Aquaculture, which breeds fish, shellfish, and aquatic plants, is one of the fastest growing food sectors in the world, providing about half of the fish that humans consume worldwide. In Europe, aquaculture production reached a 10-year high in 2017 as more fish species such as salmon, sea bass and bluefin tuna were farmed.

Fish meal and oils from small fish such as sardines and anchovies used to be the main source of feed for farmed fish. But vegetable protein sources such as soybeans or corn have been used more and more frequently in the last ten years, partly for environmental reasons.

“Fish stocks have become a finite resource and are actually protected in certain areas,” said Dr. Morten Tønsberg Limborg, Associate Professor of Evolutionary Genomics at the University of Copenhagen in Denmark. “So there has been a transition to using more and more raw materials from plants.”

However, it is not known exactly how changing the diet of farmed fish will affect their health and growth.

Fish like salmon and trout, for example, are predatory and naturally hunt other fish for food. “There were some problems trying to make them vegans,” said Jacob Agerbo Rasmussen, a graduate student at the University of Copenhagen. For example, eating soybean-based meals often slows their growth because fiber is difficult to digest.

Additionally, there has recently been a trend towards adding probiotics – cultures of live bacteria – to fish feed to promote gut health and growth. But their effects are not well understood. “Everything that happens in the gut in terms of the bacteria there and the effects of the added bacteria is just a big black box,” said Dr. Tonsberg Limborg. “It wasn’t really possible to study what happened before.”

bacteria

Dr. Tønsberg Limborg and Agerbo Rasmussen have taken steps in this direction by studying the composition of bacteria in the intestines of certain species as part of the Happy Fish project. They used new tools, like a technique that allowed them to study the genetics of bacteria in the gut, to describe the different types of bacteria and how they interact.

In one experiment, the team analyzed the intestinal bacteria of rainbow trout and Atlantic salmon, two common species, and European whitefish. The trout and salmon came from different aquaculture facilities, while the whitefish came from a wild population. They were surprised that a single bacterium had a high dominance in all fish. “We’re always talking about millions of bacteria in the human gut,” said Agerbo Rasmussen. However, in predatory salmonid fish, one type of bacteria is often dominant and present in very small quantities, he says.

It is widely believed that gut bacteria in fish are a product of their environment, where the same bacteria are in the water in which they swim. Dr. However, Tønsberg Limborg believes that their results suggest that this is not the case because the fish come from different environments. Instead, he believes that the bacteria may have evolved with the fish and that there could be a symbiotic relationship in which there is some sort of long-term interaction between them. “It’s very unlikely that this would happen by accident,” he said.

The small amount of bacteria could be related to the temperatures of their surroundings. Agerbo Rasmussen suspects that the cold water they live in could play a role. “These bacteria most likely grow slowly and take a long time (for them) to settle in the gut,” he said.

To find out if different gut bacteria had an effect, they conducted studies with probiotics. They fed three groups of rainbow trout different types of food. The first group received a standard diet, while the second received the same diet with added probiotics. The third group received the same diet as the second group, which was supplemented with fiber, which is believed to have additional benefits for the probiotic supplements.

“There were some problems trying to turn them into vegans.”

Jacob Agerbo Rasmussen, University of Copenhagen, Denmark

Probiotics

Preliminary results suggest that feeding probiotics had an effect. Although there wasn’t much of a difference in their growth, there was a change in the bacteria in their gut where other bacteria dominated. It is still unclear whether this is beneficial for the fish. There was also a shift in nutrient uptake, with probiotic bacteria appearing to enhance a rainbow trout’s ability to digest fat.

The work is a first step towards better understanding the role of added probiotics in fish feed. In the past, fish farmers only measured whether the fish had grown better when bacterial preparations were incorporated. Now they show that the effect is far more complex. “Our results will not lead to a different way of feeding farmed fish tomorrow,” said Dr. Tonsberg Limborg. “But the project showed a completely new toolbox to analyze and understand the gut microbiome of these fish.”

The definition of the nutritional requirements of different farmed fish species is also necessary in order to optimize the feed. Salmon and trout were the pioneer species in fish farming, so the guidelines formulated for them are often applied to other fish species. “It turns out they are not the same,” said Dr. Katerina Moutou, Associate Professor of Vertebrate Biology at the University of Thessaly in Greece and coordinator of the PerformFISH project.

Dr. As part of the project, Moutou and her colleagues are concentrating on identifying the specific nutritional needs of sea bream and European sea bass. They are also exploring alternative sources of protein and lipid that can be used in fish feed, such as raw materials of marine origin such as algae. Your ultimate goal is to develop new feeds with more efficient feed conversion rates that require less feed to grow a kilogram of fish. “This is important for the environment and the economic sustainability of companies,” said Dr. Moutou.

Scientists have found that supplementing plant-based diets with a small amount of selenium would encourage young sea bream to grow.  Photo credit - Zeynel Cebeci, licensed under CC BY-SA 4.0

Diets

So far, the team has managed to glimpse some of the nutritional needs of sea bream and sea bass. Several experiments were conducted with sea bream to determine the recommended dietary concentrations of certain vitamins such as A, K, D and B12 as well as copper, manganese and selenium, essential food minerals. For example, they found that supplementing plant-based diets with a small amount of selenium would encourage the growth of young sea bream. Likewise, diets low in fish meal and fish oils should be supplemented with vitamin A to improve the growth and liver health of a fish. However, adding too much vitamin can be toxic to sea bream, although higher concentrations are tolerated by the sea bass.

They also conducted experiments examining whether sea bream could adapt to novel diets, specifically examining how they metabolize fat. A trial was conducted with fish reared for breeding purposes, in which different groups were fed diets containing different amounts of fish oil. They found that genetic differences would determine how quickly the offspring of parents who ate a fish oil-free diet would grow if fed without fish oil. “We are trying to identify the genetic recipe of those fish that are better and more efficient at handling new ingredients,” said Dr. Moutou. “That way, breeding and feeding will go side by side.”

The team works with fish feed companies that have already started applying their findings. You are now able to formulate sea bream and sea bass fish feeds based on revised nutritional needs, with initial results from testing on fish farms suggesting the supplemented diets are more efficient. The new guidelines are designed to help companies select suitable new substitutes for fishmeal and fish oils, including local ingredients, which should further improve sustainability. “I think this was very important and has also broadened the range of alternative ingredients proposed,” said Dr. Moutou.

The next step will be to further optimize how much and when fish are fed by using technologies like sensors to monitor their feeding behavior. This move towards precision aquaculture would allow fish farmers to adjust feeding rates based on various factors that affect their consumption patterns, such as: B. the season. “That would reduce food waste,” said Dr. Moutou. “It will be part of what we call the fourth industrial revolution in aquaculture.”

The research in this article was funded by the EU. If you liked this article, you can share it on social media too.

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