15.6: Introduction to Fishes - Biology

15.6: Introduction to Fishes - Biology

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What you’ll learn to do: Identify characteristics of fishes

Modern fishes include an estimated 31,000 species. Jawless fishes—the hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, distinguishing them from the invertebrate chordates.

Class Osteichthyes (With Diagram) | Bony Fishes

1. The endoskeleton is cartilaginous in the embryonic stage, but in the adult forms more or less it is replaced by bones. Thus they have bony endoskeleton. They are ectothermic (cold blooded). There are about 25,000 species of bony fishes.

2. Caudal fin usually homocercal symmetrical (Gr. homos- alike).

3. The exoskeleton, if present comprises cycloid, ctenoid or ganoid scales, which are dermal in origin.

4. The mouth is terminal. Digestive tract leads into an anus. Cloaca is absent in bony fishes.

5. External nares lie on the dorsal surface of the snout. In lung fishes internal nares are also present.

6. Bony fishes have a sac-like outgrowth, the swim bladder (also called air bladder), arising from the dorsal wall of the oesophagus, which is air-filled organ, used to maintain balance and to swim up and down. In some fishes, such as Heteropneustes, it helps in respiration.

7. They have 4 pairs of gills which are covered by an operculum on each side.

8. The heart is 2-chambered (one auricle and one ventricle) and also has sinus venosus and conus arteriosus. Lung fishes have three chambered heart. (Two auricles and one ventricle). Bony fishes have well developed renal portal system. RBCs are oval and nucleated.

9. Kidneys are mesonephric. Ammonia is chief nitrogenous waste.

10. There are present 10 pairs of cranial nerves.

11. The brain bears relatively small olfactory lobes and cerebellum.

12. Lateral line system is well developed.

13. They have internal ears which helps the fish keep its balance. The nictitating mem­brane in the eye of fish is well developed.

14. Fertilization is generally external. Most forms are oviparous, some are ovoviviparous. Development is direct except in Anguilla where development is indirect with a larva lepto- cephalus. Some bony fishes show parental care.

15. Bony fishes occur in all sort of waters— fresh, marine, brackish.

Marine Fishes — Exocoetus, Hippocampus, Solea, Echeneis (Sucker fish), Lophius (Angler fish).

Fresh Water Fishes — Labeo, Catla, Clarias (Magur), Anguilla, Anabas, Mystus,

Heteropneustes Wallago, Lung fishes.

Aquarium Fishes — Betta (Fighting fish), Pterophyllum (Angel fish).

Carps have toothless mouth with scales on the body however, head is without scales. Barbels are normally absent, if present, are small or rudimentary. They are mostly herbivorous.

Carps are of two types:

(i) Major Carps:

They are bigger in size and their growth rate is faster, e.g., Labeo rohita, Labeo calbasu, Cirrhinus mrigala, Catla catla, Ctenopharyngodon idella (Grass carp),

(ii) Minor Carps:

They are smaller in size and their growth rate is slower, e.g. Labeo bata.

They do not have scales but have well developed barbels. Teeth are mostly well developed. They are carnivorous, e.g., Mystus seenghala, Clarias batrachus (Magur), Heteropneustes fossilis (Singhi), Wallago attu (Fresh Water Shark), Rita rita.

Murrels (Snake-Headed Fishes)

They have snake-like head). Scales are present both on body and head. They are carnivorous, e.g., Ophiocephalus (- Channa) punctatus. Live fingerlings (2″ size) of Channa species are used in Hyderabad once every year (in June) to cure asthma free of cost. The mouth of fingerling is filled with some herb and the patient is made to swallow it. However, no scientific explanation of this fish therapy is known.

It is found in clear and sluggish rivers and streams. The adults are chiefly herbivorous in diet. The young ones are planktivorous. Two short threads like structures, the barbels, are present. The tail is homocercal. This fish serves as a popular delicious dish.

It is a fresh water fish inhabiting all the rivers in India. It feeds on plankton and decayed vegetables. The trunk and tail bear moderate sized scales. Barbels are absent. Catla catla is an excellent food fish.

Anabas— The Climbing Perch:

It can live out of water for some time, where it respires through accessory respiratory organs lying in-front of the gills. It is carnivorous predatory fish it is very fond of eating earthworms. It is unable to climb trees. Birds may pick it up from land and drop on trees. This has led to its common name— climbing perch. The climbing perch is a good food fish.

Mystus seenghala is a common fresh water cat fish. It is carnivorous predatory fish feeding exclusively on small fishes, fish fry and prawns. Elon­gated body is without scales. The snout bears four pairs of barbels. This fish is also eaten.

Anguilla (Fresh Water Eel):

It has a long snake like body. Skin has rudimentary scales. Adult male and female fishes migrate from river to sea where they lay eggs and die. The young which hatches from egg is called leptocephalus (larva of eel). The larvae are so transparent that they are called glass fishes. They feed and grow in sea for 2 or 3 years, then enter the river and undergo metamorphosis to become adults.

Exocoetus (Flying Fish):

In fact, it does not fly but often leaps into the air up to about six metres high. It is an excellent food fish. The pectoral fins are modified into wing-like structures, with the help of which the fish glides.

Hippocampus (Sea Horse):

The neck and the head of the fish are horse-like and the tail is prehensile. The fish always swims upright in sea water. The sea-horse exhibits sexual dimorphism.

The male bears a brood pouch in which the female lays eggs and the latter remain there till they hatch. Thus, the parental care is performed by the male. The pelvic and caudal fins are absent. The dried skin of the sea horse is used for the preparation of some ornaments.

It is interesting to note that in the early development of the embryo, the eyes of these fishes are laterally situated but being a bottom dweller they are shifted towards one side, as an adaptation to water pressure.

Gambusia Affinis (Mosquito Fish):

It feeds on the mosquito larvae therefore, it is widely used to control mosquito larvae. Gambusia is also called larvicidal fish. It is a fresh water fish and shows sexual dimorphism.

There are three genera of living lung fishes: Neoceratodus, Lepidosiren and Protopterus. All have three chambered heart (two auricles and one ventricle).

1. Lepidosiren (South American Lung Fish):

It is found in river Amazon and Paraguay basin in South America. Gills are weakly developed. Respiration is supple­mented with two lungs. It undergoes aestivation during summer season.

2. Protopterus (African Lung Fish):

It lives in rivers and large lakes of tropical Africa. Gills are weakly developed. Respiration is supple­mented with two lungs. It also undergoes aestivation.

3. Neoceratodus (Australian Lung Fish):

It is found only in the Burnett and Mary rivers of Queens-land in Australia. It respires exclu­sively by gills and uses its single lung only under stress.

It is a “living fos­sil” which was taken off from the eastern coast of South Africa, on December 22, 1938 by some fishermen. Fishermen brought the specimen to Miss Courtenary Latimer, Curator of the local museum. When she failed to identify it, she sent its sketch to Professor J.L.B. Smith, an eminent Ichthyologist of Rhodes Uni­versity College at Grahams town.

He recognised it as surviving member of Crossopterygii (subclass of class Osteichthyes) and named it Latimeria chalumnii after the discoverer and locality.

Its discovery is of special interest, because it is believed that crossopterygians (fleshy finned fish) were the ancestors of the first amphibians. Latimeria is believed to be the oldest amongst living fishes. (Now this fish is not alive). It is a connecting link between fishes and amphibians.

15.6: Introduction to Fishes - Biology

Common Name: R a i n b o w T r o u t

Scientific Name: Oncorhynchus mykiss

Classification :

Phylum :
Class : Actinopleygii
Order :
Salmon formes
Family : Salmonidae

Identification : Rainbow trout has a salmon-like body shape. It is characterized from other lake and stream trout by dark spots on it tail fin. The tail fin is slightly forked, just above the anal fin which has 10-12 rays. The rainbow trout has a dark olive-colored back which gradually shades to a silvery white underbelly. The entire body is heavily speckled with pink to darker red stripes that run lengthwise along the fish's sides.

Original Distribution : Rainbow trout is native to North America in streams and lakes west of the Rocky Mountains, ranging from Alaska into northwestern Mexico. In general, its original distribution was restricted to the Pacific Ocean, the coastal drainage of North America from
Alaska to Mexico, and lakes and streams west of the continental divide in
North America. Today, endemic populations are found in most of the principal coastal watersheds of British Columbia, Washington, and Oregon. Nonanadromous rainbow trout are widespread in the Fraser River watershed, and in the Columbia River system,
except for some of the upper reaches of these systems. Rainbow trout were indigenous to the Snake River system up to Augar Falls near the present town of Twin Falls, Idho, and were distributed in 41 tributaries of the river below migration barriers, including streams in northeastern Nevada. In California, populations of rainbow trout are endemic to the Sacramento-San Joaquin system and utilize all coastal streams with favorable environmental conditions. Further south in the Gila drainage basin, rainbow trout are restricted to small headwater tributaries of the Gila and San Francisco rivers in the Mogollon Mountains of New Mexico and East Central Arizona. In the northern part of Baja California, rainbow trout are generally landlocked, although migration up to the Santa Bomingu River can occur during periods of high runoff in the winter. Thus, the native range of rainbow trout populations incorporates a very diverse array of environmental conditions.

Current Distribution : Rainbow trout has been introduced into 47 states outside of its native territories. Other introductions include the Great Lakes region, south central Canada and portions of the Great Plain states east of the Rocky mountains, and southwestern Mexico. Populations of this species can be found in waters on all continents except Antarctica. Its range extends from the Arctic circle (Alaska, Norway, Sweden) to the Equator (Ecuador, Kenya, Uganda) in the Northern Hemisphere, and from the Equator to 55 degrees south (Argentina) in the Southern Hemisphere. Because the rainbow trout is sensitive to warm temperatures it cannot survive naturally in many of these environments and is therefore reintroduced repeatedly in order to support sport fishing.

Site and Date of Introduction : This species of trout was originally stocked in the Great Lakes around 1876 when they were planted in a tributary to Lake Huron. Since then they have been widely introduced in the Great Lake Basin and around the country. Hatchery propagation of rainbow trout was probably first carried out in the early 1870's by the California Acclimatization Society at a location about 20 miles south
of San Francisco. The first recorded shipment of rainbow trout outside their native range took place in 1874 when a small consignment
of eggs from a privately owned hatchery on a tributary of the McCloud River in northern California was transferred to a private hatchery at Caledonia,
New York. From 1874 to 1879 numbers of additional shipments were sent to several locations in the US. In 1879 the US Fish Commission took over the operation on the McCloud River, moved the hatchery a short distance, and from 1880 to 1888 shipped out about
2.5 million eggs to various US federal hatcheries. From these beginnings rainbow trout were subsequently distributed throughout the US
and numerous international locations.

Mode(s) of Introduction : In most cases, rainbow trout was intentionally introduced for purposes of sport fishing into new water systems by being physically transported, in adult form, and released. Those fish subsequently bred and grew in population size. In other cases, as a result of human actions, such as altering waterways or building dams, rainbow trout could move into unexplored waterways.

Reason(s) Why it has Become Established : Rainbow trout is a highly adaptable predator species that, in foreign habitats, has the ability to out compete native fish for food resources (prey) and habitat space. Rainbow trout is prized among fishermen because it puts up a fight when hooked, sometimes leaping into the air from the water's surface while on the line. This behavior may be indicative of an aggressive nature that perhaps lets it dominate the underwater community. While some other exotic trout species, such as the brown trout, have an advantage because they spawn at opposite times from the native fish, the rainbow trout must have an inherent advantage at exploiting resources given a broadly competitive niche.

Ecological Role : Rainbow trout is a cool-to-cold water fish species that does best in freshwater systems below 70 degrees F. The anadronous form of the rainbow trout is called the steelhead. It is spawned in cold tributaries and then makees its way to salt water, or ocean ecosystems. At spawn time it migrates back up the same tributary of its birth to lay its own eggs. Anadronous rainbow trout spawn in the spring, generally April–June, and typically stay for 1𔃂 yr in the nursery stream before migrating to the ocean.
The rainbow trout are carnivorous but they do not necessarily feed on other fish alone they have a wide variety of prey, including insects, crustaceans, mollusks, and small fish. The rainbow trout's primary predator is humans, who fish the trout for sport in practically every aquatic environment. The rainbow trout has been observed living at both deep depths in stream and river columns, while also feeding close to the surface. This shows that the trout is adaptable to varied conditions in its habitat, but it also demonstrates a deviation from the behavior of natural populations, whose individuals primarily occupy the base of the water column. It has been suggested that this strange behavior is a result of 100 years of domestication of the rainbow trout in fisheries.

Benefit(s) : Rainbow trout is a hardy fish that is well adaptable in many freshwater environments. It primarily provides recreational benefits to sport fishermen and anglers.

Threat(s) : Rainbow trout is responsible for driving many native species into extinction or endangerment. They have eradicated frog species and threatened many native fish species in a variety of environments, such as the Californian golden trout and humpback chub in the Grand Canyon. Eliminating or diminishing other aquatic species in a given habitat can have drastic impacts on entire ecosystems, and the many trophic levels of terrestrial lifeforms that depend on aquatic systems for food.

Control Level Diagnosis : "medium"

Control Method : Many methods of controlling rainbow trout populations involve individual fish removal from the environment. This includes specialized fishing, in other words, making laws that restrict fisherman to only keep rainbow trout and throw back the native species. Specialized removal has been practiced in the Grand Canyon, Yellowstone Lake, and the Shenandoah valley where, in 1998, 147 individual fish were removed. Each of these programs has had varied and limited success. Because rainbow trout compete directly with other trout species, such as the brown trout, we can curb their spread by introducing them to the sprot fishing environment at times when other fish spawning rates are high. Most obviously, prevention of further introduction will of course diminish the spread of the rainbow trout.

Behnke, RJ. 1979. The native trouts of the genus Salmo of western North America. Monograph for U.S.D.A, Forest Service, Fish and Wildlife Service, and Bureau of Land Management, Lakewood, CO.

Brannon, E.L. 2004. Population structure of Columbia River Basin chinook salmon and steelhead trout. Reviews in Fisheries Science . 12 : 99-233.

Heath, D. 2001. Genetic structure and relationships among steelhead trout (Oncorhynchus mykiss) populations in British Columbia. Heredity 86 : 618-627.

Hershberger, W. 1992. Genetic variability in rainbow trout populations. Aquaculture 100 : 51-71.

MacCrimmon, HR. 1971. World distribution of rainbow trout (Salmo gairdneri). J. Fish. Res. Board Can.

Quinn, TP. 1993. Variation in life-history patterns among New-Zealand chinook salmon (oncorhynchus-tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences . 50 : 1414-1421.

Andrea Gawrylewski
Last Edited: 11/19/04
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Migration in Fishes (With Diagram)

The following points highlight the six main types of migration in fishes. The types are: 1. Latitudinal Migration 2. Vertical Migration 3. Spawning Migration 4. Feeding Migration 5. Overwintering Migration 6. Shoreward Migration.

Type # 1. Latitudinal Migration:

This is performed by fishes like barracudas (Sphyraena) and swordfish (Xiphius) of the warm tropical seas. They migrate to north in spring and to south in autumn.

Type # 2. Vertical Migration:

This is performed by many marine and freshwater fishes and is related to light, search of food, protection and also to spawning. The mackeral rises into the surface waters when there is a rich development of plankton. They eat on plankton and go to deep layers after feeding.

The swordfish, which normally lives in surface water move downwards to great depths to feed deep water fishes like scopelids. Many pelagic larvae of marine fishes perform diurnal vertical feeding migrations. They follow the vertical movements of their prey, the planktonic invertebrates which move down to great depth by day and rise to surface by night.

Many deep water fishes of the order Scopeliformes rises to spawn in the upper layers. Their eggs develop and often their larvae live feeding on the phytoplankton. Among freshwater fishes the clearest example of vertical spawning migration is that of the Lake Baikal Comephoridae. These fishes are viviparous and rise to surface from great depth of the lake to give birth to their larvae.

Type # 3. Spawning Migration:

This is the migration in fishes for breeding, and so it is related to life cycle. Spawning migration is an adaptation for ensuring the most favourable conditions for the development of the eggs and the larvae. This also gives protection to early stages of fishes from predators.

There are two major types of spawning migrations. Movement from freshwater to saltwater for spawning is called catadromous migration. The reverse movement, that is, from saltwater to freshwater is termed anadromous migration.

(a) Catadromous migration:

The most famous examples of catadromous fish is the eels, Anguilla rostrata, the European eel and Anguilla vulgaris, the American eel. For eel, the river serves as the feeding ground while die sea serves as the spawning ground. The stimulus for the start of migration in eel is the ripening of its gonads in rivers.

Before it enters the sea, the eye of the eel becomes enlarged, sometimes becoming four times as large as the eye of freshwater eels. Its face becomes sharper and its colouration changes the back becomes darker, while the belly changes from yellow to a silvery colour. The eel starts its migration in a wellfed condition.

During migration it spends enormous amount of energy so it becomes very thin. The migrating eel does not feed. Its alimentary canal degenerates considerably. Osmotic pressure of its blood rises and size of its swim-bladder decreases.

The eels migrate about 4500 km westwards from Europe or eastward from America and reach the breeding place in the Sargasso Sea off Bermuda. The adult die immediately after spawning in deep waters.

The fertilized eggs hatch out into transparent, ribbon like larvae, called the leptocephali. These were erroneously called glass fishes and placed in the genus Leptocephalus. They lead a pelagic life for a year or more and undergoes metamorphosis into elvers (glass eels).

The elvers then start ascending the rivers in shoals and grow for some years to become adult eels. The adul eel, on maturity start moving towards sea, again the cycle is repeated.

Catadromous migration is also performed by certain members of the families Galaxiidae and Gobiidae. But their migration is considerably shorter than those of the eel. They usually pass from the lower reaches of the rivers to the adjacent shallow parts of the sea.

(b) Anadromous migration:

Anadromous migrations are performed by lampreys, sturgeons, salmon, some shads, cyprinoids etc. The best examples are Atlantic salmon (Salmo salar) and Pacific salmon (Oncorhynchus nerka).

In winter, the both sexes leave their feeding ground at sea to ascend the freshwater mountain streams, reaching the identical spot where they originally grew some years ago. The total distance travelled may be even upto 3600 km, at a speed of 30-40 km. per day.

They stop feeding, alimentary tract undergoes changes into a thin thread with feebly developed pyloric caecae. Change in colour and weight too occurs. Sexual dimorphism becomes evident. The male is characterized by the possession of enlarged front teeth.

After the selection of suitable spawning grounds, the salmons segregate into pairs and produce shallow saucer-like nest where spawning takes place.

Very young salmons are known as “alevins” and they remain mostly among stones. Alevins develop into next stage called “parr” and finally to adult.

After fertilization, salmons are very exhausted. They are called “kelts”. The males seldom return to the sea. The females recover and after a period in the sea they return to breed again. This process may be repeated several times.

Some fishes do not perform significant movement like salmon. They migrate from seas to estuaries or lower reaches of river, for spawning. Such fishes are classified as fluvial anadromous (Semimigratory) fishes. Examples are many whitefishes and cyprinoids.

Many freshwater fishes leave the lakes to spawn in the river. This is called as limnodromous migration. One of the common examples of this is the whitefish Coregonus lavaretus.

Type # 4. Feeding Migration:

This is the movement from spawning or overwintering grounds to the feeding grounds. Feeding migration can be passive or active. In many fishes the feeding migration even begins in the egg stage. It is a passive feeding migration of eggs and embryos from spawning to feeding ground.

Active feeding migration is performed by many marine fishes like cod. Horizontal feeding migration of cod comprises regular journeys, going from one good feeding ground to another.

Type # 5. Overwintering Migration:

Overwintering and hibernation in fishes are a part of the life cycle of a fish. It is characterized by reduced activity, reduction or stoppage of food consumption, lack of food, poor oxygen condition, low temperature, drought etc. Overwintering do not occur in all fishes.

Overwintering migration is a movement away from feeding to wintering grounds. It occurs only in those fishes which have a wintering ground.

In the wintering ground, fish is in a state of relative inactivity and reduced metabolic rate. It requires protection against predators which are common in feeding ground. Overwintering migration is performed by marine fishes like flatfishes and freshwater fishes like grass-carp.

Type # 6. Shoreward Migration:

In this type of migration there is a temporary movement of fishes from water to land. For example, the common eel travel from one pond to another through moist meadow grass. The mud-skipper, Periophthalmus make temporary migration to land by means of modified pectoral fins. The climbing perch, Anabas migrates from water to land and even climbs trees to the height of several feet by means of the strong spines on its pelvic fins and gillcovers.

Micropterus salmoides

Largemouth bass. Photo © Richard Bejarano

Not all fish called bass are in the same family, and the largemouth bass is in the sunfish family, Centrarchidae. Originally the largemouth bass were localized to the Eastern United States, but because of their popularity as a sport fish and their ability to thrive in a variety of environments, they have been successfully exported and populated, making them one of the most widely distributed fish in the world. Bass fishing is a $60 billion dollar industry.

Order – Perciformes Family – Centrarchidae Genus – Micropterus Species – salmoides

Common Names

Common names in the English language are largemouth bass, bass, American black bass, bayou bass, bigmouth bass, black bass, bucket mouth, chub, green bass, green trout, lake bass, large-mouth bass, largemouth, largemouth black bass, line side, marsh bass, northern largemouth bass, Oswego bass, slough bass, southern largemouth, trout, and welchman. Other common names include achiga (Portuguese), achigan (French), achigan à grande bouche (French), bas dehanbozorg (Persian), bass wielkgebowy (Polish), biban cu gura mare (Rumanian), black-bass à grande bouche (French), bol’sherotyi chernyi okun’ (Russian), buraku basu (Japanese), fekete sügér (Hungarian), forelbaars (Dutch), forellenbarsch (German), huro (Spanish), isobassi (Finnish), khorshid mahi baleh kuchak (Persian), lakseabbor (Norwegian), lobina negra (Spanish), okounek pstruhový (Czech), Okuchibasu (Japanese), ostracka (Czech), ostracka lososovitá (Slovak), perca americana (Spanish), perche d’amérique (French), perche noire (French), perche truite (French), perche truitée (French), persico trota (Italian), stormundet black bass (Danish), stormundet ørredaborre (Danish), tam suy lo ue (Cantonese), zwarte baars (Dutch), and öringabborre (Swedish).

Importance to Humans

Largemouth bass are dark to light green in color with lighter sides and a whitish colored belly. Photo courtesy U.S. Department of Agriculture

Some surveys list bass fishing as the number one participation sport, while others place it in the upper five. According to U.S. Fish and Wildlife Service statistics, 43 percent of freshwater anglers fish for bass, 34 percent fish for panfish, 30 percent for catfish/bullhead, 30 percent for trout and 28 percent for crappie. Fishing is big business and a major American institution. America’s 30 million bass anglers are the basis of a $60 billion industry with one in every five fishing every year.


The largemouth bass is among the most sought after species of freshwater game fish in the world. Due to its broad range and popularity as a sport fish, numerous fishing methods have been devised to catch this fish. Numerous artificial lures have been

developed to entice these fish into biting including plastic worms, jigs, plugs, and spinners. Among the most exciting methods of fishing for the largemouth bass involves surface plugs which can incite vicious strikes on the surface where they are visible to the angler. Live bait fishing is also popular among bass fishers. Night crawlers, leeches, crawfish, and minnows are all commonly used live baits in different regions of the U.S.

Bass fishing is a popular sport. Photo courtesy U.S. Fish and Wildlife Service

Evening and morning times are usually the best times when fishing for the largemouth bass. The largemouth’s liking for heavy cover makes it a challenge to land. A hooked largemouth usually heads for the surface, the opens it mouth wide, shaking its head or jumping in an attempt to throw, or get rid, of the hook. Once it has done this, it will dive under the cover and begin to wrap the line around logs and weeds located in the water.

Largemouth bass flesh is moderately firm and has a mild flavor. If taken from lakes where the predominant cover is weeds, the flesh may have a grassy taste.


The largemouth bass is widely distributed and as such, is not listed as endangered or vulnerable with the World Conservation Union (IUCN). The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.

Geographical Distribution

World distribution map for the largemouth bass (including introductions)

This bass is one of the most widely distributed fishes in the world. This is due in large part to the popularity of bass as a sport fish which has lead to the introduction of largemouth bass populations into many areas where they are not native. Their current range includes the U.S., South Africa, Europe, Guam, Japan, Lebanon, New Zealand, and the Philippines.

The original range of the largemouth bass is most of the eastern half of the United States, however it is now found generally in the majority of the United States, including swamps, ponds, lakes, reservoirs, creeks and large rivers.


The largemouth bass lives in all types of water, including swamps, ponds, lakes, reservoirs, creeks and large rivers. The bass can even be found in estuaries. It prefers weedy oxbows and clears floodplain lakes. Since it is generally a warm water (81-86°F, (27.2°-30°C)) fish, it is seldom found at depths of more than 18.8 feet. During the winter, largemouth bass generally will move into deeper waters. In the spring, largemouth’s migrate into bays that have warmed up sooner than that of the main body of water.

During the day, largemouth bass may cruise above aquatic plants at depths of 3.1-9.4 ft, or lie under lily pads or in the shade of overhanging trees, piers, or brush. In the evening hours, largemouth bass tend to move into shallow water to feed. After night falls, they return to deeper water, where they rest on the bottom under logs or trees. In cold temperate climates, largemouth bass generally move into deeper waters during the winter months followed by movement to warmer, shallow waters in the springtime.


Distinctive Features
The largemouth bass has a large, slightly sloping mouth. Its body is slender to robust, slightly compressed laterally, and oval in cross section. The corner of the mouth extends past the eye, hence its common name.

The two recognized subspecies of the largemouth bass are the northern largemouth (Micropterus salmoides salmoides) and the Florida largemouth (Micropterus salmoides floridanus).

The Florida largemouth has slightly smaller scales than the largemouth bass in relation to the size of its body. It has 69 to 73 scales along the lateral line, compared to 59 to 65 on the northern largemouth. It grows faster and reaches larger sizes in warm waters than the northern largemouth, leading to its popularity in stocking efforts across the U.S.

The species most often confused with the largemouth bass is the spotted bass (Micropterus punctulatus). Sometimes referred to as the Kentucky bass, its range overlaps with that of the largemouth bass and has similar coloration patterns. It can be distinguished from the largemouth bass in several ways. The spotted bass has its spiny and soft dorsal fins fused. Also, its upper jaw is noticeably smaller than that of the largemouth bass and does not extend beyond the eye.

The smallmouth bass, M. dolomieu, is another popular sport fish. It is restricted to a more northern range than the largemouth with its southernmost populations in northern Alabama and Oklahoma. The smallmouth bass may be distinguished from the largemouth by several characteristics including its bronze to brown coloration, its mottled pattern of dark bands, and its noticeably smaller mouth.

Other similar species found in regions of the southeastern U.S. include shoal bass (M. cataractae), red-eyed bass (M. coosae), and Suwannee bass (M. notius), while in Texas there is a population of the Guadeloupe bass (M. treculi). The mouths of all of these species are smaller than the mouth of the largemouth bass. These four species also tend to be smaller in length with the largest species reaching a maximum length of 18.5 inches (47 cm).

The back and head are dark green to light green in color with lighter sides and a whitish belly and underside of the head. A prominent lateral stripe may be seen running from the snout through eye to the base of the tail. Towards the tail, there is a series of blotches of varying size. These blotches evolve into a solid, even stripe on the caudal peduncle. The eye is golden brown. Vertical fins lightly pigmented, paired fins generally clear caudal fin alike in young and adult. Adults from mud-bottom lakes are dark olive brown to black, with markings hardly distinguishable. Males in breeding condition tend to be darker in overall color.

The largemouth bass has well-developed pharyngeal jaws consisting of six major pads of caniform teeth in the upper pharynx and two pads in the lower pharynx. There are no teeth present on the tongue.

The average length of the largemouth bass is 18 inches. Photo courtesy U.S. Geological Survey

Size, Age, and Growth
The average length is 18 inches but the largemouth bass may attain a length of 24 inches or more. The world record largemouth bass was caught in Montgomery Lake, Georgia in 1932. The fish weighed 22 pounds, 4 ounces (10.09 kg).

Females may reach a maximum age of 9 years, while the males reach a maximum of 6 years.

Food Habits
Largemouth bass may consume small fish, insects, mosquitoes, blackfly larvae, mayfly nymphs, worms, adult insects, mussels, crayfish, snails, tadpoles, frogs, small fish, salamanders, mice, turtles. In general largemouth bass feed at all hours, but most often in the early morning or late in the day. In some cases, the prey is not completely swallowed up initially it is caught and held in the jaws and then it is sucked in.

Juvenile largemouth bass. Photo © George Burgess

The largemouth bass generally does not spawn at temperatures much below 64°F (17.8°C). Optimal spawning conditions is when the water temperature is around 18.9°-20.0°C. which correlates to between February (for the extreme southern end of its range) and July (in northern latitudes). The selection of nest sites by the male generally begins when water temperatures reach 60°F (15.6°C). The male largemouth bass begins the spawning process by clearing out a small depression in the bottom substrate. The nest are cleared in fairly shallow water on bottoms composed of sand, gravel, or pebbles. The male then attracts a gravid (egg laden) female and together they pass over his bed, releasing sperm and eggs. She may release from 2,000-145,000 eggs with much of this variation attributed to her physical size. Shortly after spawning, the female departs and the male is left to guard the developing eggs. The male will fast while he defends the eggs from predators and debris until they hatch 3-7 days later.

Upon hatching, young largemouth bass fry are transparent and 3 mm in length. After hatching, the young remain in the bottom of the nest until the yolk is absorbed from the eggs, which usually lasts a period of 6 to 7 days. They then rise from the nest, and begin feeding on zooplankton and schooling, where they are 5.9- 6.3 mm long and have changed color to become pale green. As young largemouth bass begin to grow, their diet changes to insects and other small fishes and then eventually switches to larger fishes, salamanders, crayfish, and a variety of other small organisms.

The largemouth bass may form hybrid fish by spawning with the smallmouth bass, rock bass, bluegill, warmouth, and black crappie.

Yellow perch is a known predator of juvenile largemouth bass. Photo courtesy U.S. Department of Agriculture

Largemouth bass larva and juveniles may fall prey to yellow perch (Perca flavescens), walleye (Stizostedion vitreum), northern pike (Esox lucieus), and musky as well as water birds such as great blue herons (Ardea herodias) and kingfishers (Ceryle alcyon). However, adult largemouth bass are considered top predators in many habitats where they reside and rarely become prey items due to their size, swimming speed, and protective dorsal spines.

An enemy of the largemouth bass is the bass tapeworm, which attacks the reproductive organs, resulting in sterile bass. The ectoparasitic protozoan, Scyphidia tholiformis, is one of the most common parasites. The fungus Saprolegnia may attack dead eggs. Parasites such as protozoa, copepods, roundworms, tapeworms, flatworms and leeches are common in the largemouth bass.


Lacépède was the first to describe this fish in 1802. He gave it the scientific name Labrus salmoides. In 1876, Nelson described this fish as Micropterus nigricans and in 1878 Jordan described it as Micropterus pallidus. Finally in 1884, Forbes described the fish as belonging to Micropterus salmoides, which is currently the valid scientific name.

There are two recognized subspecies the northern largemouth bass (Micropterus salmoides salmoides) and the Florida largemouth bass (Micropterus salmoides floridanus). In 1965, Smith described the subspecies Micropterus salmoides salmoides, while in 1949 Bailey and Hubbs described a second subspecies as Micropterus salmoides floridanus.

Spoilage of Fish

Spoilage of fish is a process of deterioration in the quality of fish, which changes its appearance, odour and taste. The breakdown of biomolecules like proteins, amino acids and fats in the fish are the factors responsible for fish spoilage. Thus, a fish can be spoiled by either chemical or biological degradation.

In chemical degradation, protein, fats, amino acids etc., are decomposed, whereas microorganisms carry out the biological degradation. Other than bacterial and chemical degradation, enzymatic and mechanical damage can also cause fish spoilage. There are certain factors like high moisture, protein and fat content, improper handling etc., that favouring the spoilage in fish.

The common causes of fish spoilage are bacterial contamination and chemical oxidation (protein, fats etc.). The microorganisms involved in fish spoilage refer to the SSOs (specific spoilage organisms) that result in the formation of numerous unwanted metabolites, which adds undesirable appearance, flavour, and odour to the fish.

SSOs’ growth strongly depends upon the nutritional components of fish, moisture content, high temperature etc. You will get to know the definition, causes, types, and assessment of fish spoilage in this context.

Content: Spoilage of Fish

Definition of Fish Spoilage

It refers to the contamination of fish, resulting in an undesirable change in the colour, texture, flavour, odour, appearance, etc. Spoilage of fish is also called “Putrefaction”. Fish spoilage can occur due to enzymatic degradation, bacterial degradation, chemical decomposition and mechanical damage. We can characterize the spoiled fish by observing the colour change, fishy smell, sliminess in the skin and scales, firmness of the flesh, discolouration of the backbone etc.


The action of microbes, enzymatic activity and oxidation of nutritive elements present in the fish are the common causes of fish spoilage. In addition to these, some other factors are also responsible like:

  • Improper handling
  • The high moisture content of the fish
  • Weak muscle tissue
  • Ambient temperature


Fish spoilage is generally of three types, namely autolysis, bacterial and chemical spoilage.


It refers to enzymatic degradation that results in the cell damage of fish and release of an autolytic enzyme, which degrades the cell components like proteins, fats etc. and thereby changes the flavour of fish. The changes in the flavour of fish can be due to the conversion of ATP to hypoxanthine and the decomposition of fish.

  • Conversion of ATP to hypoxanthine: This conversion adds a bitter taste, and we can estimate the degree of freshness by knowing the hypoxanthine content.
  • Decomposition of fish: It leads to belly bursting of the fish, and the action of digestive enzymes in the fish gut is the cause of fish spoilage.

Autolysis can change the appearance and odour of the fish by two ways:

  1. Cause black spot formation: Autolysis can cause black spot formation in some shrimps by showing some enzymatic action on the amino acids. The black spot occurs due to the formation of melanin pigment, which results in poor appearance.
  2. Cause foul smell: Autolysis can produce a foul smell by the proteinase enzyme degradation of muscle proteins into amino acids and other compounds like ammonia, carbon dioxide, amines, fatty acids etc. Thus, the production of secondary metabolites produces indole, skatole, etc., results in the release of a foul smell from a fish.

Bacterial Spoilage

A fish acquires a load of bacteria in the gills and on the surface. When a fish dies, the bacteria already present in the fish attack the flesh and result in the formation of undesirable products. The microbial growth in fish depends on the type of water from where it caught. The bacteria cause fish spoilage by the following means:

  1. Reducing TMAO to TMA: Reduction of trimethylamine oxide into trimethylamine produces an offensive odour.
  2. Degradation of amino acid to primary amines: It can cause food poisoning.
    • Example:
    • Histidine – Histamine
    • Glutamic-acid – Arginine
  3. Degradation of urea to ammonia: It also produces an offensive odour.

Chemical Spoilage

High temperature favours chemical spoilage. Oxidative rancidity is a common cause of chemical degradation.

Proteins split into amino acids, amines, ammonia and hydrogen sulphide by the action of proteolytic microorganisms. Carbohydrates split into acids, alcohols and gases by the action of fermentative microorganisms. Fats degrade split into fatty acids and glycerol by the action of lipolytic microorganisms.

Assessment of Fish Spoilage

We can access the quality of fish by the three consecutive methods:

Physical Method

Torrymeter is a device placed vertically, and it provides a digital reading of the fish quality, whether it is aged or fresh. From the digital readings or values of the torrymeter, we can estimate the fish freshness.

A low value indicates the presence of more bacterial mass. 10 is the highest value for the freshly caught fish, and below 3 is the value of spoiled fish. The value of 6 on the torrymeter is acceptable by the consumer.

Subjective Method

It is a sensory method assessed by the sensory organs, which represent the customer view.

Organoleptic Test

It includes the quality assessment of fish by the sense of sight, smell, touch etc.

We can check the quality of fish by using our sense of sight to examine the fish’s eyes, gills, and skin surface.

  • The eyes should be clear and vibrant. Any discolouration around the eyes and cloudiness in the eyes indicates that the fish is not fresh.
  • The gills should be red-pink in colour.
  • The skin should be shiny, not slimy.
  • The skin or the surface of fish should be clear, and there must not be any discolouration.

We can also check the quality of the fish by using our sense of touch to examine the flesh and scales of the fish:

  • The flesh should be tight, elastic, but not slimy.
  • The scales should be intact with the skin.

We can also check the quality of the fish by using our sense of smell:

  • The smell of fish should be neutral and fresh.
  • There should not be a fishy, sour or ammonia-like smell.

Biochemical Method

It includes the following methods:

  1. Proximate testing: It is a prevalent method in which the fish components like moisture, protein, lipid etc., are regularly checked from the time of fish harvesting. This method does not give a satisfactory assessment and thereby not accepted widely.
  2. Hypoxanthine value: After the death of fish, ATP (Adenosine triphosphate) splits into ADP, AMP, IMP and finally into hypoxanthine. The value of hypoxanthine increases during the storage of fish. Hypoxanthine value gives an estimate for the freshness of fish. A fish is considered to be spoiled if a hypoxanthine value reaches 7-8 micromoles/g.
  3. Trimethylamine (TMA) value: Fish contains a considerable amount of trimethylamine oxide (TMAO), but on fish spoilage, TMAO reduces into TMA. The value of TMA with a level of 1.5 mg / 100 g indicates that the fish is moderately spoilt.
  4. Ammonia production: The production of ammonia indicates the extent of spoilage.
  5. Peroxidase value: It helps in the measurement of oxygen rancidity. Peroxidase value less than 10 (indicates the good quality of fish) and a value more than 20 (indicates rancidity).
  6. Thiobarbituric acid value (TBA): It also helps us to determine the oxygen rancidity. TBA value less than 2, is accepted by the consumer.

Biological Method

It includes the total plate count method (TPC). The biological method involves a quality assessment of fish by the bacterial cell count. First, you need to grind the fish and then dilute the sample by following serial dilution. Prepare media for the growth of microorganisms present in the fish, where we can use both ordinary and selective media.

We can use agar media to enumerate the microbial mass in marine fish and tryptone glucose beef extract agar media to calculate the cell count in processed fish. Other than this, selective media like SS-agar can be used for the detection of coliform bacteria (E.coli, Shigella sp, etc.) in the fish. After media preparation, perform the pouring method and incubate the plates for 24 hours at 35-37 degrees Celsius.

Count the number of bacteria per plate by multiplying with the dilution factor. Thus, the total plate count method gives a count for the bacterial (pathogenic and non-pathogenic) population present in the fish. Hence, the total plate count method does not determine the edibility of the fish.


Acknowledgments for Ron Fricke (2021): William D. Anderson Jr., Alberto Arias, Juan Carlos Arronte, Juan Miguel Artigas Azas, Idrees Babu, Nicolas Bailly, Gustavo Ballen, Arcady Balushkin, Vânia Baptista, Esra Bayçelebi, Sergey Bogorodsky, Philippe Borsa, Pedro Bragança, Ralf Britz, John Bruner, Matthew Campbell, Emily Capuli, Fernando Carvalho, Chen Xiaoyong, Erdogan Çiçek, Courtney Cox, José de la Cruz Agüero, Cem Dalyan, David De Weirdt, Fabio Di Dario, Vahe Demirjan, Omar Domínguez-Domínguez, Laura Donin, Yury Dyldin, Joseph Eastman, Nicolás Ehemann, Hans-Georg Evers, Tilmann Fischer, João Pedro Fontenelle, Jörg Freyhof, Michael George, Daniel Golani, Caio Gomes, Adrián González-Acosta, Menachem Goren, Gustavo Hallwass, Harutaka Hata, Elaine Heemstra, John Hoover, Samuel Iglésias, Muhammad Iqbal, Isaäc Isbrücker, Laith Jawad, Howard Jelks, Arash Jouladeh Roudbar, Karuppiah Kannan, Axel Katz, Bikem Kesici, Daemin Kim, Jin-Koo Kim, Tchalondawa Kisekelwa, Stefan Koerber, Matthew Kolmann, Laishram Kosygin, Alexander Kotlyar, Maurice Kottelat, Tatia Kuljanishvili, Sven Kullander, Peter Kyne, Thomas Lachenal, Francisco Langeani, Thomas Litz, Alejandro Londoño-Burbano, Halima Louizi, Paulo Lucinda, James Maclaine, Ken Maeda, Mizuki Matsunuma, Daniel McGuigan, Marcelo Melo, Michael Mincarone, Alec Moore, Béla Nagy, André Netto-Ferreira, Cárlison Oliveira, Felipe Ottoni, Lawrence Page, Yvan Papa, Kodeeswaran Paramasiwam, Antoine Pariselle, Sabine Petri, Theodore Pietsch, Kyle Piller, Jürgen Pollerspöck, Artem Prokofiev, Patrice Pruvost, Declan Quigley, M. P. Rajeeshkumar, Arturo Ramírez-Valdez, Zachary Randall, Luiz Rocha, Cristina Rodríguez-Cabello, Pedro Romero, Miguel Rubio-Godoy, Asli Şalcioğlu, Shilpi Saha, Carlos de Santana, Christopher Scharpf, Ray Schmidt, Juan Schmitter-Soto, Erwin Schraml, Fabrizio Serena, Ilia Shakhovskoy, Bungdon Shangningam, Bakhtiyor Sheraliev, Nir Stern, Hiranya Sudasinghe, Patrick Tawil, Francesco Tiralongo, Luke Tornabene, Diego Vaz, Anyelo Vanegas Ríos, Ekaterina Vasil'eva, Thomas Vigliotta, Elena Voronina, Olga Voskoboinikova, Maximilian Wagner, Simon Weigmann, Richard Winterbottom, Baran Yoğurtçuoğlu, Cláudio Zawadzki, E Zhang, Mikhail Zhukov, Heike Zidowitz.

Acknowledgments for Richard van der Laan (2021): Felipe Abranches, Emmanuel Abwe, Erik Åhlander, Henry Agudelo-Zamora, Carlos Ardila Rodríguez, Jairo Arroyave, Muthukumarasamy Arunachalam, Nicolas Bailly, Arcady Balushkin, Yusuf Bektas, Ricardo Betancur, Flávio Alicino Bockmann, Pedro Bragança, José Carlos Brito, Marko Ćaleta, Pablo Calviño, Mac Campbell, David Černý, Brian Coad, Sebastián Cona, Kevin Conway, Neelesh Dahanukar, Fabio di Dario, Vahe Demirjian, Gaël Denys, Eddy Derijst, Omar Domínguez-Domínguez, Laura Donin, Emanuell Duarte-Ribeiro, David Ebert, Guillain Estivals, Dario Faustino-Fuster, Antonia Ford, Acacio Freitas, Jörg Freyhof, Dan Fromm, Augusto Frota, Babette van Gool, Harutaka Hata, Elaine Heemstra, Hans Ho, Jean Huber, Lily Hughes, Samuel Iglésias, Taiki Ito, Praveenraj Jayasimhan, Howard Jelks, Manda Kambikambi, Axel Katz, Daemin Kim, Seishi Kimura, Steen Knudsen, Paul van der Laan, Francisco Langeani, Sébastien Lavoué, Mao-Ying Lee, Eliseo Lescún, Thomas Litz, Thiago Silva Loboda, Diego Luzzatto, James Maclaine, Katemo Manda, Mizuki Matsunuma, Madhava Meegaskumbura, Bruno Melo, Michal Mikšík, Michael Mincarone, Guido Miranda, Marcos Mirande, Anil Mohapatra, Daniel Mokodongan, Peter Møller, Robert Myers, Dirk Neumann, Makoto Okamoto, Claudio Oliveira, Renildo de Oliveira, Larry Page, Kody Paramasivam, Murilo Pastana, Edson Pereira, Tiago Pessali, Flávia Petean, Dennis Polack, Jayasimhan Praveenraj, Divya Radhakrishnan, Filipe Rangel-Pereira, Roberto Reis, Ross Robertson, Uwe Römer, Mark Sabaj, David de Santana, Vanessa de Santis, Mudjie Santos, Sérgio Santos, Luisa Sarmento-Soares, Chris Scharpf, Erwin Schraml, Iraj Segherloo, Hiroshi Senou, Bungdon Shangningam, Bakhtiyor Sheraliev, Priscilla Caroline Silva, Dave Smith, Bill Smith-Vaniz, Gabe Somarriba, Hiranya Sudasinghe, John Sullivan, Milton Tan, Kisekelwa Tchalondawa, Andrea Thomaz, Shinichi Tomiyama, Tom Trnski, Massimiliano Virgilio, Olga Voskoboinikova, Nathan Vranken, Emmanuel Vreven, Simon Weigmann, Jeff Williams, Lei Yang, Jouke van der Zee, W. Zhou, Dan Zimberlin and Elsbeth Zwart.

Acknowledgments for Bill Eschmeyer (2021): Dave Catania, Luis da Costa, Richard van der Laan, Mikael Karlsson, John McCosker, Michael Miksik, Dirk Neumann, Rafaela Ota, Larry Page.

15.6: Introduction to Fishes - Biology

Common Name: Walking catfish (clarius catfish, freshwater catfish, thai hito, thailand catfish, alimudan, hito, hitong batukan, ikan keling, ito, kawatsi, keli, klarievyi som, konnamonni, leleh, magur, mah-gur, wagur, manguri, mangur, marpoo, musi, halimeena , pla duk dam, nga-khoo, paltat, trey andaing roueng, wanderwels)

Scientific Name: Clarias batrachus


Phylum or Division: Chordata
Class: Actinopterygii
Subclass: Neopterygii
Super-Order: Ostariophysi
Order: Siluriformes
Family: Clariidae

Identification: Walking catfish, which are scale-less, are typically a uniform shade of gray or gray-brown with many small white spots along their sides. The head is flat and wide and the body tapers to the tail. The eyes are very small and the mouth is broad with fleshy lips and numerous small pointed teeth in large bands on both the upper and lower jaw. There are four pairs of barbels, one pair each of maxillary and nasal barbels and two pairs of mandibal barbels. The fish has a lengthy dorsal and anal fin that each terminate in a lobe near the caudal fin. The pectoral fins, one on each side, have rigid spine-like elements. To move outside of water, the fish uses these "spines" and flexes its body back and forth to "walk". The walking catfish is easy to distinguish from many of the other North American catfish because it doesn't have an adipose fin.

In addition to the brown or gray-brown coloring noted above, albinos and calico morphs are also possible. However, these are uncommon in the wild. For example, in Florida the fish that escaped were albinos but today the albino is rare and descendants have generally reverted to the dominant, dark coloring.

The fish reach reproductive maturity at one year and grow up to 24 inches in their native range. However, in Florida they rarely exceed 14 inches.

Walking catfish possess a large accessory breathing organ which enables them to breath atmospheric oxygen. They are well known for their ability to "walk" on land for long distances, especially during or after rainfall.

Original Distribution: The walking catfish are a widely distributed species found across Southern Asia including Pakistan, Eastern India, Sri Lanka, Bangladesh, Myanmar, Thailand, Malaysia, Indonesia, Singapore, Borneo, Laos and the Philipines. It's hard to determine, though, to what extent this distribution comprises the native range. In Southeast Asian this fish in valued for food and it's probable that human activity is responsible for the presence of this species in parts of its current range.

Current Distribution: In addition to the locations listed above, the walking catfish has been found in the US. Specimens have been collected in multiple locations across California, the All American Canal in Arizona, widely separated bodies of water in Connecticut, the Flint River in Georgia, a lake in Massachusetts and a spring in Nevada. They are found across southern Florida. The only established, wild population is in Florida.

Outside of Florida: Tropical fish dealers in the US imported the walking catfish to be sold as pets. The walking catfish that were found in the US outside of Florida most likely came from aquarium releases (intentional or accidental).

Florida: In the early 1960's, the walking catfish was imported to Florida from Thailand for the aquarium trade. The first introductions apparently happened in the mid 60's when adult fish, imported to be brood stock, escaped from Penagra Aquarium in Broward County and/or from a truck transporting brood fish between Dade and Broward counties. In 1967, the state of Florida banned the importation and possession of walking catfish. However, this led to another release of the fish into the wild. Fish farmers in Tampa Bay who possessed the fish purposefully released them so that they would not be found in violation of the new law.

In 1968, this species was only found in three south Florida counties. However, by 1978, the walking catfish had spread to 20 counties in the southern half of the peninsula. The fish accomplished this migration by using the many hundreds of miles of interconnected canals across south Florida and by moving over land, typically during rainy nights. By the mid 1970's, the walking catfish was established in Everglades National Park and in Big Cypress National Preserve.

Mode(s) of Introduction: Since the 1960's (and possibly before) walking catfish have been imported into the US to be sold as pets. Once in the US, they either escape from their environment or are purposefully let go. On the internet, there are anecdotal stories of walking catfish owners who have lost the fish because they literally walk away. Today, the US government requires a federal permit to own one of these fish but there are still pet stores advertising them for sale.

Reason(s) Why it has Become Established: Walking catfish are hardy fish which can thrive where many other fish struggle to survive. In addition to lakes and rivers, they can be found in brackish waters or warm, stagnant, often hypoxic waters such as muddy ponds, canals, ditches, swamps and flooded prairies. They can remain dormant through periods of drought and go several months without eating. When they do eat, they consume a wide variety of prey.

In addition, walking catfish have high fecundity and the males guard the eggs and free-swimming young, giving them a better chance of survival than the native, non-protected young of other species.

Ecological Role: Walking catfish are voracious, opportunistic feeders who are mainly active at night. They consume a wide variety of prey including eggs and larvae of other fishes, small fishes, a number of invertebrates including crustaceans and insects and sometimes plant materials. In densely populated drying pools, these fish become even more indiscriminate and quickly consume most other species present.

Walking catfish of all ages and size fall victim to a wide variety of predators including other fish, reptiles, birds and mammals. They are also killed by cars when migrating en masse across streets from one body of water to another.

Benefit(s): The walking catfish can survive extended periods out of water. In its native areas, this makes it an attractive food fish which easily can be sold and traded live. The species is fished by subsistence fisherman as well as managed at commercial farming operations.

Threat(s): Walking catfish have been know to invade aquaculture farms and eat large amounts of fish stock. Fish farmers in Florida have had to put up fences or build levees to keep them out. An additional threat to catfish fisheries, specifically, is the fact that wild walking catfish carry the disease enteric septicemia (ESC) caused by the bacterium Edwadsiella ictaluri. Wild walking catfish could infect farmed catfish with the disease.

In Florida, the total impact on native species is unknown. We do know, though, that walking catfish are extremely pervasive across southern Florida and many scientist consider the introduction of the walking catfish into the area as one of the most harmful introduction in North America. Walking catfish are especially devastating in small wetland pools during the dry season where they can quickly become the dominant species. The species that appear to be most affected are native centarchids and catfishes.

The walking catfish is a tropical fish and, if introduced into other warm areas of the US, the spread of the fish could mirror what happened in Florida. Southern Texas and Hawaii are examples of two US areas that could be vulnerable.

Control Level Diagnosis: I rank this threat as medium priority. In Florida, the population is established and it's northern migration is slowed by freezing temperatures. However, it is crucial to keep this fish out of other areas where it could flourish.

Control Method: Numerous countries have "blacklisted" the walking catfish. The United States has classified all members of the family Clariidae as injurious wildlife, illegal to possess without a federal permit. It is important to keep this fish contained because, once out in the wild, the population growth could be explosive in areas where there is a mild climate. In addition, the walking catfish is very hardy. It can survive months without food, and live in water that other fish would find intolerable. Poisoning it would be very difficult because it could walk elsewhere to avoid the poisons.

Food and Feeding Habits of Fish

The inland water bodies consist of small aquariums to nursery ponds, canals, beels, haor, baors(oxbow-lake), rivers, streams, flooded lands, etc. These are called freshwater basins. The more diverse fish types are found in these water bodies.

The shape, nature, feeding habits, color, etc. vary from species to species. Their cultivation system is also different. It is important to have scientific knowledge about the nature of fish, feeding habits, diseases and so on to cultivate fish through the choice using suitable control measures.

Types of Fish

Fish are classified into the following four types based on the type of food.


This type of fish survives, grows and reproduction by eating unicellular algae, filamentous algae, small water plants, portion of higher aquatic plants, detritus along with some mud or sand. In this case, the plant materials in their food consist of about 75% or more of the total gut contents while the animal-based food varies 1-10% in its diet. For example, Labeo rohita, Catla catla, Labeo bata, Ctenopharyngodon idella, Amblypharyngodon mola, Oreochromis mossumbicus, etc.


They take large numbers of animals as food such as Copepods, Cladocerans, insects such as beetles, water bugs, damsel flies, dragon flies, larvae, mollusks, different small fishes, tadpole larvae, etc. Some notable carnivorous fishes are Wallago attu, Channa punctatus, channa striatus, Channa marulius, Channa gachua, Chitala chitala, Chanda nama, Chanda ranga, Rita rita, Glossogobious giuris, Mystus seenghala, Mystus cavassius, Ompok pabda, etc. Among them some are active predators such as Channa marulius, Channa striatus, Wallago attu, Chitala chitala, Mystus seenghala, etc.


These types of fish eat all kinds of food. Although their favorite food is insects, they also eat vegetable-based foods such as unicellular and filamentous algae, different aquatic plants when needed. Besides, they take zooplankton such as crustaceans, rotifers, insects and its larvae, mud and sands. During their young stage, most of the fish prefer to eat zooplankton. They consume varying percentage of plants and animal materials.

Among omnivorous fishes, some feed on a large amount of plant materials. Some feed on equal amount of animal and plant materials while other take a greater amount of animal foods. Some important omnivorous fishes are Cyprinous carpio, Cirrhinus cirhosus, Tor tor, Puntius ticto, Puntius sophore, Puntius sarana, Gadusia chapra, Colisa fasciatus, Eutropicthyes vacha, etc.

Plankton Feeders

Some fish species take both zooplankton and phytoplankton. They take these types of food by filtering water using their gill rackers. Gizzard shad (Dorosoma cepedianum) fry feed on zooplankton until reach the length of one inch. They become a filter feeder after losing their teeth and consume phytoplankton and some tiny invertebrates.

Menhaden (Brevoortia) is also filter feeder that prefers to feed mainly on phytoplankton. They capture phytoplankton from the water using their gill rakers. Adult menhaden can filter 4 gallons of water per minute and receive different phytoplankton and zooplankton within their gills.

A silver carp (Hypophthalmichthys molitrix) is also a filter feeder that has a special filtration capacity. They can filter though their gills and consume lots of phytoplankton and zooplankton.

Fish can also be classified into the following three types based on the niche they occupy in different water levels.

Surface Feeders

The uppermost layer of water, where sunlight enters, grows a large number of plankton which produce their food through the process of photosynthesis using their chlorophyll. At this level oxygen is even higher which is suitable for various animal organisms. Catla catla is mainly stay at this level to collect food. Silver Carp is also a resident of this level. Besides, Puntius ticto, Oygaster bacaila, Chanda ranga, Chanda nama, Glossogobious giuris,Tenualosa ilisha, Gadusia chapra, etc are notable surface feeder fishes.

Column Feeders

Some species of fishes take their food from the mid water. At this level water waves are relatively few but zooplankton, phytoplankton are available with sufficient amount of oxygen, suitable for fish. The fish that live here are neither true bottom nor true surface feeders. They mostly depend on the food of the middle layer of the water. Labeo rohita, Labeo bata, Tor tor, Puntius sophore, Mystus seenghala, Wallago attu, Mystus vittatus, etc are the column feeder fishes.

Bottom Feeders

The bottom feeder fish mainly depend on food for bottom organisms. At this level, lots of benthos live here that provides nutrients to the fishes. Labeo calbasu, Labeo gonius, Cirrhinus cirrhosus, Puntius sarana, Amblypharyngodon mola, Cirrhinus reba, Clarias batrachus, Heteropneustes fossilis, Channa striatuis, Channa marulius, etc are notable bottom feeder fishes.

The following table showing the food and feeding habits of some freshwater fishes:

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Watch the video: CONCISE NOTES ON FISH BIOLOGY (August 2022).