Feeding behaviour regulation and impact by omega-3 fatty acids during the life cycle of trout – FEEDOMEGA

The project combines long term nutritional trials in rainbow trout with behavioural, physiological and molecular approaches.

Fish are reared from first feeding to adulthood on contrasted diets: a marine based control (fishmeal/fish oil), a 100 % plant based diet, and a 100 % plant based diet supplemented with microalgal DHA. Growth, feed intake (including with self feeding devices), feed efficiency, survival and body composition are monitored throughout the life cycle. In dedicated trials, future broodstock are also fed these diets over the long term to assess effects on reproductive performance (gamete quality, fecundity, offspring viability).

At key life stages (early fry, juveniles, broodstock), targeted sampling of sensory organs (tongue, olfactory epithelium), brain regions and intestine is performed. Lipid composition, including omega 3 profiles and selected lipid mediators, is analysed in feeds and tissues using chromatographic techniques. In parallel, molecular analyses (qPCR and related approaches) are used to quantify the expression of genes involved in taste and olfactory receptors, lipid sensing, gut–brain signalling and hypothalamic control of appetite.

Behavioural readouts (self feeding activity, voluntary feed intake, response to diet switches or challenges) are integrated with these biochemical and molecular data to characterise how plant based diets and microalgal DHA supplementation modify the sensory detection of feed and the central regulation of appetite.

Finally, intergenerational effects are assessed by raising offspring from parents fed the different diets and challenging these offspring with 100 % plant based feeds. The same set of measurements (growth, feed intake, basic physiology, selected molecular markers) is applied to evaluate how maternal nutrition “programmes” the capacity of young fish to cope with plant based diets. Together, these methods provide a comprehensive picture from feed formulation to sensory perception, brain regulation, performance and reproduction.

The main results of FEEDOMEGA show that the bottleneck for 100 % plant based feeds in rainbow trout lies largely in sensory and hedonic regulation of feeding, rather than in “nutrition on paper” alone.

First, we found that very early in life, just a few days after first feeding, 100 % plant based diets already induce strong alterations in the signalling pathways of taste and olfactory receptors, both on the tongue and in the olfactory epithelium. In contrast, the expression of hypothalamic neuropeptides that regulate hunger and satiety is not markedly disturbed at this stage, indicating that the central regulatory system is in place, but receives abnormal sensory input.

The same pattern is observed later in juveniles (around 8 months of age), showing that these sensory alterations persist over time. For the first time in trout, FEEDOMEGA provides evidence that the hedonic component of feeding, that is the “desire” or “pleasure” to eat, is specifically impaired in fish fed 100 % plant based diets from first feeding. Fish appear to eat the minimum required to survive, but with a reduced motivation to consume the plant based feed.

On the reproductive side, the project shows that long term feeding of broodstock with 100 % plant based diets leads to a clear deterioration of reproductive performance (egg quality, success of reproduction). Supplementing these plant based diets with microalgal oil rich in omega 3 does not restore reproductive quality in the parents.

However, a positive effect is observed in the offspring. When females are fed a plant based diet containing microalgal oil from early life onwards, their progeny show better growth and a more favourable feeding pattern when they are themselves challenged with a 100 % plant based diet. These young fish cope better with the plant based diet in terms of feed intake and growth. This points to an intergenerational programming effect of maternal nutrition on the ability of offspring to use sustainable feeds.

Overall, FEEDOMEGA demonstrates that 100 % plant based diets can deeply and durably alter taste, smell and the pleasure of eating in trout, and impair reproduction, but also that specific strategies such as the use of microalgal oils in broodstock diets may improve the capacity of the next generation to cope with these diets.

The outstanding feature of FEEDOMEGA is its focus on what gives fish the “desire to eat” sustainable feeds. Instead of looking only at nutrients and growth, the project places the sensory and hedonic dimensions of feeding at the centre: how the fish taste and smell their food, how these signals reach the brain, and how this shapes motivation to eat throughout life and even across generations. This is a shift from classic feed formulation towards a more integrated view of the fish as a sensing, feeling organism.

FEEDOMEGA is also distinctive because it follows rainbow trout from the very first meals, through the juvenile phase and up to reproduction, and then looks at the offspring. By combining long term feeding trials, behavioural measurements, biochemical analyses and molecular tools, the project links what happens in the tank (how much and how willingly fish eat) with what happens in their sensory organs and brain. The discovery that 100 percent plant based diets mainly affect the pleasure and motivation to eat, rather than basic hunger circuits, is a key original finding.

In terms of future prospects, FEEDOMEGA provides a strong scientific basis to move from understanding to action. The results show that it will not be enough to design plant based feeds that are nutritionally “correct”. Future feeds will also need to be sensorially adapted to fish, and used in ways that take into account sensitive periods and the nutritional history of the parents.

These ideas are already being taken forward in follow up projects such as SENSOFEED. Building directly on FEEDOMEGA, SENSOFEED aims to test concrete strategies to improve the acceptance of sustainable feeds: adjusting taste and smell cues, targeting the pathways involved in the hedonic control of feeding, and using early life and maternal nutrition to prepare fish to cope better with plant based diets under realistic farming and environmental conditions.

In the longer term, the approach developed in FEEDOMEGA can be extended to other aquaculture species and combined with work on welfare and product quality. It opens the way for collaborations with feed companies to co design diets that fish really want to eat, that keep omega 3 levels high for consumers, and that reduce pressure on marine resources.

In order to maintain a sustainable aquaculture, the traditional ingredients of aquafeed, fishmeal and fish oil, must be replaced by renewable and alternative sources like terrestrial plants products. However, in rainbow trout (Oncorhynchus Mykiss), the total replacement of aquafeed led to drastically altered fish life cycle (survival, growth, reproduction, first spawning and offspring survival). Our hypothesis is related to altered feeding behaviour due to the absence of long-chain omega-3 polyunsaturated fatty acids (DHA) in these plant-based diets. Understanding feeding behaviour as well as the phenotypical, molecular mechanisms and nutritional aspects regulating feed intake of farmed fish fed alternative diets and involvement of omega-3 diet will provide extensive information about how alternative ingredients are perceived by fish.This knowledge will be crucial to develop new nutritional strategies enhancing feed efficiency and reducing nutrient losses during all life cycle of fish farmed.