THE IMPACT OF NUCLEOTIDE SUPPLEMENTATION ON THE PERFORMANCE AND IMMUNE RESPONSE OF DEPLOYMENT-SIZE BALLAN WRASSE Labrus bergylta

Introduction

Nucleotides have a wide variety of functions beyond their most recognized role as the basic building blocks of RNA and DNA. They provide and mediate energy metabolism, are key components of cell signalling, and are key enzymatic cofactors (Cosgrove, 1998), to name a few. While organisms can produce nucleotides without dietary input (Cosgrove, 1998), this process is metabolically expensive and direct provision may allow fish systems to react faster to challenges (Ringø et al., 2012). Therefore, nucleotides are generally considered semi-essential nutrients (Li and Gatlin, 2006) and beneficial during periods of rapid growth or high metabolic demands, such as larval stages or disease (Li and Gatlin, 2006). Taking their lead from human nutrition (Cosgrove, 1998; Li and Gatlin, 2006), aquaculture research has begun to explore the potential of nucleotides to be used as feed additives (Li and Gatlin, 2006). A wide range of nucleotide-based products have been used for a variety of fish species and have been shown to optimise cell proliferation, promote growth (Hossain et al., 2016), enhance immune response (El-Nokrashy et al., 2021) as well as improve gut integrity and function (de Cruz et al., 2020). However, the mode of action of nucleotides on these various functions has in many cases yet to be fully elucidated. The deployment of ballan wrasse (Labrus bergylta) from the hatchery to the net pen is a period of high stress and mortality, with increased exposure to environmental stressors and disease. Working closely with industry partners, this study will explore the use of nucleotides as health additives in developing functional feeds to support ballan wrasse during this period.

Materials and Methods

Ballan wrasse (38.4 ± 9.4g) were randomly distributed into twelve 350 L flow-through tanks (100 fish per tank). Tanks were allocated to one of four experimental diets: A control diet following the current standard commercial formulation for this species with no added nucleotides (NT0) and three treatment diets with increasing levels of nucleotide supplementation (NT1, NT2 and NT3) using the commercial product Rovimax NX (DSM Animal Health and Nutrition). Each condition was conducted in triplicate with fish fed the experimental diets to satiation for 8 weeks. At the end of the trial, samples from different tissues were collected for histological, vertebral health, molecular biology analysis as well as plasma. In order to examine the robustness of the fish after feeding the supplemented feeds, the fish were subjected to an immune challenge by intraperitoneally (IP) injecting lipopolysaccharide (LPS) of E. coli as pathogen-associated molecular patterns (PAMP) at the end of the experiment. Fish were sampled 24 hours post-LPS injection, with plasma and head kidney collected. Operational welfare indicators (OWI) were also scored at the end of the nutritional challenge and after the challenge.

Results and Discussion

Fish performance was generally good, with fish increasing weight on average 1.6 times compared to the start of the trial. After 8 weeks of feeding the experimental feeds, fish fed the non-supplemented feed did not display significant different body weight compared to fish fed nucleotide-supplemented feeds. No difference was also found in terms of survival, being 99.2 ± 0.7 % on average. The fish showed good OWI scores, though not significantly different among the experimental fish. No significant difference was found in the fish haematocrit, the hepatosomatic index, and liver histology at the end of the nutritional trial.

The preliminary results seem to indicate that nucleotide available from the non-supplemented commercial formulation covered the nutritional requirements for the general performance of ballan wrasse. Further results will be presented, including molecular markers’ response to the immune challenge, to ascertain the benefits of nucleotide supplement for the ballan wrasse subjected to an immune stressor, in order to prepare them for the cage transfer.

References

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