Global Fishing Index

Case StudY

Illuminating the world’s ‘dark fleets’

By David Kroodsma, Global Fishing Watch

Historically, what happens over the horizon in the ocean has largely been unmonitored. This obscurity has given illegal fishers uncontrolled access to offshore waters, with as much as one in every four fish thought to be caught through illegal, unreported or unregulated fishing.1

Recent advances in vessel-tracking technology have helped illuminate the activity of industrial vessels at sea. Many countries now require large vessels to broadcast their positions via an automatic identification system (AIS) or vessel monitoring systems (VMS). As a result, most fishing vessels larger than 24 metres can now be tracked with relative ease.2

The problem? Vessels trying to avoid oversight can simply turn their AIS off and ‘go dark’ – remaining hidden from surveillance systems.

But dark fleets are not completely invisible.

Optical imagery allows us to take pictures of these vessels from space using high-resolution cameras and radar (Figure 1). In the last ten years, literally hundreds of satellites have been launched that can image the ocean. With help from machine learning and big data processing, our ability to find dark fleets is rapidly improving.

The waters off North Korea provide a gripping example.

In 2017, the United Nations Security Council responded to missile tests by banning all countries from fishing in North Korea’s waters. Despite this, South Korean authorities reported hundreds of vessels passing through on their way into North Korean waters. Few of these vessels broadcasted their GPS positions through conventional tracking technology.

But these ‘dark fleets’ could only stay hidden for so long. In July of 2020, a team of researchers led by Global Fishing Watch and including scientists from Japan, South Korea, Australia and the United States, revealed that over 900 vessels originating from China were active in North Korean waters – a discovery made possible by combining AIS data and satellite imagery (Figure 2). It’s estimated that between 2017 and 2019, these vessels caught more than half a billion dollars’ worth of squid – making this the largest known case of illegal fishing by vessels originating from one country, operating in another country’s waters (i.e. foreign fishing).3


Figure 1: Detecting dark fleets using satellite technology. Estimating the size of the fishing fleet operating in North Korean waters was made possible through the combination of four different technologies: the automatic identification system data, optical imagery, low light imaging data using the visible infrared imaging radiometer suite, or VIIRS, and satellite radar.  ©Global Fishing Watch

Global Fishing Watch is now applying the same method to monitor dark fleets globally. Around the world, hundreds of thousands of large and medium-sized fishing vessels are placing their hooks and nets without broadcasting their location, either to avoid regulations or, more often than not, because they are not required to do so. By using years of satellite imagery collected from the European Space Agency, it is now possible to count and identify these dark fleets. Further, by assessing where these vessels are detected, we can model which are likely to be fishing – and, more importantly, which may be fishing illegally.

While radar and optical imagery data may help reveal the true extent of fishing activity globally, these tools do have their limitations. Satellite’s cannot take pictures of the entire ocean, and where available, imagery may be infrequent due to the satellite’s orbit. Additionally, distinguishing fishing boats from other types of vessels remains a challenge, especially in areas of high vessel traffic like east Asia and the Persian Gulf – although Global Fishing Watch’s research shows that vessels operating on the continental shelf and positioned far from shipping lanes or oil-producing regions, are often fishing vessels.

Despite these limitations, these new technologies have the power to transform how we monitor industrial fishing – by making the unseen, seen. Around the world, there are fleets like the one in North Korea, operating with little oversight and depleting fish stocks.

By combining innovative satellite imagery with existing vessel tracking data and the knowledge from fisheries agencies around the globe, vast swaths of the ocean will no longer be dark. By making invisible operations visible, we can improve fisheries management, reduce pressure on legal fishers, and help secure a more sustainable future for the ocean.


  1. Agnew, D.J., Pearce, J., Pramod, G., Peatman, T., Watson, R., Beddington, J.R. and Pitcher, T.J. (2009). Estimating the worldwide extent of illegal fishing, PLOS ONE 4, (2), p. e4570,
  2. Kroodsma, D., Mayorga, J., Hochberg, T., Miller, N.A., Boerder, K., Ferretti, F., Wilson, A., Bergman, B., White, T.D., Block, B.A., Woods, P., Sullivan, B., Costello, C. and Worm, B. (2018). Tracking the global footprint of fisheries, Science 359, (6378), pp. 904-908, [15 March 2020]
  3. Park, J., Lee, J., Seto, K., Hochberg, T., Wong, B.A., Miller, N.A., Takasaki, K., Kubota, H., Oozeki, Y., Doshi, S., Midzik, M., Hanich, Q., Sullivan, B., Woods, P. and Kroodsma, D.A. (2020). Illuminating dark fishing fleets in North Korea, Science Advances 6, (30), p. eabb1197, [10 November 2021]
david kroodsma_global fishing watch-headshotdavid kroodsma_global fishing watch-headshot

David Kroodsma
Director of Research and Innovation, Global Fishing Watch

David Kroodsma leads Global Fishing Watch’s Research Program, which is a collaboration between Global Fishing Watch and a network of over 10 research institutions. He is responsible for leading and facilitating new research projects, and he works with the GFW engineering team to develop new technologies. David has over a decade and a half of experience working with NGOs and researchers to address global environmental challenges. He has a B.S. in physics and an M.S. in earth systems science from Stanford University.