If the criterion is intelligence, culture, language or some similar feature, then the proposer has abandoned the utilitarian position, since it makes no account of these. Alternatively, the species difference may be suggested as its own support , that what makes human suffering more important is that it is human suffering. The nature of an experience is not dependant on some arbitrary feature of the being in which it is felt, like race, sex, intelligence, or membership of some specific species, so neither can the value of it be. One very simple reason why it is unlikely that fishing realises the goal of maximising total utility is that it was never meant to . But this has not been done - the purpose of fishing has always been about human interests, it has never been common to consider the interests of the fish.
Plausibility established, let's examine the situation.
There are fish, in the seas, living in populations in dynamic equilibrium with populations of other species. The system continues. Well there is still the suffering involved in the catch, and a terrible harm that is. Fish are not killed directly. The pressure changes sometimes causes internal organs to burst.