High host density, poor environment and recurring disease can suppress fish
immunity and thus reduce resistance to diseases (Reno, 1998). The average fish stocking
densities in the Upper Ping River was 99 ± 33 fish m-2 which is about double of the
recommended stocking rate. Pathogens can spread between cages on tidal currents (Anon.,
2000). The presence of sediments in the water can improve survival of pathogens by
providing protective, absorbent surfaces. Caged fish farming releases uneaten fish feeds,
feces, and soluble fish wastes into the environment. The FCR (feed conversion ratios = weight
of feed supplied/harvest weight) varies depending on management practice and technology,
with an average of two being common. Overfeeding can cause water quality deterioration
leading to stress and increased disease. Overfeeding wastes feed and money.