Northern Prawn Fishery Fishing Effort Estimated from VMS Data

The dataset comprises GIS grids of fishing effort of the Northern Prawn Fishery (NPF) estimated from vessel monitoring system (VMS) data. The VMS is operated by the Australian Fisheries Management Authority (AFMA). It periodically records the location of each vessel in the fishery. Note that the activity of the vessel at the time is not recorded. The system's primary purpose is to monitor compliance with fishing regulations, but the data can also be used for research. To estimate fishing effort from the VMS records, we estimated the track of each fishing vessel by joining successive vessel locations with lines. We divided the track into apparent trawling and non-trawling sections (e.g. based on date, time and vessel speed), and calculated the time each trawling section spent in each cell of a GIS grid covering the managed area of the fishery. This method for estimating fishing effort from VMS data has limitations. Because a vessel may have taken a path other than a straight line between the polls, our straight line path (a) may only partially match the real path of the vessel, (b) may underestimate the length of the path traversed by the vessel, and (c) may underestimate the speed of the vessel. Note that (a) and (b) would affect which particular grid cells the trawling time was allocated to, but not the total trawling time that was added to the grid. The use of vessel speed to filter out non-trawling activity is imperfect because vessels may carry out other activities at speeds less than the maximum trawling speed.本数据集包含基于船舶监控系统（Vessel Monitoring System，VMS）数据估算得到的北方对虾渔业（Northern Prawn Fishery，NPF）捕捞努力量地理信息系统（GIS）网格数据。该船舶监控系统由澳大利亚渔业管理局（Australian Fisheries Management Authority，AFMA）运营，会定期记录该渔业内每艘作业船舶的位置信息，但需注意，系统并未同步记录船舶彼时的作业活动状态。该系统的核心目的是监管渔业合规情况，但相关数据亦可用于科研用途。为基于VMS记录估算捕捞努力量，研究团队通过直线连接连续记录的船舶位置以还原每艘捕捞船舶的航行轨迹；随后将航行轨迹划分为疑似拖网作业与非拖网作业两类区段（例如依据作业日期、时间及船舶航速进行划分），并计算每一段拖网作业区段在覆盖该渔业管理海域的GIS网格各单元格内的停留时长。此种基于VMS数据估算捕捞努力量的方法存在一定局限性：由于船舶在相邻航点之间的实际航行路径未必为直线，因此我们采用的直线轨迹存在三方面局限，(a) 仅能部分贴合船舶的实际航行路径；(b) 可能低估船舶实际航行的总航程；(c) 可能低估船舶的实际航速。需注意，局限性(a)与(b)会影响拖网作业时长分配至具体网格单元格的结果，但不会改变计入网格的总拖网作业时长。通过船舶航速筛选非拖网作业活动的方式并不完善，因为船舶可能以低于最大拖网航速的状态开展其他作业活动。