• Evaluation of Statements on Fish Stress Physiology:
• Statement I: Fish can exhibit a cumulative response to repeated stressors.
• True. Repeated or chronic exposure to consecutive acute stressors (handling, hypoxia, temperature spikes) leads to cumulative physiological wear-and-tear (allostatic load), potentially progressing to tertiary stress responses including suppressed immunity, impaired growth, and reproductive arrest.
• Statement II: Repeated exposures to mild stressors can attenuate the neuroendocrine and metabolic responses to subsequent exposure to stressors.
• True. Habituation (stress conditioning / hormesis) occurs when fish repeatedly encounter mild, non-lethal stressors, resulting in diminished Hypothalamic-Pituitary-Interrenal (HPI) activation, reduced cortisol surges, and faster metabolic recovery.
• nalysis:
• (Both Statement (I) and Statement (II) are correct): Correct.

Correct option: (1) Both Statement (I) and Statement (II) are correct

• Mechanism of Branchial Nitrogenous Excretion:
• Marine teleost fishes eliminate the bulk ($\gt 80\%$) of their metabolic nitrogen as ammonia across the gills in both molecular ($NH_3$) and ionized ($NH_4^+$) forms:

• Un-ionized Ammonia ($NH_3$): Highly lipid-soluble, diffuses passively down its partial pressure gradient across apical and basolateral membranes, facilitated by branchial Rhesus (Rh) glycoproteins (Rhag, Rhbg, Rhcg).
• Ammonium Cation ($NH_4^+$): Excreted across the gill epithelium through paracellular pathways driven by the transepithelial electrical potential, and via apical $Na^+/NH_4^+$ exchange.
  • nalysis:

• (Both NH3 and NH4+): Correct (represented in text as NH3 and NHs+).

• The Master Driving Engine of Ion Transport:
• In branchial and intestinal epithelial cells, the basolateral $Na^+/K^+$-ATPase (sodium-potassium pump) couples ATP hydrolysis to pump $3\ Na^+$ ions out of the cell in exchange for $2\ K^+$ ions pumped inward.
• This active transport maintains a low intracellular $Na^+$ concentration and a negative intracellular membrane potential, generating a powerful inward electrochemical $Na^+$ gradient.
• This gradient provides the essential driving force for secondary active cotransporters:
• $Na^+/Cl^-$ cotransporter (NCC) in freshwater ion absorption.
• $Na^+/K^+/2Cl^-$ cotransporter (NKCC1 / NKCC2) in seawater ion secretion and intestinal fluid absorption.
• nalysis:
• (Na+/K+-ATPase): Correct.

Correct option: (4) Na+/K+-ATPase