Fish, especially of the bigger species such as tuna are clubbed to death after being caught in nets.

Answer

Whilst the fish remains halaal, the act of cruelty perpetrated on it is haraam. Abstaining from eating such fish relates to the domain of Taqwa. Something may be halaal, but if procured in ways in conflict with the teachings of the Shariah, it has a spiritually detrimental effect on one's Rooh (Soul). For example, an animal slaughtered not facing the Qiblah, while halaal, is spiritually repugnant. But since the hearts of people are corroded with spiritual rust and filth, these issues are beyond their comprehension.