Use of side-scan sonar for estimations of Crassostrea brasiliana (Lamarck, 1819) stocks in subtidal banks on the south coast of Brazil

Abstract Uncontrolled extractivism has led to a worldwide reduction in oyster stocks. The use of new and more efficient management tools for impacted environments must increase. The application of such tools requires previous biological and environmental knowledge of the resident populations of these bivalve mollusks. Technical difficulties are usually associated with studies of submerged oyster banks, and these difficulties result in a considerable lack of biological data on the oysters that inhabit subtidal zones. This study aimed to survey Crassostrea brasiliana (also known as C. gasar) stocks in submerged banks and to evaluate a method with which to measure the extent of the banks and identify and quantify the oysters in the banks with the use of side-scan sonar. This study was conducted on 10 oysters banks located in the subtidal zone of Guaratuba Bay, on the south coast of Brazil. The prospection of all these banks was later validated by diving, and oyster samples (n = 20/bank) were collected for species identification with a molecular method. Only one bank contained oysters that were generically classified as Crassostrea sp.; those in the remaining banks being identified as C. brasiliana. The prospected banks contained oysters of various sizes (average 1.5 m, n = 1,107) that were heterogeneously scattered in the riverbeds. The total number of oysters in the 10 sampled banks was estimated to be 21,159.13 oysters or 1 oyster/4.5 m², which represents a low oyster density in the study site. The results validate the use of side-scan sonar as an efficient means with which to prospect for oysters in banks located within subtidal zones.

摘要 无节制的采捕活动已导致全球牡蛎种群储量下降，亟需推广应用新型高效的受干扰生境管理工具。这类工具的应用需预先掌握该类双壳类软体动物定居种群的生物学与环境学相关知识。水下牡蛎礁的相关研究常伴随技术难题，这使得潮下带栖息牡蛎的生物学数据严重匮乏。本研究旨在勘查巴西牡蛎（Crassostrea brasiliana，亦称C. gasar）的水下礁体，并评估一种利用侧扫声呐测量礁体范围、识别并量化礁体内牡蛎数量的方法。本研究的采样区域为巴西南部沿海瓜拉图巴湾的潮下带10处牡蛎礁，所有礁体的勘查结果均通过潜水作业得到验证，同时每个礁体采集20份牡蛎样本，采用分子方法进行物种鉴定。结果显示，仅1处礁体的牡蛎暂归类为巨牡蛎属未定种（Crassostrea sp.），其余礁体的牡蛎均鉴定为巴西牡蛎。本次勘查的礁体中，牡蛎个体大小不一（平均体长1.5m，共统计1107只），不均匀地分布于河床中。10个采样礁的牡蛎总数量估算为21159.13只，即每4.5平方米1只牡蛎，表明研究区域的牡蛎种群密度偏低。本研究结果验证了侧扫声呐作为潮下带牡蛎礁高效勘查手段的可行性。