Migration of Fishes | Phylum Chordata

In this article we will discuss about Migration of Fishes:- 1. Definition of Migration 2. List of Migratory Fishes 3. Types 4. European Eel 5. Anadromous 6. Causes.

Contents:

1. Definition of Migration
2. List of Migratory Fishes
3. Types of Migration
4. European Eel Migration
5. Anadromous Migration
6. Causes for Fish Migration

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1. Definition of Migration:

Migration of fish is defined as a class of movement which involves a long journey to a definite area for some purpose and impels the migrants to return to the region from which they have migrated. The purpose of the journey is breeding and feeding.

Migration is a two-way journey. It includes emigration (outward journey) and immigration (return journey or inward jour­ney). The fishes are notable for migration for the purpose of spawning. The inherent pur­pose of migration is not known.

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2. List of Migratory Fishes:

The following fishes may be mentioned as migratory fishes:

Common name – Scientific name

a. Lampreys – Petromyzon marinus, Entesphenus, Lethenteron, Ceotria, Mondacia

b. Eel – Indian longfineel (Anguillabengalensis bengalensis), Short- fin eel (Anguilla bicolor bicolor), Common freshwater or European eel (Anguilla anguil­la and A. vulgaris), American eel (Anguilla rostrata), etc.

c. Hilsa shad – Hilsa (Tenualosa) ilisha

d. Toli shad – Hilsa (Tenualosa) toli

e. Herrings – Clupea

f. Salmon – Salmo salar, Oncorhynchus

g. Sturgeon – Acipenser

h. Tuna – Thunnus

i. Mackerel – Scomber

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3. Types of Migration:

Myers (1949) has classified the following types of fish migration.

a. Diadromous migration:

When the migrations occur in between freshwater and marine environments.

Diadromous type of migration can be divided into following three-types.

(i) Anadromous migration:

When migration occurs from sea to fresh­water for spawning, called anadromous migration; e.g., Atlantic salmon (Salmo salar), Hilsa shad (Tenualosa ilisha), Toli shad (Tenualosa toli), Paradise fish (Polynemus paradiseus), Flat head sillago (Sillaginopsis panijus), Sturgeon (Acipenser) and Salmon (Oncorhynchus).

(ii) Catadromous migration:

The journey of freshwater fishes to the sea for spawning, called catadromous migration; e.g., Indian longfin eel (Anguilla bengalensis bengalensis), Shortfin eel (Anguilla bicolor bicolor), Common freshwater or European eel (Anguilla anguilla, A. vulgaris), American eel (Anguilla rostrata).

(iii) Amphidromous migration:

Migration of fishes from freshwater to sea and vice versa and is not for the purpose of breeding but for the other purposes (e.g., food). The amphidromous fishes migrate regu­larly at some particular stage of the life cycle.

Marine amphidromy occurs in flat head mullets (Mugil cephalus) which spawn in the Indian seas during autumn and early winter and whose young stage spend a short period in brackish water and freshwater. They are able to survive in ponds with salinity at 87%. After spending in fresh- or brackish water they return to marine water.

b. Potamodromous migration:

Migrations of fish that occur entirely in freshwater, called potamodromous e.g., carps and trout. Trouts and carps travel long dis­tances in large shoals in search of suitable spawning grounds and return to feeding areas after spawning.

c. Oceanodromous migration:

Migration which occurs entirely in sea, called oceanodromous migration. Horizontal and vertical distribution is considered in oceanodromous migration. Many fishes undertake short distance migrations through­out their life and some fishes like herrings, cod, tuna and plaice, cover long distance migrations.

Again on the basis of the purpose, migra­tion in fish has been classified into following types:

(i) Alimental migration:

Migration occurring for the purpose of food procurement, e.g., Bombay duck (Harpadon).

(ii) Gametic or Spawning migration:

Migration by a fish for the purpose of reproduction which enables the species for better survival and proper development of the eggs, e.g., Tenualosa ilisha.

(iii) Climatic migration:

Migration takes place for the purpose of reaching a particular region to secure better climatic conditions, e.g., salmon and sturgeon etc. Swordfish (Xiphius glodius) living in tropi­cal and temperate waters, sometimes migrate north in spring and autumn to cold waters, for the suitable climatic condition.

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4. European Eel Migration:

Catadromous migration (Gk. kata = down, dramein = to run)

The life-history of common river eel or the European eel (Anguilla anguilla or A. vulgaris), and American eel (Anguilla rostrata) will repre­sent a clear idea about the catadromous migra­tion. The common river eels or European eels are found along the shores of Europe and in inland waters of countries near the shores of Europe, some of which inhabit in Iceland, the Mediterranean countries, black sea and the red sea.

The adult eels with their sexual products are not encountered in freshwater, so their biology of reproduction was quite unknown for centuries, From the time of Aristotle the diffe­rent ichthyologists tried to find out the exact spawning ground of the European river eel.

At last Danish ichthyologist Johannes Schmidt who started his investigation in 1904, ultimate­ly succeeded in 1922 to locate the spawning place of the European river eel. It was found in the Sargasso sea of the Atlantic Ocean.

The life history of the European eel is divided into 4 phases (Fig. 6.109):

a. An ordinary yellow eel representing the growing and feeding form in the river.

b. Changes of the yellow eel into the sil­very eel ready for seaward migration for spawning (breeding phase).

c. A pelagic larval phase and

d. The metamorphosis of the pelagic lar­val phase to elver or young eels.

i. First phase:

The yellow coloured variety living in fresh water represents the feeding and growing forms. With the advent of autumn, majority of yellow eels become silvery and prepare to undertake migration forwards the spawning ground, the Sargasso sea of Atlantic Ocean. The eels in the river spend about 10 to 12 years feeding partially on fish.

ii. Second phase:

During the transformation from yellow- coloured to silvery colour, the yellow eels stop feeding, eyes become greatly enlarged, the snout becomes sharper with thinner lips and the yellow colouration is replaced by a metallic silvery colour. The silver eels are recognised by having matured organs and shrunken digestive tract.

The primary development of gonads is the stimulus for the beginning of migration. The size of the eggs in the ovaries changes before migration. These silver eels first migrate down to the mouth of the rivers and then into the Atlantic Ocean. The European eels probably migrate over 6000 km between its freshwater feeding streams and its spawning ground.

The spawning ground is located in the western part of tropical waters of Atlantic Ocean between 22° and 33° N. lati­tude and 48° and 65° W. longitude, near Bermuda Islands. Spawning takes place from the end of winter to the middle of summer. After the completion of spawning the parents die.

iii. Third phase:

Eggs are laid in spring at depth of 500 to 700 meters with temperature ranging between 10-12°C. The fertilized eggs float for some time and the youngs hatch out as the pelagic larvae. The larvae are called Leptocephali.

The leptocephali are flat, glassy, leaf-like body. These tiny creatures are provided with elon­gated needle-like teeth for feeding. The gut has a straight tubular structure. The eyes are large and silvery. They now begin their long home­ward journey. At the end of first summer when they become about 25 mm in length on aver­age, are recorded in the Western Atlantic.

By the second summer the leptocephali reach central Atlantic and the size is about 50-52 mm in length on average. In the third summer they become about 72-75 mm in length and reach in the continental shelf of Europe. The larvae of eel are passively drifted by warm water current of the Atlantic Ocean.

iv. Fourth phase:

During autumn and winter of the 3rd year the leptocephali metamorphose to form elvers or glass eels or young eels. During metamor­phosis the larvae stop feeding, their flattened body become cylindrical and the needle-like teeth is replaced by new ones. The young eels when become three years old, measure about 15-20 cm long.

Then they congregate in the mouths of rivers. Here the elvers or young eels ascend the rivers, grow in size and change their colour into yellow. The males like to stay in the estuaries and the females ascend the rivers in shoals, specially at night during the spring to reach a suitable resting place. In the yellow eels teeth are lost, intes­tine shortens and the anal aperture moves for­ward.

The yellow eels spend 8-10 years on fee­ding and growing and on maturity, change into silver eels and start their perilous journey towards the Sargasso sea. The well fed silver eels first stop their feeding and leaving the rivers, empty into the Baltic Sea and gradually in the abyssal depths of the Atlantic Ocean.

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5. Anadromous Migration:

Among Indian fish, hilsa or Indian shad (Tenualosa ilisha) represents an example of anadromous migration. Its occurrence is recorded in the coastal waters of Pakistan, India, Sri Lanka, Bangladesh and Myanmar. Its presence in the estuaries, rivers and lakes (e.g., Chilka), mostly during spawning season indi­cates its anadromous migration.

During spaw­ning season the hilsa population ascends the rivers of the Hooghly, the Ganges, the Mahanadi, the Godavari, the Krishna and the Kaveri and its tributaries in the eastern region of India, and the Narmada, the Tapti and the Kali in the Saurastra coast of the west India.

Around the Indian coasts, the Hooghly-Matla estuarine system covers a major portion of the Ganga-Brahmaputra delta and is estimated to be 3,100 sq. miles which is the largest in India.

This estuary zone is mostly famous for hilsa migration. Before the con­struction of Farakka barrage on the Ganges in West Bengal the hilsa used to migrate up to Delhi during the spawning season covering a distance over 1,250 km. In Bangladesh it ascends in the rivers of Meghna, Padma, Brahmaputra and its deltaic rivers. It also migrates in the upper part of Irrawaddy river of Myanmar.

The marine distribution of hilsa in the Bay of Bengal and in the Arabian Sea is due to seve­ral factors, such as vast stretch of continental shelf, low salinity, discharge of huge amount of monsoon freshwater in the coastal region through the rivers, monsoon winds, huge abundance of planktons as food, and other favourable hydrological parameters. Oceanic properties along the Indian coasts are given below in Table 24.

In the sea, hilsa population is found along the east coast in the vicinity of rivers before the spawning season but in Gujarat and Maharashtra the hilsa is found about 12-16 km off the coast at a depth of about 20 fathoms.

There are two types of hilsa stocks — one estuarine and offshore stocks which are found in the lower region of the estuary and the fore­shore areas of the seas, and another is riverine stock which spends throughout the year in the river, mainly in the Ganges and the Brahmaputra.

The estuarine stock migrates upwardly during breeding season and after spawning they return to their natural habitat and spend throughout the year till the next breeding season to come. The riverine stock ascends in the more freshwa­ter river zone during breeding season and after spawning they come back to the lower reaches of the river and spend the rest period until the next breeding season comes.

The life cycle of hilsa is divided into 4 phases:

(i) Egg

(ii) Larval stage or Fry,

(iii) Fingerling or Juvenile stage and

(iv) Adult stage.

The size of adults differs in sexes. The females are larger than males. The size of the sexes differs in different seasons, even in the same river. In the Ganges and in the Padma different sizes of sexes have been reported by different authors. In the Hooghly estuary the mature females are recorded about 250 mm in length.

The adult hilsa are laterally com­pressed, fusiform animals, having abdomen with a keel about 30-33 scutes. There are very fine numerous, closely set gill-rakes that indi­cate for planktonic feeding habit, predomi­nantly zooplankton feeder. They mainly con­sume Cyclops, Daphnia, Moina, rotifers and protozoans. The adult hilsa in the freshwater zone of the Hooghly river are column and bottom feeders.

The hilsa is probably polygamous and fer­tilization is external. The riverine stocks of hilsa become mature in between 1-3 years of age. Matured hilsa spawns once in a year but the time of spawning differs in different parts of India. In Hooghly estuary or freshwater region of the Hooghly river spawning takes place at the onset of evening but in the Narmada spawning takes place in the early morning.

During upstream migration schooling beha­viour among hilsa is seen and males move in the upper surface layer of water and females move in the deeper water layer in river during monsoon period.

Many authors reported that during spawning period the hilsa do not feed or stop feeding but Pillay (1958) has reported that the appreciable amount of food is available during spawning period, especially of hilsa of the Hooghly region. The feeding intensity is increased considerably after spawning period.

In the spawning ground both the sexes discharge their garnets in freshwater. The tailed spermatozoa survive few hours in the water and ova are large, translucent at the marginal zone. The fertilized ova turn trans­parent after half an hour. The fertilized eggs float in water and colour is about light gree­nish yellow. The hatched ova transform into larva or fry when they become 20-40 mm in length.

The abdomen of the larva possesses 5-7 pre-ventral scutes. At about 40-45 mm in length, the dark blotches on the lateral side are seen that indicate the fingerling or young hilsa stage. The larvae are surface feeders, predo­minantly zooplanktons which constitute about 70% in their food composition. When the fry attains 100 mm in size they are called finger- lings or young hilsa.

The body of young hilsa becomes laterally compressed with keeled abdomen. The abdomen possesses 30-32 scutes. The body colour is silvery along the sides. A row of dark blotches is seen on the lateral sides of the body.

When the fingerlings become 150 mm in size, called advanced fingerlings or youngs. At that stage they con­sume small shrimps and phytoplanktons. The early fingerlings (about 80 mm) are found in the lower reaches of the river and estuary zones. The youngs or advanced fingerlings (above 150 mm) occur along the foreshore areas of the seas.

Two types of anadromous migrations are seen among Indian hilsa species. Breeding or spawning migration is seen during the south­west monsoon when the Indian and Bangladesh rivers are flooded by the monsoon rain. Another type is winter or spring migra­tion which is influenced by the certain changes of the water temperature and rain, mainly seen in the Gangetic delta.

The correlation between temperature and movement into freshwater may be a reflection of the energetic cost of migration. In the Indus river of Pakistan and Irrawaddy river of Myanmar, the hilsa migration takes place by the molten snow that creates flood in these rivers.

The adult hilsa spends most of the year in their original places except spawning season. The young and adult hilsa can tolerate certain variation of salinity. The Hooghly-Matla estuary is classified as a mixohaline range, in which salinity varies in different zones.

Mystus gulio migrates to the estuarine and freshwater zones of the Hooghly river for spawning, and Pama pama also migrates to the estuarine zone of the Hooghly river for both genetic and tropic reasons. Toli shad (Tenualosa toli) is found in the western coasts of India, ascends in rivers for spawning.

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6. Causes for Fish Migration:

Fish migration is related to several factors such as physical, chemical or biological.

i. Physical factors:

The physical factors include temperature of water, rainfall, quality of water, water depths, pressure, light intensity, photoperiod, turbidity, tides and currents.

ii. Chemical factors:

The chemical factors include pH of water, salinity, dissolve of O₂ and CO₂, types of dissolved organic and inorganic substances, and taste of water.

iii. Biological factors:

The biological factors are food, attainment of sexual maturity, endocrine behaviour and competitors and predators.