FOR the first time, scientists have been able to develop a completely vegetarian diet that works for marine fish raised in aquaculture systems, a key development in making aquaculture a sustainable industry as the world's need for protein increases.
The study, by Drs. Aaron Watson and Allen Place in the Institute for Marine & Environmental Technology at the University of Maryland Center for Environmental Science, was published in the August issue of the journal Lipids.
"Aquaculture isn't sustainable because it takes more fish to feed fish than are being produced, but a new vegetarian diet might change everything," Watson said.
Nearly half of the world's fish and shellfish supply is supplied by aquaculture — growing fish in tanks or ponds instead of catching them from the oceans or streams — and scientists have been trying to figure out how to make growing fish sustainable.
Many high-value fish such as cobia, sea bream and striped bass are predators and eat other fish to survive and grow. As a result, their food in captivity is made of a combination of fish meal and fish oil and must be caught from the wild to feed them.
In the findings, supported by another paper published in the Journal of Fisheries & Aquaculture, the team has shown that a completely plant-based food combination can support fast-growing marine carnivores like cobia and gilthead sea bream in reaching maturity just as well as — and sometimes better than — conventional diets of fish meal and fish oil made from wild-caught fish, an announcement said.
"This (vegetarian diet) makes aquaculture completely sustainable," Place said. "The pressure on natural fisheries in terms of food fish can be relieved. We can now sustain a good protein source without harvesting fish to feed fish."
Replacing the fish meal and fish oil in aquaculture diets has been a goal for researchers for decades, but they have met with limited success.
The Maryland team's research centered on replacing fish meal with a blend of plant protein sources to completely eliminate the need for fish meal and fish oil in diets for cobia and other high-value marine carnivores.
Fish meal was replaced with a food made of corn, wheat and soy. Fish oil, which is expensive and scarce due, in part, to its popularity as a health supplement for people, was replaced with soybean or canola oil, supplemental lipids from algae sources and amino acid supplements such as taurine.
(An amino acid used in energy drinks, taurine plays a critical role in the metabolism of fats, stress responses and muscle growth and is found in high levels in carnivorous fish and their prey.)
In addition to the potential to turn aquaculture into a more profitable enterprise and ease the pressure on catching wild fish, raising fish on a vegetarian diet also means safer fish to eat, with levels of polychlorinated biphenyl and mercury as much as 100-fold lower, the announcement said.