Minerals in fish: does the source matter?

Antony Jesu Prabhu, P. (2015). Minerals in fish: does the source matter? PhD thesis. Wageningen University, The Netherlands.Minerals are a group of micro-nutrients essential to fish. Meta-analysis of literature data was performed to identify the appropriate response criterion to determine the mineral requirement of fish. The meta-analysis revealed that, vertebral mineral concentrations or specific enzyme activities provide stringent requirement estimates compared to weight gain. Dietary intake forms the major route of mineral supply to fish; however fish are also capable of acquiring dissolved minerals from the rearing water. Changes in the dietary composition or mineral concentration of rearing water could have an impact on the mineral balance in fish. In this thesis, high fat diets, diets devoid, or low in fish meal and rearing systems with high water mineral concentrations were studied for their impact on the mineral balance in rainbow trout or common carp. Increased available phosphorus levels were needed (0.4% vs. 0.8%) in high fat diets to improve whole body and vertebral mineralisation as indicated by ash, P and Ca concentrations. However, supplementing phosphorus to complete plant ingredient based diets negatively affected the absorption and utilisation of micro-minerals namely Zn, Cu and Se. In rainbow trout that received complete plant ingredient based diets, the endogenous loss of Zn was higher and of Cu was lower resulting in Zn depletion and Cu accumulation in the body. Further, the hepatic metabolism of Fe and Cu was affected in rainbow trout fed the plant ingredient based diets, possibly due to the alterations in bile or cholesterol metabolism. With regard to the minimal dietary levels required, supplementation of Zn and Se were required beyond the levels recommended by NRC (2011) to maintain body balance in rainbow trout and common carp when fed complete plant ingredient based diets. Common carp reared in recirculation aquaculture systems (RAS) with high concentration of minerals in the water was able to acquire and retain minerals from water. However, only in the case of Se, they were able to compensate for a part of the dietary requirement. On the whole, higher dietary levels of P and Ca are required in the diet of fast growing rainbow trout; dietary levels of Zn and Se have to be increased beyond present recommended levels in plant ingredient based diets for rainbow trout and common carp. Low water exchange RAS has multiple effects on the whole animal physiology of fish and requires further research for better understanding.