A study finds that human activity has led to a severe decrease in Caribbean shark abundance.
Image credit: Sean Mattson (International Center for Tropical Agriculture, Cali, Colombia).
Edited by Peter M. Kareiva, The Nature Conservancy, Seattle, WA, and approved March 2, 2015 (received for review November 18, 2014)
Significance
Forage fish provide substantial benefits to both humans and ocean food webs, but these benefits may be in conflict unless there are effective policies governing human activities, such as fishing. Collapses of forage fish induce widespread ecological effects on dependent predators, but attributing collapses to fishing has been difficult because of natural fluctuations of these stocks. We implicate fishing in forage fish stock collapses by showing that high fishing rates are maintained when stock productivity is in rapid decline. As a consequence, the magnitude and frequency but not duration of stock collapses are far greater than expected from natural fluctuations. Risk-based management policies would provide substantial ecological benefits with little effect on fishery catches.
Abstract
Forage fish support the largest fisheries in the world but also play key roles in marine food webs by transferring energy from plankton to upper trophic-level predators, such as large fish, seabirds, and marine mammals. Fishing can, thereby, have far reaching consequences on marine food webs unless safeguards are in place to avoid depleting forage fish to dangerously low levels, where dependent predators are most vulnerable. However, disentangling the contributions of fishing vs. natural processes on population dynamics has been difficult because of the sensitivity of these stocks to environmental conditions. Here, we overcome this difficulty by collating population time series for forage fish populations that account for nearly two-thirds of global catch of forage fish to identify the fingerprint of fisheries on their population dynamics. Forage fish population collapses shared a set of common and unique characteristics: high fishing pressure for several years before collapse, a sharp drop in natural population productivity, and a lagged response to reduce fishing pressure. Lagged response to natural productivity declines can sharply amplify the magnitude of naturally occurring population fluctuations. Finally, we show that the magnitude and frequency of collapses are greater than expected from natural productivity characteristics and therefore, likely attributed to fishing. The durations of collapses, however, were not different from those expected based on natural productivity shifts. A risk-based management scheme that reduces fishing when populations become scarce would protect forage fish and their predators from collapse with little effect on long-term average catches.
Footnotes
- ↵1To whom correspondence should be addressed. Email: essing{at}uw.edu.
Author contributions: T.E.E. designed research; T.E.E., P.E.M., H.E.F., E.E.H., L.E.K., K.L.O., M.C.S., and C.C.S. performed research; T.E.E. and P.E.M. analyzed data; and T.E.E., P.E.M., H.E.F., E.E.H., L.E.K., K.L.O., M.C.S., and C.C.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The data reported in this paper and R codes are available at https://github.com/tessington/PNASForageFish.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1422020112/-/DCSupplemental.
Freely available online through the PNAS open access option.
Fishing amplifies forage fish population collapses
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