Introduction
Farmed ballan wrasse (Labrus bergylta) are produced as cleaner fish in salmon aquaculture and this relatively new industry faces several challenges in its pursuit of producing sustainable, high quality, robust fish. Slow growth (Brooker et al., 2018) and a high frequency of nephrocalcinosis and skeletal abnormalities (Cavrois-Rogacki et al., 2021; Fjelldal et al., 2021) are crucial challenges. While there are likely many factors involved, identifying the nutritional requirements of this species will be essential to improving performance and skeletal health. Phosphorus is an essential element of skeletal tissue and is involved in a wide range of metabolic processes (Antony Jesu Prabhu et al., 2013) making it a key target for dietary optimisation to improve both skeletal health and growth. Vitamin D3 (VD) plays an important role in Ca and P uptake and directly interacts with osteoblasts to regulate mineralization (Lock et al., 2010). It has also been implicated in the development of nephrocalcinosis in mammals (Letavernier and Daudon, 2018). This study aims to investigate the impact of 3 levels of P and 2 levels of VD on juvenile ballan wrasse performance and skeletal health.
Materials and Methods
Ballan wrasse (4.3 ± 1.1g) from Otter Ferry Seafish Ltd. (Tighnabruaich, Argyll, UK) were randomly distributed into 18 100L flow through tanks (140 fish per tank; 2520 fish in total). Tanks were allocated to one of six experimental diets, formulated to two different levels of VD (1500 IU/Kg or 3000 IU/Kg) and three levels of P (1.1, 1.9 and 2.7 %; Table 1) in a 2x3 factorial design. Each condition was conducted in triplicate and fish were fed the experiment diets to satiation for 10 weeks.
After 10 weeks, 33 fish per tank were weighed, measured, and welfare scored before storing in 10% neutral buffered formalin for x-ray, whole mount staining and histology analysis. Growth results were analysed with a two-way ANOVA after testing for assumptions followed by a Tukey’s post-hoc test. Welfare scores were analysed using the Kruskal-Wallis test followed by a Dunn’s test.
Results
After 10 weeks, growth (SGR) was significantly impacted by significantly impacted by P (two-way ANOVA, p = 0.003) but not VD3 (P = 0.599). The highest P level (2.7%) performed the significantly better than the low P condition (figure 1).
There was a high frequency of pectoral fin erosion across all groups (90-98% with a score of 1 or greater), though in both VD groups the high P conditions scored the highest, with median scores of 1.5 compared to 1 in the other groups (Kruskal-Wallis, p < 0.01).
Chemical composition analysis of the final formulated diets showed lower P and higher than expected VD3, resulting in no low P/low VD3 diet.
These general performance and welfare indicators will be complemented with radiological analysis which is currently in progress to investigate vertebral health and nephrocalcinosis frequency.
Discussion
The nutrient requirements for this species have yet to be established and most commercial diets currently in use contain ~2% P and ~2000 UI/kg. This study suggests higher levels of P may improve growth performance. However, P requirements for optimum bone mineralisation are often higher than the levels required for optimum growth (Antony Jesu Prabhu et al., 2013) and pectoral erosion scores shows initial indications that this level of P may not be optimum for welfare.
Acknowledgements
This study was part of the Ballan+ project, a Sustainable Aquaculture Innovation Centre (SAIC) collaboration between the University of Stirling, Biomar, Otter Ferry SeaFish Ltd., Scottish Sea Farms and MOWI.
References
Antony Jesu Prabhu, P., Schrama, J.W., Kaushik, S.J., 2013. Quantifying dietary phosphorus requirement of fish - a meta-analytic approach. Aquac Nutr 19, 233–249.
Brooker, A.J., Papadopoulou, A., Gutierrez, C., Rey, S., Davie, A., Migaud, H., 2018. Sustainable production and use of cleaner fish for the biological control of sea lice: Recent advances and current challenges. Veterinary Record 183, 383.
Cavrois-Rogacki, T., Drabikova, L., Migaud, H., Davie, A., 2021. Deformities prevalence in farmed ballan wrasse (Labrus bergylta) in relation to hatchery origin and life stage. Aquaculture 533.
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Letavernier, E., Daudon, M., 2018. Vitamin d, hypercalciuria and kidney stones. Nutrients 10, 1–11.
Lock, E.J., WaagbØ, R., Wendelaar Bonga, S., Flik, G., 2010. The significance of vitamin D for fish: A review. Aquac Nutr 16, 100–116.