Conservation International has just issued a report saying that fish farms provide a more eco-friendly way of producing protein than cows, pigs, or chickens.
Leaving cruelty and killing considerations aside, the reasons [PDF link] are compelling:
Fish score well in terms of food conversion efﬁciencies. They convert a higher percentage of the food they eat into consumable protein. This efﬁciency is partly a reﬂection of the fact that ﬁsh being cold blooded, do not use energy to maintain a high body temperature. As ﬁsh live in an aquatic environment they do not need as extensive a skeleton as terrestrial livestock and so there are more portions available as food. Efﬁciencies are most notable for fish species that feed low in the food chain, primarily on vegetable-based feed.
A very interesting quote from Conservation International’s Sebastian Troëng :
Aquaculture is most likely to meet the growing demand for animal products with the least demand on ecosystems. It would be better still if more people became vegetarian, but that looks unlikely.