1. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA
2. Center for Ecological Research, Kyoto University, Hirano, 2-509-3, Otsu, 520-2113, Japan
3. Institute of Oceanography, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei, 10617, Taiwan
4. Institute of Marine Environmental Chemistry and Ecology, National Taiwan Ocean University, 2, Pei-Ning Road, Keelung, 20224, Taiwan
5. Southwest Fisheries Science Center, National Marine Fisheries Service, 8604 La Jolla Shores Drive, La Jolla, California 92037, USA
6. Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way NE, Seattle, Washington 98115, USA
7. Division of Biology, Faculty of Natural Sciences, Imperial College London, RSM Building, South Kensington Campus, London SW7 2AZ, UK
8. 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).

Abstract

It is now clear that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. Here we distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. We find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does not arise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics. This finding has implications for resource management as an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels.