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30 adult rainbow trout (0. mykiss) (202.1 ± 3.5 g SEM) in a fresh water recirculation system were fed the first test diet containing small radio-opaque glass beads 'ballotini' (0.6-0.9 mm) until all fish were satiated. The fish were re-fed at different time intervals (t= 0, 4,8, 12,24,30 and 36 hours) with the second test diet containing large size of glass beads (1.16-1.40 mm) and X-rayed. The amount of feed consumed by fish for each time interval was calculated following the quantification of the number of different size of glass beads in the developed X-ray sheets and the appetite return of trout was modelled. A first order equation best explained the appetite revival data (Feed Intake = 106.26* (1-e-0.079 time)). The times for 50 and 95 % of return of appetite in rainbow trout were observed as 8 and 28 hours, respectively. It was suggested that incorporation of markers of different sizes into various types of feed allows the amounts of each type of feed presented either simultaneously or as discrete meals offered within a 12 -hour-period.

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