Fish biology

Fisheries Science (B.F.Sc) · Aquaculture · free study notes for exams, placements and interviews

In short

Fish biology is the study of the structure, function, classification, reproduction and growth of fishes — the biological foundation on which every aquaculture decision rests. Knowing gill function, osmoregulation, gonadal maturity and growth patterns is what lets a farmer choose species, decide stocking size, time induced breeding and judge whether a crop is performing. In Indian universities it carries the heaviest weight in B.F.Sc core papers (Fish Anatomy and Physiology, Fish Taxonomy, Biology of Finfish and Shellfish) and in ASRB NET, ICAR-JRF/SRF and state fisheries officer examinations. Employers — ICAR institutes (CIFA Bhubaneswar, CIFE Mumbai, CIFRI Barrackpore, CMFRI Kochi), state fisheries departments, hatcheries and feed companies — test it in technical rounds through maturity staging, fecundity estimation, species identification and condition-factor interpretation.

Explained

Classification and Indian cultivable species

Phylum Chordata → Superclass Pisces, split into Class Chondrichthyes (cartilaginous: sharks, rays, skates; placoid scales, 5–7 gill slits, no swim bladder, internal fertilisation, claspers) and Class Osteichthyes (bony fishes; operculum-covered gills, swim bladder, mostly external fertilisation).
Osteichthyes → Subclass Actinopterygii (ray-finned) → Teleostei, which includes essentially all cultured food fishes. Sarcopterygii (lungfishes, coelacanth) is of academic interest only.
Binomial nomenclature: genus + species, italicised, author and year follow — *Labeo rohita* (Hamilton, 1822). Brackets round the author mean the species was later moved to another genus.
Indian major carps (IMC): *Labeo rohita* (rohu, column feeder, detritus/plant matter), *Catla catla* / *Labeo catla* (catla, surface zooplankton feeder, upturned mouth), *Cirrhinus mrigala* (mrigal, bottom detritus feeder). Their non-overlapping feeding niches are the basis of composite carp culture.
Exotic carps: *Hypophthalmichthys molitrix* (silver carp, phytoplankton), *Ctenopharyngodon idella* (grass carp, macrophytes), *Cyprinus carpio* (common carp, bottom omnivore).
Air-breathing / minor species: *Clarias magur*, *Heteropneustes fossilis* (singhi), *Anabas testudineus*, *Channa striata* (murrel) — accessory respiratory organs allow culture in low-oxygen water.
Others of commercial weight: *Pangasianodon hypophthalmus* (pangas), *Oreochromis niloticus* (tilapia — regulated, only approved strains/all-male), *Lates calcarifer* (seabass), *Chanos chanos* (milkfish), *Etroplus suratensis* (pearlspot, Kerala state fish).
Key identification characters: fin formula (counts of spines in Roman, soft rays in Arabic — e.g. D. iii/12), lateral-line scale count, barbel number, mouth position (terminal, sub-terminal, inferior, superior), caudal fin shape, scale type.

External morphology, skin and skeleton

Body forms: fusiform (fast swimmers), compressiform (laterally flattened — catla), depressiform (dorsoventrally flattened — rays), anguilliform (eel-like), filiform, globiform.
Fins: paired pectoral and pelvic (steering, braking, lift); median dorsal, anal (stability, anti-roll), caudal (propulsion, thrust). Adipose fin in some; finlets in tunas.
Caudal fin types: protocercal (larval), heterocercal (shark — unequal lobes), homocercal (teleost — externally equal), diphycercal, isocercal.
Scales: placoid (sharks — dermal denticles), cosmoid (fossil), ganoid (rhomboid, *Lepisosteus*), cycloid (smooth margin — carps), ctenoid (comb-like margin, spiny — perches). Scales grow by adding circuli; crowded circuli form an annulus used for ageing.
Skin: epidermis without keratin, rich in mucus (goblet) cells — mucus reduces drag, carries lysozyme and immunoglobulin, and is the first barrier against pathogens. Dermis carries chromatophores (melanophores, xanthophores, erythrophores) and iridophores/guanophores for silvery sheen.
Skeleton: axial (skull, vertebral column with abdominal and caudal vertebrae, ribs), appendicular (fin rays, girdles), and the Weberian ossicles in cypriniforms that link the swim bladder to the inner ear for sound reception.

Respiration, circulation and osmoregulation

Gills: four pairs of holobranchs on gill arches; each arch bears two hemibranchs → gill filaments (primary lamellae) → secondary lamellae, the actual gas-exchange surface, served by a single-cell-thick epithelium. Gill rakers on the inner arch strain food and protect the filaments.
Counter-current flow: water flows across the lamellae opposite to the blood flow, maintaining an oxygen gradient along the whole length — extraction efficiency up to 80–85%, against about 25% in mammalian lungs.
Breathing mechanism: a double-pump — buccal (pressure) pump and opercular (suction) pump — keeps flow nearly continuous; fast pelagic fishes use ram ventilation.
Accessory respiratory organs: arborescent organ (*Clarias*), air sac (*Heteropneustes*), labyrinthine organ (*Anabas*), pharyngeal/buccal diverticula (*Channa*), vascularised intestine (*Lepidocephalichthys*).
Circulation: single circuit heart in series — sinus venosus → atrium → ventricle → bulbus arteriosus (teleost) or conus arteriosus (elasmobranch) → ventral aorta → afferent branchial arteries → gills → dorsal aorta. Blood passes the heart once per circuit, so systemic pressure is low.
Osmoregulation — freshwater fish: hyperosmotic to the medium (blood ~300 mOsm vs ~5 mOsm water). Water floods in, salts leak out → produces copious dilute urine, drinks almost nothing, absorbs Na⁺/Cl⁻ actively through gill ionocytes and food, glomeruli large and numerous.
Osmoregulation — marine teleost: hypoosmotic to seawater (~1,000 mOsm). Loses water, gains salts → drinks seawater, excretes scanty isotonic urine, pumps out Na⁺/Cl⁻ through gill chloride cells (mitochondria-rich cells with Na⁺/K⁺-ATPase) and Mg²⁺/SO₄²⁻ through the kidney; glomeruli few or absent (aglomerular in some).
Elasmobranchs retain urea and TMAO to stay slightly hyperosmotic and excrete salt via the rectal gland. Euryhaline species (milkfish, mullet, seabass, pearlspot) reverse gill ion transport and so tolerate wide salinity; stenohaline species cannot.
Nitrogen excretion: teleosts are ammonotelic, losing ~80–90% of nitrogen as NH₃ across the gills; elasmobranchs are ureotelic. This is why unionised ammonia accumulates fast in intensive systems.

Digestion, swim bladder, nerves and senses

Digestive tract: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → anus. Stomachless (agastric) carps have a pharyngeal pad/grinding teeth, a swollen intestinal bulb instead of a stomach, and a long coiled intestine.
Relative gut length (RGL = gut length ÷ total length): herbivores >3 (grass carp 2.5–3+, some >10), omnivores ~1–3, carnivores <1 (murrel, seabass ~0.5–0.7). A standard identification question.
Pyloric caeca increase absorptive area; the hepatopancreas of carps is diffuse liver + pancreatic tissue. Chief enzymes: pepsin (acid stomach, carnivores), trypsin/chymotrypsin, amylase (high in herbivores), lipase, cellulase mainly of microbial origin.
Swim bladder (air bladder): gas-filled sac giving neutral buoyancy at ~5–7% of body volume in freshwater fish. Physostomous — duct retained, gulps air (carps); physoclistous — closed, gas secreted by the rete mirabile/gas gland and resorbed at the oval (perches). Secondary roles: sound production and reception, accessory respiration, pressure sensing. Absent in sharks (compensated by oily liver with squalene) and in bottom dwellers.
Nervous system: brain of forebrain (olfactory lobes, cerebrum), midbrain (large optic lobes — vision dominant in most teleosts), hindbrain (cerebellum for balance, medulla); 10 pairs of cranial nerves.
Lateral line system: canals of neuromasts with cupula-covered hair cells detecting low-frequency water displacement — the "distant touch" sense behind shoaling, rheotaxis and predator detection. Ampullae of Lorenzini in elasmobranchs sense weak electric fields.
Other senses: olfaction extremely acute (homing in migratory fishes); taste buds on lips, barbels and body; inner ear with three semicircular canals plus utriculus/sacculus containing otoliths (also used for ageing); vision with rods/cones, retinomotor movement; Weberian apparatus amplifies sound in carps.
Endocrine: pituitary (gonadotropins GtH-I/II, growth hormone, prolactin), thyroid, interrenal (cortisol — stress), chromaffin (adrenaline), corpuscles of Stannius and ultimobranchial gland (calcium regulation), urophysis, pineal (photoperiod/melatonin).

Reproductive biology

Sexes: mostly gonochoristic; hermaphroditism occurs (protandry in seabass — male first then female; protogyny in groupers; synchronous in *Serranus*). Parthenogenesis/gynogenesis is used in producing monosex stock.
Sexual dimorphism: in ripe IMC males the pectoral fin is rough to touch and milt oozes on gentle pressure; females have a soft, bulging abdomen and a swollen reddish vent. Tilapia is sexed by the genital papilla (two openings in females, one in males).
Maturity stages (common six-point scale): I immature, II maturing/developing, III ripening, IV ripe, V spent, VI resting/recovering. Histological stages of the oocyte: chromatin nucleolar → perinucleolar → yolk vesicle → yolk granule → ripe/migratory nucleus.
Spawning habit: total (synchronous) spawners release all eggs in one go (IMC); batch/asynchronous spawners spawn repeatedly (tilapia, murrel, many marine fishes). Carps in India are seasonal breeders spawning with the south-west monsoon (June–August), triggered by rainfall, rising water level, temperature ~24–31 °C and falling conductivity.
Egg types: floating/pelagic buoyant (marine), demersal adhesive (common carp — needs kakaban/ hatching substrate), demersal non-adhesive, and semi-buoyant with a wide perivitelline space (IMC — hatched in Chinese circular hatcheries).
Induced breeding: carps do not spawn in confinement because the pre-ovulatory LH surge fails. Hypophysation uses carp pituitary extract; modern practice uses synthetic GnRH analogue plus a dopamine antagonist (domperidone) — marketed as Ovaprim, Ovatide, Wova-FH — typically 0.4–0.5 mL/kg to females and 0.2–0.25 mL/kg to males in a single dose, spawning 4–6 hours later at 27–30 °C. Dosages differ by product and species, so follow the label and local hatchery protocol.
Fertilisation and development: sperm enters through the micropyle → cortical reaction → water hardening → meroblastic discoidal cleavage on the animal pole → blastula → gastrula → embryo. Hatching in carps at ~16–20 hours (27–30 °C); yolk-sac larva (spawn) → absorption of yolk in ~3 days → fry → fingerling. Development rate is temperature-dependent, so hatchery timings are quoted with temperature.
Life-cycle terms: spawn (~6–8 mm), fry (~15–25 mm), fingerling (~25–100 mm), advanced fingerling/yearling, adult. Migration: anadromous (hilsa — sea to river to breed), catadromous (eel), potamodromous (carps within rivers), oceanodromous.

Age, growth and condition

Ageing methods: scale annuli, otolith rings, opercular and vertebral bones, spine sections, and length-frequency (Petersen) analysis — the last is most used in India for tropical species whose annual marks are faint for want of a sharp winter.
Growth is indeterminate in fish (continues through life, decelerating) and is modelled by the von Bertalanffy growth function. Growth is plastic, responding to temperature, oxygen, stocking density, feed and genetics.
Length–weight relationship W = aL^b: b = 3 means isometric growth and ideal cube-law form; b ≠ 3 is allometric. Condition factor (K or Kn) indexes plumpness, nutritional state and gonad development, and is used on farms to judge crop health.
Gonado-somatic index (GSI) tracks the reproductive cycle; its annual peak marks the spawning season. Hepato-somatic index (HSI) indicates energy reserves.
Fecundity is the number of ripe ova in the ovary before spawning; it rises with length and weight (IMC roughly 1–3 lakh eggs per kg body weight, varying with species, size and condition). Relative fecundity = eggs per unit body weight, which allows comparison between sizes.
Fish are poikilothermic — metabolic rate, feed intake, growth and oxygen demand all rise with temperature (roughly doubling per 10 °C, the Q₁₀ effect) while dissolved-oxygen solubility falls: the central stress of warm-water aquaculture.
Regulatory frame for biology-based decisions: fish seed quality, exotic species introduction and broodstock movement are governed by the National Fisheries Policy, state Fisheries Acts and inland fisheries rules, with the Wild Life (Protection) Act and CITES covering protected species; ICAR-NBFGR maintains fish germplasm and genetic resource data. Exact licensing requirements vary by state.

Key points

Counter-current exchange - water and blood flow in opposite directions across gill lamellae, giving up to 80–85% oxygen extraction.
Single circulation - blood passes the heart once per circuit; sinus venosus → atrium → ventricle → bulbus arteriosus → gills → body.
Freshwater fish - hyperosmotic; dilute copious urine, active salt uptake at the gills, no drinking.
Marine teleost - hypoosmotic; drinks seawater, scanty urine, salt pumped out by gill chloride cells.
Ammonotelic - teleosts excrete most nitrogen as ammonia through the gills, not the kidney.
Swim bladder - buoyancy organ; physostomous (duct, carps) vs physoclistous (closed, perches); absent in sharks.
Weberian ossicles - four bones linking swim bladder to inner ear in carps, sharpening hearing.
Lateral line - neuromast canal system sensing water displacement; basis of shoaling and rheotaxis.
Scale types - placoid, cosmoid, ganoid, cycloid (carps), ctenoid (perches); circuli and annuli used for ageing.
Agastric carps - no true stomach; intestinal bulb and long coiled gut, with pharyngeal grinding teeth.
RGL - gut-length-to-body-length ratio; >3 herbivore, ~1–3 omnivore, <1 carnivore.
Composite carp culture - works because catla (surface), rohu (column) and mrigal (bottom) feed at different niches.
Maturity stages - six-point scale from immature to resting; the field tool for brood selection.
Hypophysation - induced spawning using pituitary extract or GnRH analogue with domperidone (Ovaprim type).
IMC eggs - semi-buoyant, non-adhesive with wide perivitelline space; hatch in about 16–20 hours at 27–30 °C.
W = aL^b - b = 3 isometric; condition factor K indexes plumpness and health.
GSI - gonad weight as a percentage of body weight; peaks in the breeding season.
Poikilothermy - metabolism and oxygen demand rise with temperature while oxygen solubility falls.

Formulas and examples

Length–weight relationship: W = a L^b, or log W = log a + b log L
W = weight (g), L = total length (cm), a = condition-related intercept, b = growth exponent (dimensionless; b = 3 → isometric)

Condition factor (Fulton): K = (W × 100) / L³
W in g, L in cm; K is conventionally quoted without units (typical healthy carp ≈ 1–2)

Relative condition factor: Kn = W / (a L^b), using the population's own a and b; Kn ≈ 1 means average condition

Gonado-somatic index: GSI = (gonad weight / total body weight) × 100, in %

Hepato-somatic index: HSI = (liver weight / body weight) × 100, in %

Fecundity (gravimetric method): F = (n × G) / g
n = eggs in the subsample, g = subsample weight (g), G = total ovary weight (g); F in number of eggs
Relative fecundity: F_rel = F / body weight (eggs per g or per kg)

von Bertalanffy growth: L_t = L∞ [1 − e^{−K (t − t₀)}]
L_t = length (cm) at age t (years), L∞ = asymptotic length (cm), K = growth coefficient (per year), t₀ = theoretical age at zero length

Specific growth rate: SGR = [(ln W₂ − ln W₁) / t] × 100, in % body weight per day
Survival: S = (number harvested / number stocked) × 100, in %

Oxygen consumption rate: OCR = [(DO_in − DO_out) × flow rate] / biomass, in mg O₂ per kg per hour

Q₁₀ (temperature coefficient): Q₁₀ = (R₂ / R₁)^{10 / (T₂ − T₁)}, R = metabolic rate, T in °C

Example 1 — Condition factor. A rohu measures 32 cm total length and weighs 410 g.

L³ = 32³ = 32,768 cm³.
K = (410 × 100) / 32,768 = 41,000 / 32,768 = 1.25.
Interpretation: K above 1 indicates good condition for a growing rohu; a value sliding below ~0.9 across a sample points to underfeeding, overcrowding or parasitic load.

Example 2 — Fecundity by the gravimetric method. A ripe catla of 4.2 kg yields an ovary of 620 g. A 1.5 g subsample contains 480 ova.

Eggs per gram of ovary = 480 / 1.5 = 320.
F = 320 × 620 = 1,98,400 eggs ≈ 1.98 lakh.
Relative fecundity = 1,98,400 / 4.2 kg = 47,238 eggs per kg body weight — well below the usual carp figure of 1–3 lakh eggs per kg, so this ovary is not yet fully ripe.
Hatchery use: at 80% fertilisation and 75% hatching, expected spawn = 1,98,400 × 0.80 × 0.75 = 1,19,040 hatchlings.

Example 3 — GSI across the season. A female mrigal of 1,250 g has ovaries of 118 g in June; in October the same-size fish has ovaries of 9 g.

June GSI = (118 / 1250) × 100 = 9.44% → ripe, stage IV, ready for induced breeding.
October GSI = (9 / 1250) × 100 = 0.72% → spent/resting, stage VI.
Conclusion: the GSI peak confirms a monsoon spawning season, so brood conditioning must be completed by May.

Example 4 — von Bertalanffy growth. For an Indian major carp stock, L∞ = 100 cm, K = 0.30 per year, t₀ = 0. Find length at 2 years.

K(t − t₀) = 0.30 × 2 = 0.60; e^−0.60 = 0.5488.
L₂ = 100 × (1 − 0.5488) = 45.1 cm.
At 1 year: e^−0.30 = 0.7408 → L₁ = 25.9 cm. Annual increment in year 2 = 45.1 − 25.9 = 19.2 cm, against 25.9 cm in year 1 — the deceleration the model predicts.

Example 5 — Specific growth rate. Fingerlings stocked at 12 g reach 96 g in 90 days.

ln 96 = 4.564; ln 12 = 2.485; difference = 2.079.
SGR = (2.079 / 90) × 100 = 2.31% body weight per day.
Weight gain = 84 g; this is a healthy grow-out rate for carp fingerlings in a well-managed pond.

Feature	Freshwater teleost	Marine teleost	Elasmobranch
Blood vs medium	Hyperosmotic	Hypoosmotic	Slightly hyperosmotic
Drinking	Negligible	Drinks seawater	Negligible
Urine	Copious, dilute	Scanty, isotonic	Copious, dilute
Gill ion movement	Salt uptake	Salt excretion (chloride cells)	Limited
Special organ	Large glomeruli	Few/absent glomeruli	Rectal gland, urea retention
Nitrogen waste	Ammonia	Ammonia	Urea

Character	Catla	Rohu	Mrigal
Mouth	Upturned, wide	Sub-terminal/inferior, fringed lips	Sub-terminal, blunt snout
Feeding niche	Surface, zooplankton	Column, plant/detritus	Bottom, detritus
Head size	Large, deep body	Moderate	Small, streamlined
Barbels	Absent	One pair (small)	One pair (maxillary)
Growth rate	Fastest	Moderate	Moderate

Where it is used

Carp and catfish hatcheries (Andhra Pradesh's Kolleru belt, West Bengal's Naihati/ Hooghly seed farms): brood selection by maturity stage, induced breeding dosage, fecundity estimation and hatchling output forecasting.
Government hatcheries and state fisheries departments under PMMSY: fish seed production targets, seed quality certification, species-mix advisories for composite culture and stocking of reservoirs.
ICAR institutes and research roles: CIFA (carp breeding and genetic improvement — Jayanti rohu), CIFE (physiology, nutrition), CIFRI (reservoir stock assessment using length–frequency ageing), CMFRI (marine stock assessment and maturity studies), NBFGR (germplasm and cryopreservation of milt).
Stock assessment and fisheries management: length-at-first-maturity determines minimum legal size and mesh regulations; the monsoon fishing ban (usually 47–61 days, dates set state-wise) is grounded in spawning biology.
Feed and aquaculture input industry (Growel, Avanti, CP India, Godrej Agrovet): gut morphology and digestive enzyme profiles decide whether a species gets a herbivore or carnivore feed, and SGR/FCR trials evaluate it.
Ornamental fish trade and aquarium industry: breeding behaviour, egg type (egg-layer vs livebearer), sexual dimorphism and water-chemistry tolerance guide commercial breeding units in Kolkata, Chennai and Kochi.

Interview questions

Q: Why is gill gas exchange so much more efficient than lung exchange? A: Because of counter-current flow — water passes the secondary lamellae in the opposite direction to the blood, so an oxygen gradient is maintained along the entire exchange surface, allowing up to 80–85% extraction.
Q: How does a freshwater fish keep from swelling with water? A: It produces large volumes of very dilute urine through numerous large glomeruli, drinks almost nothing, and actively takes up sodium and chloride through gill ionocytes.
Q: Why can milkfish and seabass be farmed in brackishwater while rohu cannot? A: They are euryhaline and can reverse gill ion transport and adjust kidney output across salinities; rohu is a stenohaline freshwater species whose osmoregulatory machinery fails above roughly 5–8 ppt.
Q: Why do carps not breed naturally in a pond? A: Confinement suppresses the pre-ovulatory luteinising-hormone surge and the riverine cues (rising water, current, flooding) are absent, so final oocyte maturation and ovulation do not occur.
Q: What does Ovaprim contain and why two components? A: A GnRH analogue to stimulate pituitary gonadotropin release, plus domperidone, a dopamine antagonist that removes dopamine's inhibition of that release — together they reliably trigger ovulation.
Q: How would you select a ripe female carp in the field? A: A soft, bulging and evenly distended abdomen, a swollen reddish protruding vent, smooth pectoral fins, and no milt on pressure; cross-check with GSI or cannulation in a research setting.
Q: What does a length–weight exponent b of 2.6 tell you? A: Growth is negative allometric — the fish are getting longer faster than they are getting heavier, usually a sign of poor feeding, high density or environmental stress.
Q: Why is the swim bladder absent in sharks and in many bottom fishes? A: Sharks rely on a large squalene-rich oily liver and dynamic lift from heterocercal tail and pectoral fins; bottom dwellers do not need neutral buoyancy and benefit from staying down.
Q: Why is composite carp culture biologically sound? A: The species occupy different feeding niches and water strata — catla at the surface, rohu in the column, mrigal and common carp at the bottom, silver carp on phytoplankton, grass carp on macrophytes — so pond productivity is used fully without competition.
Q: Why does ammonia build up faster in intensive fish culture than in livestock systems? A: Teleosts are ammonotelic and dump most nitrogen directly across the gills into the water, so with high biomass and limited exchange unionised ammonia rises quickly, worsened by high pH and temperature.
Q: How are tropical Indian fishes aged if scale annuli are unclear? A: By length-frequency (Petersen/ELEFAN) analysis of large samples, supported by otolith microstructure and marginal increment studies, since weak seasonality leaves faint annual marks.
Q: What is the practical value of knowing length at first maturity? A: It sets the minimum legal catch size and mesh size and justifies seasonal fishing bans, so that fish spawn at least once before capture.

Common mistakes

Writing a double circulation for fish — teleost circulation is single, with blood passing the heart once per circuit.
Reversing osmoregulation: saying a freshwater fish drinks water and excretes concentrated urine (it is the marine fish that drinks, and its urine is scanty).
Confusing cycloid (carps) with ctenoid (perches) scales, or calling a shark's placoid scale a true scale.
Treating fecundity as eggs actually spawned — it is the count of ripe ova in the ovary, and the subsample must be scaled by total ovary weight, not by body weight.
Computing condition factor with length in mm against weight in g without the matching constant, or mixing standard length with total length between the a–b estimation and its use.
Saying common carp eggs are non-adhesive — they are adhesive and demersal, needing kakaban or hatching hapas, unlike the semi-buoyant IMC egg.
Quoting hatching time or hormone dose without the temperature or the product name — both are temperature- and formulation-dependent, and examiners expect the condition stated.