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Plant-based fish diets may harm the environment

Aquaculture diets have become increasingly plant based. In addition, the industry faces environmental changes. Recent findings in salmonids when the 2 challenges are combined, might give cause for concern.

Peyman Mosberian Tanha from the Department of Animal and Aquacultural Sciences at the Norwegian University of Life Sciences (NMBU) has recently defended his doctoral thesis on: "Interactive effects of dietary and environmental challenges on digestive function and intestinal homeostasis in rainbow trout". In his thesis, Mosberian Tanha gained new understanding into how the interaction of dietary and environmental challenges may affect the fish gut health and digestive function, causing an imbalance which may make the fish less resistant to disease and infection.

Suboptimal diet and environment

Since 1980, the aquaculture industry has experienced an annual growth rate of 8.8%. Traditionally, fish meal and fish oil have been used as the main feed resources. However, due to the rapid growth in this sector these ingredients are becoming limited. Feeding farmed fish in a rapidly expanding industry on fish meal and fish oil is unsustainable. Thus a transition towards a more plant-based diet has gradually taken place.

The inclusion of plant ingredients in fish feed containing anti-nutritional factors which may affect growth, nutrient utilisation and health of the fish is a widely debated challenge. In parallel, global warming that alters the fish’s environment is a further challenge. In his PhD thesis, Mosberian Tanha has incorporated the two-pronged challenge of dietary changes and environmental shifts in the fresh water system on performance and health of salmonids. By manipulating the water exchange rate, leading to a lower oxygen level, the effects of global warming and high-intensive farmed fish can be simulated.

Mosberian Tanha: “When challenged, fish has more trouble to digest feed” Photo: Norwegian University of Life Sciences
Mosberian Tanha: “When challenged, fish has more trouble to digest feed” Photo: Norwegian University of Life Sciences

“Individually, the challenge of a plant-based diet and suboptimal environment may not be a problem. However, taken together, I found that environmental changes associated with change in several water quality parameters may directly or indirectly affect the health and performance of the fish”, Peyman Mosberian Tanha says.

Weakened “first line of defence”

Tanha’s research approach has been to look at how dietary and environmental factors interact with gastrointestinal mucosa and how these interactions may be harmful to the mucosal barrier. “The mucosa has developed a barrier function which prevents the penetration of microorganisms and other undesirable substances. This function has a crucial role in maintaining gut homeostasis. Any challenge to the mucosal barrier beyond its tolerance adversely affects the function and integrity of this first line of defence, which in turn results in a disturbance of gut homeostasis”, Tanha says. He found that feeding a challenging plant-based diet reduced the digestibility of both lipid and starch. When the fish were also subjected to a suboptimal environment, the digestibility of lipid and starch was further reduced. Mosberian Tanha also found that the plant-based diet led to enteritis (inflammation) in the hind-gut of the fish, but the degree of enteritis did not increase further at suboptimal environment. Fish that were exposed to the same environmental challenges, but fed a more optimal diet, did not show any adverse effect on nutrient digestibility or gut health. “This demonstrates that there is a connection between the diet and the fish capability to digest it under environmental challenges”, Tanha says.

Optimal diet to reduce stress

The study has been conducted in cooperation with Wageningen University in the Netherlands. Based on the results, Tanha foresees a need for further insights. “Various species of fish may respond differently to the challenges. It is therefore important to gain more knowledge on mucosal barrier function and integrity in different fish and how the interaction of dietary and environmental challenges may affect mucosal barrier integrity and gut homeostasis”, Tanha says. An implication of Tanha’s findings is the potential to improve performance and health of fish in a suboptimal environment by tailoring optimal diets to cope with the two-pronged challenges.

By Liv Røhnebæk Bjergene, Norwegian University of Life Sciences

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