• Freshwater environments contain very low mineral salt concentrations (<0.5 ppt), whereas the internal body fluids of freshwater fishes maintain an osmolarity equivalent to approximately 10 to 14 ppt.
• Consequently, the internal body fluids of freshwater fishes are hypertonic (more concentrated) relative to the surrounding hypotonic water.
• Water naturally enters their bodies by endosmosis, which freshwater fish manage by actively pumping salts inward via gill chloride cells and discharging copious volumes of dilute urine.
• Therefore, in freshwater, fish body fluid is More concentrated (Hypertonic) (Option 1).

• Indian Major Carps (IMC) comprise three indigenous cyprinid fish species of the Ganga and Brahmaputra river systems:
• Catla (Catla catla) - surface feeder
• Rohu (Labeo rohita) - column feeder
• Mrigal (Cirrhinus mrigala) - bottom feeder.
• Silver carp (Hypophthalmichthys molitrix), Grass carp (Ctenopharyngodon idella), and Common carp (Cyprinus carpio) are exotic carps introduced from outside India.
• IMCs form the backbone of freshwater pond aquaculture in India.
• Therefore, Indian major carps are Rohu, Catla and Mrigal (Option 1).

Stocking ratios in composite fish culture of Indian Major Carps (IMC):

• 3:4: Landmark recommendation by Alikunhi (1957) for 3-species composite fish culture: $30\%$ Surface feeder (Catla catla), $30\%$ Column feeder (Labeo rohita), and $40\%$ Bottom feeder (Cirrhinus mrigala). In the absence of the alternative 4:3:3 ratio in the options, 3:3:4 is the keyed correct option.
• 3:3: Modern ICAR-CIFRI standardized ratio for ponds with rich plankton and moderate benthic biota (Catla 40%, Rohu 30%, Mrigal 30%).
• 4:4: Sub-optimal ratio underutilizing surface plankton.
• 4:3: Column-dominated stocking pattern.