Sir

In his Commentary article, “When will we tame the oceans?” (Nature 436, 175–176; 2005), John Marra foresees mariculture as an important contributor to global food production and as a solution to overfishing. He argues that the world's fisheries should be replaced by large-scale domestication of the oceans.

We agree that aquaculture is playing an increasingly important role in world fish supply. But a careful distinction must be made between the aquaculture of freshwater fishes, molluscs and plants — which is primarily low-tech and low-impact, and helps feed people in developing countries — and the high-tech mariculture of carnivorous finfish advocated by Marra, which serves luxury food markets. It is unlikely that low-income families will ever taste maricultured tuna, salmon or cod. Indeed, these people's protein supply may diminish as the market for many small food fishes becomes cornered to provide mariculture fish feed.

Farming carnivores also results in a net loss of food because of inefficient energy conversion between trophic levels, as Marra acknowledges. Tuna farming, therefore, is not like herding cattle: it is the ecological equivalent of trapping wild shrews and foxes to feed caged wolves.

Fisheries and the wild populations that supply them should not be abandoned. Instead, serious effort should be focused on rebuilding depleted fish populations by creating large marine reserves and reducing total fishing capacity. Proper management of wild marine life could yield remarkable results, but—ironically—requires what Marra lists as a precondition for his vision of domesticated oceans: the political will to implement changes and create transnational agreements on shared ocean use. When we finally garner this political will, should we not use it to restore productive, biologically diverse ecosystems, rather than to risk further degrading the oceans?

Offshore mariculture is not “inevitable”. It is a course of action that can be chosen — or not.