Evaluation of the Status and Risk of Over Exploitation of the Pacific Billfish Stocks Considering Non-Stationary Population Processes
December 14, 2019
Here we develop a modelling framework to explore fish populations that could exhibit non-stationary behaviour and then evaluate the consequences on the future risk of overexploitation for three Pacific billfish stocks.
Fish population processes could exhibit non-stationary behaviour as a stochastic biological process with temporal autocorrelation that may be influenced by environmental changes.
Here we developed a Bayesian autoregressive state-space surplus production modelling framework to explore potential non-stationarity in population processes.
We then evaluated the consequence of non-stationary population processes on the future risk of overexploitation for three Pacific billfish stocks (striped marlin, Kajikia audax; blue marlin, Makaira nigricans; and swordfish Xiphias gladius) that are formally assessed on a regular basis by a Regional Fisheries Management Organization in the Pacific Ocean.
The results showed evidence of non-stationary population processes for Western and Central North Pacific Ocean (WCNPO) striped marlin, and to a lesser extent, Pacific blue marlin and WCNPO swordfish.
Trends in the theoretical maximum sustainable yield and intrinsic growth rate were observed as oscillating regimes for swordfish, and as long-term directional changes for striped marlin.
The non-stationary population processes did not strongly influence the forecasted biomass trend at the current catch level for any of the three stocks.
However, the future risk of overexploitation (Prob[B < BMSY]) was sensitive to changes in the population processes for striped marlin (increased the risk by 20%).
This work illustrates that the inclusion of non-stationary population processes could impose challenges for developing a stock rebuilding plan and provides a framework to account for non-stationary population processes for the billfish stocks in the Pacific Ocean.
Chang YJ, Winker H, Sculley M, Hsu J. 2019. Evaluation of the Status and Risk of Over Exploitation of the Pacific Billfish Stocks Considering Non-Stationary Population Processes. Deep Sea Research Part II: Topical Studies in Oceanography, 104707. https://doi.org/10.1016/j.dsr2.2019.104707.