Introduction
Palatants are often used in aquaculture to enhance feed palatability and improve feeding efficiency. However, there is a lack of methodological approaches to evaluate fish feeding responses, and conventional metrics such as feed intake may not fully capture all effects, particularly those related to pre-ingestive sensory responses. This study evaluated the olfactory and gustatory responses of rainbow trout (Oncorhynchus mykiss) fed diets supplemented with a classic marine hydrolysate (MH) and a non-animal-based palatability enhancer (PE), using both traditional intake measurements and computer vision-based behavioural analysis.
Materials and Methods
Three isoproteic (37%) and isolipidic (16%) diets were tested: a control (CON) and diets supplemented with either a marine hydrolysate (MH; 1.50%) or a non-animal-based palatability enhancer (PE; 0.15%). Fish were reared in a recirculating aquaculture system and subjected to two experimental phases: optimal conditions and stress conditions (hypoxia–crowding and thermal stress). Feeding behaviour was assessed through feed intake measurements and computer vision analysis using a YOLO-based model. Behavioural metrics were classified as indicators of gustatory responses (pellet ingestion dynamics and swimming activity during feeding) and olfactory responses (fish aggregation in the aroma zone and distance to the feed source prior to ingestion). Statistical analyses included ANOVA and generalized additive mixed models (Wood, 2017)
Results and Discussion
Daily feed intake did not differ significantly among diets under either optimal or stress conditions (P > 0.05), indicating the limited sensitivity of conventional metrics. However, behavioural analyses revealed clear diet-specific effects. The MH promoted faster pellet ingestion and increased swimming activity, suggesting enhanced immediate feeding stimulation. In contrast, the PE consistently increased fish aggregation near the feed source, reduced fish-to-feed distance, and maintained stronger attraction under stress conditions, particularly under thermal challenge.
Notably, the non-animal-based PE showed a more sustained and robust effect on olfactory-driven behaviours across conditions, supporting its role in enhancing feed detection and feeding motivation over time. These findings indicate that while MH primarily stimulates short-term ingestion responses, PE modulates broader sensory pathways, contributing to improved feed acquisition efficiency, especially under suboptimal environmental conditions. Such effects are not captured by feed intake alone but are critical for understanding complex feeding responses. Computer vision-based behavioural quantification further enabled high-resolution assessment of these responses (Bradski, 2000).
Figure 1. Behavioural (gustatory and olfactory) responses of rainbow trout fed diets supplemented with marine hydrolysate (MH) or a non-animal-based palatability enhancer (PE) under stress conditions. (A) Pellet ingestion dynamics under thermal stress; (B) fish aggregation in the aroma zone under thermal stress; (C) distance of fish to the feed source under hypoxia and crowding stress. Data are presented as modelled means over time.
Conclusion
Computer vision-based metrics reveal that palatants can significantly influence feeding dynamics beyond conventional intake measurements. The non-animal-based PE, compared to the marine hydrolysate, demonstrated a more consistent and robust effect on fish attraction and feeding-related behaviours, particularly under stress conditions. These results highlight the importance of integrating behavioural tools to better characterize and differentiate palatant effects in aquafeeds.
Acknowledgment
This study was conducted in collaboration with the University of South Bohemia and supported by Lucta S.A.
References
Bradski, G. (2000) The OpenCV Library. Dr. Dobb's Journal of Software Tools. 120: 122–125.
Wood, S.N. (2017) Generalized additive models: an introduction with R. Chapman and Hall/CRC. https://doi.org/10.1201/9781315370279