The footprint of bottom-trawl fishing

A paper out this week in PNAS (open source), Amoroso et al. 2018, quantified the footprint of bottom trawl fishing around the world using highly-refined data for the first time. This new data should give us a better understanding of fishing impacts and improve fishery management.

Bottom trawling is when a net is weighted down to the seafloor then dragged across the bottom to scoop up fish. Bottom trawling lands about 19 million tons of fish per year – about a quarter of all wild-caught seafood.

Bottom trawl-500
Courtesy Marine Stewardship Counsil

Nearly all bottom-trawling occurs on continental shelves or slopes—the areas off the coast of landmasses covered in shallow water that eventually slope down into the deep sea.

A key concern with bottom trawling is damage to plants and animals that live on the seafloor. Species can be damaged or killed by the passage of a trawl, but species that anchor to the seafloor like corals, sponges, and seagrasses are particularly sensitive.

In Amoroso et al. 2018, researchers used a combination of satellite tracking using Vessel Monitoring System (VMS) data and logbook data on start and stop locations of trawl tows from captains and fishery observers to get highly precise information about the extent of bottom trawling & dredging on continental shelves for 24 ocean regions in North America, South America, Africa, Australia and New Zealand.

Researchers used several calculations to get an accurate estimation of trawling. The first step involved drawing gridlines on the ocean to break the ocean down into smaller cells to work with. Then, within each gridded cell, area covered by each trawl was calculated using VMS and logbook data. The area covered by trawling within each cell was summed across all trawls in a region and a total “swept area” was estimated. This swept area can be mapped, and for specific cells, a swept area ratio—the ratio of the swept area to the non-trawled area of the cell was calculated. Because many areas are trawled many times a year, the swept area ratio for some square may be 5, 10, or even 20.  From this mapping the total amount of area not trawled, trawled once, or trawled multiple times can be calculated.

With a swept area ratio, cell resolution matters tremendously. Larger cells tend to artificially exaggerate the estimation of fished areas. Results will differ dramatically between a cell of 31002 km versus a cell that is 12 km. The smaller the cell, the more accurate the estimate. In Amoroso et al. 2018, researchers used a 1-32 km cell in estimates, a stark difference from 10002 km cells used in previous estimates of bottom trawling footprints.

Researchers only included regions in which 70% of total bottom trawling catch could be accounted for. This excluded some data-poor regions with high amounts of trawling, notably several in Asia, but is still the most comprehensive and accurate study of global bottom-trawling footprint every performed.

Results

The extent of trawling varied dramatically by region. For example, just 0.4% of the seafloor off Southern Chile was trawled compared to 80% of the Adriatic Sea off the coast of Italy.

Overall, 14% of the seafloor was trawled in the areas where high resolution data were available over the 2-6 year study period. Europe had the highest trawling footprint, while Austalia & New Zealand had footprints below 10%. An imporant finding was that trawling happened in the same places each year—trawl “hotspots” remained hotspots.

AmorosoFigure2
Mean interval between trawling events and the proportion of unfished area at depths 0-1000 m around the world. From Amoroso et al. 2018

“For those regions where bottom-trawling footprints were less than 10 percent of the seafloor area, fishing rates on bottom-dwelling fish stocks almost always met international sustainability benchmarks. But when footprints exceed 20 percent, they rarely met them,” explained one of the paper’s co-authors, Simon Jennings. This has important sustainability implications; with a better understanding of the extent of bottom trawling, fishery managers can alter management plans based on each region.

Fig_3_Amoroso_et_al
Proportions of the total area of each region trawled at different frequencies. Region code numbers increase as regional SAR decreases. From Amoroso et al. 2018

Despite the lack of some data-poor regions, this research can be used to inform fishery management around the world explains co-author Ray Hilborn, “We are able to use this method to make reasonably good estimates of the impact of trawling in places where we don’t have fine-scale spatial data.”

Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW. He thinks a lot about how environmental perception influences environmental policies.

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