臺灣近海黑鮪族群變異與耳石抽樣策略之應用

Abstract

太平洋黑鮪 （Thunnus orientalis） 為臺灣近海鮪延繩釣漁業的經濟物種之一，其主要產卵場位於西北太平洋菲律賓海與琉球群島海域及日本海。半個世紀以來，因資源過度開發造成太平洋黑鮪族群量嚴重下降，因此有必要加強基礎研究，以利漁業管理。藉由分析耳石穩定性氧同位素值時序列變化發現臺灣漁獲的太平洋黑鮪大多孵化於西北太平洋海域（85%），而孵化於日本海的個體所佔的比例較少（15%），代表太平洋黑鮪傾向返回到出生的海域進行產卵。分析今年的資料發現，臺灣捕獲的太平洋黑鮪大多以雄魚的比例較高，平均體長的變化由2013至2015年中小型魚（小於200公分）的比例開始增加，使2013-2015年平均體長下降，但2016年的小型產卵個體有減少趨勢。再者根據2010-2016年漁獲量的分布，發現於北緯24度以南的太平洋黑鮪體長顯著大於北緯24度以北之太平洋黑鮪體長。2015年於東港及南方澳漁港採集827組黑鮪耳石進行年齡判讀，將年齡所對應的尾叉長建立Age-Length Key，估算2015年的太平洋黑鮪的年齡組成。同樣再利用2011-2015年共收集1719組耳石資料以此估計2002-2015年臺灣於西北太平洋海域所捕獲太平洋黑鮪之年齡組成。結果顯示，2002-2008年平均年齡為11-12歲。2009-2012年平均年齡增加到16-17歲。2013-2015年，6-10歲的個體開始增加並逐漸加入該漁場。由於採集大量耳石需投入非常多的經費及人力。因此透過分析三種耳石採樣方法的效益，分別為Random Otolith Sampling（ROS）, Equal Otolith Sampling（EOS）和Reweighting Sampling （REW），並且以耳石樣本數100, 200, 400, 500和1000個進行模擬，用以評估最小有效耳石樣本數。結果顯示三種方法皆隨樣本增加，所評估出的年齡組成精準度皆增加，其中以REW用於年齡組成估計結果之精準度最佳，當樣本數超過500個之後，能減少的不確定性非常的少，因此500個耳石樣本數為最適合的樣本數，可做為未來的採樣參考依據。

Pacific bluefin tuna (Thunnus orientalis, PBF), a high-valued species, is heavily exploited and its biomass reached the historically low level. The natal origin of each PBF can be traced and determined by otolith stable isotopic compositions. Totally, 26 otoliths were analyzed and the result suggested most PBF collected in Taiwan were hatched in the northwestern Pacific Ocean (85%) with a small portion (15%) hatched in the Japan Sea. This study examined the demographic changes of the PBF in 2002 to 2016 and the outcomes may improve the management of this species. PBF smaller than 200 cm were rarely found in the tropical areas(south of 24°N) and the mean sizes of the PBF caught in tropical areas were significantly larger than those caught in the subtropical areas (north of 24°N, t-test , p < 0.001). These results suggested that the choice of spawning latitudes of the PBF was influenced by the size of spawning adults. The sex ratio of smaller (smaller than 200 cm) and intermediate size classes (200-230 cm) was female dominated (50-75%) while larger fish (larger than 240 cm) were male dominated (62-100%). A classical Age-Length Key (ALK) based on the ages and lengths of 892 fish were applied to the length frequencies of the catches in the northwestern Pacific spawning grounds to estimate the annual age compositions of the PBF. Ages of the spawners concentrated in 7-20 years old (range = 4-28 years), corresponding to fork lengths between 200 and 240 cm. In 2013 to 2015, 2005-2009 year-cohorts (age 6-10) increased in relative abundance while the strong 1994 and 1996 year-cohorts decreased because they were near the end of their life span. We further evaluated the efficiency of three otolith sampling methods: the random otolith sampling method (ROS), the equal otolith sampling method (EOS) and the reweighting otolith sampling (REW) by simulating approach using different sample size. The uncertainty reduced as the sample size increased for all three methods, however, the REW provides higher efficiency than the ROS and EOS methods. Very little uncertainty can be reduced when the sample size increase to 500, which might be the minimum effective otolith sample sizes. The REW methods and 500 otoliths can be regarded as a guild line for the future study and management.