Use of a <Emphasis Type="Italic">Noctiluca</Emphasis>-killing bacterium <Emphasis Type="Italic">Marinobacter salsuginis</Emphasis> strain BS2 to reduce shrimp mortality caused by <Emphasis Type="Italic">Noctiluca scintillans</Emphasis>

The ability of an algicidal bacterium Marinobacter salsuginis strain BS2, isolated from shrimp pond water, to reduce shrimp mortality was investigated under laboratory conditions. When two species of shrimp (Penaeus monodon and Litopenaeus vannamei) (body length 1.5–1.8 cm) were cultured together with the dinoflagellate Noctiluca scintillans, nearly 80 % of the shrimps died within 7 days. However, when bacterial strain BS2 was also added to the culture, N. scintillans was killed within 48 h, and shrimp survival rates on the 7th day improved from 23 to 87 % for both P. monodon and L. vannamei. The bacterium BS2 alone had no effect on shrimp condition. Under conditions of increased dissolved oxygen, the effect of using BS2 was greater, and shrimp survival rates improved even more dramatically, from 26 to 98 %. These studies provide the first evidence that the use of killing bacteria, isolated from shrimp culture water, can suppress harmful algal blooms (HABs) and thus restore the efficiency of shrimp production. The control of HABs in this way in shrimp culture farms would be a major benefit for shrimp production.     

Isolation and characterization of Noctiluca-killing bacteria from a shrimp aquaculture pond in Thailand

To control harmful algal blooms (HABs), in particular dinoflagellate Noctiluca scintillans, which causes damage to shrimp production, algicidal bacteria were isolated from shrimp pond water and screened for their killing effect against N. scintillans under laboratory condition. Among 260 bacterial isolates, 10 showed killing effects on N. scintillans. Out of 5 strains showing relatively stronger killing activity, 4 strains belonged to the Gammaproteobacteria group. Among these, strain BS2 (identified as Marinobacteria salsuginis) showed the strongest Noctiluca-killing activity. In the present study, we also investigated bacterial effects against growth of other phytoplankton species. Strain BS2 killed only N. scintillans but not any other phytoplankton (Heterosigma akashiwo NIES-6, Chatonella antique NIES-1, Chaetoceros ceratosporum, Prorocentrum lima, and Dunaliella sp.). Results of the present study lead us to expect prevention of damage by HABs, in particular dinoflagellate N. scintillans, in shrimp farming and mitigation for shrimp production in ponds using Noctiluca-killing bacteria.     

Nitrogen dynamics in the settlement ponds of a small-scale recirculating shrimp farm (<Emphasis Type="Italic">Penaeus monodon</Emphasis>) in rural Thailand

Rural shrimp farmers in Thailand are being encouraged to adopt practices that will reduce the quantity and improve the quality of their effluent. A simple and cheap option for small-scale shrimp farmers is to use settlement ponds to store and remediate discharge water before being re-used. We undertook a detailed study of the settlement ponds in a small-scale commercial black tiger shrimp farm typical of rural Thailand. We found that over a 9-week period, following the harvest of one of the two farm production ponds, total nitrogen (TN) concentrations in the water column were reduced by 30%, with the greatest removal (56%) occurring during the fifth week. There was a 10% increase in dissolved organic nitrogen (DON) concentrations during the trial. Sediments were a source of total ammonia nitrogen (TAN), and the re-mineralisation rate was the highest in the first two settlement ponds. Coconut fronds added to two of the four settlement ponds to increase the surface area available for microbial activity were found to provide a site for microbial re-mineralisation of TAN, the photosynthetic uptake of TAN and oxidised nitrogen (NO _x ) and nitrification. The water column was a net assimilator of TAN through autotrophic uptake. This study has shown that settlement ponds are capable of reducing water column N concentrations; however, sediment must be managed to reduce re-mineralisation during successive cropping cycles. In addition, coconut fronds were shown to improve N removal, although they should be periodically removed to maintain efficiency.     

Water Quality and Phytoplankton Communities in Intensive Shrimp Culture Ponds in Kung Krabaen Bay, Eastern Thailand

Water quality and phytoplankton communities were studied in 20 intensive shrimp culture ponds and in the inlet and drainage canals at Kung Krabaen Bay, Eastern Thailand during the first shrimp crop of 1992. The grow-out ponds were categorized into two groups: low stocking density (<60 PL/m²) and high stocking density (>60 PL/m²). The results showed that there were no significant differences in water quality between the two stocking density groups. Phytoplankton collected using a 60-μm mesh net consisted of 79 genera with concentrations ranging from 1,822 to 72,527 cells/L from the first month up to the time of shrimp harvesting. Water quality deteriorated in high and low stocking density shrimp ponds, which had an influence on abundance and diversity of net phytoplankton communities. Biochemical oxygen demand and ammonia-nitrogen were most closely related to abundance of net phytoplankton communities during the shrimp grow-out period. Salinity, temperature, and dissolved oxygen appeared to play an important role in phytoplankton community variation in inlet and drainage canals. Phytoplankton community structure in drainage canals showed more variation than in ponds. The management implications of the results and recommendations for further studies are also considered.