Bottom trawl fishing footprints on the world's continental shelves

Abstract

Bottom trawlers land around 19 million tons of fish and invertebrates annually, almost one-quarter of wild marine landings. The extent of bottom trawling footprint (seabed area trawled at least once in a specified region and time period) is often contested but poorly described. We quantify footprints using high-resolution satellite vessel monitoring system (VMS) and logbook data on 24 continental shelves and slopes to 1,000-m depth over at least 2 years. Trawling footprint varied markedly among regions: from <10% of seabed area in Australian and New Zealand waters, the Aleutian Islands, East Bering Sea, South Chile, and Gulf of Alaska to >50% in some European seas. Overall, 14% of the 7.8 million-km² study area was trawled, and 86% was not trawled. Trawling activity was aggregated; the most intensively trawled areas accounting for 90% of activity comprised 77% of footprint on average. Regional swept area ratio (SAR; ratio of total swept area trawled annually to total area of region, a metric of trawling intensity) and footprint area were related, providing an approach to estimate regional trawling footprints when high-resolution spatial data are unavailable. If SAR was ≤0.1, as in 8 of 24 regions, there was >95% probability that >90% of seabed was not trawled. If SAR was 7.9, equal to the highest SAR recorded, there was >95% probability that >70% of seabed was trawled. Footprints were smaller and SAR was ≤0.25 in regions where fishing rates consistently met international sustainability benchmarks for fish stocks, implying collateral environmental benefits from sustainable fishing.

Keywords: effort; fisheries; footprint; habitat; seabed.

Fig. 1.

Relationships between the spatial resolution of effort data and the trawling footprint (approach A, grid cell based; in the text) for depth ranges of 0–200 and >200–1,000 m. Region codes follow Fig. 3 and Table 1. Three regions are not represented in Right (depths of 200–1,000 m), because these regions are predominantly <200-m deep.

Fig. 2.

Mean interval between trawling events and the proportion of unfished area at depths 0–1,000 m for regions in (A) the Americas, (B) Europe, (C) Australasia, and (D) Africa. Black lines indicate boundaries of study regions, pale blue tones indicate depths of 0–200 m in the study regions, darker blue tones indicate depths of 200–1,000 m in the study regions, and all deeper areas and areas outside study regions are shown in white. In all numbered regions, the proportion of bottom trawling included in this analysis exceeds 70% of total activity (Table 1). Region codes follow Fig. 3 and Table 1.

Fig. 3.

Proportions of the total area of each region, at depths of 0–200 and >200–1,000 m, trawled at different frequencies. Region code numbers increase as regional SAR decreases.

Fig. 4.

Relationship between the regional SAR and the trawling footprint (approach C, assumes uniform spread in grid cells; in the text). (Left) Symbol sizes indicate the proportion of total fishing activity recorded in each region (all >70%), and numbers in symbols identify the regions listed in Fig. 3 and Table 1. (Right) The black line is the fitted relationship footprint =SAR/(b + SAR); dark blue shading indicates 95% confidence intervals for model fit, and light blue shading indicates 90% prediction intervals for footprint.

Fig. 5.

Relationships between the relative rate of fishing mortality and the regional SAR by region. Circles denote the ratio of fishing mortality (F; mean 2010–2012) to the F_MSY reference point for individual bottom dwelling stocks. The black horizontal dashed line indicates F/F_MSY = 1, usually treated as a desirable upper limit on fishing rates by managers. One value of F/F_MSY > 8 for a Mediterranean stock in a region where the regional SAR is 7.93 is excluded for clarity.