In an attempt to combat stress in aquaculture animals, the Interdisciplinary Centre for Marine and Environmental Research (CIIMAR) is proposing a food supplement for fish that raises serotonin levels, among other benefits, acting as a ‘stress buffer’. The proposal follows on from the doctoral work of student Diogo Peixoto and promises other advantages such as strengthening the immune system of these animals.

At a time when the world's population is expected to exceed 9.8 billion by 2050, aquaculture has emerged as the answer to human food needs, with the sector growing at an above-average rate. However, aquaculture carries many challenges including maintain fish welfare.

Animal stress is one of the biggest challenges in the aquaculture sector. This stress is caused by transport, high animal densities, overfeeding, water quality and handling procedures such as selection and vaccination. It has an impact on animal welfare and can also reduce growth rates and resistance to disease. As well as compromising animal health, it also leads to high economic losses in the sector.

It was in this context that doctoral student Diogo Peixoto, under the guidance of CIIMAR researcher Rita Azeredo, studied the advantages of supplementing the diet of farmed fish with tryptophan, an essential amino acid with the capacity to be metabolized into various compounds, including serotonin, which in humans is known as one of the “happiness hormones”.

The role of tryptophan in combating stress

“In aquaculture, diseases most often appear after the fish have been subjected to a stressful situation. There are several strategies to combat this problem, some more successful than others. In our laboratory we have used immunonutrition as a strategy to improve animal welfare,” explains Benjamín Costas, leader of CIIMAR's Aquatic Animal Health team and principal investigator of the IMMUNAA project, funded by the Foundation for Science and Technology (FCT), where the PhD student's work was integrated.

“The main aim of my PhD was to understand the role of tryptophan in the immune response of sea bass, especially during situations of inflammatory response and resistance to disease,” explains Diogo. To this end, different research strategies were applied, which coincided in the same conclusion: tryptophan supplementation can mitigate the negative effects of stress, keeping energy metabolism stable and generating happier and healthier animals. “As fish are unable to synthesize tryptophan internally, its supplementation in feed is essential,” adds the researcher.


Health through nutrition

Today, the concept of improving animal health through better nutrition is widely accepted. This is as true for humans as it is for other animals, and it is a concept also implemented in aquaculture today. In this sense, “research and innovation efforts have increased significantly in the field of immunonutrition. Proper nutrition is essential not only for maintaining growth, but also for supplying nutrients to the immune system, inflammatory processes and stress,” explains Benjamín Costas.
“Adequate feed and diets provide optimal health, which can be achieved by linking nutrition to immunity, along with other areas such as biochemistry, physiology, microbiology and pathology,” adds Diogo Peixoto. In aquaculture, the use of supplemented or functional feeds has been implemented with the inclusion of nutrients that boost immunity. Functional amino acids, such as tryptophan, are good candidates for supplementation in feed, improving health and survival, working as a complementary strategy, for example, to vaccination in aquaculture.

Does more serotonin make fish happier?

Although there is no scientific evidence that serotonin plays exactly the same role as the “happiness hormone” as it does in humans, tryptophan is an essential amino acid that is the precursor of various metabolites, with varied effects on modulating the stress response, for the synthesis of the neurotransmitter/neuro modulator serotonin, the hormone melatonin, among other molecules with an impact on the health and well-being of animals. It also plays a very important role in regulating the antioxidant system, behavioral responses and immune function. After intestinal absorption, tryptophan metabolism can be used for protein synthesis and/or metabolized by three enzymes, whose metabolic pathways (serotonin and kynurenine) have important implications for both immune and neuroendocrine responses. Although a direct link between serotonin production and the animals‘ level of happiness has not been proven, Diogo Peixoto's work has shown that tryptophan supplementation can mitigate the negative effects of stress by increasing the animals’ levels of well-being and immune stability. In short, they are happier and healthier animals.

Is supplementing the same as medicating?

Tryptophan supplementation can be used in two ways: therapeutically or prophylactically. It can be used therapeutically when administered to fish that are already under stress (for example, at high stocking densities). In these cases, “the supplemented tryptophan will act on the fish's hypothalamic-pituitary-interrenal axis, reducing stress and, in this sense, it works as a therapy,” explains Diogo. “It can also have a therapeutic effect if administered after the onset of an inflammatory process, helping to negatively regulate the inflammatory response,” he adds.

However, in aquaculture, fish are affected by many stress factors derived from farming conditions. Even if these conditions are anticipated, the fish will have to be vaccinated, transported or eventually become ill. These fish, when fed feed supplemented with tryptophan, can deal with these situations more advantageously, as this amino acid acts as a buffer to the effect of stress, helping to neutralize its immunosuppressive effects. In this way, the effect of tryptophan can be considered preventative, because its use avoids subsequent health problems or ineffective vaccination. However, this perspective is still the subject of debate among the scientific community.

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