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

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 ac cording to the Lake Kasumigaura Standard (crude protein <35%, digestible en ergy >3.5 kcal/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 o ne of the Rye experimental diets to satiation, three times a day for 12 wee ks, at 20.0+/-1.6 degrees C. T-P from each group was determined based on wa ter extractable P and the rate of P absorbed and retained in carp. The grow th performance was highest in fish fed the diet containing 25% fish meal fo llowed by 30%, although there was no marked difference between the treatmen ts 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 re duced by decreasing dietary fish meal levels, and ranged from 5.9 for the 1 0% 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 estimate d to be 20-25%, under the present feeding conditions, substituted by a comb ination of blood meal and defatted soybean meal.