ICAR AIEEA PG 2024 - Fisheries Science

Bony fishes (teleosts) are predominantly ammonotelic organisms. They excrete 80% to 90% of their nitrogenous metabolic waste in the form of toxic, highly soluble un-ionized ammonia ($NH_3$), which readily diffuses passively across the branchial gill epithelium into the surrounding water.

In mitochondrial $\beta$-oxidation of saturated even-numbered fatty acyl-CoA chains, each repetitive four-step cyclic cleavage sequentially releases a 2-carbon fragment in the form of Acetyl-CoA ($CH_3-CO-SCoA$), which directly enters the Krebs citric acid cycle for ATP generation.

In nutritional bioenergetics, dietary energy partitions through progressive losses:

$$\text{Gross Energy (GE)} \xrightarrow{-\text{Fecal loss}} \text{Digestible Energy (DE)} \xrightarrow{-\text{(Urinary + Gill branchial loss)}} \text{Metabolizable Energy (ME)}$$

Because energy is lost in nitrogenous urinary and branchial excretions, Digestible Energy is strictly greater than Metabolizable Energy ($\text{DE} \gt \text{ME}$).