Introduction
Zinc (Zn) plays the essential role in vertebrates including fish as it could exert various physiological function such as catalytic, structural and regulatory (Lall and Kaushik, 2021). Due to the limited availability of marine resources, aquatic feed increasingly utilizes plant-based resources as ingredients substitutes. However, compared to fish meal, plant protein contains lower Zn level (Prabhu et al. 2016). Besides, plant-based resources contain antinutritional factors such as phytate, which can bind with Zn2+ during absorption and thus reduces the Zn utilization of fish. On the background of applying plant-based sources in aquatic feeds, there are two strategies to meet the Zn requirements of Atlantic salmon: increasing the level of dietary Zn and increasing the availability or utilization of Zn. Increased dietary Zn levels can improve the welfare of Atlantic salmon, but this strategy poses environmental challenges since less than 15% of the dietary Zn is retained in the body. In this regard, the present study aimed to investigate the interaction between dietary Zn and fat on Zn utilization in Atlantic salmon.
Material and Methods
Four experimental diets were produced by BioMar with difference levels of fat (high fat (HF, 35%) or low fat (LF, 32%)) and Zn (high zinc (HZ, 180 mg kg -1) or low Zn (LZ, 120 mg kg -1)), coded as HFHZ, HFLZ, LFHZ and LFLZ respectively. In trial 1, Atlantic salmon (initial weight: 741 ± 77.3g) was fed one of four experiment diets at Matre Research Station by indoor tank for 4 weeks and then the fish plasma were collected at different post-prandial time points (0, 2, 4, 8, 14, 24, 36h) for Zn level analysis. The calculating method of area under the curve (AUC) in trial 1 was based on the principle of trapezoidal rule (Prabhu et al., 2014). In trial 2, Atlantic salmon were fed with above description feeds at the GIFAS, Norway in outdoor net pens from approximately 1 kg to 4.5 kg. Whole fish and different tissues such as plasma, liver, head kidney, muscle, bone were sampled mid-way and at the end of the trial for Zn retention analysis. The Zn analyses of the feed and fish samples were performed through ICP-MS.
Results
In trial 1, the post-prandial profile of plasma Zn showed a peak 24h after the meal. Salmon fed the high-Zn diets had higher baseline plasma Zn than those fed the low-Zn group (Figure 1). Furthermore, significantly higher plasma Zn-AUC value was found in low-fat diet group compared to high-fat diet group. In trial 2, Atlantic salmon in high-Zn group had high Zn level in plasma, but not in liver, muscle, bone or whole fish at mid sampling. At the end of the trial, fish in high-fat and low-Zn (HFLZ) group had the lowest Zn level in their plasma and liver (Figure 2). The low-Zn groups had a drop of around 30-50% of their Zn levels in different tissues between mid and final sampling.
Discussion and conclusion
Sub-optimal dietary Zn supplementation may not reduce the fish growth, but it did impact on the Zn level in tissues and whole fish (Antony Jesu Prabhu et al., 2018). Therefore, growth may not be the only criteria on Zn statues evaluation in fish. Antony Jesu Prabhu et al., (2014; 2016) proposed that the AUC of post-prandial plasma Zn could reflect the absorption and utilization of minerals while Zn retention in whole-body and vertebra might be the better indexes in long-term experiment. In the present study, compared to the high-fat diet group, low-fat dietary intervention fish obtained the higher AUC value of post-prandial plasma Zn in trial 1 as well as higher plasma and liver Zn concentration in trail 2, indicating low-fat diet could enhance the Zn absorption and utilization in Atlantic salmon. The present study provides a new perspective on improving Zn utilization in Atlantic salmon. Other findings in this experiment will be presented along with further data from ongoing analyses.
Reference
Antony Jesu Prabhu, P., Geurden, I., Fontagné-Dicharry, S., Veron, V., Larroquet, L., Mariojouls, C., Schrama, J.W., Kaushik, S.J., 2016. Responses in micro-mineral metabolism in rainbow trout to change in dietary ingredient composition and inclusion of a micro-mineral premix. PLoS One 11, e0149378.
Antony Jesu Prabhu, P., Schrama, J.W., Fontagné‐Dicharry, S., Mariojouls, C., Surget, A., Bueno, M., Geurden, I., Kaushik, S.J., 2018. Evaluating dietary supply of microminerals as a premix in a complete plant ingredient‐based diet to juvenile rainbow trout (Oncorhynchus mykiss). Aquac. Nutr. 24, 539–547.
Lall, S.P., Kaushik, S.J., 2021. Nutrition and Metabolism of Minerals in Fish. Animals 11, 2711.
Prabhu, P.A.J., Schrama, J.W., Mariojouls, C., Godin, S., Fontagné-Dicharry, S., Geurden, I., Surget, A., Bouyssiere, B., Kaushik, S.J., 2014. Post-prandial changes in plasma mineral levels in rainbow trout fed a complete plant ingredient based diet and the effect of supplemental di-calcium phosphate. Aquaculture 430, 34–43.