50 JUNE 2013 • WORLD AQUACULTURE • WWW.WAS.ORG A Survey of Parasites of Wild Rohu in Trimmu Headworks, Pakistan Muhammad Ashraf and Asma Zafar Rohu Labeo rohita is one of the most important species of inland fisheries of Pakistan, contributing about 35 percent of total stocking and 23 percent of total aquaculture production in the region. Fish farmers in South Asia prefer to stock this fish as their main aquaculture species because it fetches a high price and has an excellent consumer acceptance related to particular meat qualities. The growth of rohu is exceeded only by catla among the Indian major carps. Rohu feeds on phytoplanktons, zooplanktons, detritus and vegetable debris. Rohu mature in the second year and the spawning season extends from late May to early August. It breeds in natural waters during the monsoon season but does not breed in culture ponds unless induced by reproductive hormonal preprations. Rohu inhabits the fresh waters of Pakistan, Bangladesh and Burma and has been introduced in many other countries (Agrawal 1994). Rohu is polycultured with other Indian and Chinese major carps under semi-intensive culture, obtaining sustenance mainly from natural food produced by organic and inorganic fertilization. Supplementary feed is also added to overcome nutrient deficiencies. With rapid increase in fish demand, a trend is now emerging to intensify fish production. Persistant water shortages and diminishing available land has further promoted this tendency. This development can be beneficial to fish culturists but also brings health risks and increased chances of diseases. Disease-causing pathogens proliferate as fish culture intensifies. High stocking density and water quality deterioration provide a favorable environment for bacterial and parasitic diseases. Intensification of fish culture favors a variety of diseases because of intimate contact of the animals with each other, close proximity to invading organisms, and mixing and overlapping of leftover feed and excretory matter. Culture water, therefore, serves as the most convenient mode of movement of pathogens to different fish species living in the same environment. Parasites in Fish Culture Parasites are a group of organisms that may or may not cause illness in fish, depending on a number of factors. Some parasites depend on their hosts for compounds. Parasitic damage to the host varies with type and intensity of parasites present. Parasites in freshwater fishes are primariliy protozoans, myxozoans, helminths (platyhelminths, nematodes and acnthocephalans), hirodineans and crustaceans (Tonguthai 1997). Most parasites are opportunistic and may be omnipresent time in a culture unit or on fish hosts in low numbers and only cause disease when fish are stressed. Parasites multiply rapidly under favorable conditions, distressing fish health, causing high mortality and inflicting economic lossees (Snieszko 1974, Klesius and Rogers 1995). Most parasitic diseases occur as a result of poor water quality. The presence of parasites in the culture environment may disturb the health-disease balance and disturb immune systems, where fish become feeble and lethargic and stop feeding. Parasite infestation hampers fish production and disrupts the economics of fish farming. Economic losses from ectoparasite infestation result from direct harm to the fish and from disfigurement, which renders fish grown unsuitable for sale, imposing large losses on farmers (Piasecki et al. 2004). Knowledge of the type of parasites, their habitat, seasonal and spatial distribution and underlying mechanisms of effects on hosts is an essential pre-requisite for implementing preventative measures against parasitic infestations. Size Variation and Parasitic Attack Parasitic attack is directly related to the fish size and specific organs. Roberts (2012) observed a high density of parasites on gills, attributed to the greater contact of gills with surrounding water. There is higher parasitie incidence in females than males, which might be related to the physiological condition of females, especially gravid ones (Ugwuzor 1997). Fingerlings can be more susceptible to infection because of their immature immune system (Anderson 1974). Differences in prevalence of infection between juveniles and adults may be a result of changes in their diet from phytoplankton, zooplankton, weeds, and seeds to insect larvae, crustaceans and worms in juveniles and adults. Collection of Fish and Identification of Parasites Rohu were collected from Trimmu Headworks, an important water reservoir in Pakistan. Collected fish were categorized into four groups based on weight (1-20, 21-40, 41-60 and 61-80 g) for detailed examination of parasites. Parasites showed variations in prevalence and presence with size and organ of fish. In general, there is a positive relationship between parasitism and and size. On the smallest fish, protozoans occurred the most and crustaceans occured the least on the smallest fish. Nematodes and digeaneans were totally absent on skin (Table 1). Crustaceans, digeneans and nematodes were totally absent, trematode incidence was uniform irrespective of the size of fish. The incidence of protozoans decreased as fish size increased (Table 2). As in the gills, crustaceans and digeneans were totally absent and all other parasites were present in quite variable intensity on the peritoneum (Table 3). Parasitic fauna was more numerous in the gut than other organs (Table 4). Protozoans, crustaceans and digeneans were totally absent on the liver (Table 5). There was little variation in density of nematodes and trematodes. Unlike other organs both parasitic species decreased in intensity on liver with increase in size of fish (Table 5). Unlike Banerjee and Bandyopadhyay (2010), trematodes
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