Introduction
Fish production from aquaculture has expanded greatly during the last decades, and aquaculture is recognized as a major food production industry (FAO, 2020). However, concerns about environmental issues, sustainability and animal welfare in aquaculture are increasing. About sustainability of aquaculture, major concern is related to use of fish meal and oil in fish feeds that are coming from wild caught fish (Naylor et al., 2021). In this study, we evaluate the effects of an innovative diet (with replacement by a new yeast-based protein ingredient) on the health and welfare indicators of European sea bass (Dicentrarchus labrax), a key species of the European marine aquaculture. In this work, we monitored level of stress molecular indicator (HSP70), physiological blood parameters of interest (e.g. haemoglobin, cortisol, glucose, lactate, lysozyme), as well as the growth performances of sea bass reared in sea cages fed the innovative diet. In addition, a sub-sample of fish has been implanted with accelerometer tag for continuous monitoring of acceleration, a proxy of energy expenditure (Carbonara et al., 2021), for about two months.
Materials and methods
European sea bass were monitored from May 2020 to July 2021. Feeding trial started on August of 2020. Six experimental cages were used (n=~2200 fish per cage) in the farm of AVRAMAR at Palairos (Greece). Fish were fed two feeds; a commercially available control feed which was offered in three cages and the experimental innovative- diet which was offered in the remaining three cages. Innovative diet was based on the control diet but with reduced fishmeal inclusion aiming to reduce FIFO (Fish in:Fish out) ratio. Throughout the experimental period fish were fed ad libitum, 2 – 4 times daily depending on their weight and the water temperature. Sea water parameters such as water temperature and dissolved oxygen were recorded daily. Over the trial, three samplings were carried out at three different times (T0: in May 2020 ‘control’ before starting the feeding, T1: in April 2021 and T2 in July 2021). At the three sample times, growth measurements and blood and organs samples were carried out. At T1, n=5 fish per cage were tagged using accelerometer tags with ’tailbeat mode’ algorithm, allowing to measure fish acceleration (m/s2), which is a proxy of energy expenditure (Carbonara et al., 2021). At each sampling point, fish were gently caught from the rearing tanks or cages and bathed into anesthetic (clove oil) for 2-3 min before proceeding to blood sampling. The blood samples were taken from the caudal vein, and used to assess the basal levels of the following physiological indicators: cortisol, glucose, lactate, hematocrit, hemoglobin, red blood cell count (RBCC), noradrenaline, adrenaline and lysozyme. The quantification of these parameters was performed as described in Carbonara et al., (2019). Also, the levels of total proteins, prealbumine, albumin, alfa1, alfa2, beta1, beta2, Gamma and Immunoglobulin M in plasma were assessed according manufacturer instructions. Organs (spleen, liver, kidney, gills and brain) were also sampled to further quantify HSP70 though quantitative rRT-PCR.
Results and discussion
At the end of the trial, fish growth rates differed between treatments only in the final sampling; the average weight of fish fed the commercial diet was significantly higher compared to the average weight of fish fed the innovative diet (p<0.05). Similarly, SGR was found greater in fish fed the commercial diet than fish fed innovative one (p<0.05). Despite the significant greater feed consumption from the commercial group (p<0.05), the FCR, K-factor and the total mortalities showed no significant differences between feed treatments
Based on the physiological sensors data, sea bass displayed a diurnal pattern regarding acceleration; they were more active during daytime than nighttime. This was consistent with what has been observed in previous experiments where fish were more active during feeding at daytime (Carbonara et al., 2021). Only a tendency for significant difference between the two feeds (p=0.057) has been observed (i.e., fish fed innovative diet tended to display lower acceleration; Figure 1.a). When looking at the distribution of swimming activity values recorded by sensor, fish fed commercial diet displayed greater number of low activity values than fish fed innovative one, and oppositely fish fed commercial diet displayed lower number of low activity values than fish fed innovative one (Figure 1.b). This could be indicative of greater use of anaerobic metabolism for fish fed commercial diet.
Finally, regarding all the physiological parameters measured in plasma during the trial, only few were affected by the innovative diet. In more details, we found lower level of cortisol, glucose and hemoglobin in sea bass feed innovative diet than fish fed commercial one (p<0.05). Also, we measured greater level of total proteins in sea bass feed innovative diet than fish fed commercial one (p<0.05 for all). Some change in stress parameters (i.e. cortisol, glucose) has been measured punctually in one of different sampling times but levels were returned to similar level in other sampling time, suggesting that this change should be linked more to sampling time than diet.
Overall, this study provides a global assessment of sea bass physiology under innovative diet coping with sustainability challenges of European aquaculture. At the end of trial, the average weight difference between the two diet treatments was 30g, a weight that can be gained between 10-20 days depending of year period in the Mediterranean Sea. In addition, the health and welfare of sea bass was not significantly affected by this feeding. In conclusion, such innovative feed could be promising for the future of sustainability in the European aquaculture.
Acknowledgments
This research was supported by H2020 FutureEUAqua, under grant No. 817737.
References
Carbonara, P., Alfonso, S., Dioguardi, M., Zupa, W., Vazzana, M., Dara, M., et al. (2021). Calibrating accelerometer data, as a promising tool for health and welfare monitoring in aquaculture: Case study in European sea bass (Dicentrarchus labrax) in conventional or organic aquaculture. Aquac. Reports 21, 4–13. doi:10.1016/j.aqrep.2021.100817.
Carbonara, P., Alfonso, S., Zupa, W., Manfrin, A., Fiocchi, E., Pretto, T., et al. (2019). Behavioral and physiological responses to stocking density in sea bream (Sparus aurata): Do coping styles matter? Physiol. Behav. 212, 112698. doi:10.1016/j.physbeh.2019.112698.
FAO (2020). The State of World Fisheries and Aquaculture 2020. Sustainability in action. Rome. doi:https://doi.org/10.4060/ca9229en.
Naylor, R. L., Hardy, R. W., Buschmann, A. H., Bush, S. R., Cao, L., Klinger, D. H., et al. (2021). A 20-year retrospective review of global aquaculture. Nature 591, 551–563. doi:10.1038/s41586-021-03308-6.