應用年齡結構生產量模式進行南大西洋劍旗魚之資源評估

Abstract

大西洋鮪類保育委員會（ICCAT）針對南大西洋劍旗魚進行資源評估時，通常是使用剩餘生產量模式（ASPIC)來評估該系群的各個生物參考點(Biological Reference point, BRP)，但是年級群間的成長和死亡並無法在這單一的生產量關係式中被適當的解釋。有鑑於此，本研究使用可同時將各年齡之成長、成熟與漁具選擇性納為考量因子，以解釋年級群間的成長和死亡的「年齡結構生產量模式(Age-structured production model, ASPM)」以進行南大西洋劍旗魚族群之資源評估。 南大西洋劍旗魚主要為日本、台灣、西班牙、巴西與烏拉圭之延繩釣漁業 所漁獲，本研究利用上述國家延繩釣漁業之歷年南大西洋劍旗魚之CPUE與南大西洋劍旗魚之總漁獲量資料進行模式分析。基本模式分析結果顯示南大西洋劍旗魚F2008/FMSY為0.89，SSB2008/SSBMSY略小於1（0.91），表示南大西洋劍旗魚曾經處於過度開發情形，目前漁獲死亡率有緩和的現象。若將總許可漁獲量設定為11,000公噸或以下未來十五年資源狀況會慢慢趨於穩定或回復。 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。結果顯示自然死亡率，CPUE及漁獲量誤差會對模式估計結果產生顯著的影響，未來應加強這方面研究，以進一步獲得更為正確的資源評估結果。

Traditionally ICCAT uses the surplus production model (e.g., Schaefer 1957) to calculate the biological reference points when performing stock assessment on the southern Atlantic swordfish stock, but the growth and mortality between the age classes cannot be explained by this single production function. In this study, a age-structured production model (ASPM) which can include the information of selectivity, weight-at-age, maturity-at-age and natural mortality information was used for the stock assessment of swordfish in the South Atlantic Ocean. The ASPM model was fitted to the catch-rate data for the Japanese, Taiwan, Spain, Brazil and Uruguay longline fisheries, as these fleets take the bulk of the catch of swordfish in the Sorth Atlantic Ocean. The results of this study showed that the Southern Atlantic swordfish stock’s was slightly overfished by F2008/FMSY equal to 0.89 and SSB2008/SSBMSY was slightly smaller than 1（0.91）. A total allowable catch of 11,000 tons or <11,000 tons was suggested to recover stock status for the projected fifteen years. Monte Carlo simulation was used to evaluate the impact of errors in input parameters on the quantities of management interest. The sensitivity analysis showed that errors in natural mortality, CPUE, and catch have the substantial effect on the stock status evaluations. Future research on these fields is highlight and should be enhanced to reduce the uncertainty of the stock assessment.