Sep 10 2018

Wait, So How Much of the Ocean Is Actually Fished?

September 10, 2018 — The following is excerpted from an article published today by The Atlantic. More information about this paper is available at Sustainable Fisheries UW

How much of the world’s oceans are affected by fishing? In February, a team of scientists led by David Kroodsma from the Global Fishing Watch published a paper that put the figure at 55 percent—an area four times larger than that covered by land-based agriculture. The paper was widely covered, with several outlets leading with the eye-popping stat that “half the world’s oceans [are] now fished industrially.”

Ricardo Amoroso from the University of Washington had also been trying to track global fishing activity and when he saw the headlines, he felt that the 55 percent figure was wildly off. He and his colleagues re-analyzed the data that the Global Fishing Watch had made freely available. And in their own paper, published two weeks ago, they claim that industrial fishing occurs over just 4 percent of the ocean.

How could two groups have produced such wildly different answers using the same set of data? At its core, this is a simple academic disagreement about scale. But it’s also a more subtle debate that hinges on how we think about the act of fishing, and how to measure humanity’s influence on the planet. “I think this discussion really shows how little we know about the world’s oceans and why making data publicly available is so important for stimulating research,” says Kroodsma.

As ships traverse the oceans, many of them continuously transmit their position, speed, and identity to satellites. This automatic identification system was originally developed to prevent collisions, but by training Google’s machine-learning tools on the data, the Global Fishing Watch (GFW)—a nonprofit founded by Oceana, SkyTruth, and Google—can identify different kinds of fishing vessels, and work out where they’re dropping their lines and nets.

They quantified that activity by dividing the oceans into a huge grid of around 160,000 squares. Each of these squares had sides that span half a degree of latitude, and an area of around 3,100 square kilometers. And in 2016, around 55 percent of them included some kind of fishing activity. “You’re dividing the ocean into 160,000 cells, and you’re seeing fishing activity in half of them,” says Kroodsma. “One reason this resonated with people is that however you slice it, that’s impressive.”

Actually, it’s misleading, says Ricardo Amoroso from the University of Washington. He and his colleagues had spent years trying to measure the impact of trawlers and based on their early results, the GFW’s estimates smelled fishy. The problem is that they divided the ocean into such large squares that if a single boat drops a net in an area the size of Rhode Island, that area would count as “fished” in a given year.

Fortunately, the GFW made all their data freely available, so Amoroso’s team could analyze it at finer resolutions. When they divided the ocean into smaller squares that are 0.1 degrees of latitude wide and take up around 123 square kilometers, they found that fishing activity occurs in just 27 percent of them. And when they used even smaller squares that are 0.01 degrees wide and 1.23 square kilometers in area—the size of a city block—just 4 percent of the ocean is “fished.”

Read the full story at The Atlantic

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