Ever since i was fishing as a child i wondered if fish felt pain and if this was true, is fishing cruel? As someone with a degree in science, I knew what I had been taught to do and set about researching this further. Reviewing many studies from authoritative organisations and university testing, I found my answer.
Angling and Fishing when practiced responsibly is not cruel to fish. Whilst fish may feel, they do not feel pain. This is the main reason fishing is not cruel. Catch and release fishing must be done responsibly. Needless deaths of fish is not what anyone in the sport wants. Setting a hook early, especially on species such as pike, is incredibly important.
If you are interested in this answer, you may want to review the studies I cite for yourself.
What is pain
Generally, the most common definitions of pain follow these criteria.
- Subjects body must be able to detect noxious stimuli which trigger an involuntary response. This can happen without the feeling of pain. This usually involves the transmission of a signal along a chain of nerve fibers from the site of a noxious stimulus.
- Subject must have an emotional response. It is impossible to measure pain in other creatures, even other humans. So argument-by-analogy is used. Meaning if an animal reacts in a similar manner than we do, it is assumed that it feels pain.
Nervous system in fish
When it comes to pain in fish, the nervous system is the easiest area to prove fish do not feel pain.
There are 2 types of nerve fibres which can be related to pain in fish. Type C and Type A. So we can directly compare this to humans
Type C Nerve Fibres
Pain associated in humans is burns, toothaches and crushing.
Type A Nerve Fibres
Pain associated in humans is cold, pressure and acute pain.
Do Fish Feel Pain
In man and other vertebrates C fibres are distributed throughout the nerve and can comprise 50% (cat, man) to 65% (frog ) of total fibre type.
In the rainbow trout the C fibres were found in discrete bundles and only comprised 4%  total fibre type.
In humans, the normal distribution of C fibres in cutaneous nerves is 83%  and in rare cases humans who cannot feel pain due to congenital insensitivity to pain  have a density of 23-28% .
Out of all the fish which have been tested, the highest density found is around 5% in carp . This is enough evidence to currently say that fish do not have the ability to feel pain.
Whilst A-delta-type fibres are common in some fish, which trigger avoidance reactions. 
While fish cannot feel pain, it is important anglers responsibly fish and avoid unnecessary death in fish populations. In reality, the anglers I know are more well versed in keeping aquatic ecosystems healthier than an average person. This education and in touch with nature attitude actually helps fish populations.
These types of discussions drive me crazy because the answer is: It doesn’t matter.
Humans have caught fish for thousands of years. It is a natural activity for many people. Recreational fishing is an old and legitimate sport practiced by thousands of people in the USA. Most fishermen and women are better environmentalists and conservationists than the people trying to ban fishing as cruel. Fishermen pay money to purchase fishing licenses and they pay money to register their boats, some of which goes into conservation and enforcement of wildlife laws. People fish, in part, to get out and experience and enjoy nature. They value this enough to pay to keep it available for everyone.
Asking if fishing is cruel to fish is akin to asking if putting a worm on the hook to catch the fish is cruel to the worm. It doesn’t matter, it’s a worm, it’s a fish. Is it cruel to swat a fly or mosquito? Is it cruel to kill a mouse or rat that is in your house? No, it is irresponsible not to. All are disease vectors and catching and releasing any of them outside just pushes the problem onto someone else.
I have caught fish and eaten them. I have caught fish and released them. I have caught fish and kept them in an aquarium (not game fish, that is generally illegal). I have caught fish and preserved them for ichthyological research. I have euthanized sick, deformed or old aquarium fish, usually with a sharp blade severing the spine. I have also bred and put more live fish into the aquarium hobby than I have killed, caught and eaten combined. I don’t consider myself a cruel person. I would never hurt a person who didn’t deserve it and I avoid harming dogs, cats, other pets and wildlife unnecessarily.
People have a right to have their own opinions, but what concerns me is when they try to force their opinions on all of us in the form of laws banning activities they don’t like. There are people trying to outlaw fishing and the keeping of aquarium fish on the grounds that it is cruel to fish. I will not try to argue that fish don’t feel pain when hooked, but I will point out that their brains are much simpler than ours and fish probably don’t have consciousness. Most people who have dogs believe that dogs “live in the moment”. They don’t brood over the past or worry about the future. A dog’s brain is much more similar to ours than a fish’s. I don’t believe that there is evidence that other than some birds and mammals, animals are no more than organic automatons. They behave as programmed by evolution with limited ability to modify behavior through experience. Once a fish is released, provided it is not gut-hooked, it quickly goes about its business as if nothing happened. When we learn that most flounders die when released, for example, we educate people to only fish for flounder if they are going to eat them. Unlike trout or bass, I believe that most people who fish for flounder intend to eat them anyway.
All that considered, I do believe it is possible to be cruel to animals, even to insects such as a fly. Swatting and killing a fly is OK, but pulling the wings off a fly is not. Keeping a fish in an aquarium is OK, but keeping a fish and not caring for it properly is not. Catching a fish and releasing it is OK, but catching a fish and throwing it on the ground to die is not. Purposeful cruelty should concern us because of what it says about that person, not because a fly or a fish has rights.
Richard Pierce, Marine Biologist
 Young, R.F., Fiber spectrum of the trigeminal sensory root of frog, cat and man determined by electron microscopy, In D.L. Anderson and B. Matthews (Eds.), Pain in the Trigeminal Region, Elsevier, Amsterdam, 1977, pp. 137–160.
 Sneddon, L.U. (2002). “Anatomical and electrophysiological analysis of the trigeminal nerve of the rainbow trout, Onchorynchus mykiss”. Neuroscience Letters. 319 (3): 167–171. doi:10.1016/S0304-3940(01)02584-8. PMID 11834319.
 Guo, Y.C., Liao, K.K., Soong, B.W. et al. (2004) Congenital insensitivity to pain with anhydrosis in Taiwan: a morphometric and genetic study. European Neurology 51, 206–214.
 Rose, J.D.; Arlinghaus, R.; Cooke, S.J.; Diggles, B.K.; Sawynok, W.; Stevens, E.D.; Wynne, C.D.L. (2012). “Can fish really feel pain?” (PDF). Fish and Fisheries. 15 (1): 97–133. doi:10.1111/faf.12010.