1. Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0202, USA
2. Southwest Fisheries Science Center, National Marine Fisheries Service, 8604 La Jolla Shores Drive, La Jolla, California 92037-1508, USA
3. Division of Biology, Faculty of Natural Sciences, Imperial College London, RSM Building, South Kensington campus, London SW7 2AZ, UK
4. Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK

Correspondence to: George Sugihara¹ Correspondence and requests for materials should be addressed to G.S. (Email: gsugihara@ucsd.edu).

The separation of the effects of environmental variability from the impacts of fishing has been elusive, but is essential for sound fisheries management^{1, 2, 3, 4, 5, 6, 7}. We distinguish environmental effects from fishing effects by comparing the temporal variability of exploited versus unexploited fish stocks living in the same environments. Using the unique suite of 50-year-long larval fish surveys from the California Cooperative Oceanic Fisheries Investigations⁴ we analyse fishing as a treatment effect in a long-term ecological experiment. Here we present evidence from the marine environment that exploited species exhibit higher temporal variability in abundance than unexploited species. This remains true after accounting for life-history effects, abundance, ecological traits and phylogeny. The increased variability of exploited populations is probably caused by fishery-induced truncation of the age structure, which reduces the capacity of populations to buffer environmental events^{1, 5, 8, 9}. Therefore, to avoid collapse, fisheries must be managed not only to sustain the total viable biomass but also to prevent the significant truncation of age structure^{1, 5, 8, 9}. The double jeopardy of fishing to potentially deplete stock sizes and, more immediately, to amplify the peaks and valleys of population variability⁷, calls for a precautionary management approach^{10, 11}.

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