Disease incidence in fish hatcheries and nurseries of north-central region of Bangladesh

Abstract

The study documented the disease incidences and the operation and management practices in fish hatcheries and nurseries in five upazilas of Mymensingh and Gazipur Districts. A two step data collection scheme was followed: initially a Preliminary Survey of 50 fish farms, both nurseries and hatcheries, based on interview of fish farmers and subsequently, a year round monitoring of fish farms by paying quarterly field visits. During monitoring phase a total of 10 hatcheries and nurseries, two from each selected upazila, were sampled for water quality and documenting disease incidences. In addition, farm operation and management practices were also monitored. The water quality monitoring was limited to parameters pertinent to aquaculture. The pH values in nursery and stock ponds varied from 4.5 to 10.6 with an average of 7.57 ± 1.25. Dissolved oxygen levels varied from 3.1 to 12.0 mg/l (average: 5.91 ±2.49 mg/l). Alkalinity and hardness levels fluctuated between 21.0-160 mg/l and 31.0 – 150 mg/l ( average: 33.67 ± 24.43 mg/l and 45.41 ± 22.12mg/l), respectively. The ammonia and nitrite levels varied from 0.2 to 3.0 (average: 1.06±0.89) mg/l and 0.01 to 0.6 (average: o.13±0.14) mg/l, respectively. The observed low pH and dissolved oxygen levels recorded in some farms are far below the recommended levels for fish culture. Similarly, high levels of ammonia and nitrite concentrations are far above the recommended levels. Water quality parameters showed strong variations between individual farms, could be related to farm management activities practiced in individual fish farms. The seasonal variations were also observed in water quality parameters. Both dissolved oxygen and pH measured low during October-March period. Higher levels of nitrite and ammonia were recorded during October-March and April-June periods, respectively. Low pH and high levels of ammonia and nitrite are toxic to fish. Low oxygen concentrations and low pH and high levels of ammonia and nitrite recorded in the present study are likely to adversely affect the survival, growth and immunity of fish resulting in increased disease burden on fish. Water quality also varied between different sampling areas. Nine different types of disease symptoms were observed affecting all species (7-9 species) of fish cultured in both in nursery and stock ponds. However, carps and Thai pangas were observed more susceptible to diseases. Depending on seasons, disease incidences were observed in 0-42% sampled fish farms. Highest disease incidences were observed during April-June and October-December periods in 33% sampled ponds, in each case. Of the sampled upazila, disease incidences were comparatively higher in fish farms in Kapasia upazila. In 25% sampled ponds disease affected less than 25% fish population in April-June period, while 50% and 75 % fishes were affected by disease in 8% ponds, in each case. Disease caused fish mortalities in 0-25% affected nursery ponds and 0--42% affected stock ponds. Less than 25% fish mortalities was observed in 25% ponds and up to 75% mortalities caused in only 8% nursery ponds. In case of stock ponds, upto 25% fish mortalities were observed in 58 % affected ponds, while up to 50 % fish mortalities occurred in 11% ponds. Various medicines and chemicals are used in treating fish disease in farms. More than one treatment methods are used in most fish farms. About 25% fish farms used medicine bought from market, about 4.5% farms used common salt, about 7.9% farms used potassium permanganate and less than 1% farmers used herbal medicine, while about 52% farmers used lime in case of disease outbreak. Overall, the stocking density in nursery ponds ranged from 20-200000 fry-fingerlings/decimal with an average of 14133±42880 fry-fingerlings/decimal and that of stock pond varied from 6-9000 fingerlings/decimal with an average of about 1341±1839 fingerlings/decimal pond area. Depending on seasons, 55-86% fish farms fed packed feed to fish, while in stock ponds packed feed was used by 14-58% fish farms. The rest of the farms used feed prepared on farm using locally available ingredients. In average, about 515 g feed/decimal was supplied to nursery ponds. High rate of feeding was done during January-March and low rate of feeding was done during October-December. Similarly, about 500g feed/decimal was supplied to stock ponds. In case of nursery ponds, depending on seasons, 33-83% farms fed more than once in a day, while 17-58% farms fed once a day. More frequent feeding was done during April-June period. In case of stock ponds, depending on seasons, more frequent feeding (50-75% ponds) was done during July-September and April-June periods. Water exchanges are done once in a quarter in 50-80% nursery ponds. However, in some nursery ponds water exchanges are done 2-3 times in a quarter. Almost a similar trend in water exchanges were also observed in case of stock ponds. Water exchanges are done more frequently during April-June and October-December periods. Liming of ponds was a regular activity in the fish farms under the study. Depending on seasons, 77-100% sampled fish farms used lime in their farms. Of these, 18-54% ponds were limed once in a quarter, while 17-52% pond applied lime more than three times in a quarter. Overall average rate of liming in nursery ponds accounted to 300.26±274g/decimal and that for stock ponds was 314.22 ±257g/decimal pond area. Higher rate of liming (423g/decimal) was done during January-March period, while lower rate of liming was done in October-December period. However, 75-92% farms used lime at the rate below 500g/decimal and 8-25% farms used lime at the rate of up to 1000g/ decimal pond area. Of the nine species of fish examined, only six species were found infected with metazoan parasites. Histiostrongylus coronatus was found in Anabas testudineus with a prevalence of 9% and intensity of 100, Pangasius spp. was found infected with Argulus sp,. with a prevalence of 25% and an intensity of 0.25. Clinostomum piscidium was isolated from Labeo rohita with a prevalence of 20% and an intensity index of 0.5, Heteropneustes fossilis was found infected with Aurgulus sp. with a prevalence of 14.28% and an intensity index of 100. Puntius spp was found to be infected with a Digenian parasite with a prevalence of 16.66% and intensity 0.5, Labeo gonius. was found infected with a Clinostomum complanatum parasite with a prevalence of 50.00% and an intensity 100. The prevalence of parasites was more in October-December period. It appears from the study that poor water quality and lack of undertaking proper prophylactic measures are linked to observed disease incidences in the fish farms. The final conclusion drawn from the present study is that poor farm management and lack of awareness about disease control among fish farmers are underlying causes for disease outbreaks in fish ponds. It is suggest to develop a guideline on improved fish farm management with emphasis on disease management issues and create awareness among fishers about controlling of fish diseases.