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Effect of Dams and Weirs on Fisheries

Definition: In principle, dams, weirs, barrages and anicuts are created by constructing stone

masonry or concrete bunds across a stream. All the above stated structures raise the water level

for facilitating diversion of flow.

Dams usually comprise a massive concrete wall built across a stream at strategic, usually rocky

sites, leading to the formation of vast reservoirs upstream of the bunds, often stretching into

hundreds of miles.

A weir; as distinguished from a dam, connotes the discharge of water over its crest or through

wide openings.

The term barrage is of French origin and is often applied to weirs provided with sluice openings.

An anicut is a word of Tamil origin meaning ‘dam building’. It is usually a low barrage built

primarily for irrigation purposes.

In U.S.A., any barrier placed across a river is called a dam, the term weir being

specifically used to denote the movable wickets or gates forming part of the dam and employed

for regulating the flow and the level of water.

Damming of streams led to the formation of impoundments. Dams are constructed for

multiple purposes viz. water storage for irrigation, for industrial and domestic uses, flood control,

hydroelectric power generation, for navigation, recreation, development of fisheries and sports

fishing.

Petts (1989) identified four phases in the era of modern river modification:

Phase-I (from 1750 – 1900): During this time lot of regulation, schemes were implemented in

many of the large European rivers for navigation, flood control and utilization of flood plain

land.

Phase-II (from 1900 – 1940): This is the period of development of technology to built great dams

specially in North America, Europe and Southeast Asia.

Phase-III (from 1950 – 1980): This is the time of maximum activity in dam building worldwide.

The activity reached such a height that in late 1970s dams over 15 m in height were being

completed at the rate of over 700 per year world-wide.

Phase-IV (from 1980 till date i.e. up to 1989 when author had reported): The pace of dam

building has slowed down to about 500 per year worldwide. It is estimated that by the year 2000,

over 60% of the total stream flow in the world will be regulated.

Dr. Subhendu Datta

Sr. Scientist

CIFE, Kolkata Centre, India

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EFFECTS OF DAMS AND EFFECTS OF DAMS AND EFFECTS OF DAMS AND EFFECTS OF DAMS AND WEIRS ON THE FISHERIWEIRS ON THE FISHERIWEIRS ON THE FISHERIWEIRS ON THE FISHERIES OF RIVERES OF RIVERES OF RIVERES OF RIVER

Fish and their habitat are considerably affected by river valley projects. Even though lotic sector

of the reservoir maintains a fluviatile ecosystem, the lentic zone and the bays sustain a lucstrine

ecosystem. The reservoir develops certain features of its own. The dam as a whole alters the river

hydrology both up and down streams, making a very new environment. The quality of

impounded water varies from watershed to watershed depending on soil quality, human

interferences and climate conditions. To a large extend it also depends on the morphometric

characters of the reservoirs like shape of the basin, area, mean depth and the regularity of the

shoreline. There are positive as well as negative impacts of reservoirs on fisheries.

Positive factors effecting the fish production due to impoundment:

(1). Increased primary productivity: In running water plankton population is generally very low,

due to turbidity restricting population of sunlight. Clarity of water is generally very high in most

of reservoirs as reservoirs act as silt traps and hence the suspended matter settles down at the

bottom. During the filling period of reservoirs (initial 2–3 years), there is usually an initial spurt

of plankton and benthic communities due to the increased availability of nutrients released from

the decay of submerged vegetation. This trophic burst is also because of the saprogenic (like to

feed on dead organic matter) lacustrine species filling the vacant niches created by the

disappearances of saprophobic (don’t like to feed on dead organic matter) riverine animals. This

led to increased primary production. The reservoir yielded very high production after damming

the rivers. In most of the Indian reservoirs, from first year of impounding there appeared a

phytoplankton bloom which persisted in many reservoirs like Stanley reservoir, Bhavanisagar,

Amaravatty etc of southern India.

2. Reservoirs act as sanctuary: There are many instances where reservoirs acted as sanctuaries,

example for this are Barilius bola in Tiaiya (on river Damodar), Osteobrama vigorsii and Mystus

krishnensis in Nagarjuna sagar (on river Krishna) Tor khudree in Sivagisagar (on river Krishna)

etc. The inundated tree tank can act as substrates for thick periphyton (organisms that live by

attaching to the stem and leaves of freshwater plants) growth which promotes the production of

fishes like Labeo rohita which feed on periphyton.

3. Availability of food is increased: Water level fluctuations in reservoirs benefit the fishery,

when water inundates the land area along with vegetations. Abundant food is made available for

the fishes due to the decomposition of the vegetation, which releases nutrients to the water for

the growth of biotic communities.

Negative role of impoundments due to the construction of dam on fisheries:

The problems, which may arise for fishery due to the construction of a dam, are associated with

the unfavourable physico-chemical conditions of water, unavailable food and feeding areas,

barrier for fish migration, damage of spawning grounds, excessive growth of aquatic weeds and

change in species composition of fish.

(1). Unfavourable physico-chemical conditions: The Physico-chemical conditions of a reservoir

depends on the prevailing climatic conditions including air temperature, wind velocity, rain fall

etc. Consequent to the dam construction and reservoir formation, substantial morpho-ecological

changes occur in the original river both above and below the dam site. These include conversion

of running water into a water body of slow discharge characteristics and radical transformation

of long established ties and inter-relationship between organisms. During summer, in the static

condition of the reservoirs, surface water gets heated up and the bottom layer remains unaffected.

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When the bottom is warmed up, decomposition of organic matter is accelerated and the results

will be release of nutrients. The thermo cline does not allow a mixing up of rich nutrients at the

bottom layer, which was locked up at the bottom. In lotic condition, mixing up of different layers

of water permits equal distribution of nutrients. The amount of rain determines the rate of inflow

into the reservoir hence plays a vital role in bringing in the replenishment and nutrient

enrichment. In reservoirs, inflow rate is affected as the rainfall in the catchment’s of the rivers

situated hundreds of kilometer away from the reservoirs.

The productivity of reservoir is determined by its depth also. In shallow reservoirs, the

greater part of their water is the euphotic zone facilitating greater and circulation of heat and

nutrients and hence higher productivity. In deep reservoirs, the organic matters accumulate in the

bottom and become unavailable at the photosynthetic zone. The spillway discharge removes the

oxygenated clear water at the top layer leaving the oxygen-deficient, turbid bottom water. The

deep dwawdown also removes the decomposing materials including nutrients.

The oligotrophic tendencies shown by some of the reservoirs are mainly due to the poor

nutrient status and other chemical deficiencies. In most cases, poor water quality is a direct

reflection of the catchment soil. All reservoirs in Kerala show low primary productivity and poor

plankton abundance. The reservoirs in Kerala also recorded low specific conductivity (<50

µmhos) and total alkalinity (<50 mg/l).

(2). Unavailable food: Damming of a river denies the free flow of silt which causes its deposition

to the bottom which is unfavourable for benthic invertebrates and this reduces the production of

benthivorous fishes. Sudden changes in water level, inflow and outflow directly affect the

benthic communities viz. plankton, benthos and periphyton pulses which coincide with the least

level fluctuations. Storage and release of water from dams are governed by its primary objectives

like power generation, irrigation, flood control etc. The spillway discharge dislodges the standing

crop of plankton. Lack of plankton affects the planktivorous fish production.

(3). Damage of spawning grounds: The rapid water level fluctuations damage the spawning

grounds of many fishes. Silt deposition on fish eggs increases egg mortality and thus decrease

spawning success. Aquatic littoral vegetation often provides the very substrate within which or

on which eggs are laid and may protect eggs from wave action and erosion. Heavy siltation

destroy the nesting materials i.e. aquatic vegetation, therefore, these reservoirs do not provide

good spawning habitat for species dependent upon vegetation. Draw down also minimizes the

amount of vegetation available for spawning, especially in turbid reservoirs. Many fishes have a

tendency to swim against the current and spawn. In reservoir condition, such facility is available

only at upper reaches of the reservoir where to some extent lotic condition exists. The fishes,

which cannot adapt themselves to the changed lacustrine condition, will perish.

(4). Excessive growth of floating vegetation: The lentic condition of reservoirs encourages the

growth of micro as well as macro vegetations. However, Indian reservoirs are almost free from

such problems. In African reservoirs, Kariba and Volta, floating aquatic weeds especially water

hyacinth even choked the water. This condition prevents the light penetration, loss of nutrients

and loss of water due to high rates evaporation. This affects the growth of other biotic

communities in the reservoir. Excessive growth of Salvinia (30% of lake area) in lake Kariba

leads to the low (6 12 kg/ha/year) fish production.

(5). Change of fish fauna: Riverine fish fauna is subjected to a series of habitat changes such as

water current, turbidity levels, fishing pressure, loss of breeding grounds and changes in fish

food organisms due to lake formation. The species, which could withstand these changes only

survive others perish. In many reservoirs transplantation of fish from other basins take advantage

of the vacant niches and the introduction of exotic species led to the changes in species spectrum.

Many of the indigenous fish fauna, which were relished by the natives, gave way to the

introduced species.

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(6). Hindrance to fishing: The trees, which are left behind when a reservoir is filled pose a major

problem to fishing and operation of boats in the reservoir. In reservoirs, gill nets are in maximum

use due to the hindrance of submerged trees to the operation of other gear.

(7). Multiplication of fish parasites: The changes from lotic environment of river to lentic

condition of reservoir with slow current and higher temperature favour the multiplication of

parasites. This can affect fishery in the long run.

(8). Effects on fish migration: Dams, weirs and barrages act as physical barriers to migrations,

tending to prevent access of fish to their usual breeding, rearing and feeding grounds. The denial

of migration may result in permanent and irrevocable reduction of fish stocks ranging from

lowering the levels of abundance to complete extermination. The niche so created may be filled

by undesirable species. Anadroumous fish tend to settle down. Considerable reduction of flows

in the residual rivers tailing below the dam; significantly alters the ecology of the spawning

grounds, which even dry up. The reduction of water levels in the residual rivers results in the

formation of shallow areas, which impede or obstruct fish movements. In cases where dams and

weirs etc. are constructed in estuaries, due to reduction in the discharge of water and the wetted

perimeter, changes occur in temperature and salinity regimes of brackish waters and the current

velocities and the direction at the mouth of the rivers. The latter essentially constitutes the

directive factor for migration of fishes, particularly the anadromous varieties resulting in

successful migration or total failure of runs. Whenever dams, weirs etc. are constructed in

regions beyond the areas of natural occurrence of economic species, whether of non-migratory or

migratory habitats, or located for above the routes of migration of anadromous fish, the effect of

such constructions are of no consequence to the fisheries.

FISH MIGRATIFISH MIGRATIFISH MIGRATIFISH MIGRATIONONONON

Generally fish restricts their movement within small territorial limits and do not go out of their

home ranges. However, a few species travel long distances moving from fresh to seawater or

vice versa. Migrations of fishes are usually an active though sometimes passive, consists of mass

movement from one habitat to another. Fishes migrate to the region where they find the

conditions they require at the particular phase of their life history which sets in towards the end

of their migration.

DEFINITION: Migration may be defined as the mass movement of fishes on temporary or

permanent basis by the influence of various physical, chemical or biological factors.

Migration may takes place in a vertical direction, as from the deeper to the surface water,

or it may be in a horizontal direction, either upstream or downstream.

Physical factors include (i) the bottom materials (ii) the depth of water (iii) pressure (iv)

temperature (v) light intensity and photoperiod (vi) current and (vii) turbidity.

Chemical factors include (i) salinity (ii) pH (iii) smell and (iv) taste of water.

Biological factors include (i) sexual maturity (ii) blood pressure (iii) food (iv) physiological

clock (v) memory (vi) endocrine glands (vii) presence or absence of predators and competitors.

Availability of food is one of the major factors that are responsible for large-scale

migration of many species of fishes going out in search of feeding areas. Similarly higher

temperature of the seawater in summer provides a stimulus to salmon for seaward migration

when temperature in river raises the move upstream for spawning. Salinity is also an important

factor. Most of the fresh water fishes are intolerant to salinity changes (therefore, osmotically

conservative) and don’t undertake large-scale migrations, confining themselves to freshwater

only. These fishes are called stenohaline, for example Labeo and Trout of river and lake.

However, a few species like hilsa, salmon, and Anguila adept themselves to large-scale salinity

changes and are euryhaline in nature. The intensity and duration of light: Some fishes are

attracted towards light and can be trapped by placing lights at suitable point. Lampreys and

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sturgeons migrate during night. Herring migrate during full moon. The water current

considerably influences the direction of movement of fishes. Eggs and fry are passively

transported along with the current to their breeding grounds. After spawning spent salmon are

carried by the river currents towards the sea. Smell and memory: It has been shown

experimentally that salmon returns to the same area of the river when adult for spawning, where

its own hatching and early development took place. The state of maturity of gonads and the

conditions of the endocrine glands are also important factor governing migration.

Pattern of fish migration:

The young stages leave the spawning ground ‘A’ for the nursery grounds at ‘B’. From the

nursery grounds, the juveniles go to the adult stock o their feeding grounds at ‘C’, and the mature

and ripe fish move from the feeding grounds back to the spawning grounds at ‘A’. Then the

spent fish return to the feeding grounds ‘C’. The migration pattern is believed to be related to the

currents. The young stages drift with the current to the nursery grounds. The spawning migration

is against the current and the spent fish returns to the feeding ground with the current.

Types of migratory movements:

(a). Denatant movement: Denatant means swimming or migrating with the current. For example-

the movement of the pelagic eggs, larvae and spent fishes.

(b). Contranatant movement: Contranatant swimming or migrating against the water current. For

eggs the migration of adult fishes towards spawning grounds.

Methods of migratory movements:

A fish makes migratory movements by following methods:

(1). By drifting: The fishes are passively carried by water currents. This is called ‘drift’. This

may result in the “directional movement”, if the overall movement of water is in one

direction.

(2). Random locomotary movements: The locomotary movements that are random in the

direction lead to a uniform distribution or to an aggregation. If fishes are released from a

point in a uniform environment and spread out in all directions, the process is called dispersal

and leads to uniform distribution of the species.

(3). Oriental swimming movements: The fishes swim in a particular direction (i) either towards

or away from the source of stimulation, (ii) At some angle or to a imaginary line running

between them and the source of stimulation.

Types of migration:

(i) Alimentary or feeding migration: The movement of fishes away from the spawning or

wintering ground to feeding ground in search of food and water. In many fishes, the feeding

C. Feeding area

A. Spawning area B. Nursery area

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migration begins in the egg stage. The transport of the pelagic eggs and the free embryos

from the spawning area to feeding grounds is a passive feeding migration. Passive feeding

migration occurs in marine fishes like Arab Sea barbell and fresh water fishes like grass carp,

Chinese reach etc. Active feeding migration occurs in Clupea.

(ii) Gametic or spawning migration: The movement of fishes away from feeding or wintering

grounds to the spawning grounds for the reproduction. In majority of fishes the start of

spawning migration is usually related to the attainment of a definite stage of maturity and the

appearance of a definite hormonal activity. The reaction of fish to its external environment

are altered i.e. there appears a new natural stimulus, which acts as a signal for start of

migration. The spawning migration has evolved as an adaptation for ensuring the most

favourable conditions for the development of the egg and larvae and especially for the

protection of the early stages from the predators. Some marine fishes living in deep waters

rise to spawn in the upper layers of water while some goes into deep waters to spawn.

Spawning migration precedes the time of spawning.

(iii)Wintering or climatic migration: It is a movement away from spawning or feeding grounds to

the wintering grounds to secure climatic condition, which is more suitable. This is connected

with the attainment of a definite environmental condition and fat contents of the fish body. It

occurs in the adult fishes which have a wintering ground, e.g. the Bream in the Arab sea,

makes an over wintering migration and winters in rivers while the immature individuals stay

and feed in the sea during winters. This migration is performed by migratory, semi-migratory

marine and fresh water fishes.

(iv) Osmoregulatory migration: Migration to maintain the osmotic concentration of body fluid.

Osmotic concentration of the fluid rises or falls due to the desiccation or excessive fluid entry

due to changes in the salinity regimes of the water.

Terms used to describe fish migration:

(1) Diadromous fishes: These are truly migratory fishes, which migrate between sea and fresh

water and are of three types:

(a). Anadromous: Diadromous fishes, which spend a major part of their lives in the sea, but

migrate to the fresh water during breeding period for spawning e.g. Salmon, Sea lamprey, Hilsa.

They travel long distances in the sea and run up to the river to spawn in fresh water. After egg

laying the spent fishes, return to their feeding places in the sea.

(b). Catadromous: Diadromous fishes, which spend a major part of their lives in fresh water,

but migrate to the sea for breeding purposes. The fresh water Anguilla, travels several thousand

miles from the rivers and reach the spawning grounds in the sea, where after egg laying the

adults die and the young larvae drift back towards the fresh water taking 3 years to reach the

rivers where they undergo metamorphosis, changing from Leptocephalus larval stage to elvers

and ascend the rivers. Here they become adults and on reaching maturity, start their seaward

migration again.

(c). Amphidromous: Diadromous fishes in which migration from fresh water to the sea or vice

versa, is not for the purpose of breeding but occurs regularly at some other definite period of the

life cycle e.g. Gobies [these are fresh water fishes, breed in the water where they live. Order:

Perciformes, Sub order, Gobioidei; example. Glossogobius giuris (Ham.). Fam.; Gobiidae;

example. Gobius striotus (Day)].

(2). Potamodromous fishes: Truly migratory fishes whose migrations remain confined to the

fresh water, e.g. the carps and the trout travel long distances in rivers in search of spawning

grounds, where after egg laying they return to the feeding area.

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(3) Oceanodromous fishes: Truly migratory fishes, which live and migrate in the sea. Fishes like

herring (clupea), mackerels (scomber) and the tunners (thunnus) travel long distances in the sea

to deposit their eggs and later return to the feeding grounds.

Advantages of migration: According to Nokolsky, migration is an adaptation towards

abundance. The spawning or nursery grounds may not have enough food to maintain both the

mature and immature members of a large population. Hence, it would be an advantage to have

separate spawning, nursery and feeding grounds. The fact that many commercial species are

migratory supports the view that migration is an adaptation to towards abundance. Further, there

appears to be some advantage to a species, whose adults returns to spawn in an area where the

environmental conditions were similar to those under which they themselves survived when

young. A return to the parent spawning grounds provides a means by which these favourable

conditions may be exploited. Thus, a better egg and larvae survival would lend to a greater

number of spawners in a given area.

Apart from migration, which is the link in the life cycle, many fishes perform mass

movements in various biological conditions and which most frequently bear a protective

character. Examples of such movements are movements away from the coastal zone during the

stormy weathers and the movements of the fishes away from the lakes and bays when the water

level in the rivers falls.

Effect on hilsa migration: The Indian shad, Hilsa ilisha was hitherto considered a typical long

distance anadromous migratory fish of the Indian waters of high economic value. In recent years,

however, evidence has become available of the existence of some non-migratory resident stocks

of hilsa. Hilsa ascends for breeding in shoals from sea into the Ganga-Brahmaputra, Mahanadi,

Godavari, Krishna, Cuvery, Narmada and other streams. Two runs are recognized, one, the post-

monsoon run and the other, late winter run, the former being the major. In both cases, the fish

returns to the estuary after breeding in freshwater and so do the young, which requires semi-

saline environment for growth and survival.

Consequent to the construction of barrages on the Godavari, Krishna and Cuveri, hilsa

migration got restricted to the portions of the rivers below the anicut and the fisheries bearing on

these stocks declined considerably. In the stretches of the rivers above the anicut, hilsa fishery

has been rendered practically non-existent. The drastic decline in hilsa catches in the Hoogly

estuarine system is the result of failure of runs caused by the removal of essential directive

factors in the form of adequate velocities of flow and volume of discharge due to dams of the

Damodar valley corporation. Reduction in the spawning area of the fish (both for monsoon and

winter runs of hilsa) in the residual Rupnarayan and Damodar rivers has led to the decline of

hilsa fishery. In Pakistan, the Ghulam Mahammed Barrage which was constructed in 1954 on the

Indus near Hyderabad (Sind), 291 Km upstream from river mouth, deprived hilsa of

approximately two-thirds of their previous spawning area.

Effect of Farakka barrage on hilsa fishery and some other freshwater fishes and prawn species: Collapse of hilsa fisheries occurred in middle and lower stretches, after commissioning of

Farakka barrage in 1975 (middle stretch: Kanpur to Patna, lower stretch Patna to Dhulian).

Before construction of Farakka barrage, the hilsa fishes migrate up to Kanpur during post-

monsoon for breeding purposes. Due to construction of Farakka barrage, the hilsa failed to

migrate above Farakka barrage and this is one of the main reasons for depletion of over all

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fisheries (lower fish catch) in the middle and lower stretches of Ganga. However, hilsa fisheries

have shown some improvement during last two-three years (due to artificial fecundation of hilsa

at Farakka and stocking of hilsa young). In pre-Farakka barrage period (1958-74), the yield of

hilsa at Allahabad ranged from 7.87 to 40.61 tones, at Buxar from 7.38 to 113.36 tones and at

Bhagalpur 1.47 to 9.79 tones. While at post-Farakka period the catch has declined to 0.13 to 2.04

tones, 0.07 to 2.06 tones and 0.01 to 2.18 tones respectively at the above centers. Based on

average annual hilsa landing; it has declined from, at Kanpur 31.97 t to 0.60 t, at Allahabad

19.30 t to 1.04 t and at Bhagalpur 3.95 t to o.68 t after commissioning of Farakka barrage.

On the contrary, many fold increase in fish yield has been observed in the estuarine zone

during post-Farakka barrage period. The average annual prawn and fish yield from the estuary

increased from 9481.5 tones during pre-Farakka barrage period (1966-67 to 1974-75) to 33341

tones during post- Farakka barrage period (1984-85 to 1994-95) and further to 42703.2 tones

during 1996-97. With the commissioning of Farakka barrage and higher flow of freshwater in the

estuary; the general habitat for hilsa in the estuary has improved for its migration, breeding and

growth, resulting in its increased landing from 1457.1 tones in 1975 to 5045.8 tones in 1995-97.

The species now spawn in the entire freshwater zone of the estuary certain freshwater fishes and

prawn species viz. Eutropiichthys vacha, Chepisoma grama, Rita rita, Wallago attu, Aorichthys

seengala, A. aor, Catla catla, Labeo bata etc. have made their appearance in the entire upper

estuarine zone up to Uluberia and these species were not reported prior to pre-Farakka barrage

period up to this extent. The freshwater zone of the Hoogly estuary is a potential source of hilsa

and prawn (particularly M. rosenbergii) seed.

Fish-Ways/Fish Passes/ Fish Ladders/ Fish-Lock: When a dam is constructed, it acts as a barrier in the movement of fishes. Therefore,

population of migratory fishes will diminish, for example, collapse of hilsa fishery above the

Farakka barrage after its commissioning in 1975. One of the remedies commonly proposed for

blockages to migrations caused by dams is the construction of fish ways/fish passes/fish ladders

etc. Only anguillides (eels) appear capable of surmounting even some very large dams such as

Kariba (in Africa) without the assistance of specially constructed by-pass structures. Special by-

pass structures are constructed to help the migratory fish to negotiate the dam height and to cross

this artificial obstacle (dam) in the river streams. These are in broad term called fish-ways.

Natural fish-ways are channels or a series of connected pools. Artificial fish-ways are of various

types:

(1). Fish-pass: When fish migrate pass the dam or barrage through special tunnels or channels,

those structures are called fish-pass. Designs of various fish-passes are as follows:

(a) Simple sluice or inclined chute: This is a simple channel type structure, either with or without

some modification to decrease velocity of water and with or without resting pool. Inclined

sluice is used in U.S.A. for alewives to negotiate the dam.

(b) Denil design: This is a narrow channel with closely spaced baffles set at an angle with the

axis of the channel. The baffles with parts of the channel walls and bottom from secondary

channels while they leave free a relatively large portion of the channel for the straight main

flow. The main advantage of denil design is the energy dissipation of the fast flowing water

through closely spaced baffles resulting in slower flow.

(c) The deep-baffled channel: This design is suitable for low gradient and is suitable for extreme

variations in inflow and water level. Single slot vertical baffle fish-way is a modified denil

type of fish-way.

(2). Fish ladder (The pool and jet fish-way): Oldest and most widely used of all types. It

consists of a series of pools in a stepped arrangement from tail water to headwater, connected by

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short rapid or low falls. Each pool provides adequate resting place for the ascending fish. The

effort required on the part of the fish is intermittent alternating between exertion and partial rest.

The flow from pool to pool may be over solid obstacles or through notches or orifices in the

obstacle or a combination of both. In negotiating an ordinary over fall (low falls), the fish usually

swim up the falling jet. The pools and jet design is suitable for mild slopes only and the tendency

to jump from pool to pool is limited to certain species of fish. When the obstacles have

submerged orifices (generally they are placed on the opposite side of the successive obstacles),

the fish generally prefer to use them.

(3). Fish-lock: It has mechanical gates with arrangements for the regulation of flow of water into

and out of the lock.

Fish passes/ladders in use on impounded temperate zone rivers for the salmon worked so

satisfactorily because their design has been optimized (by many years of study and experience)

for one or a small number of target species.

To be effective in a particular situation a fish ladder/pass must fulfill two main criteria:

(a) It must satisfy the complex behavioural factors of the one or more fish species requiring the

access, including such factors as the form of the standing wave downstream of the structure.

(b) It must have sufficient capacity to handle the number of individuals present in the migrating

stock(s).

In addition, the fish must have two characteristics:

(a) The fish whose protection is sought should have the habitat of leaping in the air or

swimming up-hill.

(b) There should be the habit of spawning in the upper reaches only; when a fish can spawn

in the sections of the river in the plain it cannot possess the urge to go uphill.

When the above two conditions do not exist, there is no case for fish pass/ ladders.

The type of fish ways must be adjusted to the behaviuor and physiology of fish in

overcoming obstacles. Before a fish way is constructed the migratory habits of the fish, condition

of water level, behaviuor of the fish, natural and hydraulics of the stream must be studied. Fish

ways are expansive to built and operate. The flow properties of the fish way should be such that

the effort required on the part of the fish to negotiate the fish way remains well under the limit of

the capability of the fish to do so. Before providing a fish way, its cost should be weighed against

the capitalized value of the present or potential resources that require it.

Reference books:

1. Thornton, Kent W., Kimmel, Bruce, L. and Payne, Forrest E. (1990). Reservoir

Limnology: Ecological perspectives. Chapter 8. Perspective on fish in Reservoir

Limnology, p. 209-225.

2. G. K. Vinci. (1999). River valley modifications and their impact on environment and

fisheries. In Ecology, Fisheries and Fish stock assessment of Indian rivers. M. Sinha, M.

A. Khan and B. C. Jha. (Eds.) Bulletin No. 90. CICFRI, Barrackpore, W.B. p. 47-53.

3. V. G. Jhingran. (1991). Fish and fisheries of India. Chapter 4.3. Dams and their effect on

fish migration. P. 108-111.

4. FAO Fisheries Technical Paper No. 262. Chapter 8: Management of River Fisheries.

Model questions:

1. Write an essay on impact of dams and wires on the migration of hilsa.

2. Describe the effect of Farakka dam on the migration of hilsa.

3. Write an essay on the effect of dams and weirs on fish and fisheries.

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4. Write an essay on fish migration.

5. Short notes: (a). Fish ways

(b). Fish pass.

(c). Fish ladder.

d. Anadromous and Catadromous fishes

e. Dams and Weirs

f. Stenohaline and Euryhaline fishes

g. Feeding migration and Spawning migration of fish

7. Write long notes on negative effects of dams and weirs on fisheries

8. a). Describe briefly negative role of impoundments due to the construction of dam on

fisheries.

b). Write in short different by-pass structures constructed to help the migratory fishes to

cross the artificial obstacle (dam) in the river streams?

c) Describe different types of migration of fishes. What are anadromous and catadromous

fishes?