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. 2013 Oct 1;110(40):15943-8.
doi: 10.1073/pnas.1314472110. Epub 2013 Sep 16.

Predicting overfishing and extinction threats in multispecies fisheries

Matthew G Burgess  1 , Stephen Polasky, David Tilman
Affiliations

Predicting overfishing and extinction threats in multispecies fisheries

Matthew G Burgess et al. Proc Natl Acad Sci U S A. .

Abstract

Threats to species from commercial fishing are rarely identified until species have suffered large population declines, by which time remedial actions can have severe economic consequences, such as closure of fisheries. Many of the species most threatened by fishing are caught in multispecies fisheries, which can remain profitable even as populations of some species collapse. Here we show for multispecies fisheries that the biological and socioeconomic conditions that would eventually cause species to be severely depleted or even driven extinct can be identified decades before those species experience high harvest rates or marked population declines. Because fishing effort imposes a common source of mortality on all species in a fishery, the long-term impact of a fishery on a species is predicted by measuring its loss rate relative to that of species that influence the fishery's maximal effort. We tested our approach on eight Pacific tuna and billfish populations, four of which have been identified recently as in decline and threatened with overfishing. The severe depletion of all four populations could have been predicted in the 1950s, using our approach. Our results demonstrate that species threatened by human harvesting can be identified much earlier, providing time for adjustments in harvesting practices before consequences become severe and fishery closures or other socioeconomically disruptive interventions are required to protect species.

Keywords: assessment; early warning; mechanistic; overharvesting; preventative management.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
General theory and simulated example. A–D illustrate the theoretical framework and E provides an illustrated example. Each panel assumes r = 2FMSY for all species, and all but D assume constant vulnerabilities (V). A illustrates how species’ relative vulnerabilities determine relative depletion, which combined with effort determines long-term abundances for populations (open circles). Higher effort pushes the outcome farther down the set of possible abundances determined by species’ relative vulnerabilities (blue, black, and yellow lines). B and C illustrate how relative vulnerabilities and profitability (B) or regulatory constraints (C) (dark red lines) jointly determine long-term species abundances (solid circles). C assumes that the fishery is managed to harvest species 2 at MSY. D illustrates the theoretical relationship between the long-term exploitation rate of species i (Fi*/FMSY) and the average long-term exploitation rate of the key species (formula image), as determined by the long-term Ti value (Ti*), shown for Ti = 2 (blue), Ti = 1 (black), and Ti = 0.5 (yellow). For Ti = 2, species i will be harvested to extinction when the key species is harvested at MSY or overfished. In contrast, for Ti = 0.5, the key species would be harvested to extinction before species i would be overfished. E shows time trends of the eventual threat index (Ti(t)), mortality (Fi(t)/Fi,MSY), and abundance (Ni(t)/NMSY) for the case of a by-catch species (i) caught in two fisheries whose technologies and relative fleet sizes do not change and where the key species in each fishery is harvested at MSY. Extinction of species i is predictable in year 0 whereas extinction causing mortality does not occur until year 26. Growth equations and parameter values for E are provided in Materials and Methods.
Fig. 2.
Fig. 2.
Exploitation histories and estimable T values. A comparison of eventual threat index values [T (3-y geometric mean), red] to the combined fishing mortality rates (UCombined) from longline, purse-seine, and pole-and-line fisheries as a fraction of the mortality rate producing MSY (UMSY), (UCombined/UMSY, purple), for each population. Levels indicating high extinction threats (T, UCombined/UMSY = 2) (solid black lines), high overfishing threats (T, UCombined/UMSY = 1) (dashed black lines), and low overfishing threats (T = 0.5) (dotted black lines) are shown.
Fig. 3.
Fig. 3.
Gear-specific threats. Fishing gear-specific estimates of threats (3-y geometric mean T values summed only over fisheries within each gear type) are shown. T values of 2 (solid line) (high extinction threat), 1 (dashed line) (high overfishing threat), and 0.5 (dotted line) (low overfishing threat) are highlighted. The shift in longline fishing toward deeper sets beginning in the 1970s led to a reduction in the threat caused by the shallow-set longline fishery to all populations and introduced a threat from the deep-set fishery to striped marlin populations. Purse-seine fisheries have recently begun to pose a threat to bigeye and yellowfin tuna.
Fig. 4.
Fig. 4.
Assessment histories and earliest identifiable threats. Shown is a comparison of when threats could have been identified by estimating T (T > 1: red triangles) vs. when populations were first assessed as overfished (N < NMSY) or subject to overfishing (F > FMSY) in stock assessments (purple diamonds). The populations’ abundance trends are shown (black curves), each scaled to its maximum value in the series. Estimates of NMSY (dashed lines) and dates when the eventual threat index no longer would have predicted an overfishing threat (T < 1) (green triangles) are also shown.
See this image and copyright information in PMC

Comment in

  • Forecasting fisheries collapse.
    Gaines SD, Costello C. Gaines SD, et al. Proc Natl Acad Sci U S A. 2013 Oct 1;110(40):15859-60. doi: 10.1073/pnas.1315109110. Epub 2013 Sep 24. Proc Natl Acad Sci U S A. 2013. PMID: 24065819 Free PMC article. No abstract available.

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