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Assam ASSEB Higher Secondary Skill Subject Fishery Sciences study materials
Assam ASSEB Higher Secondary Skill Subject Fishery Sciences study resources

Assam ASSEB Higher Secondary Skill Subject Fishery Sciences practice questions

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Question 1Introduction to Fisheries and Aquaculture

In fishery science, which term describes the practice of rearing aquatic organisms under controlled or semi-controlled conditions for human use?

  1. Sea ranching
  2. Capture fishery
  3. Aquaculture Correct answer.
  4. Ranching

Explanation

Aquaculture is the farming of aquatic organisms such as fish, crustaceans and molluscs under managed conditions. Capture fishery, by contrast, relies on harvesting naturally occurring wild stocks rather than rearing them.

Question 2Introduction to Fisheries and Aquaculture

Which of the following is classified as a finfish rather than a shellfish in Indian aquaculture?

  1. Crassostrea oyster
  2. Perna mussel
  3. Rohu (Labeo rohita) Correct answer.
  4. Penaeus shrimp

Explanation

Finfish are aquatic vertebrates with fins, and rohu is a major Indian major carp reared widely in ponds. Shrimp, oysters and mussels are invertebrates grouped as shellfish in fisheries classification.

Question 3Introduction to Fisheries and Aquaculture

A pond manager wants to know the term for the total weight of fish present per unit area of water at a given time. Which term applies?

  1. Condition factor
  2. Fecundity
  3. Primary productivity
  4. Standing crop Correct answer.

Explanation

Standing crop (or standing stock) expresses the biomass present per unit area or volume at one moment in time. It differs from productivity, which measures the rate of biomass production over time.

Question 4Introduction to Fisheries and Aquaculture

Which water quality parameter is most critical for oxygen supply to fish in a still, heavily stocked pond?

  1. Total hardness
  2. Dissolved oxygen Correct answer.
  3. Salinity
  4. Total alkalinity

Explanation

Dissolved oxygen is the directly limiting gas for fish respiration, and in still, densely stocked ponds it can fall sharply at night. Alkalinity, hardness and salinity influence productivity and osmoregulation but do not supply oxygen directly.

Question 5Introduction to Fisheries and Aquaculture

Which fish is a classic example of a freshwater air-breathing species used in Indian aquaculture and often cultured in swampy, low-oxygen ponds?

  1. Clarias magur (magur catfish) Correct answer.
  2. Sardinella longiceps (oil sardine)
  3. Tenualosa ilisha (hilsa)
  4. Tor putitora (golden mahseer)

Explanation

Clarias magur possesses accessory air-breathing organs that allow it to survive low dissolved oxygen, making it suitable for swampy ponds. The other species listed are either fast-flowing river fish or marine pelagic forms.

Question 6Introduction to Fisheries and Aquaculture

Which of the following best describes a composite fish culture system?

  1. Holding fish in cages within a river
  2. Rearing a single fast-growing species at very high density
  3. Culturing fish together with paddy in the same field
  4. Stocking several species with different feeding niches in the same pond Correct answer.

Explanation

Composite fish culture stocks species with complementary feeding habits, such as surface, column and bottom feeders, so that food resources at all levels are used. This polyculture approach raises total yield per pond compared with monoculture.

Question 7Introduction to Fisheries and Aquaculture

A farmer stocks catla, rohu and mrigal together in a pond. Which species is primarily a surface feeder in this combination?

  1. Cirrhinus mrigala
  2. Catla catla Correct answer.
  3. Clarias batrachus
  4. Labeo rohita

Explanation

Catla is a surface and mid-water feeder with an upward-directed mouth, so it exploits plankton near the surface. Rohu feeds in the mid-water column and mrigal is a bottom feeder, which is why the three complement each other.

Question 8Introduction to Fisheries and Aquaculture

Which pond fertilization practice mainly aims to boost natural plankton production rather than to feed fish directly?

  1. Feeding with pelleted supplementary feed
  2. Applying organic manure and inorganic fertilizers Correct answer.
  3. Applying lime to raise pH
  4. Applying common salt to the water

Explanation

Manures and inorganic fertilizers supply nitrogen, phosphorus and organic matter that stimulate phytoplankton and zooplankton, the natural food of carp. Lime corrects soil and water pH, and pelleted feed is taken directly by fish.

Question 9Introduction to Fisheries and Aquaculture

Which method is used to determine the age of a fish by examining a bony structure that shows annual growth rings?

  1. Hepatosomatic index
  2. Bioaccumulation assay
  3. Scale reading Correct answer.
  4. Gonadosomatic index

Explanation

Scales, otoliths and some bones deposit growth rings that reflect seasonal growth, allowing age estimation by ring counting. Indices such as gonadosomatic and hepatosomatic ratios describe organ proportions, not age.

Question 10Introduction to Fisheries and Aquaculture

In fish breeding, which term refers to the process of injecting hormones such as pituitary extract to induce spawning in broodstock that do not spawn naturally in captivity?

  1. Aeration
  2. Hypophysation Correct answer.
  3. Grading
  4. Acclimatization

Explanation

Hypophysation is induced breeding by injecting pituitary gland extract or synthetic hormones to trigger ovulation and spawning. Acclimatization, grading and aeration are husbandry operations unrelated to hormone-induced spawning.

Question 11Introduction to Fisheries and Aquaculture

Which structure is used to incubate fertilized fish eggs under controlled flow so that eggs are kept moving and oxygenated in a hatchery?

  1. Earthen nursery tank
  2. Nursery pond
  3. Hatchery jar (incubation jar) Correct answer.
  4. Floating cage

Explanation

Hatchery jars, often of the MacDonald or similar type, hold eggs in an upwelling water flow that keeps them suspended and oxygenated until hatching. Nursery ponds and cages are used for rearing fry or older fish, not for egg incubation.

Question 12Introduction to Fisheries and Aquaculture

A hatchery manager observes that eggs in an incubation jar are clumping and developing fungus. Which immediate action is most appropriate?

  1. Reduce water flow to settle the eggs
  2. Stop aeration and keep the water still
  3. Add more eggs to the jar
  4. Increase water flow and remove dead eggs Correct answer.

Explanation

Fungal growth and clumping usually arise when dead eggs remain and water movement is inadequate. Increasing flow keeps eggs separated and oxygenated, while removing dead eggs reduces the fungal inoculum.

Question 13Introduction to Fisheries and Aquaculture

Which of the following is a key difference between a natural pond ecosystem and a managed aquaculture pond?

  1. Natural ponds always have higher fish biomass than managed ponds
  2. Managed ponds involve deliberate stocking, feeding and water management Correct answer.
  3. Natural ponds require daily liming and fertilization
  4. Managed ponds rely entirely on natural food with no human intervention

Explanation

Managed aquaculture ponds are deliberately stocked and receive inputs such as feed, fertilizer and water regulation to raise yield. Natural ponds depend on ambient nutrient cycles and are not routinely managed for maximum fish output.

Question 14Introduction to Fisheries and Aquaculture

Which statement about the relationship between stocking density and fish growth in a pond is most accurate?

  1. Growth increases indefinitely as density rises
  2. Lower density always gives lower total pond yield
  3. Growth is unaffected by density if feed is provided
  4. Beyond an optimum, higher density reduces individual growth Correct answer.

Explanation

At low density, growth per fish is high, but as density rises competition for food and oxygen intensifies, and individual growth declines beyond an optimum. Total yield may still rise with density up to a point before water quality limits intervene.

Question 15Introduction to Fisheries and Aquaculture

A fish farmer finds that pond water turns green with dense phytoplankton but dissolved oxygen drops sharply before dawn. What is the best explanation?

  1. Phytoplankton respire at night, consuming oxygen without photosynthesis Correct answer.
  2. Bacteria stop consuming oxygen at night
  3. Algae stop respiring during darkness
  4. Dissolved oxygen rises at night due to algal photosynthesis

Explanation

During darkness, phytoplankton continue respiration and consume oxygen but cannot photosynthesize to release it, so dissolved oxygen falls toward dawn. Dense blooms make this night-time oxygen drop more severe.

Question 16Introduction to Fisheries and Aquaculture

Which of the following is a primary function of aeration in intensive fish culture ponds?

  1. Raising total alkalinity
  2. Removing nitrogen from feed
  3. Increasing dissolved oxygen and reducing stratification Correct answer.
  4. Increasing water temperature

Explanation

Aeration mixes the water column, breaks thermal and chemical stratification, and adds oxygen for respiration. It does not raise alkalinity or temperature and is not a nitrogen-removal process from feed.

Question 17Introduction to Fisheries and Aquaculture

A pond has a total alkalinity of about 15 mg/L as CaCO3. Which management interpretation is most reasonable?

  1. The pond is low in alkalinity and may benefit from liming Correct answer.
  2. The pond has adequate alkalinity for good carp production
  3. Alkalinity has no bearing on pond productivity
  4. The pond is too alkaline and needs acid treatment

Explanation

Pond waters with alkalinity below roughly 20 mg/L as CaCO3 are considered low and often respond poorly to fertilization. Liming is commonly recommended to raise alkalinity and buffer pH swings.

Question 18Introduction to Fisheries and Aquaculture

In a polyculture pond, a sudden algal crash is followed by a rise in ammonia and a drop in dissolved oxygen. Which sequence best explains this cascade?

  1. Ammonia rises because fish stop feeding
  2. Algal death adds oxygen, which raises ammonia
  3. Dead algae decompose, bacteria consume oxygen and release ammonia Correct answer.
  4. Oxygen falls because algae begin photosynthesis

Explanation

When a bloom collapses, dead algal cells are decomposed by bacteria that consume dissolved oxygen and release ammonia as a by-product. The combined oxygen demand and ammonia load can stress or kill fish.

Question 19Introduction to Fisheries and Aquaculture

Which statement about the difference between extensive and intensive aquaculture is most defensible?

  1. Extensive systems rely mainly on natural food and low inputs, while intensive systems use high stocking and formulated feed Correct answer.
  2. Extensive systems always use cages and raceways
  3. Intensive systems have lower stocking density than extensive systems
  4. Intensive systems rely only on natural pond food

Explanation

Extensive aquaculture depends largely on natural productivity with minimal supplementary inputs, whereas intensive systems use high stocking densities, formulated feed, and aeration. The two differ fundamentally in input level and control.

Question 20Introduction to Fisheries and Aquaculture

A hatchery technician needs to transport carp fry over several hours. Which practice best reduces stress and mortality during transport?

  1. Filling the container completely to exclude air
  2. Using clean, well-oxygenated water with moderate stocking and controlled temperature Correct answer.
  3. Raising water temperature sharply before loading
  4. Adding excess feed before transport

Explanation

Fry transport succeeds when water is clean and oxygenated, density is moderate, and temperature is kept stable and preferably cool to lower metabolic demand. Excess feed fouls the water and sharp warming raises oxygen demand.

Question 21Fish Biology, Anatomy and Physiology

In fish, the chamber of the heart that receives deoxygenated blood from the body via the sinus venosus is the:

  1. Atrium Correct answer.
  2. Ventricle
  3. Bulbus arteriosus
  4. Conus arteriosus

Explanation

Blood from the body drains through the sinus venosus into the atrium, which is the first true cardiac chamber in the fish heart. The atrium then passes blood to the ventricle, which pumps it onward.

Question 22Fish Biology, Anatomy and Physiology

The single-loop circulation of most teleost fishes is best described as:

  1. Blood passes heart to body to gills to heart
  2. Blood flows from gills directly back to the gills
  3. Blood passes through the heart twice per circuit
  4. Blood passes heart to gills to body to heart Correct answer.

Explanation

Teleosts have a single circulation in which the heart pumps venous blood to the gills for oxygenation, and the oxygenated blood then flows directly to the systemic tissues before returning to the heart. There is no separate pulmonary circuit.

Question 23Fish Biology, Anatomy and Physiology

Which structure in a bony fish is primarily responsible for maintaining buoyancy without active swimming?

  1. Swim bladder Correct answer.
  2. Kidney
  3. Spleen
  4. Liver

Explanation

The swim bladder is a gas-filled sac whose volume is adjusted to match the fish to the surrounding water density, allowing neutral buoyancy. Gas is added or removed via the gas gland and oval body.

Question 24Fish Biology, Anatomy and Physiology

Fish gill filaments bear numerous thin plates called lamellae whose main function is to:

  1. Grind food before swallowing
  2. Detect water vibrations
  3. Store excess nitrogenous waste
  4. Provide a large surface for gas exchange Correct answer.

Explanation

Secondary lamellae greatly increase the respiratory surface area and are richly supplied with capillaries so that oxygen uptake and carbon dioxide release occur across a thin epithelium. The countercurrent flow of water and blood enhances exchange efficiency.

Question 25Fish Biology, Anatomy and Physiology

The lateral line system of a fish is primarily sensitive to:

  1. Changes in light intensity
  2. Magnetic field direction
  3. Water movements and pressure gradients Correct answer.
  4. Dissolved oxygen concentration

Explanation

Neuromast organs in the lateral line detect low-frequency water displacements and pressure gradients, enabling schooling, prey detection and predator avoidance. This mechanosensory system is distinct from the inner ear.

Question 26Fish Biology, Anatomy and Physiology

Fish scales that are thin, flexible, overlapping plates with a comb-like posterior margin are called:

  1. Placoid scales
  2. Ctenoid scales Correct answer.
  3. Ganoid scales
  4. Cycloid scales

Explanation

Ctenoid scales have small tooth-like ctenii on their exposed posterior edge, giving a rough texture, and are typical of spiny-rayed fishes. Cycloid scales lack ctenii and have a smooth rounded margin.

Question 27Fish Biology, Anatomy and Physiology

In most teleost fishes, the organ that regulates ionic balance and water movement between the body and the environment is the:

  1. Kidney together with gill ionocytes Correct answer.
  2. Swim bladder
  3. Gall bladder
  4. Spleen

Explanation

Osmoregulation in teleosts depends on the kidney and on mitochondrion-rich ionocytes in the gill epithelium, which actively take up or excrete ions depending on whether the fish is in fresh or salt water. The two organs act together to maintain blood osmotic concentration.

Question 28Fish Biology, Anatomy and Physiology

A freshwater teleost tends to gain water across its gills and lose ions to the dilute medium. Which combination of responses best maintains its internal balance?

  1. Scanty urine and passive ion loss without compensation
  2. Copious dilute urine and active ion secretion into water
  3. Scanty urine and active ion uptake from water
  4. Copious dilute urine and active ion uptake from water Correct answer.

Explanation

Freshwater fish are hyperosmotic to their environment, so water enters osmotically and must be expelled as large volumes of dilute urine, while ionocytes actively absorb sodium and chloride from the water. This combination offsets both the water load and the diffusive ion loss.

Question 29Fish Biology, Anatomy and Physiology

A marine teleost is placed in fresh water. Which physiological change is expected?

  1. Its urine volume decreases sharply
  2. It stops drinking and loses ions while gaining water Correct answer.
  3. It drinks more water and excretes excess salts
  4. Its gill ionocytes secrete more sodium into the water

Explanation

In sea water the fish drinks to replace water lost osmotically and excretes excess salt; moving to fresh water reverses the gradient, so drinking ceases, water enters and ions are lost. The fish must switch to ion uptake and dilute urine production.

Question 30Fish Biology, Anatomy and Physiology

Countercurrent flow in the fish gill refers to:

  1. Water and blood flowing in opposite directions across the lamellae Correct answer.
  2. Blood flowing in two directions within one lamella
  3. Water flowing only during inhalation
  4. Water and blood flowing in the same direction across the lamellae

Explanation

In the gill, water passes over the secondary lamellae in the opposite direction to the blood, maintaining a diffusion gradient along the whole exchange surface. This allows oxygen extraction far above the 50 percent ceiling of concurrent flow.

Question 31Fish Biology, Anatomy and Physiology

Which feature of the fish heart allows it to pump blood efficiently despite having only one atrium and one ventricle?

  1. Valves between chambers prevent backflow Correct answer.
  2. Blood is pumped only when the fish surfaces
  3. The ventricle has four separate chambers
  4. The heart contains no muscle tissue

Explanation

Valves at the sinoatrial and atrioventricular junctions, and in the outflow tract, maintain one-way flow so that each contraction ejects blood forward. Without these valves, backflow during relaxation would reduce pumping efficiency.

Question 32Fish Biology, Anatomy and Physiology

A fish has a spiral valve in its intestine. This structure most likely functions to:

  1. Store gas for buoyancy
  2. Increase absorptive surface area and slow passage of food Correct answer.
  3. Pump blood to the gills
  4. Detect prey movements

Explanation

The spiral valve is a folded helical ridge in the intestine, common in sharks and rays and in some primitive bony fishes, that increases surface area for absorption and lengthens the path of digesta. It is part of the digestive tract.

Question 33Fish Biology, Anatomy and Physiology

In sharks, the rectal gland is specialised for:

  1. Production of red blood cells
  2. Storage of urea
  3. Secretion of excess sodium chloride Correct answer.
  4. Production of digestive enzymes

Explanation

The rectal gland of elasmobranchs secretes a concentrated sodium chloride solution into the rectum, helping the fish eliminate salt taken in with food and seawater. Urea retention in the blood, not the rectal gland, maintains osmotic balance.

Question 34Fish Biology, Anatomy and Physiology

The Weberian apparatus of otophysan fishes (such as carps and catfishes) connects the swim bladder to the inner ear and serves to:

  1. Regulate buoyancy by gas secretion
  2. Pump blood to the brain
  3. Store sperm during breeding
  4. Transmit sound vibrations for hearing Correct answer.

Explanation

The Weberian apparatus is a chain of modified vertebrae and ossicles linking the swim bladder to the inner ear, improving the detection of sound and high-frequency vibrations. It is an auditory specialization rather than a buoyancy device.

Question 35Fish Biology, Anatomy and Physiology

Which type of fish scale is found in sharks and rays and is structurally a dermal denticle?

  1. Ganoid scale
  2. Ctenoid scale
  3. Cycloid scale
  4. Placoid scale Correct answer.

Explanation

Placoid scales are tooth-like structures with a pulp cavity, dentine and an enamel-like cap embedded in the skin of cartilaginous fishes. Their shape reduces drag and they are homologous to vertebrate teeth.

Question 36Fish Biology, Anatomy and Physiology

A researcher finds a fish whose swim bladder is connected to the oesophagus by a duct. This fish is best described as:

  1. Agnathous
  2. Elasmobranch
  3. Physoclistous
  4. Physostomous Correct answer.

Explanation

A physostomous swim bladder retains a pneumatic duct to the gut, allowing the fish to gulp or release gas at the surface. In physoclistous fishes the duct is lost and gas is regulated by the gas gland and oval body.

Question 37Fish Biology, Anatomy and Physiology

The ampullae of Lorenzini in sharks and rays detect:

  1. Sound waves
  2. Dissolved salts
  3. Weak electric fields Correct answer.
  4. Water temperature only

Explanation

These gel-filled pores open to electroreceptors that sense the weak bioelectric fields produced by prey muscles and nerves. This electroreception helps sharks locate hidden prey.

Question 38Fish Biology, Anatomy and Physiology

The spiracle of many sharks and rays functions mainly as:

  1. A brood pouch for eggs
  2. A site of urea excretion
  3. An opening for sperm transfer
  4. An accessory opening for water intake to the gills Correct answer.

Explanation

The spiracle is a reduced first gill slit that admits water to the gill system, particularly important in bottom-dwelling rays where the mouth may be buried in sediment. It is not part of the reproductive or excretory system.

Question 39Fish Biology, Anatomy and Physiology

A teleost fish shows a very high proportion of red, myoglobin-rich muscle along the lateral line. This suggests that the fish:

  1. Is a continuously cruising swimmer such as a tuna or mackerel Correct answer.
  2. Uses anaerobic glycolysis for all swimming
  3. Relies on burst escape responses only
  4. Is a sedentary bottom dweller that ambushes prey

Explanation

Red muscle is rich in myoglobin, mitochondria and capillaries and supports sustained aerobic swimming, so a high proportion is typical of active, continuously cruising fishes. White muscle, by contrast, powers short bursts anaerobically.

Question 40Fish Biology, Anatomy and Physiology

A fish is observed to have a heterocercal caudal fin in which the upper lobe is larger and the vertebral column extends into it. This condition is typical of:

  1. Lungfishes only
  2. Larval amphibians
  3. Sharks and other elasmobranchs Correct answer.
  4. Most advanced teleosts

Explanation

A heterocercal tail, with the notochord or vertebral column turning upward into the larger upper lobe, is characteristic of sharks and rays and generates lift as well as thrust. Teleosts typically have a homocercal tail with externally symmetrical lobes.

Question 41Fish Biology, Anatomy and Physiology

In a physoclistous fish rapidly brought up from deep water, the swim bladder may overinflate and rupture. The best physiological explanation is that:

  1. The oval body actively absorbs gas during ascent
  2. Gas cannot escape quickly because there is no duct to the gut Correct answer.
  3. The gas gland secretes gas faster during ascent
  4. The fish swallows air at the surface

Explanation

Physoclistous fishes lack a pneumatic duct, so gas must be resorbed through the oval body at a limited rate. During rapid ascent the expanding gas cannot be removed fast enough, causing overinflation, so such fish must ascend slowly.

Question 42Fish Biology, Anatomy and Physiology

Which functional pairing correctly matches a fish sensory organ with its adequate stimulus?

  1. Olfactory rosette with dissolved chemical cues Correct answer.
  2. Semicircular canals with dissolved oxygen
  3. Lateral line with light intensity
  4. Ampullae of Lorenzini with water pressure

Explanation

The olfactory rosette bears receptor neurons that respond to dissolved chemical stimuli carried in the water, mediating smell. Each other listed organ is paired with a stimulus it does not detect.

Question 43Fish Biology, Anatomy and Physiology

A fish living in a warm, fast-flowing stream has a higher metabolic rate and greater oxygen demand than a related species in a cold, stagnant pool. Which adaptation would most directly support the higher demand?

  1. A larger swim bladder with a pneumatic duct
  2. Greater gill surface area and more secondary lamellae Correct answer.
  3. Reduced heart size with fewer valves
  4. Lower haemoglobin concentration in blood

Explanation

More gill surface area and numerous secondary lamellae increase the area available for oxygen diffusion, matching the higher metabolic demand of the active stream fish. Warm water also holds less oxygen, making efficient gill exchange more important.

Question 44Fish Biology, Anatomy and Physiology

In elasmobranchs, blood urea concentration is high and is retained rather than excreted. The main advantage of this strategy is that it:

  1. Provides a nitrogen store for protein synthesis
  2. Neutralises stomach acid
  3. Raises internal osmolarity toward that of sea water Correct answer.
  4. Increases buoyancy by forming gas in the swim bladder

Explanation

Retaining urea and trimethylamine oxide raises blood osmolarity close to that of sea water, reducing the osmotic influx of water and the need for continuous drinking. This is a key osmoregulatory adaptation of cartilaginous fishes.

Question 45Fish Pond Management and Water Quality

In a composite fish culture pond, which combination of species is correctly described as a surface feeder and a column feeder respectively?

  1. Grass carp and silver carp
  2. Catla and rohu Correct answer.
  3. Rohu and mrigal
  4. Mrigal and catla

Explanation

In composite culture of Indian major carps, catla is a surface feeder with an upward-directed mouth, while rohu feeds in the mid-water column. Mrigal is a bottom feeder, so it occupies a different niche and does not compete directly with the other two.

Question 46Fish Pond Management and Water Quality

The greenish colour of pond water in a well-managed carp pond is mainly due to which plankton group?

  1. Zooplankton
  2. Green algae (Chlorophyceae) Correct answer.
  3. Blue-green algae
  4. Diatoms

Explanation

A healthy green colour in managed carp ponds is usually produced by green algae, which are palatable and support zooplankton production. Dense blue-green algal blooms give a dull blue-green scum and often indicate poor water quality.

Question 47Fish Pond Management and Water Quality

Which instrument is used to measure the transparency of pond water in a fish pond?

  1. Turbidimeter
  2. Secchi disc Correct answer.
  3. Salinometer
  4. Hach kit

Explanation

A Secchi disc is lowered into the water until it just disappears and the depth is recorded as a measure of transparency. Transparency reflects the amount of suspended plankton and silt in the pond water.

Question 48Fish Pond Management and Water Quality

Which chemical is commonly applied to pond water to neutralize excessive acidity during liming?

  1. Sodium chloride
  2. Calcium carbonate (agricultural lime) Correct answer.
  3. Ammonium sulphate
  4. Potassium permanganate

Explanation

Agricultural lime, mainly calcium carbonate, is applied to raise the pH of acidic pond water and improve alkalinity. It also helps release phosphorus from bottom sediments and improves the buffering capacity of the water.

Question 49Fish Pond Management and Water Quality

Which of the following is the most important reason for applying cow dung or poultry manure to a nursery pond before stocking spawn?

  1. To increase water transparency
  2. To promote plankton growth as natural food Correct answer.
  3. To lower dissolved oxygen
  4. To increase water hardness

Explanation

Organic manures decompose and release nutrients that stimulate phytoplankton and zooplankton, which are the natural food of spawn and fry. This is why nursery ponds are manured before stocking so that the young fish find food immediately.

Question 50Fish Pond Management and Water Quality

In fish pond management, 'plankton bloom' in a pond is best indicated by which observation?

  1. Brown muddy water after rain
  2. Water with heavy surface foam
  3. Greenish water with reduced Secchi reading Correct answer.
  4. Clear transparent water with visible bottom

Explanation

A plankton bloom shows as greenish water with reduced transparency because of dense algal growth. It indicates active primary production that supports fish food organisms, though excessive blooms must be monitored.