GROWTH AND SURVIVAL OF ROHU Labeo rohita SPAWN IN BIOFLOC SYSTEM AT DIFFERENT STOCKING DENSITIES

Rohu (Labeo rohita) is the most preferred freshwater fish species cultivated in Indian subcontinent. During nursery rearing of rohu, a survival of 30 - 40% of fry is normally achieved in earthen nursery ponds and still higher survival levels of 50 - 60% is achieved in concrete nursery tanks. The survival can be improved by minimizing predation and by carrying out culture in controlled indoor system. Therefore, an attempt was made to rear rohu spawn in biofloc culture system which provides nutrition on continual basis, requires less water, less labour and yields high survival and production.

A 20-day lab scale experiment was performed to investigate the growth and survival of rohu (Labeo rohita) spawn in biofloc system at different stocking densities. The experiment was conducted in 100 L circular HDPE indoor tanks. Spawn in the size range of 6 to 8.5 mm length and 1.5 to 1.57 mg weight were stocked at five different densities, viz. 1 (D₁), 2 (D₂), 3 (D₃), 4 (D₄) and 5 (D₅) spawn L^-1 in biofloc based units with four replicates for each treatment, following completely randomized design. The treatment with stocking density of 1 spawn L^-1 without biofloc and aeration served as control. Sugar (42.45% C) was added daily into the biofloc treatments as a carbon source at an estimated C:N ratio of 20:1. The spawn were fed powdered groundnut oil cake at a feeding rate of 400% of body weight for first 5 days and then 800% of body weight for next 15 days.

Among the treatments, the treatment D₁ showed significantly (P < 0.05) better growth performance in terms of length gain, weight gain and specific growth rate after 20 days duration (Table 1). The survival rates were significantly (P < 0.05) higher in D₂ and D₁ compared to the other treatments.

Emphasizing on the parameters like size attained by fry (20-25 mm) and yield of appropriately sized fry per unit area, the stocking density of 2 spawn L^-1 is found to be optimum for nursery rearing of rohu in biofloc system for a period of 20 days.