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Quantifying the effects of trawling on the seabed biota of the NPF
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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.