Control of predators, weed animals and pests
Pests and predators seldom feature in aquaculture research, but the losses caused by them are often much higher than generally recognized. It is reported that a pelican can consume between 1 and 3 tons of fish in a year. According to du Plessis (1957), ten breeding pairs of cormorants will catch about 4.5 tons of fish in a year. Herons may cause losses of up to 30–40 per cent of fry and juvenile fish in a pond farm. A heron may consume as much as 100kg fish per year. Bird predation in shrimp ponds is reported to decrease production by about 75 per cent in Texas (USA). The losses caused by mammalian predators like otters are even greater (as much as 80 per cent of the stock), as they generally kill much more than they can eat. The measures that are presently available are only partially effective, as predatory birds and animals very soon find ways of circumventing control measures employed in aquaculture farms. Many of the pests are difficult to control in large farm areas and most control measures require continued application, involving employment of considerable labour.
Several species of predatory fish may gain access to aquaculture farms through water supplies or along with seed brought into the farm. Water management in farms, such as periodic draining and preparations for introduction of new stock, offers opportunities to the farmer to exercise a reasonable amount of control on predatory fish. Outdoor nursery ponds, where the post-larvae and fry are susceptible to predation not only by predatory fish, but also by insect larvae, notonectids, etc. and amphibians such as frogs, it is relatively easy to adopt control measures like the spread of oil emulsions to prevent aerial breathing of insect larvae or fencing to prevent entry of frogs. The control of avian and mammalian predators is more difficult.
Among bird predators, cormorants, fish eagles, herons and kingfishers are considered to be the worst. If adequate protective measures are not taken, large flocks of cormorants can drive fish into shallow areas by flapping their wings, and prey on them in large numbers. The shallow waters of tropical coastal ponds provide ideal conditions for birds to prey on dense stocks of cultured species. Herons and egrets are the major predatory birds in Texan shrimp ponds; grebes and shore birds (Charadriiformes) as well as gulls (family Laridae) are mainly competitors for food, andprey on shrimps only when the water level is very low.
Several methods of controlling bird predation are practised with varying degrees of success. Small ponds and raceways can often be covered with nets or wire-mesh, but it is intriguing to see how some birds learn in the course of time to gain access through such protective covers. Devices like flash guns, sirens, klaxon horns, gongs, scarecrows, bamboo rattles and bells have all been tried with initial success. In small nursery ponds in Southeast Asia, farmers sometimes run lines of string on poles set in the pond and attach pieces of bright-coloured cloth or metal to the string to scare birds. An ingenious device consisting of a windmill with mirrors that revolve and flash brilliantly has been used in Malaysia to scare birds with some success, but it appears that if kept in the same area for a period of time its scaring effect is considerably reduced. Obviously, these devices can only be an adjunct to continued vigil by conscientious watchmen. Watching is made more difficult by the fact that some of the birds, like herons, are active primarily during the night. Some of the fish-eating birds may be protected by law, but at least the others can be shot or caught with spring traps. They can also be poisoned or their nests destroyed.
Frogs and toads have been reported to destroy the larvae and juveniles of fish, particularly of tilapia in African ponds. Some of the aquatic snakes prey on juvenile fish. Other predators are crocodiles, alligators and large lizards. All these can more easily be prevented from entering farms with proper fencing and by keeping the pond banks and surrounding areas free from dense growths of vegetation. The snapping turtle has been found to prey on catfish, but other turtles usually only compete with fish for space and food.
Otters (Lutra and Aeonyx) are probably the most destructive of the mammalian predators.
They live in the immediate vicinity of water and burrow into the banks under the roots of trees. Otters are nocturnal in habit and hunt for fish mainly on clear nights. They attack relatively large fish, eat the best parts and leave the rest. The recommended control measure is to catch
them with special otter traps. Large traps with sturdy solid teeth have to be used as otters can easily escape from smaller traps. The traps should be set in passages generally used by the animals to enter the farm; the passages can be identified by the otters’ webbed footprints and excrement. Hunting them from their holes with the help of trained otter dogs and proper fencing of the farm are other means of control.
Among the losses sustained by predation should be included poaching by man, which is extremely difficult to prevent. This problem is experienced worldwide, but its severity varies with the system of culture (e.g. pond culture, cage culture, raceway culture, etc.) and the socio-cultural background of the neighbourhood communities. The risk becomes greater when the crop is ready for harvest and the culture system makes it easy to catch large quantities in a short period of time with little effort, as in cage farms. On the other hand, in intensive farming systems using limited space, it will be possible to exercise greater vigilance than in large pond farms covering hundreds of hectares. Traditional anti-poaching measures include the employment of reliable watchmen, use of trained watchdogs, placing hidden obstructions in ponds to prevent seining and fencing of farm areas. In recent times, several types of burglar alarms and even electrified fencing have been tried with varying degrees of success.
Removal of weed fish, that is species of fish which compete for space and food with the cultured species, is a common practice in all types of aquaculture, and in confined areas it is often possible to achieve considerable success in this. In open-water culture systems, like those of molluscs, only very limited success can be expected. In confined aquaculture waters, weed fish gain access generally during early stages of their life history with incoming water. By the use of filtering devices at the intake, the entry of wild fish and other aquatic animals can be reduced to a large extent. There are different types of filters that can be used for the purpose. Lee (1973) describes two types of filters used in catfish farms: a saran sock filter and a box filter.
Sock filters are cylindrical in shape, made by sewing together two pieces of saran screen (generally 3.7m long and 0.9m wide) with a drawstring closing arrangement at each end. The inlet pipe is placed inside one end of the filter and the drawstring tied tightly around it. The other end is also tightly tied to prevent escape of fish or other animals. The filter is cleared regularly to remove the catch. Box filters may be made to float or fixed permanently below the inlet. They are constructed of wooden frames and screens. Common dimensions are about 2.5m long, 0.9m wide and 0.6m deep. The bottom of the box is made of screen, reinforced at intervals with wooden boards. Most filters, however, would not be able to prevent the entry of small larvae and eggs of weed species. So, when necessary, some other measures such as selective fishing or selective toxins will have to be used, to eradicate them from the farm.
Certain species of snails, particularly those belonging to the family Cerithidae, are major competitors for food in fresh- and brackish-waters when they occur in abundance. Large numbers of them enter farm areas as larvae and grow and multiply rapidly. They affect the growth and abundance of benthic algal complexes, which are especially important in coastal ponds of Southeast Asia. If their numbers are high, they disturb the benthic algae by loosen-ing the sediments. The pond can become very muddy and the algal complex may break loose from the bottom on windy days and float to the surface. The wind and waves waft them to the pond bank, where they may settle and decompose, producing large amounts of hydrogen sulphide. Very dense populations (up to 34 tons/ha) of snails have been reported from coastal ponds in Indonesia. The application of 12–15kg/ha of nicotine (commercial tobacco dust) or 15–18kg/ha of saponin on the pond bottom after drainage of the pond is reported to be effective in controlling snails in coastal ponds (Tang, 1967). Manual or mechanical removal of snails can also be effective, if properly done after the ponds are drained.When the pond cannot be drained, application of Bayluscide (5,2-dichloro-4-nitor-salicylicaniline-ethanolamine), at a concentration of 3ppm in the pond water, has been suggested (Tang, 1967). But Bayluscide and other commercial preparations have residual effects for varying periods of time.
Polychaete worms are serious pests in coastal ponds. They live in burrows on the pond bottom and make the soil porous, reducing the water-holding capacity of the ponds. The growth of desirable algae is also hindered. Drainage of the pond seldom helps to eradicate the worms. Phenol has been used in the Philippines (Pillai, 1962) to control these worms after partial draining. The pond has to be flushed once or twice with fresh tidal water, before algal growth commences again and the pond becomes suitable for stocking fish. Tang (1967) reported that 2ppm nicotine or 3ppm Bayluscide are also effective in controlling polychaete worms after the water has been drained. Chironomid larvae which compete for food with benthic-feeding fish in coastal ponds can be controlled by repeated application of technical Y-BHC at a concentration of 0.08–0.1ppm (Tang and Chen, 1959). But it imparts an unpleasant odour to the fish. There is also the likelihood of the larvae developing resistance to this gamma isomer.
Among the pests in coastal aquaculture farms, probably the most noxious ones that affect the safety of the farm itself are crabs. Leakage of water from the dikes and consequent problems in maintaining the required water levels in ponds are often caused by holes made by burrowing crabs. In coastal areas, crabs are found in great abundance and so the damage that they do to the farm can be immense. Water flowing through crab holes can cause the complete collapse of dikes. Predators and weed animals can gain access to the farm through the holes. Swimming crabs (Portunidae) have been found to be serious predatorsin shrimp ponds. Net-cages in open waters are often damaged by crabs, resulting in the escape of fish from the cages.
Because of the damage that crabs do to pond dikes, farmers spend considerable time and effort in reducing crab populations in pond farms and neighbourhood areas by using chemicals or special trapping devices. Jordan (1957) reported that repeated fortnightly spraying with a 10 per cent suspension of technical BHC (containing 6.5 per cent gamma isomer) was effective in controlling Sesarma and Sarmatium species, without affecting fish. ASEAN (1978) reported the use of the insecticide ‘Sevin’. It is
mixed with ground-up fish and made into small balls which are then placed in crab holes above the water line. Sevin is toxic to shrimps and if it has to be used in holes below the water line in shrimp ponds the holes should be closed, so that the shrimps will not have access to the chemical. Another means of killing crabs is by applying calcium carbide in crab holes and pouring water into it to wet the carbide, in order to produce the lethal acetylene gas (ASEAN, 1978). Tobacco dust and several other toxic materials and insecticides have also been used as contact poisons to kill crabs.
The so-called burrowing shrimp (Thalassina) is another pest in certain areas, where it damages dikes by burrowing. The presence of Thalassina is easily detected by the occurrenceof high mounds at the entrance to their holes, which are above the water line. The methods adopted for controlling crabs can be used for killing Thalassina as well. Special trigger-type traps have also been found to be useful in catching them.
Muskrats (Ondatra) and field rats dig large burrows in banks and dikes and can thus weaken the structures. Suggested ways of controlling them are by capture with traps and trapnets or by shooting.
The various fouling organisms that grow on the water control structures of ponds and cages in open-water areas can also be considered as pests in aquaculture. Regular cleaning and drying is essential to keep these in good condition. Wooden structures should be made of treated wood or painted with preservative paints that are not toxic to cultured animals, to reduce the problem. Greater use of cement concrete to build water control structures when possible will also help to reduce fouling problems.
Even though the selective treatments to eradicate predators, weed fish or pest animals described above are valuable in combating specific individual infestations, there are several pesticides and poisons that could be used to eradicate some or all of the predators, weed fish and pests simultaneously. The use of natural products like teaseed cake and derris powder has been very popular among aquaculturists for
this purpose, because these are not harmful to man in small amounts and lose their toxicity in water within a short period. The use of chlorinated hydrocarbons (e.g. DDT, Endrin, Chlordan, gamma BHC), although effective, has to be avoided because of their long-term residual effects. On the other hand, organophosphate pesticides like Gusathion do not leave a toxic residue for more than two weeks or so after application.
Teaseed cake or saponin is widely used in fish and shrimp ponds in Asian countries to control pests and predators. Teaseed cake is the residue of the seeds of Camellia drupisera after extraction of oil, and generally contains 10–15 per cent saponin. A dose of 216kg teaseed cake together with 144kg quicklime per hectare is recommended for application in an aqueous solution on pond bottoms after reducing the water level. It is effective in killing weed and predatory fish, as well as snails and crabs. Tobacco dust, the active component of which is nicotine, can also be used as a fish toxicant to eradicate unwanted species.
Rotenone is another plant product that is widely used as a toxicant to clear aquaculture waters. It can be used as derris powder which contains 4–8 per cent of the active ingredient rotenone (C23H22O6). Different dosages have been recommended. When applied at levels of 0.5ppm, the toxicity disappears within about 48 hours (Hall, 1949). Lunz and Bearden (1963) found 1.5ppm concentration of rotenone effective in controlling undesirable fish in shrimp ponds. Alikunhi (1957) reported on the safe use of concentrations of up to 20ppm, and under tropical temperatures the toxicity continues from 8 to 12 days. Fresh derris roots are more effective than dry roots or their powder, because of higher rotenone contents. Roots should be cut into small pieces and soaked overnight in water. Soaked roots are pounded and the crushed roots are replaced in the water in which they were soaked and squeezed to press out as much of the rotenone as possible. The extract is applied in the pond at the rate of
4g roots per m3 water.
Many chemical fish toxicants, particularly Endrin, Dieldrin and Aldrin, have been used to clear aquaculture farms of predators and pests. Pillai (1972) has summarized the use of DDT, 2,4-D and sodium cyanide in the eradication of predators and pests. A lethal concentration of DDT is reported to be 0.03g/l and that of 2,4-D 0.13, ml/l of water. Repeated washing of the pond will be required after treatment to remove toxic effects. In the case of sodium cyanide, the lethal concentration is 1ppm and the toxic effects are reported to disappear under pond conditions after about 96 hours. The agricultural weedicide sodium pentachlorophenate (PCP-Na) is recommended for use in shrimp ponds to kill predatory and weed fish, as the lethal concentration of 0.5ppm of the chemical for fish is below that for shrimps. Sodium pentachlorophenate decomposes when exposed to direct sunlight and toxicity is reduced by 90 per cent after about three hours. As in all other chemical treatments, the water level in the farm should be reduced as low as possible before treatment. The aqueous solution of the weedicide should be evenly distributed and as soon as the fish are killed fresh water should be let in to dilute its concentration.