Occurrence and percent cover of the colonial invasive tunicate Didemnum vexillum from near-seafloor drift transect video imagery and high-resolution digital still images in the western Bay of Fundy, Canada

Funded under DFO's Marine Conservation Targets Program, this optical imagery benthic survey documents the occurrence and estimated percent cover of the invasive colonial tunicate, Didemnum vexillum in seven drift-camera transects in the 'Head Harbour/West Isles Archipelago/The Passages' Ecologically and Biologically Significant Area (ESBA, ~113km2) in the western Bay of Fundy, New Brunswick, Canada. Occurrence data was derived from the use of high-resolution still images (n=386) taken periodically throughout each transect, and simultaneous continuous high-definition video. Video was divided into 20-second segments (here, we report the start location of each segment within a transect) and when D. vexillum was present in a video segment, frequency of occurrence was classified using a modified SACFOR abundance scale reporting coverage as common (continuous coverage/patches throughout the video segment), occasional (individual colonies of various sizes encountered >5 times throughout the video segment), or rare (small, isolated colonies encountered ≤5 times throughout the video segment). A video segment was deemed unusable and removed from the dataset if there was too much turbidity, or if the camera position was too high off-bottom to reliably image the seafloor. For still images, when D. vexillum was observed in an image, colony percent cover was categorized as >50%, 26-50%, 6-25%, or ≤5% of the images field of view (FOV). Distance travelled and distance between still images (m) was calculated using ArcGIS tools. FOV was estimated by measuring the length and width of a subset of still images and video frame grabs in ImageJ2, using 10-cm lasers for scale. FOV was standardized for each reported altitude, and area sampled (m2) along a continuous video segment was estimated by multiplying the average FOV by the distance travelled in that segment. Didemnum vexillum was found in 44% of the area sampled at depths from 34 to 118m, deeper than previous reports globally of ~80m.

本光学影像底栖调查获加拿大渔业与海洋部（DFO）海洋保护目标计划资助，记录了加拿大新不伦瑞克省芬迪湾西部「黑德港/西群岛群岛/海峡」生态与生物重要区域（Ecologically and Biologically Significant Area，ESBA，面积约113平方千米）内7条漂流摄像样带中入侵性群居被囊动物Didemnum vexillum的出现情况与估算盖度。出现数据来源于各条样带中定期采集的高分辨率静态影像（共386张）与同步连续高清视频。视频被切分为20秒片段（本研究报告各片段在样带内的起始位置）；当片段中出现Didemnum vexillum时，采用改良SACFOR丰度分级标准对出现频次进行分类，将盖度划分为常见（片段内连续覆盖或呈斑块状分布）、偶见（片段内遇到多种大小的独立群落超过5次）、罕见（片段内遇到小型孤立群落≤5次）。若视频片段存在严重浊度，或相机离底过高无法可靠拍摄海底，则该片段将被判定为无效并从数据集中移除。对于静态影像，若其中观测到Didemnum vexillum，则将群落盖度按其占影像视场（Field of View，FOV）的比例划分为>50%、26%-50%、6%-25%及≤5%四个等级。移动距离与静态影像间的间距（单位：米）通过ArcGIS工具计算得到。视场通过ImageJ2软件对部分静态影像与视频帧截图的长宽进行测量估算，以10厘米激光点作为比例尺。视场根据各报告的拍摄高度进行标准化；连续视频片段的采样面积（单位：平方米）通过平均视场与该片段内移动距离相乘估算得到。本调查中，Didemnum vexillum在34至118米水深的44%采样区域中被发现，该水深比全球此前报道的约80米更深。