Global patterns of marine mammal, seabird, and sea turtle bycatch reveal taxa-specific and cumulative megafauna hotspots

Abstract

Recent research on ocean health has found large predator abundance to be a key element of ocean condition. Fisheries can impact large predator abundance directly through targeted capture and indirectly through incidental capture of nontarget species or bycatch. However, measures of the global nature of bycatch are lacking for air-breathing megafauna. We fill this knowledge gap and present a synoptic global assessment of the distribution and intensity of bycatch of seabirds, marine mammals, and sea turtles based on empirical data from the three most commonly used types of fishing gears worldwide. We identify taxa-specific hotspots of bycatch intensity and find evidence of cumulative impacts across fishing fleets and gears. This global map of bycatch illustrates where data are particularly scarce--in coastal and small-scale fisheries and ocean regions that support developed industrial fisheries and millions of small-scale fishers--and identifies fishing areas where, given the evidence of cumulative hotspots across gear and taxa, traditional species or gear-specific bycatch management and mitigation efforts may be necessary but not sufficient. Given the global distribution of bycatch and the mitigation success achieved by some fleets, the reduction of air-breathing megafauna bycatch is both an urgent and achievable conservation priority.

Keywords: fisheries bycatch; gillnets; longlines; trawls; trophic downgrading.

Fig. 1.

Bycatch intensity for (A) seabirds, (B) marine mammals, and (C) sea turtles for records from 1990 to 2008. Three gear categories (gillnet, longline, and trawl) are represented by separate symbols, and symbol size is scaled to reflect the proportional amount of observed fishing effort for each record. Bycatch data records that did not have reported observed effort are shown in Fig. S2.

Fig. 2.

Geographic regions used for analysis: 1, northeast Pacific; 2, southeast Pacific; 3, eastern Tropical Pacific; 4, northwest Atlantic/Caribbean; 5, northeast Atlantic; 6, southwest Atlantic; 7, eastern Atlantic; 8, Mediterranean; 9, western Indian Ocean; 10, eastern Indian Ocean; 11, northwest Pacific; 12, southwest Pacific; and 13, southern Ocean/Antarctica.

Fig. 3.

Calculated cumulative bycatch intensity (Eq. 1) for all taxonomic groups and gear types. Bycatch intensity values were used to generate a raster surface from an inverse weighted distance function for polygons with available data. Interpolated values are displayed using the same color ramp as in Fig. 1. Numbers represent the number of data records represented within each polygon.