Alternative methods for nutrition research on the southern bluefin tuna, Thunnus maccoyii: in vitro digestibility

Southern bluefin tuna is an important mariculture species in Australia but the development of manufactured feeds has been limited by the lack of detailed information on their nutrition. Although pelleted feeds have been developed progress has been slow. Feeding trials are difficult, time consuming and very expensive, thus complementary research methods using in vitro digestibility were developed and assessed in order to more rapidly achieve a viable manufactured feed. The in vitro crude protein and dry matter digestibility of ingredients and feeds using three enzyme systems based on purified commercial, Atlantic salmon or southern bluefin tuna digestive enzymes, were compared. Crude protein digestibility was higher than dry matter digestibility and values were highest using commercial enzymes, then using crude extracts from Atlantic salmon and lowest using crude extracts from tuna. However, the three enzyme systems gave similar relative values and were significantly (P < 0.05) correlated with each other. Atlantic salmon were used as a surrogate species to compare apparent in vivo with in vitro digestibility and, depending on the calculation of digestibility, there were positive and significant correlations (P < 0.001) between animal based protein meals for all the enzyme systems. In vitro digestibility values are now used to screen feeds and ingredients, before they are selected for use in feeding trials with tuna, and to provide digestibility values for formulating tuna feeds.