Turning Fish Vegetarian

iStock 000001246180XSmall square Turning Fish VegetarianAquaculture is helping to meet people’s growing demand for seafood, and fish farms often use fishmeal and fish oil as feed. But carnivorous fish can be coaxed to accept a more sustainable vegetarian diet, researchers have shown in a PLoS ONE study.

Worldwide seafood consumption is rising fast, and fish farms have stepped in to fill the supply gap. Many farms have partially replaced fish-based feed with plant products. But meat-eating fish that are forced to go completely vegetarian may grow poorly or be less healthy.

The authors reasoned that since wild fish can adapt to changes in diet through evolution, farmed fish should be able to do the same. The team started by feeding rainbow trout strictly plant-based meals. In a process called selection, the researchers sorted the fish by size every one to four months and kept the biggest ones. The large fish then bred to produce the next generation of trout.

Next, the researchers fed the offspring either a plant-based or fish-based diet. They did the same with “control” fish, whose parents ate only the fish-based food and didn’t undergo selection.

The vegetarian fish had a lower survival rate and body weight than meat-eating fish. But the selection process improved the offspring’s chances of survival, the team found. While 77 percent of the selected fish’s plant-eating offspring survived at 145 days post-fertilization, only 66.9 percent of the control group’s plant-eating offspring did. Selection also appeared to increase body weight among the vegetarian fish.

The results show that “domesticated populations of rainbow trout have the genetic potential to adapt to major dietary changes,” the authors write. With more rounds of selection, the vegetarian fish’s survival and body weight could come closer to those of meat-eating fish. The team cautions that some ingredients used in this study’s plant-based diet aren’t sustainable, however, so they will eventually need to be replaced with more environmentally-friendly alternatives. Roberta Kwok | 3 October 2012

Source: Le Boucher, R. et al. 2012. Selection for adaptation to dietary shifts: Towards sustainable breeding of carnivorous fish. PLoS ONE doi: 10.1371/journal.pone.0044898.

Image © Tootles | iStockPhoto.com

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