Threatened Fishes of the World: Tor tor Hamilton 1822 (Cyprinidae)

Abstract

Common names: Mahseer, Mohashol, there are many different names in various parts of India. Conservation status: Vulnerable and threatened (Ameen et al. 2000). Importance: T. tor is an economically valuable species and a very important recreational angling fish on the Indian subcontinent including Nepal, Sri Lanka and Turkey. It forms an important part of reservoir fisheries in India and the coldwater fisheries along the Himalayas. It is a potential candidate for the development of an openwater fishery as well as aquaculture. T. tor is the most common Himalayan mahseer and a very attractive sportfish, with excellent food value. Identification: D 12 (3/9), P 19, V 9, A 7–8 (2–3/5), C 19, L 1.22–27, Ltr 4/2. Head slightly shorter than depth, dorsal profile more sharply arched than ventral profile, lips thick fleshy with continuous labial fold across the lower jaw, mouth is small and its gape does not extend below the eyes, snout pointed, jaws of about the same length, two pairs of barbels, maxillary ones slightly longer than rostral ones but shorter than eye, interorbital space flat. Dorsal fin opposite to or slightly in advance of ventral fin, dorsal spine smooth, shorter than body depth, pectoral reaching pelvic as long as head excluding snout, pelvic shorter and when laid flat not reaching caudal-fin base, caudal deeply forked and lateral line complete. Predorsal scales 9. There is a well-developed scaly appendage in the axil of each pelvic fin. The length of head 4 and depth of body 3–3.7-times SL. Silvery green or greyish green dorsally, with pinkish sides replaced by greenish gold above and light olive green below, lower fin reddish yellow. Maximum size 152.0 cm. Distribution: Subtropical (15–30 C), 29 N–20 N, it is distributed in Asia where it is a popular sport fish in India, Pakistan, Bangladesh and Nepal. It has also been reported from Sri Lanka, Afganistan, Mayanmar, Turkey, Bhutan, China and Papua New Guinea. It occurs in large rivers, lakes and reservoirs. Abundance: No recent estimate is available. Desai (2003) reported a steady decline in abundance in reservoirs in India. Habitat and ecology: Inhabits riverine pools and lakes and also in streams with good flows and a rocky bottom where they attain the best growth. They are benthopelagic, potamodromous and occur in tropical freshwaters (15–30 C) at depths of up to 15 m. Adults have an omnivorous diet and feed on small fish, insects, molluscs, zooplankton, debris, sand & mud, fish scales & bones, fruits, chironomid larvae, water beetles, crustaceans, filamentous algae and macrophytes. Juveniles mainly consume insects. Reproduction and spawning: Size at maturity ranges from 28–50 cm for males and 34–38 cm for females. They exhibit sexual dimorphism. T. tor migrate towards headwaters at the start of the rainy season (June–August) to breed over gravel and stone beds and move downstream at the end of the rainy season to perennial wetlands. Monsoon floods and physico-chemical factors induce mahseers to migrate to their spawning grounds and act as the triggers for reproduction. They can spawn from March to September (Shrestha 1999) when water temperature is 20.0–28.0 C with a high dissolved oxygen content (>9.5 mgl). Eggs are released on a gravel substratum in rivers. Fecundity ranges between 10,000 and 100,000 depending on fish size. Hatching takes 60–80 hours after being incubated at 19.5–20.4 C. Although different mating behavior is recorded in other mahseer species, only a long courtship is reported for T. tor (Desai 2003). Threats: Populations in open waterbodies (rivers/lakes) are decreasing due to various forms of human impacts such as high fishing pressure, loss of habitats resulting from river damming for hydroelectricity and other developmental works, natural disasters, pollution and reclamation of wetlands. Fishermen harvest T. tor during the critical migration period. Conservation action: Artificial propagation has been successfully done in Bangladesh and India and domestication is under trial (Desai 2003). Many aquaculture trials have shown that this species is a promising candidate for coldwater aquaculture. Openwater ranching and stock enhancement with artificially reared fish is under experimental trial in Nepal and India (Petr & Swar 2002). Conservation recommendation: There is an urgent need for surveys on the abundance, distribution and ecological requirements over the natural range of this species. It is necessary to identify why this species is declining in numbers and measures should be taken to conserve the ecological conditions of the rivers where this fish still occurs. Extensive studies should be done on different aspects of domestication, artificial propagation, and rearing of the fish. Finally, augmentation of the natural stock should be done through habitat improvement and stock enhancement and ranching.