DESTRUCTIVE METHODS OF FISH CATCHING

There are many methods of fish catching which are destructive in nature and harmful to fisheries.

Some of these methods are discussed below:

Unselective gears: In the broadest sense, all fishing is environmentally damaging to a greater or

lesser degree. Many fishing gears are unselective with respect to both size and species in the

catch. Compared with gill nets, for example, trammel nets are believed to be unselective in that

they catch a wide size range of individuals and a much larger number of different species. For

this reason, the use of trammel nets is banned in some areas. Trawl nets with heavy ground

chains, and steel dredges, are known to be highly destructive to the sea floor and its epifauna.

Even the continual use of light trawling gear may prevent the settlement of marine benthos.

Much development in gear technology is aimed at reducing the catch of small individuals

and non-target (by-catch) species. In trawl nets, the use of square meshes rather than the more

conventional diamond-shaped ones has been shown to reduce the capture of small individuals.

To reduce the number of by-catch species caught, special devices have been designed to

incorporate into net designs. Such devices, which funnel unwanted or threatened by-catch

species out of trawl nets, are often referred to as TEDs (Trawl Efficiency Devices, Trash

Eradication Devices, or even Turtle Exclusion Devices). It is now mandatory to fit exclusion

devices to trawl nets used in some fisheries, including the turtle exclusion device fitted to

Norwegian pandalid shrimp trawlers (Figure 1).

Figure 1. A Norwegian-designed fish exclusion device fitted ahead of the codend of a shrimp

trawl. The catch of fish and shrimp is forced through a funnel against a grid, which deflects fish

(small cod and haddock) out of the net through a triangular outlet. Shrimp pass through the grid

into the codend.

Dr. Subhendu Datta

Sr. Scientist

CIFE, Kolkata Centre, India

In many fisheries, steps are being taken to replace destructive gears with those, which are less

environmentally damaging. New types of lighter trawling gear are being designed to skim the sea

floor rather than dig it up. Steel dredges, which are towed to catch some species of scallops

(Pecten), may eventually be replaced with trawl nets.

Catching of juveniles and brooders: Indiscriminate catching of juveniles due to poor

availability of good size fishes resulted deterioration in the quality and quantity of spawn and

this ultimately affecting the standing fish stock. For example, the exploitation of juveniles and

spawnners is considered severely jeopardizing the spawn abundance in Ganga river system at

Buxer, Patna & Bhagalpur from 1984-1990. Also during the rainy season brooders are caught

during their spawning migration.

Explosives and Poisoning: In some countries, use of explosives and poisons to disable and

capture fish represents a serious threat to ecosystem and the long-term viability of fisheries.

These destructive fishing methods include the use of toxic plants, commercially available

poisons such as bleaches (sodium hyplochlorite), insecticides and explosives. These destroy the

fish fry and other aquatic organisms forming food for the fish.

Poisoning: Poisonous plant material is traditionally used to catch fish in many tropical countries.

In Western Samoa, for example, fish poisons are derived from the root of the climbing vine

Derris elliptica and the nut of the coastal tree Barringtonia asiatica, which are ground into a

paste and wrapped in small parcels made of leaves; fishers drive fish into the shelter of a

preselected coral head where two or three parcels of poisonous materials are placed. More

seriously, commercial poisons, including bleaches, are poured into pools isolated at low tide to

capture small coral fish. Although a number of chemicals and plant derivatives are available in

the market, which are poisonous for fish, only a limited number of such toxicants are safe and

suitable for eradicating predatory and unwanted fish from cultural ponds. Mahua oil cake (250

ppm or 2500 Kg/ha/m of water), bleaching powder (350-500 Kg/ha/m of water) and anhydrous

ammonia (20-30 Kg/ha/m of water) are considered suitable. Root powder of Derris elliptica

contains 5% rotenone and oil cake of Mohua (Basia latifolia) contains 4-6% of active ingredient,

the saponia. Both saponia and rotenone have strong pscicidal activity. From an economic point

of view the poisoning in perennial tank should be done where dewatering is not possible during

pre-monsoon season when the water level is usually low, requiring the minimum quantity of

poison material. The date of poisoning should be fixed three weeks before the anticipated date of

stocking. Seasonal ponds, which dry up during summer months should not be treated with fish

toxicants.

Poaching; the problem in cultural ponds: Poaching is perhaps the biggest problem in freshwater

aquaculture. High value and ready market for carps make them more prone to poaching. The

widely used gear for poaching are cast nets, gillnets and small drug nets. Except these nets, some

very highly toxic pesticides are often used. These are; highly toxic, fat soluble and persistent

organo-chlorine insecticides e.g. isodrin, endrine (epoxide of isodrin), aldrin, dieldrin (epoxide of

aldrin) etc. Among all of these, endrin is most toxic to fishes. Its LD50 value for fishes is about

0.0003 ppm (at 0.001 ppm kills all fishes). Tafadrin-20 is a commercial formulation of endrin,

which contains 20% endrin as active ingredient used by some farmers 6 weeks before the

stocking to clear the nursery pond. (LD50 value for isodrin is about 0.0015-0.0025 ppm.) In

several countries, water protection regulations make it use on land in the proximity of water

subject to permission. Because of its high toxicity, the MRL in most of the European and

overseas countries is 0 ppm but in Canada it is 0.1 ppm. To prevent poaching in stocking ponds,

placing the branches of trees and bamboo twigs or stretching and fixing barbed wire in criss-

cross manner along the sides, occasional rowing of boats with hooks or barbed wire hanging

from its keel to detect gill nets, fencing the farm with barbed wire and employing the services of

watch man are suggested.

Dynamiting: It is employed for catching the fish in lakes and reservoirs near the pool.

Dynamiting kills each and everything from egg, spawn, fry, and fingerlings to adult in the water

body. This causes great losses to standing stock as well as reduces the composition and total

production in the next year. Explosives are either thrown from a canoe into a school of fish such

as mullet, or set on coral where fish have been encouraged to gather by setting baits. Explosives

and severe poisons are many times more damaging to small animals, such as fish larvae and

coral polyps, than they are to large fish. Destroyed coral reefs result in low fish production and

may not recover for many years. Dynamited reefs in the Philippines, for example, have taken an

average of 38 years to recover.

Electric discharge: In commercial fishing, A.C. and D.C. are used in fresh water, while I.C.

(Interrupted current) is used in seawater. Electrical fishing operates on the principle of

concentrating fish in a limited area between the electrodes, under the influence of electric field

and then collecting the fish by netting or fish pumping. As such electric current as employed in

fishing is harmless for, it does not affect the growth, viability and reproductive capacity of the

fish, and it does not influence the food of the fish. However, electro fishing is an art. The

operator’s skill in matching the machine to the fish species and to the water matter very much in

determining the efficiency of the whole operation. Efficiency also depends upon a number of

environmental factors; colour of water lowers the visibility of fish. Conductivity and temperature

of water affect ion-flow and this is in turn the extension of the electric field about the electrode.

Excessive electicity produced due to the carelessness may result in spinal damage in fish

captured.

To check all above-mentioned destructive methods of fish catching many rules and

regulation are laid down under the Indian fisheries Act 1897 (Govt. of India, 1956) such as (for

details please see the original act):

(i) Mesh regulation: The minimum mesh size for nets permitted is 30 mm so that smaller fish

are not caught.

(ii) Restriction on size of the fish to be caught: The minimum size of the fish to be caught vary

in different states from 225 to 305 mm so that every fish get a chance to breed at least once

in its lifetime. Restrictions are also imposed on the sale of undersized fish.

(iii) Restriction on types of gear: In Delhi, since 1948 restriction have been imposed on fishing

except with rod and line, hand line and long line from July 1 to August 30, every year.

(iv) Restriction on total catch: Licenses are issued to fishermen in order to restrict the total catch

of fish.

(v) Closed season: Fishing is prohibited during spawning season or during any season of poor

quality of fish.

(vi) Declaration of sanctuaries or protected waters: Certain parts of the river and reservoirs,

which constitute breeding and nursery grounds of fishes are declared as sanctuaries and are

closed for fishing for certain periods or all the year round.

(vii) Leasing of lakes and reservoirs in alternate years: This is done in order to give the fish a

chance to breed and to make up the natural stock of these water bodies.

(viii)Prohibition on indiscriminate fishing: Fishing by means of fixed engines (fixed engine

means net, cage, trap or other contrivances fixed in the soil for tracing the fish) or

construction of weirs is prohibited.

(ix) Ban on the use of explosives or poisonous substances: For fishing use of explosives or

poisonous substances is prohibited

As population increase place greater demands on fisheries resources and the aquatic

environment, the role of fisheries technologists will assume an even greater importance. The

challenge will be to devise fishing gear and methods, which don’t threaten the environment or

non-target species, yet catch the target species in the most economically efficient manner.

Model questions:

1. Enumerate different destructive methods of fishing practiced at present and how far these

can be prevented.

2. a). Enumerate different destructive methods of fishing practiced at present.

b). Describe briefly different rules and regulation, which are laid down under the Indian

Fisheries Act, 1897 to check destructive methods of fish catching.