Growth and survival of the oyster Crassostrea echinata in subtidal tray aquaculture, Missionary Bay, Hinchinbrook Island, north Queensland

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.

本研究选取昆士兰州北部欣钦布鲁克岛传教士湾红树林潮汐水道内的5个位点，开展棘牡蛎（Crassostrea echinata）潮下托盘养殖的生长与存活特性初步研究。其中2个位点位于河道上游远端，与传教士湾沿岸水体交换能力较弱，且受淡水影响极小；另有2个位点同样处于上游远端，但根据既往研究经验，其所在区域在季风期可能受到显著淡水影响；最后1个位点位于小型潮汐水道内，可与传教士湾水体直接交换，且预计雨季盐度较低。

1981年11月15日，每个位点投放240只幼牡蛎，随后自1981年11月至1983年1月期间，以约每月1次的频率评估牡蛎的生长与死亡率。同步采集水文参数数据，包括盐度、温度、溶解氧、颗粒有机碳以及溶解无机营养盐（硝酸盐、亚硝酸盐、磷酸盐与铵盐）。采样方案为：在每个位点的高潮时刻开始，以10分钟间隔开展约2小时的连续监测，该采样周期用于估算牡蛎在涨潮至落潮阶段所经历的平均环境条件。

本研究作为棘牡蛎潮下托盘养殖在红树林水道中开展的可行性探索，旨在达成以下目标：1. 监测潮下托盘养殖模式下牡蛎的生长与死亡情况；2. 识别与牡蛎生存力相关的环境参数，为后续优化养殖条件的深入研究提供参考依据。

所有位点均采用悬挂于浮动浮筒的方式投放幼牡蛎（单只牡蛎总重约8g）。浮筒由100mm口径PVC管材通过90°弯头连接制成，为1.3×0.6m的矩形浮体。每个浮筒下方距水面0.5m处悬挂4个养殖托盘。托盘由直径8mm的轧制钢框架支撑，网面为网孔尺寸1cm²的挤出尼龙网，整体尺寸为0.6×0.3×0.05m。托盘配有固定网盖，以防牡蛎因潮汐流动或捕食者逃逸。

4根尺寸为100×50mm的木杆分别打入溪床，位于矩形浮筒的两侧，且高出低潮线4m，形成有效导轨，使浮筒可随潮汐升降，同时保持水平位置固定。将铁链连接于两根相对的木杆与浮筒角部，铁链长度需确保落潮时托盘不会沉至泥滩中，如此一来，牡蛎仅在最低潮时段会短暂暴露于空气中。

每个托盘容纳60只牡蛎，单个浮筒总计投放240只，养殖密度为333只/㎡。该密度通过每月以同规格活牡蛎替换死亡个体维持。替换用个体取自位于开普弗格森的澳大利亚海洋科学研究所（AIMS）港湾内的亲贝种群，该亲贝种群同样饲养于类似的浮动浮筒系统中，初始种群采自AIMS码头。所有托盘均定期清洁以去除附着生物。