The practice of catch and release fishing (which uses a flattened barb and more careful hook removal techniques, so as not to kill a species of fish which is endangered and should be preserved) is a debatable practice in and of itself. While it is the law of the land (or perhaps the water) in much of Canada for many different species, it is actually outlawed in Switzerland. While some people argue that it is a positive strategy for preserving the species who repopulate the most slowly, some other people argue that it causes the fish pain. So far, there has been no conclusive evidence that fish feel pain, and no evidence that they do not. However, one thing is for certain: fish which are caught while deep sea fishing are almost never going to survive once they have been pulled up to the surface, even if they are not removed from the water.
When a fish is removed from a depth of more than about 30 feet of water, they suffer from a fate similar to scuba divers who come up to the surface too quickly. While in humans this is called the bends, in a fish it is called barotrauma. Simply put, the fish’s body is not able to adjust its internal pressures to the much lower pressure of shallow water, much less to the far lower pressure that is present outside of the water. With this kind of trauma, a fish’s eyes bug out, and its diving bladder bulges out of its front.
If fish feel pain, this has got to feel absolutely terrible for them. And in most cases, even if you promptly release the fish it is destined to die soon. After all, with its swollen diving bladder, it can not go back to the depth at which its internal pressure is healthy for it. And in that case, the pressure is simply going to cause its tissues to be damaged because they can not expand the way that the air inside of their bodies does. For all intents and purposes, the poor thing explodes.