ýou are assuming a perfect world, a strong NZD, and no competition from other countries.
the market here is also thin, and illiquid, and so the normal supply-demand conditions will not work.
retailers do not and have never created markets. they merely exploit existing markets, unexplored niches, and think of ways to differentiate products that already exist in markets, by cost leadership, product differentiation etc etc strategies.
Alan is right when he said
if importers bought a whole lot of fish and competed with the larger markets out there, it would lead to higher prices in NZ, and when the prices increase immediately with the new imports, fewer people would buy fewerfish... poor wholesalers now have illiquid inventory, that loses value everyday; plus the variable and fixed costs = importers, wholesalers (more often that not are the importers), retailers and then the poor consumers get done over.
the rules that you have used, while are fundamentally correct; simply do not apply to this trade, and if they did, we would have capitalised on this a long time ago.