In a recent Nature article, Dr Christian NK Anderson of the Scripps Institution of Oceanography and colleagues found that fished populations can fluctuate more than unharvested stocks. While they are unsure as to why, they have ruled out some possible explanations through this study.
The researchers examined three major competing mechanisms for this phenomenon by using 50 years’ worth of records of fished and unfished species from the California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record, established by California sardine fishers after that fishery collapsed in the 1940s.
The three hypotheses for why fished populations are more susceptible to fluctuation are:
• Variable fishing pressure directly increases variability in exploited populations
• Commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly
• Age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates
In California Current fisheries, the authors found no evidence for the first hypothesis, limited evidence for the second, and strong evidence for the third, indicating that increased temporal variability in the population arises from increased instability in dynamics. This finding has implications for resource management, providing as 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.
