Effect of dietary fish meal levels on environmental phosphorus loading from carp culture

Abstract

The present study was carried out to reduce the total phosphorus loading (T-P) from carp by replacing dietary fish meal with alternate proteins (blood meal and defatted soybean meal). The experimental diets were formulated according to the Lake Kasumigaura Standard (crude protein <35%, digestible energy >3.5kcal/g) and contained fish meal at the levels of 10, 15, 20, 25, and 30%. The dietary total phosphorus (P) contents were between 0.9 and 1.4%, and water extractable P (available P for carp) ranged from 0.55 to 0.70%. Duplicate groups (n=50) of carp (average weight 2.2±0.3 g) were fed one of the five experimental diets to satiation, three times a day for 12 weeks, at 20.0±1.6°C. T-P from each group was determined based on water extractable P and the rate of P absorbed and retained in carp. The growth performance was highest in fish fed the diet containing 25% fish meal followed by 30%, although there was no marked difference between the treatments except for the 10% fish meal group. The feed gain ratio ranged between 0.96 and 1.14. The values for T-P (kg/t carp production) were effectively reduced by decreasing dietary fish meal levels, and ranged from 5.9 for the 10% fish meal diet to 9.6 for the 30% fish meal diet. However, low fish meal diets with high amounts of alternative proteins resulted in high nitrogen loading, therefore, a suitable level of fish meal in carp diet was estimated to be 20-25%, under the present feeding conditions, substituted by a combination of blood meal and defatted soybean meal.