ENHANCING PERFORMANCE OF LOW FISHMEAL DIETS IN RAINBOW TROUT Oncorhynchus mykiss UNDER HIGH DENSITY CONDITIONS THROUGH UMAMI-MEDIATED GUT SENSING MECHANISMS

Introduction

The reduction of fishmeal (FM) inclusion in aquafeeds is a key priority due to its limited availability, cost volatility, and sustainability concerns. However, lowering FM levels often impairs feed intake and performance in carnivorous fish. Fish possess highly sensitive gustatory systems capable of detecting amino acids (AA), key drivers of umami taste associated with protein-rich ingredients. Importantly, taste receptors are also expressed along the gastrointestinal tract, where they participate in nutrient sensing and regulation of digestion and appetite via gut–brain signalling pathways (Angotzi et al., 2022). Therefore, dietary strategies targeting both oral and post-ingestive chemosensory mechanisms may help maintain performance in low FM diets.

Materials and methods

A 91-day feeding trial was conducted with rainbow trout (initial body weight: 29.1 ± 2.1 g) under high-density conditions (29 to 53 kg m^-3at the start and end of the trial, respectively) in aerated freshwater (14.2ºC). Fish were distributed into triplicate tanks (50 fish tank^-1) and fed three diets (43% crude protein and 24% crude fat): a positive control (PC), a negative control (NC), and NC supplemented with 0.25% of a functional palatability enhancer (FPE) containing umami taste-stimulating substances. The PC contained 15% high quality FM and 5% soy protein concentrate (SPC), while the NC replaced 50% of FM and all SPC and wheat gluten with higher levels of less palatable and less digestible ingredients such as feathermeal hydrolysate, corn gluten meal, guar meal and sunflower meal. Growth performance, feed intake, feed conversion ratio (FCR), protein efficiency ratio (PER), and apparent digestibility coefficients (ADC) were evaluated. Additionally, expression of nutrient sensing and gut function markers (taste receptors and signalling effectors, enteroendocrine peptides, AA receptors and tight junction genes) was analysed in the foregut of 24h fasted fish at the end of the trial.

Results and Discussion

Reduction of FM in the NC diet negatively affected feed intake and growth performance compared to the PC. Supplementation with FPE significantly improved feed intake and growth (final body weight and specific growth rate, SGR), partially restoring these parameters (Figure 1A,B). Feed efficiency (FCR and PER) followed a similar trend, worsening in the NC and partly restored by FPE supplementation. Protein digestibility, but not energy or lipid digestibility, was significantly reduced in the NC compared to the PC, and significantly improved by supplementing the NC with FPE (Figure 1C).

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Figure 1. Performance indicators including feed intake along the experimental period (A), and specific growth rate (B) and apparent digestibility coefficient (ADC) of protein (C) at the end of the trial.

At the molecular level, FPE supplementation induced an upward trend in the expression of tas1r1 (umami receptor subunit), significantly reduced itpr3 (taste-signalling effector) and increased cck (a regulator of satiety, pancreatic secretion and peristalsis), suggesting activation of gastrointestinal chemosensory pathways. Moreover, expression of tight junction proteins cldn3 and cldn15 was significantly up-regulated in the FPE, suggesting a tighter, more efficient gut barrier, potentially associated with improved gut function.

In conclusion, dietary inclusion of a FPE mitigated the negative effects of a less palatable and lower quality diet, including lower FM and less digestible ingredients, by improving feed intake and growth performance in rainbow trout. These effects were likely mediated, at least partly, by gastrointestinal sensing and umami taste receptor signalling, which may explain the increased expression of cck and protein digestibility. Previous studies similarly showed that dietary inclusion of umami receptor stimulants has the potential to improve appetite and physiological responses to low FM diets in rainbow trout (Calo et al., 2024). Overall, these findings support this type of sensory additive as a promising approach to sustain performance in increasingly sustainable aquafeeds under real-world farming conditions.

Acknowledgment

We thank Almudena Martinez (Lucta S.A.) for performing the gene expression analysis.

References

Angotzi R., Leal E., Puchol S., Cerdá-Reverter J.M., Morais S. (2022) Exploring the potential for an evolutionarily conserved role of the taste 1 receptor gene family in gut sensing mechanisms of fish. Anim. Nutr., 11: 293–308. DOI: 10.1016/j.aninu.2022.08.010.

Calo J., Comesaña S., Fernández-Maestú C., Blanco A.M., Morais S., Soengas J.L. (2024) Impact of feeding diets with enhanced vegetable protein content and presence of umami taste-stimulating additive on gastrointestinal amino acid sensing and feed intake regulation in rainbow trout. Aquaculture, 579: 740251. DOI: 10.1016/j.aquaculture.2023.740251.