ALBERT

Notes: Abstract The effects of blooms of the cyanobacterium Trichodesmium spp. on penaeid prawn larvae were examined using in situ and laboratory rearing experiments and plankton surveys in Albatross Bay, Gulf of Carpentaria, Australia. The in situ experiments demonstrated that, during a bloom of Trichodesmium spp., larvae of the prawn Penaeus merguiensis did not develop beyond the first protozoea stage, and survival was low compared with times when diatoms were dominant in the same study area. Laboratory experiments confirmed the in situ results. None of the prawn larvae fed Trichodesmium sp. in laboratory experiments developed beyond the first protozoeal stage. In contrast, 94% of prawn larvae fed the green flagellate Tetraselmis suecica successfully developed to the second protozoea stage. Electron microscopy of larvae gut-contents revealed that Trichodesmium spp. were ingested by larvae but were of no nutritional value, resulting in starvation. A 7 yr plankton survey, from 1985 to 1992, showed that minimum abundance of prawn larvae occurs during the annual summer blooms of Trichodesmium spp. and that maximum abundance of prawn larvae generally occurs just after the bloom. There was a negative correlation between the abundance of larvae and the abundance of Trichodesmium at individual sites, one offshore and one inshore, indicating that the blooms affect the survival of larvae. We conclude that variations in both timing and magnitude of Trichodesmium blooms are important determinants of prawn larvae abundance in Albatross Bay.

Notes: Abstract The relationship between Penaeus merguiensis protozoea larvae and their phytoplankton diet was examined using seasonal plankton surveys and in situ rearing experiments. Larval abundance, phytoplankton community structure, and chlorophyll a concentration in Albatross Bay, Gulf of Carpentaria, were monitored monthly for 2 yr. Larval abundance peaked in November (spring) and March (autumn), at which times diatoms were the most abundant group in net samples of phytoplankton and in the guts of larvae. During November 1989 and March 1990, larvae were reared in nylon mesh enclosures positioned throughout the water column at three depths: 0 to 3 m, 3 to 6 m and 6 to 9 m. Overall, larval survival and gut fullness were both higher in November than in March. In both months, larval survival was lower at the surface than at other depths. This correlated with lower chlorophyll a concentrations, but lower total cell densities were not detected. During the in situ experiments, diatoms were the most abundant phytoplankton group in the water column and in the guts of larvae and, therefore, appeared to be the principal diet of larvae. Pigment analysis demonstrated that while gut contents generally reflected the composition of the phytoplankton community, the larvae were not feeding exclusively on diatoms. They also ingested green algae and possibly seagrass detritus. The in situ experiments demonstrated that the predominantly diatom flora in Albatross Bay can provide a nutritionally adequate environment for prawn larvae even at seasonally low levels. It is unlikely, therefore, that starvation is a major cause of mortality of P. merguiensis larvae during either of the biannual peaks in their abundance in Albatross Bay, Gulf of Carpentaria.

Notes: Current shrimp pond management practices generally result in elevated concentrations of nutrients, suspended solids, bacteria and phytoplankton compared with the influent water. Concerns about adverse environmental impacts caused by discharging pond effluent directly into adjacent waterways have prompted the search for cost-effective methods of effluent treatment. One potential method of effluent treatment is the use of ponds or raceways stocked with plants or animals that act as natural biofilters by removing waste nutrients. In addition to improving effluent water quality prior to discharge, the use of natural biofilters provides a method for capturing otherwise wasted nutrients. This study examined the potential of the native oyster, Saccostrea commercialis (Iredale and Roughley) and macroalgae, Gracilaria edulis (Gmelin) Silva to improve effluent water quality from a commercial Penaeus japonicus (Bate) shrimp farm. A system of raceways was constructed to permit recirculation of the effluent through the oysters to maximize the filtration of bacteria, phytoplankton and total suspended solids. A series of experiments was conducted to test the ability of oysters and macroalgae to improve effluent water quality in a flow-through system compared with a recirculating system. In the flow-through system, oysters reduced the concentration of bacteria to 35% of the initial concentration, chlorophyll a to 39%, total particulates (2.28–35.2 µm) to 29%, total nitrogen to 66% and total phosphorus to 56%. Under the recirculating flow regime, the ability of the oysters to improve water quality was significantly enhanced. After four circuits, total bacterial numbers were reduced to 12%, chlorophyll a to 4%, and total suspended solids to 16%. Efforts to increase biofiltration by adding additional layers of oyster trays and macroalgae-filled mesh bags resulted in fouling of the lower layers causing the death of oysters and senescence of macroalgae. Supplementary laboratory experiments were designed to examine the effects of high effluent concentrations of suspended particulates on the growth and condition of oysters and macroalgae. The results demonstrated that high concentrations of particulates inhibited growth and reduced the condition of oysters and macroalgae. Allowing the effluent to settle before biofiltration improved growth and reduced signs of stress in the oysters and macroalgae. A settling time of 6 h reduced particulates to a level that prevented fouling of the oysters and macroalgae.

Notes: The present study compares the relative costs of stocking ponds with postlarvae from wild or domesticated Kuruma shrimp, Penaeus japonicus (Bate), broodstock. Wild broodstock were obtained from a commercial trawler. Domesticated broodstock were reared to harvest size (wet weight = 25 g) on a commercial farm and then transferred to controlled environment tanks where they were grown to a final wet weight of 50 g. The costs of stocking ponds with postlarvae were calculated from the observed reproductive output, the costs of purchasing wild broodstock and the costs of domesticated broodstock production in the controlled environment facility. Domesticated P. japonicus spawned comparable numbers of eggs to wild broodstock of similar size, but hatching success was significantly lower. A total of 12 domesticated P. japonicus broodstock would be needed to produce the postlarvae to stock a 1-ha pond, compared to only six wild broodstock. However, the much higher relative costs of wild broodstock means that the cost of using their postlarvae would be Aus$851 per pond compared to Aus$390 for domesticated broodstock. The present authors conclude that the use of domesticated P. japonicus broodstock could be a cost-effective alternative to wild broodstock in Australia and in other countries where P. japonicus is farmed.

Notes: Strategies for sampling sediment bacteria were examined in intensive shrimp. Penaeus monodon (Fabririus), ponds in tropical Australia. Stratified sampling of bacteria at the end of the production season showed that the pond centre, containing flocculated sludge, had significantly higher bacterial counts (15.5 × 109 g-1 dw) than the pond periphery (8.1 × 109g−1 dw), where the action of aerators had swept the pond floor. The variation in bacterial counts between these two zones within a pond was higher than that between sites within each zone or between ponds. Therefore, sampling effort should be focused within these zones; for example, sampling two ponds at six locations within each of the two zones resulted in a coefficient of variation of ± 5%. Bacterial numbers in the sediment were highly correlated with sediment grain size, probably because eroded soil particles and organic waste both accumulated in the centre of the pond. Despite high inputs of organic matter added to the ponds, principally as pelleted feeds, the mean bacterial numbers and nutrient concentrations (i.e. organic carbon, nitrogen and phosphorus) in the sediment were similar to those found in mangrove sediments. This suggests that bacteria are rapidly remineralizing particulates into soluble compounds. Bacterial numbers were highly correlated with organic carbon and total kjeldahl nitrogen in the sediment, suggesting that these were limiting factors to bacterial growth.

Notes: Shrimp pond effluent water can contain higher concentrations of dissolved nutrients and suspended particulates than the influent water. Consequently, there are concerns about adverse environmental impacts on coastal waters caused by eutrophication and increased turbidity. One potential method of improving effluent water quality prior to discharge or recirculation is to use bivalves to filter the effluent. In this study, we examined the effects of the Sydney rock oyster, Saccostrea commercialis (Iredale & Roughley), on the water quality of shrimp pond effluent. Effluent from a shrimp farm stocked with Penaeus japonicus (Bate) was pumped directly into 34-L tanks stocked with different densities of oysters. Combinations of live and dead oysters were used to test the effects of three different densities of live oysters (24, 16 and 8 live oysters per tank). The concentrations of total suspended solids, the proportion of organic and inorganic matter, total nitrogen, total phosphorous, chlorophyll a and the total number of bacteria in the pond effluent water were determined before and after filtration by oysters. The oysters significantly reduced the concentration of all the parameters examined, with the highest oyster density having the greatest effect. Shrimp pond effluent contained a higher proportion of inorganic matter (72%) than organic matter (28%). The organic component appeared to be mainly detritus, with chlorophyll a comprising only a minor proportion. Filtration by the high density of oysters reduced the effluent total suspended solids to 49% of the initial level, the bacterial numbers to 58%, total nitrogen to 80% and total phosphorous to 67%. The combined effects of settlement and oyster filtration reduced the concentration of chlorophyll a to 8% of the initial effluent value.

Notes: The abundance and species composition of zooplankton assemblages were examined in a commercial Penaeus monodon (Fabricius) aquaculture pond in subtropical Australia. Numbers and biomass of zooplankton in the surface tows (142 µm mesh) varied from 2080 L−1 (2466 µg L−1) to 〈 1 L−1 (15 µg L−1). Peak zooplankton abundance and biomass occurred at the beginning of the shrimp farm season. The small copepod Oithona australis Nishida was dominant during these peaks in abundance although other species were common, including small, low-salinity copepods in the first year and larger marine copepods in the second year. Irrespective of taxa or size, zooplankton abundance declined rapidly after the stocking of P. monodon postlarvae, indicating high levels of non-selective predation. For the remainder of the grow-out season, mean zooplankton abundance was below 1 L−1 in the first season and below 3 L−1 in the second season. The factors that maintain the abundance of these zooplankton assemblages at these relatively low levels are poorly understood but may include a deterioration pond water quality. At these lower levels of abundance, the copepods Acartia pacifica Steuer and Acartia sinjiensis Mori were relatively common during the middle phase of the production season, with barnacle nauplii increasing in abundance during the later stages of the grow-out season. Analysis of fixed effects showed that there were significant differences in total zooplankton abundance between farm production seasons and occasion within season, and time (day or night). Analysis of random effects demonstrated that the dominant source of variation, in total zooplankton abundance, was day-to-day changes within sampling occasions. The complexity of temporal and spatial patterns in the abundance, distribution and composition of zooplankton assemblages in shrimp ponds presents significant challenges in designing sampling programmes that accurately quantify temporal or spatial trends. Our results have shown that sampling for more than four consecutive days, at more than one site, is necessary to accurately assess such trends.