That's a great question.
Most fish feeds now have some vegetable proteins and vegetable lipids in them, so it's common practice, to some degree. One of the concerns is whether you are changing the nutrient composition of the animal and how does it taste? What the industry generally does is this. A few months before an animal will be harvested for market, it is put on finishing feed, and that feed then influences both the flavour as well as the fatty acid composition of the animal. Within a few months you can convert an animal back to what it would have been had it been reared on that diet all its life. So it's quite an interesting practice.