Introduction
Climate change brings a significant challenge to aquaculture systems, especially in cage farming where rising water temperatures can negatively affect fish health and performance. Increasing water temperatures over the past decades have been reported for the Mediterranean Sea (Pastor et al., 2020), inland waters like the Black Sea in Turkey (Sakalli & Başusta, 2018), and lakes and rivers in Italy (Gavioli et al., 2024) . Global warming is not expected to halt in the near future and will increasingly affect aquaculture . In rainbow trout (Oncorhynchus mykiss), heat stress can impair immune function and gut health, and reduce nutrient digestibility. The associated decline in performance poses a challenge for the productivity in aquaculture systems. With the dietary supplementation of a nutritional emulsifier based on polyethylene glycol ricinoleate (PEGR), an improved digestion, particularly of fats and oils, can be obtained. Moreover, our previous research already demonstrated microbiota shifts due to extreme heat in farmed gilthead sea bream were partially reversed by nutritional intervention with PEGR (Domingo-Breton et al., 2024). This study, evaluated the potential of a PEGR-based nutritional emulsifier to mitigate the effects of heat stress in rainbow trout.
Material and Methods
A 12-week feeding study was conducted at the trial facilities of ABT Innovia in Malta with a total of 600 rainbow trout (initial weight 111.77g) . Fish were randomly assigned to twelve tanks across four dietary treatment groups: Control, 750 ppm PEGR-based nutritional emulsifier (Volamel Extra produced by Nukamel BV, the Netherlands - 750V), 1000 ppm nutritional emulsifier (1000V) and 1250 ppm nutritional emulsifier (1250V). Fish were fed their respective experimental feeds as 4.5 mm pellets three times daily, at a fixed rate based on body size. Water temperature was maintained at 16 ± 2°C during the first eight weeks of the trial, and then gradually increased by 1°C per day to a final temperature of 22 ± 2°C to induce heat stress . Growth performance and health parameters were monitored and statistically analyzed with ANOVA, using post-hoc multiple comparisons with Tukey HSD test.
Results
No significant differences between treatments were observed in the thermoneutral phase d uring the first eight weeks (Figure 1) .
Under heat stress, both treatments with nutritional emulsifier (750V and 1000V) showed significantly higher growth and lower FCR compared to control (Figure 2). Survival was numerically better, while hepatosomatic index (HSI), and viscerosomatic index (VSI) showed a significant increase in the diets with the nutritional emulsifier.
For the overall 12 weeks trial period, 1000 ppm of nutritional emulsifier showed a significant improvement in the FCR (p<0.05) and a trend towards higher growth (p=0.1). Survival and K-factor (condition factor) were unaffected across treatments.
Discussion
During heat stress a lot of changes in metabolism take place and can be accompanied by changed levels of HSI and VSI and can be harmful for fish health. A decrease in performance is often recorded, resulting in production losses for aquaculture systems. N utritional emulsifiers based on PEGR can influence the gut dynamics of carnivore fish. This trial showed that addition of a nutritional emulsifier can mitigate the negative effects on growth performance and feed efficiency of carnivore fish under heat stress challenge . More research is needed to see the effect on other mechanisms in the fish, such as digestibility, immunity and antioxidant capacity.
Conclusion
Inclusion of a nutritional emulsifier based on PEGR helped maintain growth and feed efficiency under heat stress conditions in rainbow trout. This suggests that such emulsifiers can be a valuable nutritional strategy to improve resilience in aquaculture systems facing rinsing temperatures and heat stress, maintaining production to reasonable levels.
References:
Domingo-Bretón, R., Cools, S., Moroni, F., Belenguer, A., Calduch-Giner, J., Croes, E., Holhorea, P., Naya-Català, F., Boon, H., & Pérez-Sánchez, J. (2024). Intestinal microbiota shifts by dietary intervention during extreme heat summer episodes in farmed gilthead sea bream (Sparus aurata). Aquaculture Reports, 40, 102566. https://doi.org/10.1016/j.aqrep.2024.102566
Gavioli, A., Castaldelli, G., Trasforini, S., Puzzi, C., Gervasio, M. P., Granata, T., Colombo, D., & Soana, E. (2024). Global Warming and Fish Diversity Changes in the Po River (Northern Italy). Environments, 11(10), 226. https://doi.org/10.3390/environments11100226
Pastor, F., Valiente, J. A., & Khodayar, S. (2020). A Warming Mediterranean: 38 Years of Increasing Sea Surface Temperature. Remote Sensing, 12(17), 2687. https://doi.org/10.3390/rs12172687
Sakalli, A., & Başusta, N. (2018). Sea surface temperature change in the Black Sea under climate change: A simulation of the sea surface temperature up to 2100. International Journal Of Climatology, 38(13), 4687–4698. https://doi.org/10.1002/joc.5688