Water Thermocline Confirms Susceptibility of Tilapia Cultured in Lakes to Streptococcus agalactiae
M. N. A., Amal, Saad, Mohd Zamri, Abd. Rashid, Zulkafli, Abdullah, Siti Zahrah, Misri, Sairizal, Ramley, B., Shahidan, H. and Mohd Yusoff, Sabri (2010) Water Thermocline Confirms Susceptibility of Tilapia Cultured in Lakes to Streptococcus agalactiae. Journal of Animal and Veterinary Advances, 9 (22). pp. 2811-2817. ISSN 1680-5593
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A study was conducted on water quality profiling to confirm susceptibility of tilapia cultured in lakes to Streptococcus agalactiae infection. A total of 1,010 and 719 tilapias of different sizes were collected from two lakes; the Kenyir and Pedu lakes, respectively. They were randomly sampled for a period of 24 months. Swabs of brain, eye and kidney were streaked directly onto blood agar before S. agalactiae was identified by the API 20 STREP kit, Slidex Strepto-kit and PCR technique. The water temperature (thermocline) and dissolved oxygen profiling were determined at 1 m intervals for up to 20 m deep. Water clarity and flow rate were also recorded using Secchi disk and a current meter. S. agalactiae was successfully isolated from both lakes throughout the year, ranging between 2 and 78%. Isolation was more frequent during the hot and dry months of both years. During this period, the mean water temperature was >29 degrees C for up to 8 m deep due to the significantly (p<0.05) clearer water and slow rate of water flow that allowed deeper light penetration and enhanced heat retention. Water thermocline that showed a drop in water temperature was observed only in Kenyir lake at >12 m deep. This and the slow water flow kept the water temperature at 4 m deep where tilapias under the cage culture system were kept to remain high causing stress to tilapia and increases susceptibility to S. agalactiae. Dissolved oxygen profiling, however remained high at >5 mg L-1 for up to 8 m deep and did not give adverse effects to susceptibility of tilapia to S. agalactiae.
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