What can a fish feel?

Jonathan Balcombe writes about the debate over whether fishes can feel pain. One of the main points of What a Fish Knows is that, yes, fishes can feel pain and can suffer—as a vegetarian, he notes that human beings don’t treat fishes very nicely, and he doesn’t understand why some people don’t eat meat but eat fish. He aims to write a book on behalf of fishes.
However, Balcombe doesn’t write in an emotional, sentimental kind of way, but bases the book on scientific facts, research, and studies, and now and then sprinkles the narrative with some anecdotes.
The fact that a fish doesn’t have a neocortex doesn’t mean that it doesn’t have consciousness and therefore doesn’t feel pain. Look at birds, the bird-brain myth has long been debunked—they can use tools, remember for months locations of thousands of buried objects, recognise a neighbour’s voice, categorise objects, and so on.
Balcombe says:

“If any animal without a neocortex is nevertheless conscious, it disproves the notion that a neocortex is required for consciousness. As such, it is no basis for a claim that fishes are unconscious. ‘There are many ways to get to a complex awareness’, says the neuroscientist Lori Marino of Emory University. ‘To suggest that fishes cannot feel pain because they don’t have sufficient neuroanatomy is like arguing that balloons cannot fly because they don’t have wings.’
Or that humans cannot swim because they don’t have fins.” (Part III)

He then spends the next few pages writing about the scientific experiments on trouts and zebrafishes. Fishes’ responses to pain are not mere reflexes. The experiments provide evidence that a fish can “feel both initial, sharp pain and the lasting pain that follows”, responds to different kinds and levels of pain in different ways, in pain may be “distracted” and unable to perform normal survival behaviours, can improve with painkillers (“Morphine belongs to a family of drugs called opioids, and fishes are known to have an opioid-responsive system”), can withstand higher temperature with morphine before reacting to it, and is willing to pay a cost for pain relief.
Whether fishes feel pain is a question of cardinal importance, because it forces us to question the practice of catching and eating fishes, even if it doesn’t change our eating habits.
Reading Jonathan Balcombe’s book, I myself am more interested in whether fishes can experience other kinds of feelings. He argues that they can, and do.
For example, they can feel pleasure—the pleasure of touch.

“Fishes often touch one another in pleasurable contexts. Many court with rubbing or gentle nips. Cleanerfishes curry favour with their valued clients by caressing them with their fins as a means to strengthen the cleaner-client relationship. Moral eels and groupers approach familiar divers and receive strokes and chin rubs.” (Part II)

Sharks, rays, and skates also “show pleasurable responses to touch”. Balcombe mentions the story of manta rays that enjoy bubble massages (the diver “swims beneath them and bellows bubbles from her SCUBA regulator”). There’s also a similar story with zebra sharks.
He goes on:

“Besides touch, there are many other ways that fishes may derive pleasure. Food, play, and sex spring to mind. And then there’s comfort for its own sake. Southern bluefin tunas in the waters of Australia spend hours rolling on their sides, catching the sun’s rays. It’s not known for sure why they do this. 1 possibility is that they are sunbathing to raise their body temperature, which in turn helps them swim and react faster, making them more efficient hunters. I expect the warmth of the sun also feels good to a tuna, for pleasure evolved to reward useful behaviours.” (ibid.)

According to What a Fish Knows, fishes can feel pain, fear, and stress, and they may seek relief. They have social lives and can feel compassion. They can have curiosity. They can also feel joy, and enjoy play.
I will not write more details, because I think you should read Jonathan Balcombe’s book. It is a good book, an enjoyable read full of fascinating facts.
But it’s interesting to talk about fishes being playful and feeling joy. Fishes don’t get much sympathy probably because they don’t have facial expressions and make sounds in the way that mammals do, and they seem boring—as a pet, there isn’t much you can do with a fish, like with a dog or a cat. But they can enjoy play.
Gordon M. Burghardt, an ethologist at the University of Tennessee, studies fish play, and defines play as follows:

“1. It does not achieve any clear survival purpose, such as mating, feeding, or fighting;
2. it is voluntary, spontaneous, or rewarding;
3. it differs from typical functional behaviours (sexual, territorial, predatory, defensive, foraging) in form, target, or timing;
4. it is repeated but not neurotic; and
5. it takes place only in the absence of stressors, such as hunger, disease, crowding, or predation.” (Part III)

An example is fishes riding air bubbles. I’ve found this video:



That is play.
Fishes can have object play, social play (even interspecies social play), and solitary play.
Another example could be fishes jumping and leaping. Why do they jump? We don’t know.
Balcombe says: 

“Mobula rays aren’t motivated by fear when they hurl their large bodies (up to a 17-foot wingspan and a ton in weight) skyward in leaps of up to 10 feet before splashing down with a loud slap. […] They do it in schools of hundreds. Most of their leaps are calculated to land them on their bellies, but sometimes they do a forward somersault, landing on their backs. Males seem to be the initiators, so some speculate that there might be a courtship role. Other scientists think it might be a parasite removal strategy. Whatever its function, I posit that the rays are enjoying themselves.”

Here’s a video of mobula rays leaping:



Doesn’t that look fun?
Fishes are awesome.