Why farmed fish need more veggies in their diet

Consumer demand for sustainable farmed seafood is growing, but the food supply for the salmon in the pens is running thin – there are simply not enough fish in the sea.

“Historically, fishmeal [ground-up wild fish] has been a relatively inexpensive and highly nutritious [fish food] source,” says Ian Forster, an aquatic animal nutrition scientist for Fisheries and Oceans Canada.

However, according to The State of World Fisheries and Aquaculture, a 2012 report by the United Nations' Food and Agriculture Organization, fish farming has been growing by almost nine per cent each year for the past 30 years – to the point where demand for fish food is exceeding supply.

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The price of fishmeal has been rising steadily as a result, and changes in ocean currents during El Niño years cause the already maxed-out fish stocks to dwindle even further, sending prices skyrocketing periodically.

Feed manufacturers are trying to find ways to maximize the amount of fishmeal and oils they can get from the heads and tails and trimmings that would normally be discarded by the seafood industry.

But in order to shift our dependence away from wild-caught fish stocks and protect fish farmers against the year-to-year variability in feed prices, scientists are racing to develop alternative sources of food.

The trick is finding a way to get these finicky farmed fish to expand their edible horizons.

Basically, “We’re trying to feed carnivores – salmon and trout – plant products,” says Murray Drew, associate dean and professor at the University of Saskatchewan's College of Agriculture and Bioresources.

Reluctant omnivores

The research aimed at replacing fish meal and oil with locally grown plants builds on fish-farming practices that have been around for several decades.

According to Jason Mann, managing director of the fish feed manufacturer EWOS Canada, when fish farming was a new industry in the mid-1980s, the food given to farmed fish was about 85 per cent processed wild-caught fish (fishmeal and fish oil) and 15 per cent wheat.

Today, the ingredients list of any feed often has more than 13 different ingredients in order to include all the nutrition the fish need, says Mann.

“Most of the feed producers are still heavily dependent on wild fish that are caught and ground up into pellets. But almost all the feeds have some supplemental lipids and fats just to make the food go further,” says Colin Brauner, professor of zoology at the University of British Columbia.

The trick is getting the balance of nutrients right. Researchers and feed manufacturers are focusing their efforts on using locally grown alternative proteins and oils while maintaining both the health of the fish and the nutritional value of its fillets.

At Fisheries and Oceans Canada, research is being done into alternative ingredients like “canola, soybean, poultry meals, corn-gluten meal – these are [all] common ingredients used in agriculture feeds,” says Forster.

Corn-gluten meal and pet-food-grade poultry byproduct meal are also often used as “highly nutritious sources of protein” in fish feed, says Mann.

But according to Drew, not all plant ingredients are edible by fish. Some of the plants actually have “anti-nutritional factors.”

For example, soybeans have an anti-nutritional factor that inhibits trypsin – a vital enzyme in both human and fish stomachs that breaks down proteins. To neutralize this problem, soybeans must be roasted or cooked before they can be eaten by either fish or humans.

By using combinations of ingredients, feed manufacturers like EWOS Canada have already reduced the total marine content - which includes fishmeal and fish oil - in their feeds to by “anywhere from 18 to 30 per cent,” says Mann, raising the plant-based content to more than 70 per cent.

Tastes like chicken

Unfortunately, a fish that accepts a plant-based diet but shows no problems with their health and welfare, “might not taste as good and they might not have the same nutritional quality,” says Brauner. It’s like the old saying goes, you are what you eat.

“If you just give the fish vegetable oil, you can run into a thing where they don’t taste like fish anymore, they taste like chicken,” says Drew.

High concentrations of omega-3 fatty acids are the reason that the fish on our plate has the taste that consumers expect. Omega-3s are also the heart-healthy fat that we all need plenty of in our diet.

Researchers are looking at ways to supply fish with omega-3s without feeding them other fish. Mann says that flax oil is very high in omega-3s, for example, but it’s expensive and hard to get in large quantities.

Canola oil has lower levels of omega-3s, but in Canada, it’s inexpensive and easy to get in large amounts, he says.

Brauner’s research shows that “we could replace up to 70 per cent of fish [oil] with canola oil with no negative effects on growth or environmental tolerance,” but these fish would not have the full “fishy” flavour in their fillets.

Another approach is to raise fish primarily on plant-based nutrients, and feed them fish protein in the weeks before they are harvested.

“What the industry will do is after the [fish have] been reared on a diet that has a greater proportion of plant-based material, they’ll put them on what’s called a 'finishing diet' for a few months,” says Brauner

“The point of that is that it converts the animal back to almost what it would have been if it had been reared on this diet its whole life,” says Brauner.

Forster says that “some species of fish are more able, more willing, to accept alternative ingredients than others,” so researchers are still searching for a balanced mix of ingredients to keep the fish, and the fillets we get from them, healthy and full of omega-3s.

Understanding what fish are able to eat, and using different combinations of all the available substitute food sources, may bridge the gap between supply and demand of fish feed as farming continues to grow.