CLASS OF FISH

Class: Placodermi
•    The placoderms dominated the waters of the late Devonian period and died out completely in the late Carboniferous period (about 350 million years ago).
•    Body was typically fish-like with bony armour.
•    Jaws were present, but not properly formed.
•    Paired and unpaired fins were present.
•    Cranium was partially ossified.
•    Vertebral column had neural and haemal arches.
•    Notochord persisted throughout life.
•    Example: Dunkleosteus spp. (Figure 2.2).

Figure 2.2. A. Dunkleosteus skull present at Sam Noble Museum, Oklahoma, USA. 

B. An imaginary drawing of Dunkleosteus.

Class: Elasmobranchii
•    Mostly marine and includes largest fishes that ever lived.
•    Endoskeleton is cartilaginous which is partially calcified.
•    There are 5-7 pairs of gills without operculum.
•    There is no air bladder and lungs.
•    Spiracle is generally present.
•    Skin is naked or covered by small placoid scales, composing toothlike denticles that give the skin an abrasive texture.
•    Mouth is generally ventral.
•    Dorsal fins are rigid.
•    Fins are without rays. Tail is heterocercal.
•    The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm (Figure 2.3).
•    Intestine has a characteristic spiral valve.
•    A cloaca is present.
•    Fertilization is generally internal.
•    Examples: Carcharodon carcharias (White shark) (Figure 2.4), Rhincodon typus (Whale shark), Scoliodon laticaudus (Spadenose shark), Manta birostris  (Ray) (Figure 2.5), Raja erinacea (Skate).

                       Figure 2.3. Male and female sharks, (A) Male with claspers, (B) female without claspers.

                                                      Figure 2.4. Carcharodon carcharias

                                                          Figure 2.5. Manta birostris

Class: Holocephali
•    This class includes deep sea and carnivorous fishes.
•    Endoskeleton is cartilaginous.
•    There are 4 pairs of gills with operculum.
•    Spiracle is absent.
•    There is no air bladder.
•    Skin is naked or covered by small placoid scales.
•    Mouth is generally ventral.
•    There is no cloaca.
•    In male, a pair of claspers is present posterior to the pelvic fins as in Elasmobranchii. Besides these, a frontal clasper is present on the dorsal surface of the head of some species known as tenaculum.
•    Examples: Chimaera spp. (Ratfish), Harriotta spp. (Spookfish) (Figure 2.6).

                                          Figure 2.6. A. Chimaera spp., B. Harriotta haeckeli

Class: Dipnoi
•    Mostly found in freshwater and commonly known as lungfish.
•    The air bladder is modified into lung which serves for aerial respiration. Gills are also present.
•    Notochord is persistent and unconstricted. Skeleton is largely cartilaginous.
•    Skin is covered by cycloid scales.
•    Operculum is present and there is only one external branchial aperture.
•    Paired fins are lobate, cylindrical, with joined median axis.
•    Internal nostrils are present.
•    Intestine has a spiral valve.
•    A cloaca is present.
•    Examples: Protopterus spp. (African lungfish) (Figure 2.7), Lepidosiren spp. (American lungfish).

                                                          Figure 2.7. Protopterus spp.






Class: Teleostomi
•    The endoskeleton is bony, and are known as bony fishes.
•    There are 4 pairs of gills; the 5th pair of gill arch is modified into tooth bearing lower pharyngeal.
•    Operculum is present.
•    Spiracle is absent.
•    Air bladder is usually present.
•    Skin is generally covered by cycloid or ctenoid scales.
•    Branchial lamellae are supported by a double row of branchial rays.
•    Fins are with fin rays. Tail is homocercal or diphycercal.
•    No spiral valve in the intestine.
•    There is no cloaca.
•    Fertilization is generally external.
•    Examples: All fishes found in Nepal.

The class Teleostomi is divided into two sub-classes, Crossopterygii and Actinopterygii.

 Sub-class: Crossopterygii
•    This is the rare (almost extinct) sub-class, which is characterized by the presence of lobed paired fins covered with scales, and are known as lobed-fin fishes.
•    There are two dorsal fins.
•    The inner nostrils are present.
•    The air bladder is modified into lung.•    The scales are cosmoid type.
•    This sub-class is not represented in Nepal, and includes some rare marine fishes.
•    Examples: Latimeria (Figure 2.8), Coelocanthus.

                                                                      Figure 2.8. Latimeria


Sub-class: Actinopterygii
•    This sub-class contains most of the bony fish species that exist today.
•    The paired fins are unlobed and without scales.
•    There are generally single dorsal fin.
•    The inner nostrils are completely absent.
•    The external nostrils are present.
•    The scales are cycloid or ctenoid type.
•    This sub-class is represented by all fish species in Nepal.
•    Examples: Labeo rohita, Tor putitora, Catla catla (Figure 2.9), Oreochromis niloticus (Figure 2.10) and all other fish species found in Nepal.

                                                              Figure 2.9. Catla catla

                                                        Figure 2.10. Oreochromis niloticus

DISTINGUISHING CHARACTERISTICS OF EACH LEVEL

   Kingdom: Animalia
•    Absence of cell wall.
•    Heterotrophic.
•    Plastids and vacuoles are generally absent and centrioles and lysosomes are present.
•    Reserve food is in the form of glycogen.

    Phylum: Chordata
•    Presence of notochord.
•    Presence of gill slits.
•    Presence of dorsal tubular nerve cord.

    

     Sub-Phylum: Craniata
•    Presence of brain box (cranium).
•    Vertebrae surrounding nerve cord.

    Division: Gnathostomata
•    Presence of true jaw.
•    Presence of paired nostrils.

    Super-class: Pisces
•    Aquatic habitat.
•    Streamlined body.
•    Respiration by means of pharyangeal gills.
•    Locomotion by means of fins.
•    Body is generally covered by scales.
•    Presence of internal ear.

PARTR 2:- TAXONOMY OF FISHES OF NEPAL

The word taxonomy is derived from two Greek words, ‘taxis’  means arrangement and ‘nomia’ means method. Thus, the taxonomy can be simply defined as the science of classifying biological organisms. Fish taxonomy is concerned with the identification and classification of fishes to the level of species or subspecies, following the conventional principles of basic taxonomy.

The Swedish Botanist Carolus Linnaeus (1707-1778) is regarded as the father of modern taxonomy, as he developed a system known as Linnaean classification for categorization of organisms and binomial nomenclature for naming organisms. However, the word taxonomy was first coined by a French Professor A. P. de Candolle as ‘Taxonomie’ in 1813. Taxonomy has three stages; (i) alpha taxonomy, which deals with the description of new species and its arrangement in comprehensive genera, (ii) beta taxonomy, which works out the relationships to the level of species, and (iii) gamma taxonomy, which emphasis intraspecific variations and their evolutionary relationships. The term ‘taxon’ (plural taxa) refers to the taxonomic group of any rank which is sufficiently distinct to be worthy of being assigned to a definite category.

2.1. Nomenclature of fishes
Nomenclature is the scientific naming of fish or any organisms. Common names frequently vary so they are not reliable for identification. Every living organism has only one unique and internationally recognized scientific name. For example, common carp is called zoologically Cyprinus carpio all over the world. The nomenclature of zoological species is regulated by the International Code of Zoological Nomenclature (ICZN).

Whenever a species or genus is newly described, certain rules of zoological nomenclature have to be taken into consideration. In binomial nomenclature, the name of a species consists of two words, the first word denoting the genus and the second word denoting the species (e.g., Cyprinus carpio). Sometimes a species also has a subspecies, in which the full scientific name becomes trinomial. In case of trinomial nomenclature, the name of a species consists of three words, the first word denoting the genus, the second word denoting the species and the third word denoting the subspecies (e.g., Cyprinus carpio specularis). The genus name is always a noun and written with an initial capital letter while species and subspecies name are generally an adjective and are always written in small letters. The genus, species and subspecies name should be either underlined separately or written in Italic font. When a subgenous name is used, it is placed within parentheses between the geneus and species names [e.g., Channa (Ophiocephalus) striatus]. To indicate many species within a genus, the abbreviation ‘spp.’ is sometimes used and should not be italicized.

In taxonomic classification of fishes, the name of the order generally ends with ‘formes’, of the family with ‘idae’, of the subfamily with ‘inae’ of the tribe with ‘ini’. When the species is described for the first time, its name should be written, for example, as Neoanguilla nepalensis sp. nov., while a new genus should be expressed, for example, as Neoanguilla gen. nov. When the said species is cited by another author, it should be written as Neoanguilla nepalensis (Shrestha) to indicate the author of the species.

2.2. Classification and identification of fishes of Nepal
Fishes have been classified in different ways from time to time. The earliest classification was proposed by J. Muller (1844) who proposed seven sub-classes under the class ‘Pisces’. Later, Berg (1940) and Romer (1959) have given detailed classification of fishes, which have generally been accepted worldwide. In this book, we follow the classification given by Berg (1940), who divided the Super-class (Series) Pisces into seven classes: (1) Acanthodii, (2) Coccostei, (3) Pterichthys, (4) Elasmobranchii, (5) Holocephali, (6) Dipnoi, and (7) Teleostomi (Figure 2.1). Of these, the first three classes are completely extinct and are collectively known as Placodermi (meaning plate-skinned). Thus, the modern fishes are represented only by four major classes, i.e., Elasmobranchii, Holocephali, Dipnoi and Teleostomi. Out of these, the fishes of Nepal belong to the sub-class Actinopterygii of the class Teleostomi. Berg has divided the sub-class Actinopterygii into a series of 59 orders. Out of these, only 11 orders are represented in freshwater ecosystem of Nepal.

The total number of fish species represented in Nepal has been controvercial and is a subject of study. Different fish biologists have reported different number of fish species present in Nepal, which ranged from 228-232. According to Shrestha (2008), a total of 232 species belonging to 114 genera under 36 families and 11 orders existing in culture or natural water bodies of Nepal. Recently, Shrestha and Pandit (2012) have described one more exotic species, Pangasius hypophthalmus (Sauvage), which has been commercially cultured in the terai region of Nepal since few years. This indicated that the total number of fish species present in Nepal might be more than 232.

1.4. HARMFUL FISHES

Actually, the harm caused by fishes is negligible in comparison to their benefits. Certain marine fishes like sharks and rays are extremely dangerous, and cause injuries and even death to swimmers and fishermen. They also damage the nets of fishermen. Some fishes (e.g. stonefish: Synanceia verrucosa) inflict wounds with their stings and insert poison that is very painful. Some electric fishes such as Torpedo are capable of generating electric shocks to man. Some fish species are the intermediate hosts of various parasites causing diseases in human and other animals.

Like marine fishes, some freshwater fishes also carry different types of toxic substances and these when consumed cause food poisoning or have carcinogenic effects. Some of these are as follows.

a. Ootoxin: This type of toxin is present in the eggs of some fishes such as catfish (Clarias spp.), asala (Schizothorax spp.) and common carp (Cyprinus carpio). Mature fishes have more ootoxin in their eggs. Consumption of such eggs without proper cooking might cause headace, diarrhoea, vomiting and fever.

b. Zootoxin: Certain catfishes such as Heteropneustes fossilis have zootoxin in their pectoral fin spine when they are alive. It inflicts by pectoral spine which is extremely painful and cause swelling of wounding place with fever. There is no harm if these fishes are eaten by proper cooking. 

c. Haemotoxin: This type of toxin is found in the blood of some fishes. Certain species of eels (Anguilla spp.) have been reported to carry haemotoxin in their blood. Consumption of such fishes without proper cooking might cause vomiting and diarrhoea.

d. Tetradoxin: It is also known as puffer poisoning and is caused by some species of puffer fishes (Tetraodon spp.). The mature puffers contain a toxic substance called tetradoxin in the skin, liver, intestine and eggs. Tetradotoxin is neurotoxic and inhibits neural transmission even at relatively low concentrations (~2mg). These fishes when consumed without removing such organs lead to gastrointestinal disorders and neurological disorders such as paralysis, convulsions and respiratory failure leading to death. Thus, only highly trained and certified chefs are allowed to prepare this fish for consumption; a common practice in Japan where pufferfish are considered a delicacy.

e. Heavy metals: Fishes that live in industrial polluted water contain large amount of heavy metals and chemical toxins such as mercury, lead, cadium, dioxins and phthalates. Predatory and carnivorous fishes that feed on smaller ones generally contain higher amounts of such heavy metals (e.g., shark, tuna, freshwater bass, catfish, trout, etc.). Poisoning form such heavy metals and chemicals affects brain development and the nervous system of the developing foetus and child. The U. S. Food and Drug Administration (FDA) has recommended that the consumption of fish be limited during pregnancy and chieldbearing, with some fish such as tuna, swordfish and tilefish to be completely avoided.

1.3. ECONOMIC IMPORTANCE OF FISH

Fishes are one of the most important group of vertebrates serving as food for human. They possess a great economic, nutritional, medicinal, industrial, aesthetic and religious values as well as providing employment for millions of people in the world. They contribute to food security in many regions of the world, providing a valuable supplement for diversified and nutritious diets. Consumption of fish has several health, nutritional, environmental and social advantages over other terrestrial animal meat. The importance of fishes can be discussed under the following headings.

I. Food value
Fish serves as an important food for human. Edible tissues of fish are appreciably greater than that in chicken, pig and sheep/goat. For example, approximately 65% of the raw weight of finfish is eaten, compared with 50% of chicken and pigs, and 40% of sheep/goat; fish are supported by water, but terrestrial animals and birds require comparatively strong bones so they spend their substantial energy into the growth of the bones, which cannot be consumed as food. The total estimated fish production of the world in 2012 was 158 million metric tons with a per caput consumption around 19.2 kg. Similarly, the total estimated fish production of Nepal in 2013/14 was 64,900 metric tons with a per caput consumption of 2.3 kg.

II. Nutritive value
Fish is highly nutritious. It provides tasty, low calorie meal but is a good source of high quality protein. Fish is an almost zero-carbohydrate food, good for diabetes and other such patients. The protein content in fishes varies from 15-30% on wet weight basis and 60-80% on dry weight basis. The protein of fish is highly digestible and with well-balanced amino acids. Fish are low in fat and cholesterol. Fish is a good source of Vitamins- A, B and D and also offers a good source of calcium, iodine, fluorine, magnesium and zinc. Fish are rich in poly-unsaturated fatty acids containing Omega-3. Two poly-unsaturated fatty acids present in fish, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), collectively known as Omega-3, are essential fatty acids. They cannot be produced in human body, but are essential in the diet. These poly-unsaturated fatty acids can help to reduce the cholesterol level in the blood, thus minimize the risk of heart attack.

III. Medicinal value
Fish is low in fat, high in protein and an excellent source of Omega-3 fatty acids. Regular consumption of fish can reduce the risk of various diseases and disorders. Some research findings indicate the following:

Asthma: Children who eat fish are less likely to develop asthma.

Brain and eyes: Fish rich in Omega-3 fatty acids can contribute to the health of brain tissue and the retina of the eye. The IQ level of children whose mother consumed about 340 g fish per week during pregnamcy was found higher than non-fish eaters. Similarly, breastfed babies whose mothers eat fish have better eyesight, perhaps due to the Omega-3 fatty acids transmitted in breast milk.

Cancer: The Omega-3 fatty acids in fish reduce the risk of many types of cancers by 30 to 50 percent, especially of the oral cavity, oesophagus, colon, breast, ovary and prostate.

Cardiovascular disease: Eating fish every week reduces the risk of heart disease and stroke by reducing blood clots and inflammation, improving blood vessel elasticity, lowering blood pressure, lowering blood fats and boosting good cholesterol.

Dementia: Elderly people who eat fish or seafood at least once a week may have a lower risk of developing dementia, including Alzheimer’s disease.

Depression: People who regularly eat fish have a lower incidence of depression. Depression is linked to low levels of Omega-3 fatty acids in the brain.

Diabetes: Fish may help people with diabetes to manage their blood sugar levels.

Prematurity: Eating fish during pregnancy may help reduce the risk of delivering a premature baby.

Further, it is observed that different fishes are used as Ayurvedic medicines which help in treatment of duodenal ulcers, skin disease, night blindness, weakness, loss of appetite, cough and cold, bronchitis, asthma, tuberculosis, etc.

IV. Fish products
a. Fish meal: The dried and ground preparation of unused or trash fish is called fish meal and is a highly nutritive product that makes an excellent feed for poultry, pig, cattle and fish. Fish meal contains about 60-70% crude protein, 2-15% oil and 10-20% minerals. The protein of fish meal is highly digestible and contains all amino acids. It is also very rich in vitamins, calcium, phosphorous and iodine.

b. Fish oil: Fish oil is of two kinds, body oil and liver oil. The oil extracted from the whole body of the fish is called fish body oil. For the extraction of body oil, the fishes are minced, steamed, and then pressed for oil recovery, then subject to filter for refine. It is very rich in iodine. The fish body oil is mostly used in manufacture of paints, varnishes, soaps, lubricants, candle, printing inks, etc. It is also used in dressing of leather, tanning of skin and smearing the surface of boats for longer preservation.

The oil obtained from the liver of fishes is called fish liver oil. For the extraction of liver oil, livers are chopped and boiled in water. The oil is then skimmed out from the surface of the water and purified. Fish liver oil has medicinal value. It contains 55-75% fat and 5-10% protein. It is very rich in Vitamin A and D. Shark (Carcharhinus spp.) and Cod (Gadus callarius) liver oil are well known in pharmaceutical industries.

c. Fish silage: Fish silage is a liquid or semi-liquid product made from whole or part of the fish that are liquefied by the action of enzymes in the fish in the presence of an added acid (Hydrochloric or Sulfuric acid). The enzymes break down fish proteins into smaller soluble units, and the acid helps to speed up their activity while preventing bacterial spoilage. It is a highly nutritive animal feed. It contains about 15% protein; however, the composition of fish silage depends on the fish species from which it is made.

d. Fish flour: Fish flour is made up of dried and powdered fish. It is a superior quality of fish meal which is used for human consumption. It is considered an ideal protein source to supplement diet. It can be mixed with wheat or maize flour and used for enriching the nutritive value of bread, biscuits, cakes, etc. It is also known as hydrolyzed protein of fish.

e. Fish manure and guano: Low grade, unedible fish and offal from the fish-canning factories, fish-salting plants, fish-filleting plants and other fish processing industries are utilized to prepare the fish manure. These residues are dried, ground, mixed with ash and converted into manure, which contains a high percentage of nitrogen and phosphorous.

The word ‘guana’ means dung in Spanish language. Fish guano is prepared from the material left over after extracting oil from the fish. It has high content of nitrogen (8-10%), and has been found more effective than animal manure.

f. Fish glue: The gelatinous adhesive material obtained from the connective tissues of skin and bones of certain fish, principally cod fish, is called fish glue. It is used in gummed tape, letterpress printing plates, blueprint paper and adhering the wood, leather, glass, etc.
g. Isinglass: Isinglass is a substance obtained from the dried swim bladders of fish. It is a high-grade collagen used mainly for the clarification of wine, beer and vinegar. It can also be cooked into a paste for specialized gluing purposes. For making isinglass, the air bladder is removed from the fish, washed thoroughly and flattened by beating. The beaten pieces are then dried for the preparation of isinglass. It is in the form of shiny powder. Isinglass is generally made from sturgeon and cod fish.

h. Fish leather: The coarse scaly skins of several fishes (e.g., sharks and rays) are used for manufacturing, polishing and smoothing materials in place of sand paper. The dried and processed skin is also used for preparing high-quality shoes, moneybags, suitcases, belts, phone cases, etc. Fish leather is an eco-friendly alternative to the typical exotic leathers such as crocodile and snake that threaten endangered species.

The production of fish leather is a long and complicated process. For this purpose, skins from larger fish is collected and soaked in brine for a day then salted and again put in brine containing 10% hydrochloric acid. After that, skin is scraped on the surface and tanned by usual process. Actually, the exact details of the process are a closely guarded secret as only few manufacturers in the world are able to make it to a high standard after years of experimentation and fine-tuning.

i. Fish fin: The fins of the sharks and rays are used to make tasteful sauce and soups. It is a popular soup item of Chinese cuisine usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture.

j. Fish caviar: Caviar is a high value fish product. It is a salt-cured eggs of certain species of fish such as sturgeon, salmon and trout. Traditionally the term caviar refers only to the eggs from some species of wild sturgeon in the Caspian and Black Sea. Depending on the country, caviar may also be used to describe the eggs of other fish such as salmon, trout, lumpfish and whitefish. Caviar is considered a delicacy and luxurious food and is eaten as a garnish or a spread.

For preparation of caviar, ovaries are first removed from the sedated female sturgeon and passed through a sieve to remove the membrane. Then the eggs are rinsed to wash away impurities. The eggs are then ready to become caviar by adding a precise amount of salt for taste and conservation. The fresh preparation is tasted and graded according to quality. Finally, the shiny eggs are packed into lacquer lined tins that will be further processed or sold directly to customers. It can be sold as fresh (non-pasteurized) or pasteurized, with pasteurization reducing its culinary and economic value. The price of caviar generally depends on age and species of the fish. For example, caviar of 60 to 100 years old Iranian beluga sturgeon costs about $35,000/kg. Caviar from salmon and trout are cheaper.

k. Fish pearls: The material obtained by scraping the silvery coating of the scales of certain fishes is used for polishing the hollow glass beds. These beds are then filled with wax and marketed as artificial pearls which are used as jewelry.

l. Fish insulin: Insulin is extracted from the pancreas of large sized fishes such as sharks. Prior to the introduction of biosynthetic insulin, insulin derived from sharks and other fishes was extensively used for glycemic control in human.

V. Biological control
Many species of carnivorous and larvivorous fishes prey upon insects and their larvae in water. These fishes can be used to control harmful insects, mosquito larvae, etc. Gambusia affinis is a well known fish for mosquito control, hence called mosquito fish. Other larvivorous species are Brachydanio spp., Rasbora spp., Puntius spp., Esomus spp., Colisa spp., Barilius spp., Chela spp., etc. They help in biological control of dengue, malaria, filaria, encephalitis, etc.

Similarly, the herbivorous species like grass carp (Ctenopharyngodon idella), tilapia (Oreochromis spp.), silver barb (Puntius gonionotus), etc. are used to control aquatic weeds and vegetations.

VI. Sports and games
Sport or recreation fishing is the fishing for pleasure or competition. Sport fishing can be done in a variety of ways. The most common form of sport fishing is done with a rod, line and hooks with baits, called angling. The most common sport fishes of Nepal are sahar (Tor spp.), asla (Schizothorax spp.) and rainbow trout (Oncorhynchus mykiss).

VII. Decorative value
Many species of colored fish are kept in aquarium, ocenarium, ponds and lakes for decoration or ornamentation. The common ornamental fish species kept in aquarium in Nepal are gold fish (Carassius spp.), gourami (Colisa spp.), zebra fish (Brachidanio spp.), guppy (Poecilia spp.), fighting fish (Betta splendens), koi (Cyprinus carpio), etc.

VIII. Employment opportunity
Fisheries and aquaculture sector provide, either directly or indirectly, a great employment opportunity for millions of people around the world. In 2012, about 500 million people were directly engaged in the world, part time or full time, in production of fish, either by fishing or in aquaculture. Fisheries and aquaculture sector in Nepal is relatively small, which provides employment to about 0.6 million people in 2013/14.

1.2. GENERAL CHARACTERISTICS OF FISH

characteristics 
All the fishes are built on a fundamental plan and possess following characters in general.
1. Fishes are aquatic, cold-blooded (poikilothermic) vertebrates.
2. The body of fish is generally fusiform and streamlined, but in puffers (e.g. Tetraodon spp.) the body is globe shaped and in eels (e.g. Anguilla spp.) is of serpentine form. In spite of many variations in shape, the ground plan of the body is bilaterally symmetrical with prominent lateral line system.
3. The body of the fish is generally covered by tough skin armored by variety of scales.
4. The appendages of the fish comprised of the fins, which are generally paired (pectoral and pelvic fins) and unpaired (dorsal, anal and caudal fins). All fins are supported by dermal fin rays. These fins constitute the main locomotory organ.
5. Mouth is generally situated anteriorly in the head, and the anus is in the second half of the overall length of the body just in front of the anal fin.
6. Respiratory organs are generally in the form of gills and other accessory respiratory organs.
7. Nostrils are paired and do not open into the pharynx, except in lung-fishes and lobed-fishes.
8. Skeleton is in the form of notochord, connective tissues, bones, cartilages, and non-bony scales.
9. The digestive tract is well developed (with mouth, oral cavity, pharynx, oesophagus, stomach, intestine, and anus).
10. Heart is generally two-chambered, pump with one auricle and one ventricle. The circulation is a single circuit, i.e. heart-gills-body-heart.
11. The kidneys are paired, longitudinal structures that lie above the body cavity close to the vertebral column.
12. Brain is well developed with ten pairs of cranial nerves.
13. External ear is completely absent but internal ear with well-developed semi-circular canals.
14. Sexes are separate. Most are oviparous and some are viviparous or ovoviviparous.
15. Fertilization is generally external, i.e. the male releases sperm over eggs as they escape from the body of the female.
16. Embryonic membranes are absent.
17. Eggs are large with much yolk.
18. Development is direct, i.e. without any metamorphosis.

PART1:- INTRODUCTION TO FISH

1.1. General concepts on fish

Fish
Fishes are aquatic cold-blooded vertebrates which breathe by means of gills and propelling and balancing themselves by means of fins that are supported by dermal fin rays. However, some species are well known for their ability to leap over the sea surface and fly long distances using their fins as wings. In addition, other species can live out of the water for short period, walk to migrate to other water bodies using accessory respiratory organs, and some species do not have scales on their body. The word fish is sometimes used more broadly to include any edible animal living in water. In this book, we limit to fish in the narrow sense which includes animals belong to only super-class ‘Pisces’. Fish are also called Pisces or Ichthys.

Fish or Fishes

The term ‘fish’ can be used boh as a singular or plural when talking about fish of the same species. For example, one salmon is a fish, three salmons are three fish. On the other hand, the word ‘fishes’ is usually used as the plural of fish when taking the multiple species of fish. For example, one salmon is a fish, but one salmon and one tilapia are two fishes.

Ichthyology
(GK: Ichthys or Ikhthus – fish; logos – study). Ichthyology is the branch of zoology devoted to the study of fishes. In simple words, the study of fishes is called Ichthyology. This includes bony fishes (Osteichthyes), cartilaginous fishes (Chondrichthyes), and jawless fishes (Agnatha).

Fish diversity
Fishes are the most diverse group of vertebrates, with more than one-half of the total vertebrate species. It is believed that fish began to evolve about 480 million years ago. They can be found in nearly all aquatic environments, from high mountain streams to the abyssal and even hadal depths of the deepest oceans. According to FishBase (2014), there are about 32,800 species of fishes, of which about 10,000 are freshwater fish. While a majority of species have probably been discovered and described, approximately 250 new species are officially described by science each year. These figures probably underestimate the exact numbers, as more species continue to be described. Thus, there may be well over 40,000 species of fishes worldwide. Out of total fishes, about 96% are bony species and rest are cartilaginous species. Many types of aquatic animals named ‘fish’, such as Shellfish, Crayfish, Jellyfish, Starfish, Cuttlefish, etc. are not a true fish. They, and some aquatic mammals like Whales, Dolphins and Pinnipeds (Seals and Walruses) are not a focus of Ichthyology. 

Fisheries
The industry or occupation devoted to the catching, processing and selling of aquatic organisms from natural water resources are the fisheries activities. According to FAO (1988), aquatic organisms, which are exploitable by the public as a common property resources with or without appropriate license are known as fishery. For example, Marine fisheries, Riverine fisheries, Lake fisheries, Reservoir fisheries, fishing on wetlands, etc. In general, the fisheries activities can be divided as capture fisheries, enhanced fisheries and culture fisheries or aquaculture.