About 15 fish species have been introduced in Pakistan. Out of these species, swordtails, guppies and goldfish have been imported for ornamental purposes, although gambusia was brought for mosquito control. Two species of trout referred to as semi-exotics were introduced for sport fishing. The three Tilapia species were imported to culture in the saline waters of the waterlogged areas. The common carp (Cyprinus carpio) and three Chinese species viz., the silver carp, grass carp and bighead carp were imported by the Punjab Fisheries Department to enhance fish yield per unit area.
With exotic species, it has been observed that these fish have proven effective in achieving the targets for which they were introduced into the freshwater ecosystem of Pakistan. The trout have bred very successfully in the hilly regions. Since the brown trout cannot tolerate higher temperatures, the Punjab Fisheries Department has introduced it into the Murree Hills. Three species of Tilapa (Oreochromis aureus, O. niloticus, and O. mossambicus) are well established in the saline waters of Okara, Bahalwalnagar, Bahawalpur and Rahim Yar Khan. The Tilapia fish is internationally known as the ‘aquatic chicken’ and is relished everywhere in the world. Generally, its size does not grow to more than 200-300 gms, but in Pakistan, this species has been known to grow upto 2-3 kg. This fish has become very popular due to its delicious taste. It is best suitable for culture in saline waters. Tilapia can thrive well, even in half seawater concentrations. The common carp is another fish, which is cosmopolitan, and is found everywhere. This fish has the quality to survive even in adverse aquatic conditions. It can feed on any kind of food present in the water, and breeds naturally in small confined waters. It can breed more than once a year depending upon the temperature of the water. The fish grows to a good size in a year and can survive even in unattended fishponds. The three Chinese carps (Ctenopharyngodon idellus, Hypophthalmichthys molitrix, and Aristichthys nobilis) have surprisingly helped in enhancing the fish yield per unit area. The grass carp that feeds on macro vegetation like buffalo dung has a high growth rate, even more than that in its native China. In Pakistan, this fish lives very amicably with the indigenous culturable species (Catla catla, Labeo rohita and Cirrhina mrigala). Likewise, the silver carp and bighead carp also grow to a very big size, even more than our own culturable species. The average annual weight attained respectively by our culturable species for Catla catla is 2-3 kg, Labeo rohita (not available) and 1.0 to 1.5 kg for Cirrhina mrigala. The grass carp grows to about 2-3 kg, the silver carp is 4-5 kg, and the bighead is 4-5 kg. There has been a significant increase in our yield per unit area because we put all these fish together in our ponds (composite culture, polyculture) where they reside peacefully.
The mosquito fish (Gambusia affiniss) has been used as an ornamental fish more than it has been used for the control of mosquitoes. Its population size has significantly reduced, perhaps due to the degradation of its habitat.
14.7.1 Impacts of Exotic Fish
Until now, apart from three species, viz., the common carp, Tilapia and silver carp, no detrimental effects have been manifest on our local fish. The common carp is a pond breeder, sometimes breeding twice a year, and has the habit of burrowing at the bottom of and along the dikes of ponds. This makes the water muddy and turbid. Consequently, the gills of all cultured fishes are choked and the process of photosynthesis is retarded. Moreover, its prolific breeding results in a large increase in its population that feeds at the cost of other desired fish. The Tilapia, a prolific breeder, competes for space with others’ and its own spawn. Hence, the productivity of the fishpond is reduced.
The third species observed to have affected other indigenous fish is Catla catla. It is known to greatly affect the silver carp. Although Catla catla feeds on aquatic organisms and the silver carp feeds on aquatic plants, the latter is responsible for the reduction of the population of the former. A reason for this may be the silver carp’s active filtering of the maximum quantity of aquatic plants, thus disrupting the food chain for the production of aquatic organisms, as they need to feed on the plants. Table 14.1 gives information on the exotic fish found in Pakistan.
Freshwater Fish Species with Decreasing Populations