Question

Nitrogen waste excreted in the form of $NH_3$ in fish amounts to

• A. 50 %
• B. 65 %
• C. 95 %
• D. 83 %

Hint:

Remember: Gills are the primary "kidneys" for nitrogenous waste in fish. $NH_3$ is the dominant waste product because it requires the least energy to produce and is easily flushed away by the water.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question focuses on the nitrogen metabolism and excretion in teleost (bony) fishes, specifically asking for the percentage of total nitrogenous waste that is excreted as Ammonia ($NH_3$).
Detailed Explanation:

• Ammonotelism: Most teleost fishes are "ammonotelic," meaning they excrete the majority of their nitrogenous waste as ammonia. Ammonia is a byproduct of protein catabolism and is highly toxic, but it is also highly soluble in water.

• Excretion Path: Unlike terrestrial animals that convert ammonia to urea or uric acid to save water, fishes can easily diffuse ammonia directly into the surrounding water through their gills.

• Percentage Contribution: While the exact percentage can vary based on the species, diet, and environment, research shows that in a typical freshwater or marine teleost, approximately 80% to 90% (with a mean value of 83%) of the total nitrogen is excreted as ammonia. The remaining small percentage is excreted as urea, creatine, and other compounds, mostly through the kidneys (urine).

• Environmental Impact: In aquaculture, this high level of ammonia excretion is the reason why water exchange or biofiltration is critical to prevent "ammonia spikes" that can kill the fish.

Step 2: Final Answer:
In fish, nitrogen waste in the form of ammonia amounts to approximately 83%.