ICTHYOLOGY is the scientific study of fish. Fish, because of the possession of notochord belong to the phylum chordata. They are most numerous vertebrates. About 20,000 species are known to science, and compare to other classes, aves 98,600species and mammals 8600species, reptiles 6,000 spandamphibians 2,000species.Fish also in various shape and forms from the smallest niamoy17mmT.L the giant whale shark that measures 15m and heights 25 tonnes.
Fish are poikilothermic cold blooded animals that live in aquatic environment Most fish , especially the recent species, have scales on their body and survive in aquatic environment by the use of gills for respiration. Another major characteristic of a typical fish is the presence of gill slits which cover the gills on the posterior.
FISH TAXONOMY.
Everyone is at heart a taxonomist whether by virtue or necessity or because of mere curiosity.
- To know/identify the difference component in a fish That is to name and arrange.
- To study the population dynamics in a (Number of each species in a population.)
- Important in fish culture propagation – to know the species of fish that is most suitable for
- To exchange information to people in other parts of the world living known that both are dealing on the same
- Reduce confusion as same Latin word generally acceptable worldwide are used while vernacular names differ form one location to
- Information on the discipline like genetics, ecology comparative physiology, behavior, cytology,
Biometncs and related fields are necessary in taxonomy study of meristic count tells of fish behavior and geographical location e.g. Northern clarias garie pinus have more gill rakes then southern ones and different in gill rakes number the two geographical locations collations indicate that the northern species are in general more of invert ebrate feeders and less psivorous than southern ones (Though all have long slender closely set gill rakers).
HOW TO IDENTIFY FISH.
Fisheries workers are frequently accused of veiling other subject in complicated terminology but to describe a fish properly technical terms must be used. The most common ones are shown in figure 1. A glance at the illustrations should enable on to identify most species with a fair degree of accuracy. When doubt exists other anatomical details may have to be examined in giving measurements we have used total lengths throughout.
NAMES
To know the correct name of a fish is satisfying for an angler or amateur naturalist. For professional fisheries workers it is far more important, for it might enable them to study investigations carried out on certain species, or to exchange information with people in other parts of the world, safe in the knowledge that both are dealing with the same species. Confusion can be avoided only by using the Latin scientific names for they are the same throughout the world, while vernacular names are often different in locations only fifty miles apart.
Any group of fish, or of any other animal for that matter, whose members are similar in structure and appearance and are capable of breeding among themselves belong to the same species. A genus encompasses a group of species, which are closely related to one another and are therefore usually similar in appearance. Genetic names begin with a capital letter, and both generic and specific names are italized. Genera in turn form larger groups known as families and these form sub-orders and so on. But for general fisheries work it is sufficient to trace the species back only as far as its family name which always has the ending idea.
MERISTIC FEATURES
The most vital external characteristics for identifying fish are the fin ray counts, especially those of the dorsal and anal fins. The sizes and shapes of fins, their situation on the body and positions in relation to each other are also important.
The number of spines and or rays in the dorsal and anal fins is generally the most consistent character in a species and seldom the same in different species. Each fin is made up of a number of rays, which are usually bony and flexible, and may be either simple or branched; that is, they divide or sub-divide part of the way along the ray. In many fish, strong and sharp spines replace some of the rays, especially of the dorsal fin and these are counted separately when describing a fish. It is customary to use Roman numerals for spinecounts.
Thus D XIV-XVI, 11-13; A 111, 7-10, the fin counts for Tilapia zilli, or italized indicate that this species has from 14 to 16 spines and from 11 to 13 rays in the dorsal fin, while the anal fin has 3 spines and between 7 and 10 rays. Some West African species have two dorsal fins, the posterior of which is often soft, fleshy tissue and are thus termed an adipose fin. The size and shape of the adipose fin is sometimes given as a clue to the identity of a species. The caudal fin, (or tail fin) of most fish is lobed; i.e. it is forked and has the upper and lower lobes attenuated to points. It can also be rounded or truncate.
Mouth
The position of the mouth is sometimes given when describing a species. A mouth is said to be terminal when it is at the extreme tip of the snout. As the mouth is progressively posterior to this position on the ventral side it is described as being sub-terminal sub-inferior, inferior or ventral. A mouth which is dorsal to the terminal position is said to be upturned or oblique. It is a general rule that fish with mouths in an inferior position, like most catfish and carps feed on detritus, worms, algae and bottom dwelling organisms; those with terminal mouths, such as perch and tiger fish, are usually predators or plankton feeders. Cyprinodonts and others with oblique mouths usually feed on insects or their larvae which they take from the surface of the water.
Teeth
The position and character of teeth are sometimes important in the classification of fish. Teeth may be pointed and with a varying number of cusps, according to which they are termed unicuspid, bicuspid, etc or they are said to be granular when they are numerous and flat, forming a rough surface like sandpaper. In fish such as Tetraodon and Protopterus, the teeth are coalesced into beak-like structures or massive ridges and these are usually used for crushing shells or seeds. The terms used to describe positions of teeth are: premaxillary, when teeth are situated in the front margin of the upper jaw, maxillary when on the sides of the upper jaw on a separate bone; mandibular, when on the margin of the lower jaw; vomerine, when on the front part of the roof of the buccal cavity, palatine, when further back on the palate and pharyngeal, when they are situated in the throat.
Nostril
Most fish have two nostrils on each side of the head in front of the eyes. Cichlids are exceptions, with only one on each side. Nostrils can also be situated at the end of nasal tentacles, as in Channa. Fish nostrils are usually connected to olfactory organs and have no respiratory functions.
Gills
The function of the gills is to extract oxygen from water, where it is usually abundant in a dissolved form, and to rid the blood of carbon dioxide. Gills usually consist of a series of bony hoops, to the posterior edge of which are attached numerous fine gill filaments, usually bright red because of the abundance of blood they contain. Water enters through the mouth, passes through the gill chamber and over the filaments, and is expelled through openings on each side of thehead.
The large surface area of gill filaments and their thin membranous covering allows an interchange of gases to take place as the oxygenated water passes over them. Clarias are an example of local fish which possess unusual accessory breathing organs which enable them to extract oxygen directly from the air, thus allowing them to survive for long periods out of water or in swamps and pools which become polluted when they almost disappear during the dry seasons. Gill rakers, rather like combs in appearance are found on the inside of the gill archers.
They vary in number and size in different species and are sometimes an important aid to identification. Gill rakers serve both as a sieve, preventing large particles, from reaching and perhaps damaging the delicate gill filaments and also at times as a filter to collect minute food part6icles from the water as it passes through them. It therefore generally follows that species having numerous and fine gill rakers are microphages or plankton feeders, while those with heavier and fewer gill rakers feed chiefly on larger objects. The freshness of a fish can be determined by examining its gills. Recently caught specimens have blood-red gills, but bacterial action soon causes them to become white and sticky and they begin to give off an offensive odour before the flesh of the fish does
Lateral Line
The visible part of the extraordinary sensory system of fishes, known as the lateral line consists of a series of marks or pits, usually one on each scale, running along about the midline of each side of the body and also at times on the head. Heterotis niloticus provide a striking example of a conspicuous lateral line. Some fish have a discontinuous lateral line, the anterior part often being higher on the body and more conspicuous. A few fishes have no lateral line. These external pores are connected through special sensory organs to nerves running to the brain and by means of these sense organs fishes are able to detect movements and vibrations which are far beyond their range of vision.
Scales
Most fish have their bodies and also sometimes part for their heads, fins and tail, covered by a layer of scales which are embedded below the skin. Besides this protective coat of flexible armour, fish are usually covered by a thin layer of mucous slime which helps to protect the skin from bacterial infection and lubricates the body to ease its passage through the water. The different types of scales possessed by various fish are important for identification purposes. Polypterus alone among local fishes have ganoid scales. Scales are termed ctenoid when the exposed edges are ciliated ortoothed.
The surface of fish with ctenoid scales, such as climbing perches, is always rough. Most fish have cycloid scales; that is, with the exposed margin evenly rounded, giving the skin a smooth surface. Figure 2 shows the different types of scales. For correct identification of a fish, it is sometimes necessary to count the number of scales in a horizontal line from the rear edge of the operculum to the end of the caudal
peduncle. A vertical count of the number of rows is also sometimes necessary, and figures are given for above and below the lateral line. The number of rows of scales around the caudal peduncle may occasionally be the most reliable guide to the determination ofaspecies.
The number of scales remains the same as a fish grows, but each scale increases in size. An abundance of food at certain seasons might speed up the growth rate of a fish; or perhaps growth is retarded as a result of gonad development and spawning or other factors. This stop- go pattern of growth leaves marks on the scales which are sometimes useful in determining the age of a fish. Unfortunately, these rings are more difficult to interpret and less reliable with tropical fish than they are with those from temperate waters where the seasons are more marked.
Colour
When colours are stated in the description of a fish, these refer to the colours of either a live or recently dead specimen. Identification on the basis of colour can however be quite misleading as these often differ strikingly according to the habitat, sex, breeding activity or other factors. Many members of the cichlid family rival chameleons in their ability to change colour.
Male Hemichromis fasciatus, for example, when in clear water during breeding activity, are canary yellow and have vivid black bands on the sides. At other times they are usually silvery on the sides and the bands are reduced to smaller black patehes. Colours of many fishes change after death and sometimes again after being preserved, so a museum specimen may, at first glance, appear very different from another member of the same species kept alive in an aquarium.
Sexual Differences
The sex of a fish can very seldom be determined from its external characters, but the anal fins of males and females of some species do differ in shape. In some genera – Clarias is an example – only the males possess an external genital papilla. Extreme sexual dimorphism is also evident in the cyprinodont family, where males usually have larger fins and are more colourful than females of the same species.
Ancient Fishes
Three genera of fishes which live in West African freshwaters deserve the name ancient fishes. The term ancient means only that they closely resemble fish which lived thousands of years ago and do not show the characteristics of more modern, bony fish. That they have survived means that they are no less well adapted than more recently evolved groups of fish living under today’s conditions.
Lepidosirenidae
The sub-order to which the family Lepidosirenidae belongs can be traced back to the Devonian period, some 300 million years ago. Only one species occurs in West Africa. Protopterus annectens the lungfish or mudfish. It is very easy to recognize because both the pectoral and pelvic fins are reduced to thread-like filaments and the dorsal, caudal and anal fins are continuous.
The eyes are small and so are the scales, which are cycloid and embedded in the skin. The teeth are fused into sharp tooth-plates and the largest specimens can give a nasty bite. The most peculiar feature of this fish is that it has a pair of lungs which enable it to breathe air. This ability allows it to do something which no other species of fish can do aestivate it normally lives on flood plains and when these dry up, during the dry season it secretes a thin slime around itself which dries into a fragilecocoon.
It can exist in this state for over a year, although normally it hibernates only from the end of one wet season to the start of the next. Even when the rains come and it takes to a normal life, it still continues to breathe air. This means that it can survive in swamps which are normally badly oxygenated.
Polypteridae
Polypterus is another living representative of a group of fishes which can be traced back to the Devonian period. Its thick, ganoid scales, which are rhomboid in shape and barely overlap, give the genus a primitive, armoured appearance.