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 the author: Jerôme Roy, INRAE, Jerome.roy@inrae.fr
The publisher KeAi was established by Elsevier and China Science Publishing & Media Ltd to unfold quality research globally. In 2013, our focus shifted to open access publishing. We now proudly publish more than 200 world-class, open access, English language journals, spanning all scientific disciplines. Many of these are titles we publish in partnership with prestigious societies and academic institutions, such as the National Natural Science Foundation of China (NSFC).
Water Biology and Security
Experimental study
Animals
Temporal hedonic regulation of feeding behavior in rainbow trout: a role for the opioid system under plant-based diets from first feeding onward
The authors declare that they have no competing interests.