They have a complex nervous system based around a brain and spinal cord similar to other vertebrates. They are not as intelligent as humans in terms of functioning in our social and physical environment, but they are very successful and effective in their own environment. Behavioral studies indicate that they exhibit complex forms of learning, such as operant conditioning, serial reversal learning, probability learning, and avoidance learning. Many authorities doubt that there is a significant qualitative difference between learning in fishes and that in rats. This implies an awareness and intelligence in the hunted of a level at least sufficient to challenge the hunter.
Some people believe that it is OK to catch fish as long as they are returned to the water.
But, when you think about it, it's as if one is playing with the fish. Also, handling the fish wipes off an important disease-fighting coating on their scales. The hook can be swallowed, leading to serious complications, and even if it isn't, pulling it out of their mouth leaves a lesion that is open to infection.