title: Why fishing magnifies fluctuations in fish abundance.
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.
authors.affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA.
authors.forename: Christian N K
authors.initials: CN
authors.lastname: Anderson
authors.forename: Chih-hao
authors.initials: CH
authors.lastname: Hsieh
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Sandin
authors.forename: Roger
authors.initials: R
authors.lastname: Hewitt
authors.forename: Anne
authors.initials: A
authors.lastname: Hollowed
authors.forename: John
authors.initials: J
authors.lastname: Beddington
authors.forename: Robert M
authors.initials: RM
authors.lastname: May
authors.forename: George
authors.initials: G
authors.lastname: Sugihara
mesh_terms: D017677:Age Distribution; D000367:Age Factors; D000818:Animals; D018533:Biomass; D049628:Body Size; D002140:California; D017753:Ecosystem; D005398:Fisheries; D005399:Fishes / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D007814:Larva / Q000254:growth & development / Q000502:physiology; D008954:Models, Biological; D011157:Population Dynamics; D016019:Survival Analysis