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To Make Fish Farming More Efficient, Carnivorous Fish May Become Vegetarians

It’s not just humans who are being encouraged to embrace a plant-based lifestyle anymore.
Image via Virginia Sea Grant on Flickr

It’s not just humans who are being encouraged to embrace a plant-based lifestyle anymore. Certain breeds of carnivorous farmed fish are being pushed towards vegetarianism for the sake of sustainability.

The first demonstration that such an idea could be successful came recently when a group of researchers from Baltimore announced they could grow healthy fish-eating fish by providing them with vegetarian feed. The carnivorous fish in question were cobia, a species known for its large size and appetite for flesh.

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Despite their biologically predetermined tastes, the cobia under the researcher’s care actually grew bigger and consumed less contaminants, such as mercury, than their non-vegetarian cohorts, which is good for them and good for those who want to eat them.

This is significant for the aquaculture industry, which has been looking for this sort of news for decades. As human demand for seafood grows, feeding the carnivorous fish humans often eat, like salmon, becomes less and less sustainable. Forage fish, including menhaden and sardines, are the usual feed of choice.

But as Aaron Watson, one of the researchers on the Baltimore project, noted when speaking to the University of Maryland, the problem is “it takes more fish to feed fish than are being produced.” It's a simple thermodynamic reality: As you move up trophic levels, more energy is required to fuel a creature. In other words, a fish that eats plants is more energy efficient than a fish that eats fish that eat plants.

In nature, this just means there are fewer predators than there are herbivores. But when it comes to the booming market for farmed fish, feeding large stocks of, say, salmon means finding fish for them to eat—either by catching more wild feeder fish, which could depress food availability for wild salmon, or farming feeder fish separately, which is less efficient.

The underlying cause of the aquaculture problem isn’t that the cobia are eating other fish, but rather that we are eating too much carnivorous fish. So is the answer to turn them vegetarian? It's certainly a weird position to be in, despite scientific proof that it is possible. But if demand for farmed fish continues to grow, it ultimately may be necessary to fit our own dietary desires.