The science of sustainable seafood, explained

What is the Global Footprint of Fishing?

Thanks to Global Fishing Watch, a new partnership between Oceana, SkyTruth, and Google, scientists may be getting closer to figuring out how much of the world’s ocean is fished—but discrepancies in the scale of data are producing wildly different answers.

Global Fishing Watch launched in 2016 as a way to track fishing boats around the world. The core data comes from boats’ automatic identification system (AIS), a GPS system that pings out its location every 30 seconds to satellites. Most large boats around the world (not just fishing boats) are required to have AIS onboard for monitoring purposes and safety. Global Fishing Watch collects AIS data on all boats around the world to “determine the type of ship (e.g., cargo, tug, sail, fishing), its size, what kind of fishing gear (e.g. longline, purse seine, trawl) it’s using, and where and when it’s fishing based on its movement patterns.” It is an impressive way to collect fishing data and shows some promise for curbing illegal fishing. You can read more about it here.

One of the first major publications to come out of these AIS data attempted to map the ‘global footprint’ of fishing. It concluded that 55% of the global ocean was fished. However a recent paper, using the exact same data, concluded that only 4% of the global ocean is fished. An order of magnitude difference! What is going on here?


The issue is scale. The original paper drew gridlines on the ocean every 0.5° of latitude and longitude, creating a grid of 0.5° x 0.5° cells, roughly 3100km2 at the equator, or about the size of Rhode Island. If fishing occurred inside a cell, the entire cell got marked as ‘fished.’ Doing that across the globe showed 55% of cells as fished.

However, using 0.5° x 0.5° cells the size of Rhode Island seemed a bit suspect to a group of researchers who redid the analysis with the same data on a finer scale in a paper published earlier this week. They broke the ocean into 0.01° x 0.01° cells, only 1.23 km2—about the size of city block. Using this higher resolution data, they showed a more accurate footprint of fishing that covered 4% of the cells. Figure 1 from their paper shows the difference between using 0.5° cells vs 0.01° cells.

From Amoroso et al. 2018. The images compare the footprint of fishing at 0.5° resolution (left) vs 0.01° resolution (right). The top comparison shows how lower resolution data assumes all of a protected area was fished, whereas the higher resolution shows the protected area as more effective.

The authors of the original paper defended their conclusions by noting that the most complete historical fisheries catch database uses 0.5° x 0.5° cells. However, the decision to use 0.5° x 0.5° cells in that database was made nearly 15 years ago as a balance between the available data and what was capable computationally. Comparisons to previous data are useful, but it seems like a disservice to the groundbreaking nature of AIS data collection to down-scale the data resolution in such a dramatic way.

Communicating Fisheries & Science

The differences in scale have major communication and perception implications. The original paper that used low-resolution data was picked up by over 100 news outlets and has an Altmetric score (a way to measure social reach of a scientific publication) in the top 5% of any scientific paper ever scored. It produced headlines like, “More than 55 percent of Earth’s ocean surface is occupied by industrial fishing vessels” and “New maps show the utterly massive imprint of fishing on the world’s oceans” with practically no mention of the low resolution 0.5° x 0.5° cells.

This was probably due to the press release that went out with the paper that highlighted the 55% number but left out methodological information on measurement scale. The press release also pointed out that 55% of the ocean is more than 4 times the size of land used for agriculture, a point that was well represented in news coverage. Comparing fisheries to agriculture is extremely important for understanding conservation and sustainability—food should be examined on a global scale, but when looked at in a higher resolution, the fishing footprint is less than a third of global agriculture.

The lead author of the paper, David Kroodsma, defended their numbers and impact assessment by admitting that yes, technically 4% of the ocean is directly fished, but the ecosystem impacts of fishing extend to at least 55% of the ocean considering that fish move and migrate. By this measurement, a comparison to agriculture would say that nearly 100% of land is “affected by agriculture” as roaming mammals and migrating birds are impacted by farms. There are many more precise ways to measure the ecosystem impact of fishing than choosing an arbitrary grid line (e.g. population assessments), just as there are more accurate ways of describing the ecosystem impacts of farming than looking at how much space a farm uses or animal migrations.

However, in the original paper they are not consistent in their comparison of fisheries to agriculture. They compared the diffuse, low-resolution impacts of fisheries to direct agricultural land use (as opposed to diffuse impacts of agriculture). From a global food policy perspective this is highly misleading and potentially works against global conservation efforts by encouraging people to think that food from land is more sustainable than food from the ocean, which is not necessarily true—especially when comparing protein (we’ve written quite a bit about comparing the environmental impacts of food). Better dietary policies and a more sustainable future are dependent on accurate comparisons of environmental impacts.

The issue with the original paper is that the data was not represented in proper context by the press release so most news stories covered the paper as ‘55% of the ocean is fished,’ rather than something more accurate like:

Cool new data show that 4% of the ocean is directly fished, and if you assume a 0.5° x 0.5° cell is representative of ecosystem impact, 55% of the ocean is ‘affected’ by fishing.

This is an extremely important distinction given that, according to Kroodsma, “the public [does not think] of area fished in the same way many fishery scientists do.”

What we’ve got here is failure to communicate.

Unfortunately this is all too common in fisheries and conservation. For example, the myth that all fish are going to disappear by 2048 was started by a misleading (re: bad) press release in 2006. Despite the original authors and the scientific community walking away from the notion, the 2048 myth persists in public perception today.

Another example: “fully-fished” fisheries are sustainable, but they frequently get lumped into over-exploited, unsustainable fisheries. It sounds dire to say that 93% of fisheries are either over exploited or fully fished. The reality is that 33% are over exploited (unsustainable) and 60% are fully fished (sustainable). In the 2016 election, 93% of people voted for Donald Trump or Hillary Clinton. Is it appropriate to group people who voted for either of those two candidates together as similar?

Press releases are designed to garner headlines and attention, but in science they should be helpful, accurate, and try to head off any potential misinterpretation—especially in an era where science is losing public trust and truthful news is under attack. Hopefully, the press release and subsequent headlines that stressed 55% of the ocean is fished do not result in decades-long misinterpretation.

Picture of Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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3 Responses

  1. Love the site and the idea behind it, is a much needed approach. I’m sure that many of you readers (like me) are not from the USA, so it would be great if once you use a comparison, like the size of Rhode Island, could you use a square km reference (250km2). Just make the whole issue easier to read for non Americans. Thanks

  2. Whem you learn of the discreptancies here, one is reminded of Winston Churchill’s quote:, “There are lies, damm lies and statistics”.

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