The physico-chemical characteristics of water quality, upper-layer sediment, and bacterial aspects of four grass prawn Penaeus monodon ponds, which are located in the Taitung area of eastern Taiwan (A and B ponds) and the I-Lan area of north-east Taiwan (C and D ponds), were studied. The water temperature (℃), dissolved oxygen (DO), pH value, salinity and concentration of chemical oxygen demand (COD), unionic ammonia (NH3-N), nitrite (NO2-N), nitrate (NO3-N), hydrogen sulfide (H2S) and redox potential (ORP) were selected for analysis. Water temperature ranged from 27.9 to 31.9 ℃ and pH value from 7.9 to 8.7. Salinity was from 13.4 to 25.8? and DO was over 5 mg/l. The nitrogen wastes in the culture water were NH3-N (0.03 to 0.09 mg/l), NO2-N (23.4 to 131.8 mg/l) and NO3-N (0.6-0.7 mg/l). In general, the normal growth of cultured shrimp might not be influenced by culture water, in accordance with published reports of previous studies.
The physico-chemical properties of upper-layer sediment were determined by pH value, the concentration of soluble nitrogen substance (ammonia-N, nitrite-N, nitrate-N), orthophosphate-P and ORP value. The average pH value was 7.5, which is suitable for raising shrimp, and the concentrations of soluble ammonia-N, nitrite-N, nitrate-N, orthophosphate-P were at low safe levels. The ORP values in A and B ponds were -334mV and -320mV respectively, compared to -170mV and -180mV in C and D ponds. The ORP values were significantly different (P<0.05) between the two areas. The H2S level in upper-layer sediment was higher than in the water, but there was no significant difference among the four investigated ponds (P>0.05).The higher shrimp yield was obtained in A and B ponds (1,200 kg and 900 kg/0.1ha), although their upper-layer sediment was more seriously contaminated as indicated by the lower ORP value.
Average total viable bacterial counts of upper-layer sediment ranged from 7.2x105 to 3.2x106 cfu/g in A pond, and 2.8x105 to 22.8x106 cfu/g in B pond. The amount of 1.2x105 to 1.9x106 cfu/g and 1.2x105 to 6.4x106 cfu/g revealed in C pond and D pond, respectively. The bacterial counts in the sediment between two ponds of each area were neither significantly different (P>0.05) nor among the four ponds in the two areas. Some predominantly aerobic heterotrophic bacterial genera, including Acinetobacter, Alcaligenes, Curtobacterium, Kingella, and Pseudomonas, were isolated and identified from the upper-layer sediment in the four ponds. Very low rate of vibrio was found in A (0%) and B ponds (1.66%) or in C (2.85%) and D ponds (0%), as detected from 60 samples and from 70 samples, respectively. The diversity index (DI) ranged from 2.2 to 2.93 and 1.84 to 2.91 in A and B ponds, and from 1.32 to 2.78 and 1.98 to 2.68 in C and D ponds. A major finding was that the higher production could be obtained from high level ponds (A, B) compared with less affected ponds (C, D).
Red disease had broken out in the lower level production ponds (C: 45 kg and D: 282 kg/ha) causing mass mortality to the cultivated shrimp. Bacterial pathogens were isolated and identified from the hepatopancreatic tissue of reddish shrimp, including a large quantity of vibriosis pathogens (over 50%), of which Vibrio alginolyticus, V. damsela, V. harveyi, V. parahaemolyticus and V. vulnificus formed the dominant species. In the normal shrimp, the quantity of vibrio species was only 25%. Consequently, water and upper-layer sediment quality was not a sufficient proof to estimate the survival conditions of pond shrimp in association with the vibrio species. This species rarely existed in upper-layer sediment but it could be isolated from the moribund shrimp during the culture period. On the other hand, those pathogens may have originated either from the broodstock possibly acting as vectors or from some unqualified hatcheries.
Key words: Grass prawn Penaeus monodon, Rearing condition, Bacterial disease