Welch and colleagues analysed 3.7 billion AIS messages recorded between 2017 and 2019 in the global Fishing Watch AIS dataset, identifying more than 55,000 suspected intentional disabling events in waters more than 50 nautical miles from shore, amounting to 6% (>4.9 million hours) of obscured vessel activity. Hotspots of disabling activity were located near several regions of IUU concern and transshipment hotspots, including in the exclusive economic zones of Argentina and West African nations and in the Northwest Pacific. Using individual boosted regression tree models for the four dominant gear types (squid jiggers, trawlers, tuna purse seines and drifting longlines) and a full model that included all suspected disabling events (that is, the four gear types listed above and additional gears such as gillnet and troll), Welch and colleagues found that loitering by transshipment vessels (a proxy for potential transshipment events) was the most important driver in the full model and squid jigger model and more than half of the disabling events by squid jiggers were close enough to undertake transshipment to refrigerated cargo vessels.
Although these data do not provide direct evidence of the type of activities carried out during AIS disabling events, Welch and colleagues note that an absence of AIS data contains information and can serve as a tool in the currently data-deficient fight against IUU fishing activity. For example, some patterns of behaviour suggest that ships may disable their AIS devices to hide the locations of productive fishing grounds from competitors, which could contribute to overexploitation and population decline in commercial stocks. Models such as those developed by Welch and colleagues could be used as prediction systems aiding to position at-sea or airborne enforcement and surveillance, or could be applied in combination with high-risk ports to guide IUU inspections.
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