Utilization of fish and animal by-product meals in low-pollution feeds for rainbow trout Oncorhynchus mykiss (Walbaum)

The apparent digestibilities (availabilities) of dry matter, protein, phosphorus and selected minerals in fish and animal by‐products were determined using rainbow trout Oncorhynchus mykiss (Walbaum). Blood meal (ring‐dried), feather meal and deboned fish meal had relatively high concentrations of protein, low concentrations of phosphorus and many minerals, and high digestibilities (availabilities) of these nutrients. Other animal by‐products, however, had high concentrations of minerals, including phosphorus, which are associated with the bone fraction. Availabilities of manganese and zinc in the diet were reduced by the inclusion of high‐ash animal by‐products in the diet, whereas availabilities of potassium, sodium and copper were relatively unaffected. Dietary concentrations of bone minerals (calcium, phosphorus) and ash were inversely correlated with availabilities (% of intake) of most minerals except copper in the diet. Also, dietary concentrations of bone minerals correlated inversely with the net absorption (mg g^?1 diet) of zinc, manganese and magnesium in the diet. When rainbow trout were fed diets containing incremental concentrations of fish bones, the apparent availabilities of phosphorus, calcium, magnesium and iron decreased as fish bone content in the diet increased. Reducing the bone fraction of high‐ash (high‐phosphorus) by‐product meals is therefore an essential approach to using such ingredients in low‐pollution fish feeds.