Spray-dried blood cells as a partial replacement for fish meal in diets for rainbow trout
Because many marine fisheries resources used for preparation of fish meal have reached or exceeded optimum sustained yield and attendant economic response has increased their prices, researchers have given substantial attention to replacements for fish meal in animal feeds. At the same time, there is rising concern over phosphorous (P) content of fish feed because P in excess of the dietary requirements, or in a form that is not digestible, will be eliminated and cause eutrophication problems in aquatic systems. Thus, another reason for finding replacements for fish meal, which is relatively high in total P, is to develop feeds less polluting feeds, that is feeds which contain digestible forms of P and in concentration sufficient to meet requirements for growth. Spray-dried blood is a high protein, high lysine, and highly digestible feedstuff which is also low in phosphorus. These nutritional qualities make it a desirable candidate to reduce fish meal content of the fish feeds.In this study, rainbow trout Oncorhynchus mykiss were fed either a diet containing 34.5% herring meal and 8.75% spray-dried blood cells (SBC) as a partial replacement for herring meal or a reference diet containing 47.5% herring meal and no spray-dried blood cells. During a 12-week feeding trial, differences in growth and feed conversion were not significantly different. Apparent protein digestibility was 89.7% for the diet containing SBC compared with 88.1 % for the reference diet (P=0.02), a small but statistically significant difference. As a standard procedure to meet nutritional requirements, both diets were fortified with ferrous sulfate. Iron content of the test diet containing SBC was 149% (433 mg Fe/kg compared with 291 mg Fe/kg) greater in the reference diet, yet whole body iron content of fish in both diet treatments decreased over the study, from 23.3 mg Fe/kg initially, to 16.5 mg/kg in the reference group and to 18.4 mg/kg in the test group at the end of 12 weeks.The iron content of the diet did not affect peroxide values of the feed, which remained low throughout the 12 weeks. These findings indicate that spray-dried blood cells (SBC) are suitable for partial replacement of fish meal in rainbow trout diets. Because of its higher protein content, a 8.75% substitution level of SBC replaced 27.5% of the fish meal. Because the phosphorus content of SBC is only 0.33% compared with 1.67% in herring meal, the P content of the diet was reduced 29.3%. The higher levels of iron in the diet containing SBC did not have any adverse affect on trout performance or on oxidation of the lipids in the feed.