Firstly I will say what I believe ought be sufficient for this paper to be successful. I do not, I make quite clear, have to prove that vegetarianism results in maximal utility, all consequences considered. I am not a fortune-teller - I cannot predict what all the consequences of any far-reaching action will be. Fortunately, I do not need to be: utilitarians being, I hope, reasonable people, I have only to show that, for the reader, vegetarianism will likely result in a level of utility higher than the alternative. 
I will also make clear that this paper is not meant as the "last word" on the issue of vegetarianism - by all means, let this issue be discussed. If, at any point, my case seems to lack evidence, I hope the fair-minded reader will inquire as to how much evidence is easily available for alternative conclusions, and wonder if indeed my argument is insufficient against it. And if there exist substantial arguments against my position, which I have not raised here, let them be brought them out into the light of critical examination.
I will, in both the cases of wild-caught fish and farm-produced animals, proceed in two parts - I will try to show that
(1) The industry is wrong, i.e. severely sub-optimal in its effects.I will be careful to avoid the mistake of assuming that claim 1 necessarily implies claim 2. It can fairly be expected that to maintain 1 will be a good deal easier than maintaining 2.
(2) We (individually and collectively) should stop supporting that industry.
One possible position must be overturned immediately - some people may believe that fish have no interests, that they cannot feel pleasure or pain, thus we don't need to consider them directly. I make my case as an appeal to authority, the authority being biologists. I name the Medway Report (which concludes "all vertebrates (including fish), through the mediation of similar neuropharmacological processes, experience similar sensations to a greater or lesser degree in response to noxious stimuli" ) in support, and remind the reader that I ought not need to prove that fish can feel pain, only that it seems likely that they can.
Another position must be overturned - one that is commonly the immediate successor to the above position. While it may be granted that fish do suffer, it may be denied that this suffering counts, that it has value in the same way as human suffering does. To counter this position it is only necessary to ask what the distinguishing criterion is between a human's suffering a fish's suffering. 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. One man's suffering is no worse than another's for him being a genius and the other an imbecile, so this cannot be the criterion for separating men from fishes. Alternatively, the species difference may be suggested as its own support, that what makes human suffering more important is that it is human suffering. But if we allow "reasons" like these, we cannot then reject the racist's claim that a white man's suffering is more important than other types because it is a white man's 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.
Given that me must give equal weight to the interests of fish as we would to the similar interests of men, I would now state an argument for the plausibility of the claim that fishing is wrong: One very simple reason why it is unlikely that fishing realises the goal of maximising total utility is that it was never meant to. If it had been the case that teams of well-educated scientists and solid non-speciesist utilitarians had carefully examined the evidence and suggested that we ought to (catch) fish then there would be some conflict with what I am saying here. 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. It would be the most remarkable coincidence if the maximally useful action, considering the interests of fish with equal weight of those of humans, turned out to be exactly the same one as would be maximally useful considering only the interests of humans!
Plausibility established, let's examine the situation. There are fish, in the seas, living in populations in dynamic equilibrium with populations of other species. Naturally some fish are born, and some fish die (they are eaten, diseased, or starve etc). The system continues. Now introduce man's effects: he catches some of them. Does this affect the system as a whole? It does, in some cases: some previously high-yielding areas are overfished and some species, in those areas, become commercially extinct. Is this a harm? Yes, if we assume that a fish have any net value whatsoever - if their lives involve a balance of happiness over suffering, yes; if fish are useful to others, yes, because having fewer fish available diminishes their total instrumental value. And in other cases, where quota systems ensure sustainable yields by limiting catches, what there? Well there is still the suffering involved in the catch, and a terrible harm that is. Fish are not killed directly. They become caught in the nets, and they stay there - dragged along, for some hours, under increasing weight of other bodies - until the nets are raised. The pressure changes sometimes causes internal organs to burst. The fish are then dumped in the boat and left to die from the weight of their numbers, or from suffocation or internal bleeding. It takes hours.
This death, it seems very likely to me, is a very worse one than nature offers - I guess that most would naturally be eaten by other fish or marine mammals, a fate involving only brief suffering. The death by a thousand gasps is horrific in comparison. I have little evidence - but where is the evidence that I am wrong? If there is any, bring it forth. And in any case, the potential for improvement is obvious - the fish could be euthanised, rather than left to suffocate, or internally bleed to death. So let him who says that fish are better off being caught than left to die naturally put the case for increasing the number of fish we kill, and let him also suggest the optimal technique for doing so. Let him say whether it is specific to fish, or whether he will apply his principles to the many other kinds of wildlife. And let him do all this with some bare shred of plausibility.
I say that the harmful nature of the industry places a prima facie obligation on Mr. Smith to campaign for the end of the fishing industry (the assumption being that he has resources which cannot be better spent on other campaigns ), and given the kind of society we live in, I would ask Mr. Smith how credible (and therefore effective) he thinks his campaign will be if it is known that he continues to, in practice, support the industry. He could, I suppose, opt for dishonesty, if he will take the risk of harming the public perception of his philosophy. (He cannot, of course, publicly advocate that others continue to support the industry, since this will multiply the risk of harm he is taking - he would have to be silent or a hypocrit.)
I also say (in a disappointingly non-specific argument) that there is very likely some better way for Mr Smith to spend his money, other than buying fish. Let Mr Smith put the case for the importance of his taste for fish over feeding the starving , buying medicine for the sick, paradise-engineering research, or any other cause that I care to put to him. And let him afterwards still claim to be a good man.
Supplementary reasons for the individual to refrain from fish-eating include the effects upon the agent's public image, ad hominem considerations (self-image), and a case for the more optimal disposal of capital. The capital argument alone I take to be sufficient, though hardly, I can honestly say, inspiring - being quite unrelated to the terrible harm of the industry. 
 If I use Mr Parfit's terms (see "Reasons and Persons", part 10) I hope to show that vegetarianism is clearly subjectively right, by virtue of its appearance of being likely objectively right.
 Paragraph 57, Report of the Panel of Enquiry Into Shooting and Angling, Lord Medway, et. al., 1979. Information and extracts available from Pisces here and here. See also "Do pain and fear make a hooked carp in play suffer?" and "The welfare of fish and aquatic invertibrates" at the same site.
 See MindWatching, H. and M. Eysenck, chapter 14 (cf. declining marginal utility).
 I should mention at some point that the majority of fish caught in the northern hemisphere are not eaten. Most are either of the wrong species (inedible, unsellable, or simply of a type for which the fisher has no remaining quota) - these being thrown overboard - or are caught for industrial purposes: for use as fertilizer, oil, making candles, or (in a remarkable case of inefficiency) fish-food.
 It might be assumed that the money saved from not eating fish must be spent on some other food source, but I would dispute this - the evidence is that we in western society generally eat too much rather than too little - yet, in any case, one can imagine an agriculture or arboriculture equally useful in this regard.
 This number would presumably be much larger than it would be in a "local fishing for local consumption" situation. Could this be the beginnings of an argument for smaller-scale industry, against nationalisation and internationalisation of big-business?
 Given the blatant discrimination against fish, merely for them being fish and therefore somewhat dissimilar to our normal targets for compassion, I suggest that this is quite unlikely.
 The money saved from having a simple diet being quite sufficient to make an impact on the lives of many of the world's poor.
 The (likely) very great harm of the industry is significant however, in that it makes it a prime target for our campaigning resources to close the industry down. It is only on this specific issue of one individual's eating-habits that its case is disappointingly weak.