Utilization of acid hydrolysed phosphorous from herring bone by-products in feed for Atlantic salmon (Salmo salar) start-feeding fry

Summary

The aim of this study was to test the efficacy of a new marine P ingredient from the bone fraction of herring by-products by evaluating the dietary impacts on growth, mineralization and skeletal development in Atlantic salmon fry. Five experimental diets were produced from a fish meal based diet, only differing in the dietary P source and level; low P control (5.5 g/kg soluble P), intermediate P (6.5 g/kg soluble P) and high P (8.0 g/kg soluble P) from either fish bone hydrolysate (FBH1, FBH2) or from NaH2PO4 (NaP1, NaP2). The diets were given for 168 days from start of feeding (0.17 g fish) until 24 weeks of feeding (33 g). Weight and specific growth rate did not reveal diet dependent differences except in the initial 10 week feeding period where fish fed FBH1 showed lower growth as compared to fish fed the Na–P diets. Mineralization of fish evaluated by whole body and bone ash and mineral contents showed diet dependent differences that were related to dietary P level, but not to P source. Fish fed the low P diet showed clear P deficiency signs with significantly reduced tissue ash and mineral content, reduced whole body Ca:P ratio and morphological deviation from the normal. Histological evaluation of the vertebrae after 19 weeks of feeding (15 g) revealed stagnation in cartilage development, with accumulation of mature chondrocytes in fish fed low P control and FBH1 diet. FTIR showed that fish fed FBH1 had lower mineralization and increased cross binding in the vertebral end plates. Fish fed FBH2 resembled the positive NaP1 control. Real time qPCR analyses confirmed the histological results, by showing up-regulating of col10a1 (a marker for mature cartilage) in fish fed low P diet and FBH1. Osteocalcin (a marker for mineralization) was also activated in fish fed the low P diet and FBH1, possibly indicating a compensatory regulation in response to inadequate P. The dietary impacts on histology, FTIR and qPCR analyses in the FBH1 fed fish disappeared at 24 weeks of feeding (33 g). In conclusion, the results showed that P solubilized from bone fraction of herring can be efficiently utilized for growth, mineralization and bone development in Atlantic salmon fry. However, in the initial weeks of feeding, the FBH was a less efficient P source compared to NaP. Overall, the data suggest that P from fish bone hydrolysate is more suited for fish of minimum 15 g size. Statement of relevance The phosphate (P) rock reserve is a limited resource worldwide. Increased efficiency in the utilization of P and improved recycling of P from waste and manures are examples that could reduce the industry's vulnerability to the limited P supply while also reducing the negative environmental impacts. Recycling of P from fishery offal and development of new available P ingredients to farmed fish will significantly improve the sustainability of the aquaculture industry and reduce the environmental loss. In this paper we show that P hydrolysed from herring bone by-product is as efficient and good for salmon fry as commercially available NaP-salts.