Identifying purse seine vessels via track analysis
Purse seine vessel tracks can be most easily distinguished by periods of searching for fish at higher speeds followed by periods of fishing at lower speeds.
Purse seine vessel movements can be less distinctive during fishing than longline vessel movements. The setting of a purse seine net can be done quickly, in around eight minutes¹. Therefore, unless the AIS reporting is very frequent, it is unlikely that the vessel’s detailed movements during setting will be observed.
Closing the net, hauling the net and harvesting the fish can take several hours but this will depend on the size of the net and the quantity of fish caught².
Changes in vessel course and orientation followed by a reduced speed for several hours can align to the setting and hauling of a purse seine net. The purse seine nets are up to 3km long so the area covered by this activity is fairly confined¹.
Try it out: See the movements of a purse seine fishing vessel that is likely setting and hauling its net as it changes course, forming a small semi-circle in the vessel track, and then slows down for several hours.
Purse seine vessels can sometimes use additional boats for searching out free schools of fish or setting and monitoring fish aggregating devices (FADs).
International MCS Network. (2021). The MCS Practitioners Introductory Guide to Purse Seine Fishing. Retrieved from https://imcsnet.org/wp-content/uploads/2021/11/TMT-IMCSN-MCS-guide-Purse-Seine-English-DIGITAL.pdf
Food and Agriculture Organization of the United Nations. (n.d.). Fishing Techniques Tuna Purse Seining. Retrieved December 6, 2021, from https://www.fao.org/fishery/fishtech/40/en
Identifying longline fishing vessels via track analysis
Longline vessels have distinctive vessel tracks—these reflect the setting and hauling of the vessel’s fishing gear. The setting of lines is done more quickly than the hauling.
Buoys are often associated with longline vessels and the buoys' movements can display characteristic tracks as the longlines are deployed and then hauled in.
Buoy locations can be particularly useful for vessel movement analysis when there is limited AIS reporting from the vessel itself.
Even when longline vessels move in less methodical patterns, looping tracks with variable speeds, as the lines are set and hauled are still evident.
Learn more: Identifying types of fishing vessels with RFMO vessel subtypes