Opioid system may flag poor acceptance of plant-based feed in farmed trout
Published 06 February, 2026
As aquaculture shifts toward fish-free feeds to reduce reliance on fishmeal and fish oil, producers face a recurring problem: some fish eat less and grow more slowly on fully plant-based diets. A new study in rainbow trout shows that this drop in performance is accompanied by delayed, time-dependent changes in the brain’s opioid system — a key pathway involved in food reward and palatability.
The researchers fed trout either a conventional diet containing fishmeal and fish oil or a fully plant-based diet from the very first meal, then tracked growth, feeding and brain chemistry at 5 days, 30 days and 8 months.
“Fish on the plant-based diet were lighter at every stage: about 8% lower body weight after 5 days, 17% lower after 30 days, and 33% lower after 8 months (229 g vs 344 g),” shares the study’s senior author
Jérôme Roy. “Feed efficiency was similar between diets, but feed intake was lower on the plant-based diet, by 16.3% at 30 days and by 33% across the long-term period.”
The team also measured indicators of neurotransmitter activity and the expression of opioid-related genes involved in hedonic regulation. Early on, the opioid response was limited: after 5 days, only one opioid receptor gene (oprk1) showed a diet-related change shortly after a meal. By 30 days, the plant-based group showed a clear shift, including higher serotonin turnover and changes in an opioid precursor gene (pdyn). After 8 months, multiple opioid-system genes in the telencephalon, a brain region associated with reward processing, were affected in fish fed the plant-based diet.
“Interestingly, growth and intake differences appear very early, but the brain opioid system reacts much more strongly later, suggesting a delayed engagement of reward pathways under long-term nutritional pressure,” says Roy.
“These results shine a new light on why fish-free feeds can be harder to accept,” adds lead author Elisabeth Plagnes-Juan. “Beyond nutrition, palatability matters, and opioid-related markers may help us monitor how fish perceive alternative diets across development.”
Contact author name, affiliation, email address: Jerôme Roy, INRAE, Jerome.roy@inrae.fr
Social media handles: N/A
Funder: This research was funded by the French National Research Agency (grant no. ANR-21-CE20-0009-01 “FEEDOMEGA”).
Conflict of interest: The authors declare that they have no competing interests.
See the article: Elisabeth Plagnes-Juan, Maud Martinat, Elodie Baranek, Anne Surget, Cécile Heraud, Jérôme Roy. Temporal hedonic regulation of feeding behavior in rainbow trout: a role for the opioid system under plant-based diets from first feeding onward, in press. https://doi.org/10.1016/j.watbs.2025.100529.