Five sites within mangrove tidal channels at Missionary Bay, Hinchinbrook Island, north Queensland, were used in a preliminary study of growth and survival of the oyster Crassostrea echinata in subtidal tray culture.Two sites were located well upstream, had reduced interchange with the coastal water of Missionary Bay, and occupied areas with little fresh water influence. Two further sites were also located well upstream but situated in areas which, from previous experience, were likely to experience significant freshwater influence during the monsoonal periods. The last site was located in a small channel which had direct exchange with Missionary Bay waters and which also was expected to experience lower salinities during the wet season.240 small oysters were placed at each site on November 15, 1981 and growth and mortality assessed at approximately monthly intervals up to January 1983. Hydrographic parameters (salinity, temperature, dissolved oxygen, particulate organic carbon and dissolved inorganic nutrients (nitrate, nitrite, phosphate and ammonium)) were obtained concurrently with oyster measurements. Measurements were taken at 10 minute intervals over an approximately 2 hour period from the top of the tide at each site. This sampling periodicity was used in order to estimate the average conditions experienced by the oysters during the high to low tide periods.
This research was undertaken as a feasibility study for the subtidal tray aquaculture of Crassostrea echinata within mangrove waterways.The objectives of the study were to1. monitor growth and mortality of oysters in subtidal tray culture2. identify some environmental parameters associated with oyster viability as a guide to more detailed studies of optimal culture conditions.
All sites were stocked with small oysters (ca 8g total weight per oyster) suspended from floating pontoons. The pontoons were constructed from 100mm PVC piping jointed with 90° elbows to form rectangular floats of 1.3 x 0.6m dimensions. Each pontoon supported 4 trays at 0.5m below the water surface. Trays constructed of extruded nylon mesh, 1cm2 mesh size, supported on metal frames of 8mm diameter rolled steel, had dimensions of 0.6 x .3 x .05m. The trays had tied-down lids to prevent oyster loss by tidal movement or predation. Four wooden poles (100 x 50mm) driven into the creek bed were located one at each side of the rectangular pontoon, and extended 4m above the low tide line. These provided effective guide rails within which the pontoon could rise and fall with the tide while being maintained in the same horizontal position. Chains were attached from two facing poles to the pontoon corners and were of a length that prevented the trays settling on the mud at low tide. As such the oysters were exposed only for short periods at the lowest tides.Each tray held 60 oysters totalling 240 oysters per pontoon with a stocking density of 333 oysters/m2. This density was maintained by monthly replacement of dead oysters with live specimens of similar dimensions. The replacements were drawn from a parental stock located at the AIMS harbour at Cape Ferguson. The parental stock was also maintained within a similar floating pontoon system. Original stock were removed from the AIMS pier. All trays were cleaned regularly to remove fouling.
