Comparative Growth, Economic Profitability and Effluent Quality of Shrimp–mullet Polyculture Versus Grey and Tade Mullet Monocultures in Brackishwater Ponds

The comparative economic benefits and effluent quality from shrimp–mullet polyculture (SM), grey mullet (Mugil cephalus) monoculture (GM) and tade mullet (Liza tade) monoculture (TM) were evaluated at similar production levels of 2.5 ± 0.2 tonnes ha⁻¹ over a seven-month culture period. The experiment was conducted at Raidighi, South 24 Parganas, West Bengal, India, in nine rectangular earthen ponds (0.15–0.2 ha). The ponds were prepared following standard procedures, and 20-30 percent of the water was exchanged during each lunar cycle throughout the culture period. Tiger shrimp (Penaeus monodon), grey mullet and tade mullet were stocked at 3, 0.4 and 0.8 individuals m⁻², respectively in the SM system. Grey mullet and tade mullet were stocked at 0.8 and 1.6 numbers m⁻² in GM and TM, respectively. Shrimp PL-20 (0.02 ± 0.003 g) were stocked in SM. Initial weights of grey mullet and tade mullet fingerlings were 6.23 ± 1.71 g and 4.22 ± 0.95 g, respectively. Shrimp and fish were fed formulated floating pellet feed (26 percent crude protein) at 8–2 percent of the estimated biomass. At harvest, tiger shrimp attained 41.66 ± 2.91 g. Grey mullet grew to 304.43 ± 11.09 g in SM and 336.58 ± 14.77 g in GM. Tade mullet grew to 158.25 ± 6.99 g in SM and 187.38 ± 11.49 g in TM. Total production was 2607, 2473 and 2543 kg ha⁻¹, with benefit–cost ratios of 2.21, 1.42 and 1.81 in SM, GM and TM, respectively. Effluent nutrients and plankton density were lower in SM, indicating more efficient nutrient utilisation and better economic returns.