Incorporating uncertainty into the estimation of population dynamics for Pacific bluefin tuna using Bayesian state-space models
Date Issued
2008
Date
2008
Author(s)
lee, Hui-Hua
Abstract
This study integrates quantifiable uncertainties into estimation of population dynamics for Pacific bluefin tuna. Uncertainties arise from the state equation (process error) and data link equation (observation error) were constructed within the production models. The values of unknown parameters and variables and their variability were simultaneously estimated by using Bayesian approach, when fitting the models to time series of abundance indices. Uncertainty in risk assessment of projection associated with different functional forms of production models was addressed. As a result, the Pella-Tomlinson models were more uncertain than the Schaefer models in terms of parameter estimation and management measures resulting in different risk levels. However, the population biomass and fishing mortality rates yielded similar trends between them. Since the stock biomass declined below the 97.5% quantiles of biomass at the level that yields the maximum production and the fishing pressure started to increase above the 2.5% quantiles of fishing mortality rate that produces the maximum production in 1970, the stock has been overfished. The future stock biomass was projected from the current year (2005) into the future if the current catch level is taken. The stock maintains its biomass for next 10 years but would still be in the overfished state. If the annul catch maintains at the 20,000 tons level which is lower than maximum production, the risk of collapse is low in the 5-10 years and the risk of being overfished is decreasing as time increases. However, if the annul catch increased to 40,000 tons, the biomass would drop down and the stock eventually collapsed. Because catch is fluctuated without regulation, the routine monitoring of Pacific bluefin tuna is necessary. To avoid this stock incurring the risk of collapse, my suggestion is to control annual catches around 20,000 tons and annual catches should not over than 40,000 tons.
Subjects
Pacific bluefin tuna
Bayesian approach
Uncertainty
Risk assessment
Type
thesis
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