Systematic Ichthyology BZO5T12

Syllabus topics (RTMNU)

• History, Scope and Importance of Ichthyology: Origin and development of ichthyology
• Scope of ichthyology in zoology
• Ecological and scientific importance of fishes
• Berg’s Classification of Fishes: Salient features of Berg’s classification
• Major groups of fishes
• Basis of classification
• Significance and limitations of Berg’s system
• Modern Cladistics and Nelson’s Classification of Fishes: Principles of cladistics
• Phylogenetic relationships among fishes
• Outline of Nelson’s classification
• Advantages of modern phylogenetic systems over traditional classifications
• Principles of Taxonomy and ICZN (International Code of Zoological Nomenclature): Concept and principles of taxonomy
• Hierarchical categories
• Binomial nomenclature
• Rules and significance of ICZN in scientific naming of fishes

History of Ichthyology: Origin and Development

Ichthyology (Greek ichthys = fish, logos = study) is the branch of zoology that deals with the study of fishes – their classification, structure, physiology, behaviour, ecology and economic use.

1. Ancient period: Aristotle (384–322 BC) described more than 100 kinds of fishes from the Aegean Sea and noted their gills, scales and breeding habits.
2. Peter Artedi (1705–1735): Swedish naturalist called the "Father of Ichthyology". His work Ichthyologia was published by his friend Linnaeus in 1738. He used fin rays, scales and body parts to group fishes.
3. Linnaeus (1758): Systema Naturae (10th edition) gave binomial names to fishes; this book is the starting point of zoological names.
4. Cuvier and Valenciennes: Histoire naturelle des poissons (1828–1849), a huge descriptive work on fishes of the world.
5. Louis Agassiz: studied fossil fishes; Albert Günther wrote the Catalogue of Fishes of the British Museum.
6. Indian ichthyology: Francis Hamilton (Buchanan) described fishes of the Ganga (1822) – many Indian carps carry "Hamilton, 1822" after their name. Francis Day wrote The Fishes of India (1875–1878). S. L. Hora worked on hill-stream fishes and fish distribution and is often called the father of Indian ichthyology. Later works: K. C. Jayaram (freshwater fishes of the Indian region), Talwar and Jhingran (Inland Fishes of India).
7. Modern period: L. S. Berg (1940) and J. S. Nelson (Fishes of the World) gave classifications; today DNA barcoding and molecular phylogeny are used.

Scope of Ichthyology

• Taxonomy and systematics: identification, naming and classification of fishes.
• Anatomy and physiology: respiration, osmoregulation, swim bladder, sense organs.
• Ecology and behaviour: habitats, food, migration, schooling, breeding.
• Fisheries science and aquaculture: capture fisheries, fish culture, breeding of carps (induced breeding), feed.
• Fish pathology: diseases and health management.
• Fish technology: processing, preservation, by-products.
• Conservation biology: endangered fishes (e.g. Golden mahseer Tor putitora), biodiversity surveys.
• Palaeoichthyology: fossil fishes and vertebrate evolution.

Ecological and Scientific Importance of Fishes

Fishes are the largest group of vertebrates – more than 30,000 species, about half of all living vertebrates.

1. Food chains: fishes are both predators and prey; they link plankton to birds, mammals and man.
2. Nutrient cycling: migratory fishes such as salmon carry nutrients from sea to rivers.
3. Biological control: larvivorous fishes like Gambusia affinis and Aplocheilus eat mosquito larvae.
4. Bio-indicators: fish health shows water pollution.
5. Research models: zebrafish (Danio rerio) is used in genetics and developmental biology.
6. Evolution: fishes show the origin of jaws, paired fins and the move from water to land.
7. Economy: food, livelihood of fishermen, by-products, export earnings and ornamental fish trade.

Berg’s Classification of Fishes

The Russian ichthyologist Leo S. Berg published Classification of Fishes, both Recent and Fossil in 1940. It was the most widely used classification for many years and is still taught in Indian universities.

Salient features:

1. It included both living and fossil fishes in one system.
2. Fish-like vertebrates were divided into two big groups: Agnatha (jawless) and Gnathostomata (jawed).
3. Fishes were split into many separate classes instead of the older two or three.
4. Groups were placed in an order showing increasing complexity.
5. It used the ending -iformes for orders (e.g. Cypriniformes, Perciformes), which is still used.

Major groups (outline as given in Indian textbooks):

Basis of classification: presence or absence of jaws, nature of skeleton (cartilage or bone), type of scales, number and structure of gill openings, operculum, structure of fins and tail (heterocercal, homocercal), presence of lungs/swim bladder, and fossil evidence.

Significance: clear and simple; covers fossils; used for decades in textbooks and museums; gave a firm base for later systems.

Limitations:

• Based mainly on overall similarity (external and anatomical), not on common ancestry.
• Some groups are not natural, e.g. "Agnatha" puts very different fossil and living forms together.
• Too many classes; ranks were given unequally.
• No molecular (DNA) data was available in 1940.

Principles of Cladistics

Cladistics (phylogenetic systematics) is a method of classification in which organisms are grouped only by shared derived characters that show common ancestry. It was developed by the German entomologist Willi Hennig (1950, English version 1966).

1. Clade: an ancestor and all its descendants.
2. Plesiomorphy: an ancestral (old) character, e.g. a cartilaginous skeleton.
3. Apomorphy: a derived (new) character; a synapomorphy is a derived character shared by two or more groups, e.g. jaws in all gnathostomes.
4. Monophyletic group: contains the common ancestor and all its descendants – the only kind accepted in cladistics.
5. Paraphyletic group: ancestor and only some descendants. "Fishes" is paraphyletic because tetrapods also descend from fish ancestors.
6. Polyphyletic group: members from different ancestors.
7. Outgroup comparison decides which character is ancestral; parsimony chooses the tree with the fewest changes.
8. The result is shown as a branching diagram called a cladogram.

Phylogenetic Relationships among Fishes and Outline of Nelson’s Classification

Present understanding: hagfishes and lampreys are the only living jawless vertebrates; jawed vertebrates split into cartilaginous fishes and bony fishes; bony fishes split into ray-finned (Actinopterygii) and lobe-finned (Sarcopterygii); tetrapods arose from lobe-finned fishes.

Joseph S. Nelson gave the modern classification in Fishes of the World (latest 5th edition, 2016, with Grande and Wilson). It is based on cladistic principles. Simple outline:

• Jawless (Cyclostomi): Class Myxini (hagfishes) and Class Petromyzontida (lampreys).
• Gnathostomata (jawed vertebrates):
  • Class Chondrichthyes – Subclass Holocephali (chimaeras) and Subclass Elasmobranchii (sharks and rays).
  • Class Actinopterygii – Subclass Cladistia (bichirs, Polypterus), Chondrostei (sturgeons, paddlefishes), Neopterygii (Holostei: gars and bowfin; Teleostei: most living fishes).
  • Class Sarcopterygii – Actinistia (coelacanths), Dipnomorpha (lungfishes), Tetrapodomorpha (fossil fishes + tetrapods).

Advantages of Modern Phylogenetic Systems

Concept and Principles of Taxonomy; Hierarchical Categories

Taxonomy is the science of identification, naming (nomenclature) and classification of organisms. Systematics is wider – it also studies their evolutionary relationships.

Principles: (1) every distinct species gets one valid scientific name; (2) organisms are grouped by similarity and common descent; (3) groups are arranged in ranks; (4) type specimens fix the meaning of a name; (5) names follow international rules.

Hierarchy (Linnaean ranks) – example Rohu:

Binomial Nomenclature and ICZN Rules

Binomial nomenclature (Linnaeus) gives each species a two-part Latin name: genus (capital first letter) + species epithet (small letter), printed in italics or underlined when handwritten, e.g. Catla catla, Labeo rohita.

The ICZN is the rule book for animal names; the current is the 4th edition (1999, in force from 1 January 2000), managed by the International Commission on Zoological Nomenclature.

1. Starting point: names from Linnaeus' Systema Naturae 10th ed. (1758) onwards.
2. Law of priority: the oldest available name is the valid name; later names are synonyms.
3. Homonymy: the same name cannot be used for two different animals; the later one is replaced.
4. Type concept: each species has a holotype specimen; each genus has a type species.
5. Family-group endings: family -idae (Cyprinidae), subfamily -inae, tribe -ini, superfamily -oidea.
6. Author and year: may follow the name: Labeo rohita (Hamilton, 1822). Brackets show the species was first described in a different genus (originally Cyprinus rohita).
7. Tautonyms are allowed in zoology (Catla catla).
8. Names must be Latin or Latinised and published in a proper way.

Significance for fishes: one fish may have many local names (Rohu = Rui, Tambada); the scientific name is the same worldwide, avoids confusion in fisheries data, trade, research and conservation.

Likely exam questions

1. (Long, 8 marks) Describe Berg’s classification of fishes. Mention its basis, significance and limitations.
2. (Long, 8 marks) Explain the principles of cladistics and give an outline of Nelson’s classification of fishes. State the advantages of modern phylogenetic systems.
3. (Short, 4 marks) Write a note on the history of ichthyology.
4. (Short, 4 marks) What is binomial nomenclature? Give examples from fishes.
5. (Short, 4 marks) State any four rules of ICZN.

Syllabus topics (RTMNU)

• Superclass Agnatha: General characters of jawless fishes
• Extinct ostracoderms and living cyclostomes
• Classification, morphology, and evolutionary significance of Myxine and Petromyzon
• Class Chondrichthyes: General characters and classification
• Elasmobranchii and Holocephali
• Structure and reproductive biology of cartilaginous fishes
• Class Osteichthyes – Actinopterygii: General characters and classification of ray-finned fishes
• Chondrostei, Holostei, and Teleostei
• Subclass Sarcopterygii: General characters of lobe-finned fishes
• Dipnoi and Actinistia
• Evolutionary significance in the origin of tetrapods

Superclass Agnatha: General Characters

Agnatha (a = without, gnathos = jaw) are the most primitive vertebrates – fish-like animals without true jaws.

1. No jaws; mouth is round, suctorial (sucking).
2. Paired fins absent; only median fins.
3. Notochord persists throughout life; skeleton cartilaginous.
4. Gills in pouches (marsipobranchs) opening by gill slits.
5. Single median nostril (monorhinous).
6. Skin soft and slimy, without scales (in living forms).
7. Only two semicircular canals (lamprey) or one (hagfish) in each ear.

Extinct Ostracoderms and Living Cyclostomes

Ostracoderms ("shell-skinned") were the earliest vertebrates, from Ordovician to Devonian (about 470–360 million years ago). They had a heavy bony armour of plates on the head, no jaws, and were bottom-dwelling filter or deposit feeders. Examples: Cephalaspis (Osteostraci), Pteraspis (Heterostraci), Birkenia (Anaspida). They are important because they show the earliest bone.

Cyclostomes ("round mouth") are the living agnathans: lampreys and hagfishes, about 120 species in all.

Myxine and Petromyzon: Classification, Morphology and Evolutionary Significance

Evolutionary significance:

• Living representatives of the earliest vertebrate stage before jaws evolved.
• Persistent notochord and no paired fins show primitive features.
• The ammocoete larva resembles Amphioxus (filter feeding, endostyle). The endostyle becomes the thyroid gland – evidence linking protochordates and vertebrates.
• Molecular studies show hagfishes and lampreys form one clade (Cyclostomi), sister to jawed vertebrates.

Class Chondrichthyes: General Characters and Classification

1. Endoskeleton of cartilage, often calcified; no true bone.
2. Skin with placoid scales (dermal denticles).
3. Mouth ventral; jaws with many rows of replaceable teeth.
4. 5–7 pairs of gill slits, uncovered (no operculum) in sharks.
5. No swim bladder; buoyancy by an oily liver.
6. Tail heterocercal (upper lobe longer).
7. Intestine with a spiral valve; cloaca present.
8. Males have claspers on pelvic fins; fertilisation internal.
9. Body fluids hold urea and TMAO (ureosmotic) – helps osmoregulation in sea water.

Classification: Subclass Elasmobranchii – sharks (Selachii) and rays/skates (Batoidea); Subclass Holocephali – chimaeras.

Structure and Reproductive Biology of Cartilaginous Fishes

Structure (e.g. Scoliodon, the dogfish shark): spindle-shaped body, head with ventral mouth, nostrils and lateral eyes; two dorsal fins, paired pectoral and pelvic fins, heterocercal caudal fin; ampullae of Lorenzini on snout; lateral line on sides.

Reproduction: sexes separate; fertilisation internal by claspers; few, large, yolky eggs. Three modes:

1. Oviparous: eggs laid in a horny case ("mermaid's purse"), e.g. skates, Scyliorhinus, chimaeras.
2. Ovoviviparous (aplacental viviparous): eggs hatch inside the uterus; young nourished by yolk, e.g. Squalus, many rays.
3. Viviparous (placental): the empty yolk sac forms a yolk-sac placenta with the uterine wall, e.g. Scoliodon, Carcharhinus.

Class Osteichthyes – Actinopterygii

Actinopterygii (ray-finned fishes) have fins supported by thin bony rays (lepidotrichia). General characters: bony skeleton, bony operculum over the gills, terminal mouth, swim bladder present, scales ganoid, cycloid or ctenoid, external fertilisation in most.

Subclass Sarcopterygii: Lobe-finned Fishes

General characters: paired fins fleshy and lobed, with a central chain of bones and muscles; cosmoid scales (in older forms); lungs or lung-like swim bladder; some with internal nostrils; diphycercal tail in many.

Dipnoi (lungfishes): three living genera – Neoceratodus (Australia, single lung, cannot aestivate), Protopterus (Africa) and Lepidosiren (South America), both with paired lungs. In dry season Protopterus aestivates in a mucus cocoon in mud and breathes air.

Actinistia (coelacanths): thought extinct for 70 million years until Latimeria chalumnae was caught off South Africa in 1938 (identified by J. L. B. Smith); a second species, L. menadoensis, was found in Indonesia (1997–98). A "living fossil": persistent notochord, fat-filled swim bladder, rostral organ (electroreception), ovoviviparous.

Evolutionary Significance in the Origin of Tetrapods

1. Bones in the lobed fins of Eusthenopteron match the humerus, radius and ulna of tetrapod limbs.
2. Transitional fossils: Eusthenopteron → Panderichthys → Tiktaalik (about 375 million years ago; neck, flat head, wrist-like fin bones) → Acanthostega and Ichthyostega (early tetrapods).
3. Lungs and internal nostrils allowed air breathing.
4. Lungfishes are the closest living fish relatives of tetrapods (DNA evidence).

Likely exam questions

1. (Long, 8 marks) Give the general characters of Agnatha. Compare Myxine and Petromyzon and state their evolutionary significance.
2. (Long, 8 marks) Describe the general characters of Sarcopterygii. Discuss Dipnoi and Actinistia and their significance in the origin of tetrapods.
3. (Short, 4 marks) Differentiate between Elasmobranchii and Holocephali.
4. (Short, 4 marks) Describe the modes of reproduction in cartilaginous fishes.
5. (Short, 4 marks) Distinguish Chondrostei, Holostei and Teleostei.

Syllabus topics (RTMNU)

• Integumentary System and Scales: Structure and functions of fish skin
• Epidermis and dermis
• Types of scales—placoid, ganoid, cycloid, and ctenoid
• Significance of scales
• Swim Bladder and Weberian Ossicles: Structure and functions of swim bladder
• Physostomous and physoclistous types
• Role in buoyancy and respiration
• Weberian apparatus and its role in sound transmission
• Electric Organs: Structure and functions of electric organs in fishes
• Role in electric signal production
• Lateral line system and Electroreception: Structure and function of the lateral line system
• Role of neuromasts in detecting water movements, vibrations and pressure changes
• Concept of electroreception and its role in sensing electrical signals in fishes
• Types of electroreceptors: ampullary and tuberous receptors

Structure and Functions of Fish Skin: Epidermis and Dermis

The skin (integument) of fish has two layers:

1. Epidermis (outer, from ectoderm): stratified epithelium of living cells (usually not keratinised). It contains:
   • Mucous (goblet) cells – secrete mucus.
   • Club cells – release an alarm substance when skin is injured (in carps and catfishes).
   • Granular/poison gland cells (e.g. stonefish, catfish spines) and sensory cells.
2. Dermis (inner, from mesoderm): connective tissue with collagen fibres, blood vessels, nerves. Upper loose stratum spongiosum and lower dense stratum compactum. It has chromatophores (melanophores – black, xanthophores – yellow, erythrophores – red, iridophores – silvery, with guanine crystals) and forms the scales.

Below the dermis lies the hypodermis (fat and loose tissue).

Functions of skin: protection from injury and germs; mucus reduces friction during swimming and blocks infection; helps in osmoregulation; colour and colour change (camouflage, breeding colours); light production by photophores in deep-sea fishes; skin breathing in eels; sensory functions (taste buds, lateral line); defence by poison glands.

Types of Scales

Scales are small hard plates in the skin that protect the body. Except placoid scales, they are dermal bony scales.

Cycloid and ctenoid scales together are called leptoid (elasmoid) scales. Fossil lobe-finned fishes had cosmoid scales.

Significance of Scales

1. Protection against injury and parasites.
2. Age and growth study: yearly rings (annuli) form when growth slows (scalimetry).
3. Taxonomy: number of scales along the lateral line and scale type help identify species.
4. Hydrodynamics: placoid denticles reduce drag (idea copied in swimsuits).
5. Evolution: placoid scales are homologous with teeth.

Swim Bladder: Structure, Types and Functions

The swim bladder (air bladder) is a thin-walled gas-filled sac lying dorsal to the gut, below the kidneys. It arises as an outgrowth of the foregut. It is absent in sharks and in many bottom-dwelling fishes.

Structure: wall of outer fibrous tunica externa, smooth muscle and inner epithelium. In closed bladders there is a gas gland supplied by a rete mirabile (a network of parallel capillaries working by countercurrent exchange) and an oval (a thin area that absorbs gas).

Gas secretion: the gas gland produces lactic acid and CO₂; low pH makes haemoglobin release O₂ (Root effect); the rete mirabile keeps the high gas pressure near the bladder so gas enters it.

Functions:

1. Buoyancy (hydrostatic): by changing gas volume the fish becomes neutrally buoyant and stays at any depth without effort.
2. Respiration: in Amia, Lepisosteus and lungfishes it is vascular and works as a lung.
3. Sound production: drumming muscles vibrate it (croakers).
4. Sound reception and pressure detection: via Weberian ossicles.

Weberian Apparatus and Sound Transmission

The Weberian apparatus (named after E. H. Weber, 1820) is a chain of small bones connecting the swim bladder to the inner ear. It is found in Ostariophysi – carps, minnows, catfishes, characins.

Structure: four paired ossicles, modified from parts of the first four vertebrae, from behind forward: tripus → intercalarium → scaphium → claustrum.

Mechanism: sound waves make the swim bladder vibrate → the tripus moves → vibrations pass through intercalarium and scaphium → to the fluid-filled sinus impar → transverse canal → sacculus of the inner ear → hair cells stimulated.

Role: the swim bladder acts like an eardrum and the ossicles like the middle-ear bones of mammals; hearing becomes more sensitive and covers higher frequencies.

Electric Organs: Structure and Functions

Electric organs are special organs that produce electric discharges. In most fishes they are modified muscle; in some (e.g. the knifefish Apteronotus) they come from nerve fibres.

Structure: the organ is made of thousands of flat cells called electrocytes (electroplaques), stacked in columns in a jelly-like tissue. Only one face of each electrocyte receives a nerve ending.

Mechanism (role in signal production):

1. At rest both faces are equally polarised, so there is no net current.
2. A nerve impulse releases acetylcholine on the innervated face, which depolarises and reverses (inside becomes positive).
3. The other face stays polarised, so each cell acts like a small battery of about 0.15 volt.
4. Thousands of cells in series add their voltages (electric eel – high voltage in fresh water); many columns in parallel give high current at lower voltage (Torpedo – suits salty sea water).
5. All cells fire together because a command nucleus in the brain sends synchronised impulses.

Functions: strong discharges stun prey and defend against enemies; weak discharges are used for electrolocation (finding objects in muddy water) and electrocommunication (species and sex recognition).

Lateral Line System and Neuromasts

The lateral line is a sense organ system seen as a line of pores along each side of the body, branching on the head (supraorbital, infraorbital and mandibular canals). It lies in a canal under the skin that opens to the outside through pores in the lateral line scales.

Neuromast – the sensory unit: a group of hair cells, each with many stereocilia and one long kinocilium, embedded in a jelly-like dome called the cupula, with supporting cells. Neuromasts are free on the skin (superficial) or inside canals (canal neuromasts). They are supplied by the lateral line nerves.

Mechanism: water movement → bends the cupula → stereocilia bend towards the kinocilium → hair cell depolarises → more nerve impulses; bending the other way reduces impulses. So the fish knows the direction of the flow.

Functions: detects water currents, low-frequency vibrations and pressure changes; locates prey and predators; avoids obstacles (blind cave fish); helps keep position in a school; orientation against current (rheotaxis).

Electroreception: Concept and Receptor Types

Electroreception is the ability to sense weak electric fields in water. It is passive (sensing fields made by other animals' muscles and nerves, e.g. sharks finding fish buried in sand) or active (sensing changes in the fish's own electric field caused by nearby objects, in weakly electric fishes).

Likely exam questions

1. (Long, 8 marks) What is a swim bladder? Describe its structure, types and functions. Add a note on the Weberian apparatus.
2. (Long, 8 marks) Describe the structure and mechanism of electric organs in fishes. Add a note on electroreceptors.
3. (Short, 4 marks) Describe the types of scales in fishes.
4. (Short, 4 marks) Explain the structure and function of a neuromast.
5. (Short, 4 marks) State the functions of fish skin.

Syllabus topics (RTMNU)

• Fish Products and By-Products: Fish as a source of food
• Fish oil
• Fish meal
• Isinglass
• Fish glue
• Fish manure
• Fish Migration: Types of migration—anadromous, catadromous, potamodromous and oceanodromous
• Physiological and environmental factors controlling migration
• Fish Pathology: Common fish diseases
• Bacterial, viral, fungal and parasitic infections
• Symptoms, causes and preventive measures
• Harvest Technology: Fishing Crafts and Gears
• Types of fishing crafts- non-mechanized and mechanized
• Fishing gears—hooks and lines, gill nets, cast nets, drag nets, trawls

Fish as a Source of Food

Fish is a cheap, healthy and easily digested source of animal protein for millions of Indians, especially in coastal states, Bengal, Assam and Kerala.

1. Protein: about 15–20% of fresh fish flesh; high quality with all essential amino acids (rich in lysine and methionine).
2. Fats: rich in omega-3 fatty acids (EPA and DHA) which protect the heart and help brain development; low saturated fat.
3. Vitamins: A and D (liver oils, oily fish), B₁₂.
4. Minerals: iodine (marine fish), calcium and phosphorus (small fish eaten with bones), iron, zinc.
5. Easily digestible because it has little connective tissue.

Lean fish (e.g. pomfret, cod) store fat in the liver; oily fish (e.g. oil sardine Sardinella longiceps, mackerel Rastrelliger kanagurta, hilsa) store fat in the flesh. Fish spoil fast, so they are preserved by chilling, freezing, drying (e.g. Bombay duck, Harpodon nehereus), salting, smoking and canning.

Fish Oil

Two kinds:

• Liver oil – from the livers of sharks, rays and cod; very rich in vitamins A and D.
• Body oil – from whole oily fish such as sardines and mackerel, and from processing waste.

Extraction: fish/livers are cooked or steamed → pressed → oil separated from water by settling or centrifuge → refined.

Uses: vitamin and omega-3 capsules (medicine); paints, varnishes and printing ink; soap and candles; tanning leather; lubricants; aquaculture and poultry feed.

Fish Meal

Fish meal is a dry, powdered product made from whole low-value fish or fish waste. Process: cooking → pressing (removes oil and water) → drying → grinding.

It is rich in protein (roughly 55–65%), minerals and B vitamins. Uses: feed for poultry, pigs, cattle and especially for fish and shrimp farms. In India, oil sardine and other small fishes are used on the west coast.

Isinglass, Fish Glue and Fish Manure

Isinglass: a pure, transparent form of collagen (gelatin) made from the dried swim bladders ("fish maws") of fishes such as sturgeons, catfishes (Arius), threadfins (Polynemus) and croakers. Bladders are cleaned, dried and cut into strips. Uses: clearing (fining) beer and wine, making jellies, special glues and cement, and in medicine.

Fish glue: made by boiling fish skin, bones, heads and scales, which releases collagen as glue. Uses: bookbinding, woodwork, gummed paper and tapes.

Fish manure (fish guano): fish waste and unsaleable fish are dried and powdered, or made into liquid fish fertiliser. It is rich in nitrogen and phosphorus and improves soil for crops like coconut and coffee.

Other by-products: shark fins, shagreen (shark skin leather), fish silage, and pearl essence (guanine from scales).

Fish Migration: Types

Migration is a regular, usually yearly, mass movement of fishes from one habitat to another and back, for breeding, feeding or shelter. Fishes that move between sea and fresh water are called diadromous.

Salmon young (parr) change into silvery smolts before going to sea, and adults return to their birth stream using smell (olfactory homing). Eel larvae (leptocephali) drift with currents to Europe.

Factors Controlling Migration

Physiological (internal) factors:

1. Maturity of gonads – the main drive for breeding migration.
2. Hormones – pituitary gonadotropins, thyroid hormones (smoltification), cortisol and prolactin (for changing salt balance).
3. Osmoregulatory changes – gills and kidneys adjust between salt and fresh water.
4. Fat reserves – stored energy for the long journey (salmon stop feeding).
5. Internal rhythms (biological clock).

Environmental (external) factors: water temperature; day length (photoperiod); water currents (fish swim against current – rheotaxis); salinity; rainfall and floods (monsoon triggers carp breeding); food availability; dissolved oxygen; tides and moon phases; chemical cues (smell of home stream); sun position and the earth's magnetic field for navigation.

Fish Pathology: Common Diseases

Disease outbreaks mostly follow stress – poor water quality, crowding, handling, injury and bad feed – which lowers the fish's immunity.

Preventive measures:

1. Keep good water quality – oxygen, pH, no excess organic load; change water when needed.
2. Prepare ponds by drying and liming (quick lime) to kill germs.
3. Avoid overcrowding and rough handling.
4. Give clean, balanced feed; do not overfeed.
5. Quarantine new fish; buy healthy seed from trusted hatcheries.
6. Disinfect nets and equipment (potassium permanganate solution); remove dead fish quickly.
7. Short dips in common salt or KMnO₄ solution are traditional treatments for external parasites, given on expert advice; follow current rules on drug and antibiotic use (check latest).

Fishing Crafts: Non-mechanised and Mechanised

A craft is the boat or vessel used for fishing; a gear is the tool or net used to catch fish.

Non-mechanised (traditional) crafts – moved by oars, paddles or sails:

• Catamaran (kattumaram) – logs tied together; Tamil Nadu and Andhra coast.
• Dugout canoe – scooped out of one tree trunk; Kerala, Karnataka.
• Plank-built boats – masula boat (Andhra), machwa (Maharashtra, sail boat).
• Coracle – round bamboo basket covered with hide or plastic; rivers and reservoirs.
• Rafts and small country boats in inland waters.

Mechanised crafts – fitted with engines: motorised traditional boats with outboard motors; fibreglass (FRP) boats; trawlers, gill-netters, purse-seiners, dol-netters and long-liners. Benefits: go farther, stay longer, larger catch; but cost more fuel and can overfish.

Fishing Gears

1. Hooks and lines: a baited hook on a line. Types – hand line, pole and line (tuna in Lakshadweep), long line (many hooks on one main line), troll line (towed behind a moving boat). Selective and cause little damage.
2. Gill nets: a wall of net held upright by floats on top and sinkers below. Fish try to pass through and get stuck by the gills. Drift gill nets float with current; set (bottom) gill nets are fixed. Mesh size decides fish size caught (e.g. hilsa, pomfret, seer fish).
3. Cast nets: a round net with weights on the edge, thrown by one person so it opens and falls over fish; pulled shut by a rope. Used in ponds, rivers and shallow coasts.
4. Drag nets (seines): a long net with wings and often a bag, dragged along the bottom to encircle fish – shore seine (pulled to the beach by many people), boat seine, and drag nets used for harvesting pond fish. Purse seine encircles a shoal and is closed at the bottom like a purse (sardine, mackerel).
5. Trawls: a cone-shaped bag net towed by a boat (trawler). Otter boards keep the mouth open (otter trawl). Bottom trawls catch prawns and bottom fish; mid-water trawls catch shoaling fish. Problems: by-catch and damage to sea floor; turtle excluder devices (TEDs) reduce turtle deaths.

Likely exam questions

1. (Long, 8 marks) What is fish migration? Describe its types with examples and the physiological and environmental factors controlling it.
2. (Long, 8 marks) Describe common bacterial, fungal, viral and parasitic diseases of fishes with their symptoms and preventive measures.
3. (Short, 4 marks) Write a note on fish oil and fish meal.
4. (Short, 4 marks) What is isinglass? Give its uses.
5. (Short, 4 marks) Describe gill nets and trawl nets.