Aquaculture is growing rapidly worldwide and is projected to become the major source of fish used for human consumption. A major factor that limits aquaculture reaching its full potential is an adequate supply of the raw materials necessary for formulated fish feeds. The dependence of modern aquaculture on fishmeal obtained from wild fisheries is not environmentally sustainable and replacements for fishmeal must be found. Some animal by-products are viable replacements for fishmeal, and can provide sufficent nutrition for high growth rates, but little is known about the potential of animal by-products to adversely affect fish health. The objectives of these experiments were to determine if animal by-products used in fish feeds impair immune response or alter bone physiology in cultured juvenile rainbow trout. Four animal by-product containing diets (poultry meat meal (PMM)/ PMM plus feathermeal / PMM plus bloodmeal) and two reference diets (fishmeal or soya) were evaluated to determine their effect on innate immune response, the ability of fish to cope with normal husbandry stressors, and bone physiology. PMM was then selected due to its favourable amino acid profile and high digestibility and assessed to determine if the high levels of fishmeal replacement that may be required in the future, impact the health of rainbow trout. Due to the lack of reliable indicators of bone quality and quantity in salmonids the effects of exercise and phosphorus deficiency in rainbow trout were also examined. Relative to the fishmeal control diet, fish fed diets with PMM [(PMM) 50% crude protein, by substitution], PMM plus two percent blood meal, or PMM plus five percent feather meal, did not have an impaired innate immunity (lysozyme, alternative complement, phagocytosis, intracellular respiratory burst, differential counts of peripheral blood leukocytes) or changes in bone physiology as assessed by dynamic bone histomorphometry. Higher levels of PMM (0-70% digestible protein, by substitution) caused a reduction in apparent net mineral retention of phosphorus and calcium (P<0.001), a lower vertebral bone mineral content (P<0.001) and reduced vertebral mechanical properties (compressive extension (P=0.04), Young’s Modulus (P=0.03)), but fish growth was not affected. Exercise influenced bone modelling, with exercised animals having a reduced bone area and trabecular thickness (P=0.01), increased autocentrum width (P=0.04), and higher bone mineral content (P= 0.02); however, bone mechanical properties were unaffected. Induction of genes (receptor activator nuclear factor kappa beta and osteoprotogenerin), involved in the resorption of mineralised tissue, was not observed in fish fed phosphorus deficient diets although scales were evidenced to be an important source of labile minerals. Overall our results indicate that low level replacement of fish meal by poultry meat meal, and blends of poultry meat meal with blood or feathermeal do not affect fish innate immune response, bone physiology, or growth however the greatly elevated levels of poultry meat meal that may be required in future salmonid aquafeeds could increase the risk of spinal malformations. Thus the category 3 animal by products tested are valuable fishmeal replacements for aquaculture based on the endpoints measured in this study.

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