Research on environmental factors resulting in the outbreak of mud crab (Scylla serrata) diseases in Niutianyang reclaiming coastal area of Shantou
汕头牛田洋沿海围垦区锯缘青蟹病害爆发的环境因素

Mud crab Scylla serrata is one of the most valuable shellfish and the largest crab fishery in China, which is widely cultured in brackish and seawater ponds along the coast of southeast China. In recent years, mud crab cultured in Niutianyang reclaiming coastal area of Shantou suffered from serious diseases, which resulted in high mortality and great economic loss. To study the relationship between environmental factors and the outbreak of diseases, we carried out a consecutive investigation of water physico-chemical parameters and biological factors from April to December in 2007. It was found that all the investigated factors displayed obvious seasonal variation. In July and August, 5. Serrata inhabited in a relatively high water temperature ( ≥ 28℃ ) and low salinity (≤ 6). During the period of disease outbreak ( from Sep. To Nov. ), the environmental factors deteriorated. Ammonia-N (235. 63μg·L(-1)) and nitrite-N (27. 75μg·L ~(-1)') reached a peak value in September; while nitrate-N (10. 85μg·L ~(-1)), phosphate-P (39μg·L ~(-1)) and biological factors (water heterotrophic bacteria 157. 93 157.93 x 10~5 cfu·L~(-1), vibrio bacteria 116.75 x 10~3 cfu. L(-1), sediment heterotrophic bacteria 733.25 x 10~5cfu.g~(-1) , vibrio bacteria 141.65 x 10~3 cfu·g~(-1)) reached their peak values in October. These results suggested that both the deterioration of environmental factors and the proliferation of conditional pathogens are the important reasons for the outbreak of S. Serrata diseases.