Replacement of fish meal in diets for Australian snapper, Pagrus auratus

Abstract

The potential for replacing fish meal with soybean and poultry offal meals in the diets of Australian snapper was assessed. Four diets with 50% crude protein were formulated using fish meal, soybean meal and poultry offal meal as sources of protein. The control diet had 64% fish meal. The other three diets had 30, 20 or 10% fish meal with the remaining crude protein contributed by a combination of soybean meal and poultry offal meal. Weight gain of juvenile snapper (77 g mean initial weight) decreased as the amount of fish meal decreased below 30%. The relationship was best described by the equation Y= AX/( X+ B) where Y is weight gain, X is fish meal content (%) and, A and B are fitted constants. Similar results were achieved when snapper were reared under improved conditions for growth, i.e., at water temperatures 3–5°C above ambient. At the higher water temperatures, growth rate almost doubled. Apparent digestibility coefficients for energy, phosphorus, crude protein and amino acids were determined for the diets with 64 and 30% fish meal. Small but significant ( P<0.05) differences were found for energy, phosphorus and methionine availability. However, the differences in digestible energy and available nutrients were negligible since both diets satisfied published requirements for warmwater fishes. The data on growth and FCR were used in a simple economic model which considered fish meal content of the diet, asymptotic growth rate and relative production costs. This model predicted that relative production costs were: (1) reduced substantially when asymptotic growth rates increased from 0.4 to 0.8 g per day, and (2) relatively insensitive to replacing up to 50% of fish meal in the diets by soybean meal and poultry meal.