Sex-specific age-structured model for stock assessment of swordfish (Xiphias gladius) in the North Pacific Ocean
Date Issued
2004
Date
2004
Author(s)
Wang, Sheng-Ping
DOI
en-US
Abstract
Swordfish are sexually dimorphic. However, previous assessments of the status of swordfish in the North Pacific Ocean have ignored this. A sex-specific age-structured assessment model was therefore constructed for the stock of swordfish in the North Pacific Ocean.
This model was fitted to the catch, catch-rate and length-frequency data for the Japanese high-seas and Hawaii-based longline fisheries, as these fleets take the bulk of the catch of swordfish in the North Pacific Ocean. The results of this assessment indicate that the spawning stock biomass in 2000 remains at a high (~70%) percentage of its unfished level and that the fleet-aggregated fishing intensity in 2000 was only about 12% of that at which MSY is achieved. These results pertain to a base-case analysis. Although the results of the assessment model are sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship, even the most pessimistic results of the sensitivity analyses conducted indicate that the spawning stock biomass in 2000 was larger than half of its unfished level and that the fleet-aggregated fishing intensity in 2000 was less than 25% of that at which MSY is achieved. Therefore, the swordfish stock in the North Pacific Ocean appears to be relatively stable at the current level of exploitation. Forward projections based on samples from a Bayesian posterior distribution indicate that there is negligible risk of the stock dropping below 40% of the unfished spawning stock biomass if fleet-aggregated fishing intensity remains at the current level.
The performance of the method of stock assessment applied in this study has not been evaluated using Monte Carlo simulation. Therefore, Monte Carlo simulation is used to evaluate the impact of observation errors, and errors when setting the values for some of the parameters of the model that cannot be estimated from the data available for assessment purposes. The observation error associated with the catch-rate indices has a larger impact on estimation ability than errors when measuring catches and when sampling catches for length-frequency. Improved performance, in terms of both the accuracy and precision of the estimates of quantities of management interest, can be obtained by increasing the length-frequency sample sizes. The estimates of spawning stock biomass, MSY, and (to a lesser extent) fishing intensity are substantially more biased when the assessment ignores sexual dimorphism. The ratios of current to unfished spawning stock biomass and to the spawning biomass corresponding to MSY were found to be the quantities estimated most robustly of those considered.
Subjects
性別-年齡結構模式
劍旗魚
模式評估
資源評估
北太平洋
模擬
swordfish
North Pacific
stock assessment
sex-specific age-structured model
simulation
evaluating model performance
SDGs
Type
thesis
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