Global ecosystem overfishing: Clear delineation within real limits to production

Abstract

The well-documented value of marine fisheries is threatened by overfishing. Management typically focuses on target populations but lacks effective tools to document or restrain overexploitation of marine ecosystems. Here, we present three indices and accompanying thresholds to detect and delineate ecosystem overfishing (EOF): the Fogarty, Friedland, and Ryther indices. These are based on widely available and readily interpreted catch and satellite data that link fisheries landings to primary production using known limits of trophic transfer efficiency. We propose theoretically and empirically based thresholds for each of those indices; with these criteria, several ecosystems are fished sustainably, but nearly 40 to 50% of tropical and temperate ecosystems exceed even extreme thresholds. Applying these criteria to global fisheries data results in strong evidence for two specific instances of EOF, increases in both pressure on tropical fish and a climate-mediated polar shift. Here, we show that these two patterns represent evidence for global EOF.

Fig. 1. Schematic of stock and EOF.

(A) Schematic of population overfishing. While a population is experiencing overfishing, the abundance and biomass (here as the number of fish icons) and fish size declines over time, along with many other facets related to population and fleet dynamics. (B) Schematic of EOF. Analogous to population overfishing, EOF is the result of continued fishing pressure on multiple populations, leading to sequential depletion across populations in an ecosystem over time.

Fig. 2. The three novel indicators of ecosystem overfishing for major latitudinal bands.

Values of the potential measures to delineate EOF; the Ryther index (A), the Fogarty ratio index (B), and the Friedland ratio index (C) for all of the five latitudinal bands. The solid black line represents the proposed threshold (for the Friedland ratio; data are below the threshold for Ryther and Fogarty indices and thus is not shown). N Temp, northern temperate zone; S Temp, southern temperate zone.

Fig. 3. The three novel indicators of ecosystem overfishing, for example large marine ecosystems in the tropical, temperate, and polar regions.

Values of the potential measures to delineate EOF; the Ryther index (A to C), the Fogarty ratio index (D to F), and the Friedland ratio index (G to H) for representative examples of the 65 LMEs that had an index above one of the noted threshold values (black lines) at some point during the time series. Not all of the 65 LMEs are shown. The solid black line represents the proposed threshold.

Fig. 4. Multiple characterizations of global fishing and fisheries catch demonstrating core criteria of EOF for different regions, with a particular demonstration of polar fishery stasis and increases in tropical fishing.

(A) Total, global marine capture fisheries catch of all taxa over time. Total marine capture fisheries catch of all taxa over time, as integrated into major latitudinal bands; polar (B), temperate (C), and tropical (D). All latitudinal bands have data from both northern and southern hemispheres. (E) The width at each latitudinal band (four examples given) at which total average annual marine capture fisheries catch exceeded 60 t km⁻¹. (F) Total fishing effort for exemplary latitudinal bands over time. (G) Total CPUE by latitudinal band over time. (H) The proportion of global fish catch in latitudinal bands over time. These are presented in approximate widths as degrees of longitude.

Fig. 5. Global patterns in total fisheries catches from over more than 50 years as seen in three example stanzas.

Total average annual marine capture fisheries catch (including estimates of illegal unreported and discards) of all taxa for 1950–1959 (A), 1970–1979 (B), and 2010–2014 (C).