Principle of Fish Nutrition

• Soil Liming in Carp Culture:
• Acidic pond soils ($\text{pH} \lt 6.5$) severely inhibit microbial mineralisation, fix soluble phosphorus into insoluble aluminium/iron phosphates, and depress natural plankton productivity.
• Quick Lime (Calcium Oxide, $\text{CaO}$): Applied to aggressively neutralize soil acidity, rapidly raise bottom soil $\text{pH}$ to the productive alkaline range ($6.5 - 7.5$), release fixed phosphates, and disinfect the pond bottom against pathogenic spores and parasites.
• Correct Option: Option 3 (Quick lime).

• Classification of Water Hardness:
• Water hardness is quantified as the equivalent concentration of calcium carbonate ($\text{CaCO}_3$) in $\text{mg/L}$ or $\text{ppm}$:
• Soft Water: $\lt 75\text{ ppm } \text{CaCO}_3$
• Moderately Hard Water: $75 - 150\text{ ppm } \text{CaCO}_3$
• Hard Water: $150 - 300\text{ ppm } \text{CaCO}_3$
• Very Hard Water: $\gt 300\text{ ppm } \text{CaCO}_3$
• Therefore, $75\text{ ppm}$ represents the recognized boundary dividing soft water from hard water.
• Correct Option: Option 3 (75 ppm).

• Iron Transport and Transferrin in Fishes:
• Transferrin: A monomeric serum glycoprotein ($70 - 80\text{ kDa}$) that plays a central physiological role in iron metabolism in teleosts.
• It binds two ferric iron ions ($Fe^{3+}$) with high thermodynamic affinity and transports iron systemically to erythropoietic tissues and hepatocytes.
• By sequestering free extracellular iron, transferrin also functions as an innate humoral immune barrier, denying iron to invading pathogenic bacteria (nutritional immunity).
• Correct Option: Option 1 (Transferrin).

• Mineral Roles in Fish:
• Calcium ($Ca$) and Phosphorus ($P$) are macro-minerals that together constitute over $70\%$ of the inorganic ash content of fish.
• They form the crystalline hydroxyapatite [$Ca_{10}(PO_4)_6(OH)_2$] matrix that provides mechanical strength and rigidity to skeletal bones, fin rays, scales, teeth, and crustacean exoskeletons.
• While freshwater fish can absorb sufficient calcium from the surrounding water across gills, phosphorus is poorly available in water and must be supplied in adequate quantities through the diet.
• nalysis:
• (Structural component): Correct.
• Options 2, 3, 4: Secondary or non-defining physiological functions relative to skeletal structure.

Correct option: (1) Structural component