Introduction
Fish meal has been progressively replaced by plant ingredients in salmonid feeds to good effect with respect to macro-nutrients. However, metabolic consequences of altered micro-nutrient supply is less studied. It is known that Zn and Cu are important essential trace elements and involved in several metabolic reactions . Although the role of dietary Zn and/or Cu in influencing lipid metabolism is beginning to be understood in teleost (Zheng et al., 2015; Meng et al., 2016), their practical implications for aquaculture are less clear. It was shown that LC-PUFA bioconversion in fish is improved by a multi-micronutrient supplement including Zn (Lewis et al., 2013; Giri et al., 2016). The requirement for Zn and Cu to rainbow trout is 37 mg Zn/kg feed and 5 mg Cu/kg feed, respectively. Plant based salmonid feeds are limited in Zn supply, while provide excess Cu. These dietary changes warrant investigations on the functional importance of Zn and Cu nutrition at critical windows of early life stages on their impact during later stages. In this context, we hypothesized that the metabolic capacity of fish towards utilization of dietary lipids might be altered by early nutritional history of dietary Zn or Cu.
Material and methods
The feeding trial lasted 24 weeks and had three phases. Growth was recorded every 3 weeks, mortality and feed intake were monitored daily. In Phase I, 200 first feeding rainbow trout fry (52 ± 3 mg) were fed for 6 weeks with one of the four experimental feeds with two levels of Zn (Zn-, 70 or Zn+, 130 mg/kg) or Cu (Cu-, 10 or Cu+, 20 mg/kg) in a 2 x 2 factorial design , in triplicate. At the end of phase I, ninety fish per tank were sampled for growth and body composition analysis. In Phase II, all the groups were fed with a commercial feed for the next 12 weeks. In Phase III, all the groups were challenged with the Zn-Cu- diet for another 6 weeks. At the end of Phase II and III, eight fish per tank were sampled for plasma metabolite, liver fatty acid profile and gene expression, and whole-body proximate composition analysis.
Results
Growth: Dietary Zn supplementation significantly increased mean body weight and whole-body Zn levels, while body lipid and gross energy level was reduced by Cu supplementation at the end of Phase I. In Phase II, when all groups were fed the commercial feed, no differential effect was observed on growth or body composition . In phase III, when the fish were re-introduced to the Zn-Cu- diet, growth differences became significant. Rainbow trout fed Zn supplemented diet during first feeding (Zn+ and Zn+Cu +) showed significantly higher weight gain and final weight at the end of phase III. On the contrary, they also had significantly low whole-body Zn levels.
Body trace element status: Fish fed Zn+ diets had significantly increased body Zn status at the end of phase I, which was no more apparent at the end of phase II and the contrary was significant at the end of phase III. Dietary Cu did not affect body Cu status at any stage of the experiment. Dietary Mn concentrations were reduced in fish fed Zn+ and Cu+ until end of phase II, but the effect disappeared in phase III when all the groups were fed Zn-Cu- diet.
Plasma lipid metabolites: Circulating levels of total cholesterol and phospholipids in the plasma at the end of phase III were significantly lower in fish with Zn+ early history. No effect was seen on plasma glucose, triglycerides or non-essential fatty acid concentrations.
Hepatic lipid metabolism: A lthough total lipid content was the same , the fatty acid profile in the polar lipids was affected by early stage Zn or Cu nutrition. Notably, 18:3n-6 , 18:4n-3 and 22:5n-3 were lower in fish with Zn+ history . Whereas, 20:2n-6 and 20:3n-3 were lower with Cu+ history . E xpression of genes involved in LC-PUFA biosynthesis especially those involved in the fatty acid chain elongation (elovl2 and elovl5 ) and desaturation (d6d and d9d ) were downregulated by Zn+ early history.
Discussion
In plant-based feeds, increased dietary Zn at first feeding not only improved the growth of rainbow trout fry, but also enhanced growth when sub-optimal Zn supply wa s encountered later in the juvenile stage. Growth dilution due to enhanced growth in Zn+ fed fish reduced whole body Zn levels at the end of phase III. Impact of Zn or Cu restriction at first feeding on hepatic fatty acid profile and expression of genes in intermediary metabolism at juvenile stage indicate of a possible programming effect, warranting further investigation .
Acknowledgements
This work was financially supported by AQUAEXCEL2020 research infrastructure project funded under the EU’s Horizon 2020 programme.
References
Zheng, J.-L., Luo, Z., Hu, W., Liu, C.-X., Chen, Q.-L., Zhu, Q.-L. & Gong, Y. (2015) Different effects of dietary Zn deficiency and excess on lipid metabolism in yellow catfish Pelteobagrus fulvidraco. Aquaculture, 435, 10-17.
Meng, F., Li, M., Tao, Z., Yuan, L., Song, M., Ren, Q., Xin, X., Meng, Q. & Wang, R. (2016) Effect of high dietary copper on growth, antioxidant and lipid metabolism enzymes of juvenile larger yellow croaker Larimichthys croceus. Aquaculture Reports, 3, 131-135.
Giri , S.S., Graham, J., Hamid, N.K.A., Donald, J.A. & Turchini, G.M. (2016) Dietary micronutrients and in vivo n − 3 LC-PUFA biosynthesis in Atlantic salmon. Aquaculture, 452, 416-425.
Lewis, M.J., Hamid, N.K.A., Alhazzaa , R., Hermon, K., Donald, J.A., Sinclair, A.J. & Turchini , G.M. (2013) Targeted dietary micronutrient fortification modulates n − 3 LC-PUFA pathway activity in rainbow trout (Oncorhynchus mykiss). Aquaculture, 412–413, 215-222.