大西洋黑皮旗魚之豐度指標標準化及資源評估

Abstract

黑皮旗魚為大洋性洄游魚種，主要分布熱帶及亞熱帶海域。在大西洋，黑皮旗魚為鮪延繩釣漁業混獲魚種，主要漁獲國家為日本及台灣，近年來迦納流刺網漁業漁獲量明顯增加。單位努力漁獲量(catch per unit effort, CPUE)可作為資源相對豐度指標，但由於資源時空分布及漁具演進皆會對漁業的捕獲能力造成影響，故在利用CPUE當作資源豐度指標前須將其標準化。本研究以泛線性模式(generalized linear models, GLM)進行黑皮旗魚CPUE標準化，分析日本、台灣、巴西鮪延繩釣漁業以及迦納流刺網漁業CPUE之變動。GLM模式考慮因子包含年別、月別及漁區別主效應，及月與漁區之交感效應。結果顯示黑皮旗魚的標準化CPUE大致呈現逐漸下降的趨勢。除此四種漁業外，本研究外加蒐集美國與委內瑞拉鮪延繩釣漁業、委內瑞拉流刺網漁業以及美國休閒漁業的標準化CPUE資料，進一步以非平衡生產量模式評估黑皮旗魚的資源狀況，結果顯示大西洋黑皮旗魚最大可持續生產量(maximum sustainable yield, MSY)為2538公噸，目前(2009年)相對資源量(B2010/BMSY)及相對漁獲死亡率(F2009/FMSY)分別為0.745及1.508，顯示大西洋黑皮旗魚資源已呈現過漁狀態，建議應減少黑皮旗魚的總容許漁獲量(total allowable catch, TAC)至2000公噸，以使資源回復至接近MSY的水準。

Blue marlin is a highly migratory species mainly distributed in tropical and subtropical waters of the world. In the Atlantic, blue marlin is a by-catch of longline fisheries, mostly caught by Japanese and Taiwanese longliners. The catch from gillnet fishery of Ghana increased in recent years. Catch per unit effort (CPUE) is usually used as relative abundance index in stock assessments. However, catchability is affected by spatio-temporal distribution of fish population as well as the evolution of fishing gear. Consequently, CPUE should be standardized to represent as an abundance index. In this study, the generalized linear models (GLM) were applied to standardize CPUE data of blue marlin for the longline fisheries of Japan, Taiwan and Brazil, and gillnet fishery of Ghana. The factors considered in GLM include year, month, fishing area, and the interaction term between month and fishing area. The standardized CPUE generally showed a gradually declining trend. In addition to these four standardized CPUE series, we also collected standardized CPUE series from ICCAT for longline fisheries of USA and Venezuela, gillnet fishery of Venezuela, and recreational fishery of USA. The non-equilibrium surplus production model ASPIC was then applied to assess the stock status of the Atlantic blue marlin. Results showed that the maximum sustainable yield (MSY) was estimated at 2537 tons, and B2010/BMSY and F2009/FMSY were estimated at 0.745 and 1.508, respectively, indicating that the blue marlin stock is in an overfishing and overfished status. The results suggest that the total allowable catch (TAC) of blue marlin should be reduced to 2000 tons in order to recover the stock to near the level corresponding to MSY.