Introduction
Inorganic minerals are routinely added in commercial fish feeds, and there is increasing evidence that different trace mineral and phosphorous sources have different bioavailability (Maage and Sveier, 1998; Standal et al., 1999) and physiological effects (Berntssen et al., 2018). The relative levels of naturally occurring and added trace minerals in different forms will vary in diets changing from marine to plant based with unknown consequences for fish physiology. Skin ulcers and low stress tolerance are likely to be linked to sub-optimal mineral and vitamin nutrition, whereas poor smoltification and transfer performance may be linked to inadequate essential amino acid and mineral status of the fish. Fillet quality degrading conditions, as for instance gaping, liquid losses, suboptimal pigmentation, and melanin spots may also be affected by variable mineral nutrition status of the fish. There is a trend in nutrition of farmed land animals, such as poultry and swine, to replace inorganic trace mineral sources with lower amounts of bioavailable organic trace mineral sources which results in minimizing the excreted amounts of minerals, such as copper (Cu), in the environment, but also have the potential and have shown improvements in trace mineral-associated functionalities (Abdallah et.al., 2009). Organic Se sources have been shown to be assimilated more efficiently than inorganic compounds and are less toxic (Pacitti et al., 2016; Silva et al., 2019). Also, there are studies which have reported better Se apparent availability and Se retention in fillet of salmon fed organic Se diets (Sele et al., 2018; Silva et al., 2019). Reevaluation of organic minerals requirements may be necessary as studies has shown reduced levels required when organic minerals were used (Apines et al., 2003; Lin et al., 2010; Pierri Bruno da Silva et al., 2021). For example , Cu requirements for juvenile grouper, Epinephelus malabaricus , were reduced in half when organic Cu was included in the diets (Lin et al., 2010). Regarding mineral bioavailability, emphasis should also be placed, on the release rate of minerals to the environment when difference mineral sources are used. Russel et al. (2011) outlines the excess use of Cu and zinc (Zn) in Scottish Atlantic salmon farming, with concentrations 3-4 times more than required. In the same study, they concluded that Cu and Zn elevated levels on the sediment may cause adverse effect on the local environment around the sea farm. The scope of this study was to compare lab scale and commercial production results in salmons fed diets with either organic (OM) or inorganic (IM) trace minerals in different inclusion levels and their effects on Atlantic salmon performance, skin health and tissue mineralization.
Materials and Methods
In lab scale, 18 groups of Atlantic salmon smolt with initial mean body weight 150g were fed for 12 weeks 1 of 8 experimental diets with 3 replications. At trial end mean fish body weight was approx. 450g. The diets contained either organic or inorganic mineral premixes in 4 dietary supplementation levels (Se: 1.2-1.5ppm, Cu: 10-24ppm, Mn: 55-100ppm, Zn: 80-180ppm and Fe: 300-500ppm). The trial design included an undisturbed feeding period at start and end and 3 weekly handling stress treatments half-way the experimental period. F eeding rates, FCR, biometrics, fillet and skin technical quality, tissue mineral levels and skin histology were evaluated. In the commercial production trials we used salmon of approx. 500g body weight at start to slaughter, fed 4 different diets containing either organic or inorganic minerals at two supplementation levels (Se : 0.6-0.8 ppm, Cu: 17-25 ppm, Mn: 65-85 ppm, Zn: 180 ppm and Fe: 226-275ppm) and 2 replicate cages per treatment. All other farming operations were based on common praxis including for instance lice treatments . The mineralisation of whole body and different tissues (liver, skin, fillet, gills, and spleen) was studied at trial start, mid (when fish weighed approx. 2kg) and end (4.5 kg ). Biometric and welfare measurements were taken at mid and end sampling, and liver and skin histology were evaluated at trial end. Growth and FCR at sampling points in commercial production were estimated based on collected daily farm feeding data and biomass and mean fish body weights at slaughter.
Results and Discussion
There is an increasing interest of comparing the bioavailability of organic and inorganic minerals in fish diets, but the available data are still scarce and inconsistent (Dominguez et al. 2017; Antony Jesu Prabhu et al. 2016). Our results can fill some gaps on the bioavailability of organic and inorganic minerals and highlight the difference between mineral accumulation on tissues based on their origin (organic/inorganic) and dietary level. In lab scale, we saw significantly higher fish performance , in the organic as compared to the inorganic mineral treatments. A significantly positive correlation between dietary organic mineral level and skin Zn was also observed. Whole body mineral composition did not differ for the two mineral forms, with the exception of Se which showed a tendency for higher accumulation in the organic mineral treatments, whereas a significant negative correlation between organic Cu supplementation and whole body Cu was seen. Accordingly, i n a meta-analysis by Antony Jesu Prabhu et al. (2016), it was highlighted that organic forms of Se, like SeMet and Se yeast, are more bioavailable compared to selenite; however, for other trace minerals like Zn and Mn the published results were conflicting. Comparative results with the cage trial data will be presented.
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