environments that stimulate them, including lots of things to swim through and plants to explore.

So the next time you're considering a fish as a pet, or even watching them in an aquarium, think to yourself, "What would I want if I were this fish?"