ASRB NET and ARS / SMS / STO (Agricultural Scientists Recruitment Board) · India · Agricultural Scientists Recruitment Board
Original practice questions written to the published syllabus · not past papersDiseases of fish and crustaceans, the role of water quality, aquaculture biosecurity, and the residue rules for therapeutants used in food fish.
In the exams: NAVLE aquatics 1%; China runs a separate aquatic-animal category of its national exam
How our questions for ASRB NET and ARS / SMS / STO spread across the units of Aquatic Animal Diseases, Health Care & Management. Difficulty: 9 easy, 27 medium, 30 hard.
| Diagnosis of aquatic animal disease | 8 | |
| Biosecurity in aquaculture | 8 | |
| Therapeutants and residues | 8 | |
| Parasitic diseases of fish and shrimp | 7 | |
| Degree-day withdrawal periods | 7 | |
| Water-quality diseases | 6 | |
| Viral diseases of fish and shrimp | 6 | |
| WOAH-listed aquatic animal diseases | 6 | |
| Bacterial diseases of fish and shrimp | 5 | |
| Fungal diseases of fish | 5 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
At a constant 25 °C (pKa 9.25), by roughly what factor does the un-ionised fraction of total ammonia increase when pH rises from 7.0 to 9.0?
C. About 64-fold
f(pH 7) = 1/(1 + 10^2.25) = 0.56%; f(pH 9) = 1/(1 + 10^0.25) = 36%. 36/0.56 ≈ 64. The tempting answer of exactly 100 (two pH units, 10 × 10) ignores that at pH 9 a large share is already un-ionised, so the fraction cannot keep rising tenfold; at low pH each unit is close to tenfold.
Further reading: Boyd & Tucker, Pond Aquaculture Water Quality Management; Emerson et al. (1975)
A laboratory reports un-ionised ammonia as 0.30 mg/L NH3-N. The permitted limit in the farm's standard is written as mg/L NH3 (as the whole molecule). What is the result on the standard's basis?
B. 0.36 mg/L NH3
NH3 = NH3-N × 17/14 = 0.30 × 1.214 = 0.36 mg/L. Multiplying by 14/17 (0.25) reverses the conversion, dividing by 17 alone gives 0.017, and leaving the figure unchanged ignores the different basis. The molecular mass of NH3 is 17 and of N is 14.
Further reading: Boyd & Tucker, Pond Aquaculture Water Quality Management; Emerson et al. (1975)
Which condition makes hydrogen sulphide in pond-bottom water more toxic?
B. A fall in pH
H2S is the toxic un-ionised form and dominates at lower pH, the opposite of ammonia, which becomes more toxic at higher pH. Higher DO oxidises sulphide, and alkalinity is not the determining factor.
Further reading: Boyd & Tucker, Pond Aquaculture Water Quality Management; Emerson et al. (1975)
In a large population, how many fish must be sampled to be 95% confident of detecting at least one infected fish if prevalence is 2% and the test is perfect?
B. About 150
n = ln(0.05)/ln(1 − 0.02) = −2.996/−0.0202 = 148.3, so 149, usually quoted as 150. About 300 is the sample for 1% prevalence; about 50 corresponds to 6% prevalence, and 20 to about 14%. The rule of three (n ≈ 3/p = 150) gives the same answer.
Further reading: WOAH Aquatic Animal Health Code, chapter 1.3; WOAH Manual of Diagnostic Tests for Aquatic Animals
Survivors of Taura syndrome in P. vannamei typically show:
B. Multiple irregular melanised lesions in the cuticle
The recovery phase of Taura syndrome shows black melanised cuticular lesions; the acute phase has a red tail fan and soft shell. White opaque muscle suggests infectious myonecrosis; white spots suggest WSSV; bent rostrum suggests IHHNV.
Further reading: Lightner (ed.), A Handbook of Shrimp Pathology and Diagnostic Procedures
The salmon louse of sea-farmed Atlantic salmon is:
C. Lepeophtheirus salmonis
L. salmonis (with Caligus species) is the sea louse of salmon farming. Argulus and Lernaea are mainly freshwater parasites; Ichthyobodo is a flagellate.
Further reading: Roberts (ed.), Fish Pathology, 4th ed.; Noga, Fish Disease: Diagnosis and Treatment, 2nd ed.
Which of these mollusc pathogens is WOAH-listed?
A. Bonamia ostreae
Infection with Bonamia ostreae (a haplosporidian of flat oysters) is listed, as are Marteilia refringens and Perkinsus marinus. The others are not mollusc pathogens on the list.
Further reading: WOAH Aquatic Animal Health Code, chapter 1.3; WOAH Manual of Diagnostic Tests for Aquatic Animals
What does 'SPF' mean when applied to shrimp broodstock?
D. Free of specified pathogens, as shown by a defined surveillance programme
Specific-pathogen-free refers to documented freedom from named pathogens; it says nothing about resistance, which is what SPR (specific-pathogen-resistant) lines are selected for. Shrimp cannot be conventionally vaccinated.
Further reading: Lightner (ed.), A Handbook of Shrimp Pathology and Diagnostic Procedures
A cage was fed 1,500 kg of feed; biomass rose from 200 kg to 1,400 kg at harvest, and 100 kg of fish died and were removed during the cycle. What is the biological FCR (counting the weight of dead fish as gain)?
A. 1.15
Biological FCR = 1,500 ÷ (1,200 + 100) = 1,500 ÷ 1,300 = 1.15. The economic FCR, using harvested gain only, is 1.25. Subtracting the dead fish from the gain (1,500/1,100 = 1.36) is the wrong direction; 0.87 is the inverse.
Further reading: Standard aquaculture arithmetic
A pond of 0.2 ha with an average water depth of 1.2 m is to be treated with a chemical at a prescribed 0.5 mg/L. How much active ingredient is needed?
D. 1.2 kg
Volume = 2,000 m² × 1.2 m = 2,400 m³. Mass = 0.5 g/m³ × 2,400 = 1,200 g = 1.2 kg. Treating 0.2 ha as 200 m² gives 120 g; treating it as 20,000 m² gives 12 kg; ignoring depth (2,000 × 0.5) gives 1.0 kg. Pond treatments depend on accurate depth measurement — confirm against the product label and your national formulary.
Further reading: Boyd & Tucker, Pond Aquaculture Water Quality Management
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