Extraction and Classification of Longline Fishing Trips from Vessel Monitoring Systems Data with Sequential Recording Gaps
نویسندگان
چکیده
The evaluation of fish stocks exploitation is a complex task that comprehends the analysis of several types of data including information on the spatial and temporal dynamics of commercial fishing fleets. Satellite-based Vessel Monitoring Systems (VMS) is one of the best independent data sources available that can provide information on fishing fleet operations. This type of system allows the transmission of data from a satellite-tracking device installed on board to a land-based Fisheries Monitoring Centre. The data transmitted typically contains information on date/time, longitude and latitude coordinates and vessel’s speed and course. Aiming to characterize the fishing operations of the Portuguese deep-water longline fleet targeting black scabbardfish (Aphanopus carbo Lowe, 1839), to identify its main fishing areas and also to calculate fishing trip durations, IPIMAR has been analysing VMS data provided by the Portuguese General Fisheries Inspections Service (IGP). In general, data points from the Portuguese VMS are recorded in a constant time lag of 10 min. However, owing to several technical and non-technical problems related with the transmission device installed on board, vessel’s datasets can exhibit large time differences ( > 4 hours) between consecutive data points. These temporal discontinuities pose serious difficulties to the analyst when trying to track down vessel’s operations at sea. The present paper presents a software toolkit composed by a set of procedures developed using Oracle PL/SQL and Oracle SPATIAL tools to extract and classify fishing trips held by the longline vessels from VMS discontinued data. The set of procedures was implemented taking into account ancillary information about the fishing operations characteristics and the number of fishing hauls held by fishing trip. The toolkit enabled to identify fishing operations held at sea by fishing trip, namely setting and hauling of the gear and their duration. The validation of the number of trips processed by the toolkit for each vessel was conducted by comparative analysis with daily landings records data, assuming that each landing record corresponds to a unique fishing haul. From a total of eight vessels analysed in the period 2000-2004, the toolkit was able to process 76% of the total fishing trips held with landings on black scabbardfish.
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