北太平洋劍旗魚之性別-年齡結構模式及資源評估

Sex-specific age-structured model for stock assessment of swordfish (Xiphias gladius) in the North Pacific Ocean

王勝平(Sheng-Ping Wang)

指導教授 ： 孫志陸

國立臺灣大學/理學院/海洋研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.01959

摘要

劍旗魚之成長與性成熟在雌雄間具有顯著差異，然而在過去有關北太平洋劍旗魚之資源評估研究均忽略此一重要因子。有鑑於此，本研究建構 "性別-年齡結構評估模式"，並用以進行北太平洋劍旗魚族群之資源評估。
北太平洋劍旗魚主要為日本遠洋鮪延繩釣漁業及以夏威夷為基地之延繩釣漁業所漁獲，本研究利用此兩種延繩釣漁業之歷年北太平洋劍旗魚漁獲量、釣獲率及體長頻度資料進行模式分析。根據基本模式分析結果，顯示北太平洋劍旗魚於2000年產卵親魚量仍維持在相當高的水準，約為未開發產卵親魚量之70%，總漁業利用率僅為最大持續生產量（MSY）水準下總漁業利用率之12%。在敏感度分析方面，雖然分析結果會因自然死亡率及加入量與親魚量關係式之曲度（steepness）的設定值而產生差異，然而即使是最保守的評估結果也顯示於2000年產卵親魚量仍高於未開發產卵親魚量之50%，總漁業利用率仍低於MSY水準下總漁業利用率之25%。因此，北太平洋劍旗魚資源在目前的漁業利用下仍維持在相對穩定的狀況。根據貝氏後驗分布（Bayesian posterior distribution）進行未來之預測，結果顯示若未來之總漁業利用率維持在目前之水準下，未來產卵親魚量低於未開發產卵親魚量之40%的風險是微乎其微。
本研究亦利用蒙地卡羅模擬法（Monte Carlo simulation）模擬評估資料觀測誤差及部份模式參數預設值誤差對模式估計結果所產生之影響。結果顯示相較於漁獲量及體長頻度之觀測誤差，釣獲率之觀測誤差會對模式估計結果造成較大之影響，而增加體長頻度資料之樣本數可以改善模式估計結果之正確度及精確度。結果亦顯示，若在資源評估時忽略了性別因子，會對產卵親魚量、MSY及漁業利用率之估計值造成相當大的誤差。而目前之產卵親魚量與未開發產卵親魚量及與MSY水準時之產卵親魚量的比值則為模式分析結果中最為穩健的估計量。

關鍵字

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

並列摘要

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.

並列關鍵字

swordfish ； North Pacific ； stock assessment ； sex-specific age-structured model ； simulation ； evaluating model performance

參考文獻

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