2010至2023年臺灣周遭海域太平洋黑鮪漁獲動態之研究

Abstract

太平洋黑鮪（Thunnus orientalis）屬於鯖科鮪屬的大型洄游性魚類，是臺灣近海相當重要的漁業資源之一，除了臺灣之外，太平洋黑鮪的高經濟價值也面臨其他國捕撈，包括日本、南韓、美國及墨西哥，也造成太平洋黑鮪處於過度捕撈的狀態，為了恢復太平洋黑鮪資源，WCPFC與IATTC的保育管理措施開始對太平洋黑鮪限額捕撈，並制定各國的捕撈額度限制。預期此措施將改變太平洋黑鮪群，因此本研究將驗證此措施對於臺灣周邊海域的太平洋黑鮪的影響，研究結果將能提供制定相關漁業管理措施的重要基礎。從捕獲海域圖顯示太平洋黑鮪主要捕獲於臺灣東部、東南部海域、以及東北部釣魚台附近，其他地區雖也有漁獲但是較為零星。臺灣捕獲的太平洋黑鮪從2011的九百多尾，至2020年達四千多尾2023年更增加至將近一萬尾，顯示捕獲量已有明顯增加。本研究以耳石定齡法，Age-Length Key估算近13年來太平洋黑鮪產卵群的年齡組成，結果顯示太平洋黑鮪年齡有年輕化的趨勢，2011年的年齡多數集中在14-18歲，2014、2015年時，分佈出現雙峰的狀態，於6-10歲及16-20歲有兩個峰波，2020-2023年的年齡則多集中於8-12歲之間。體長分佈也有相同的情況發生，線性混合模型的結果顯示太平洋黑鮪的體長與年份有關聯，且太平洋黑鮪所有年齡別體長隨時間下降的趨勢，模型的分析結果也顯示出近幾年間太平洋黑鮪的成長率有下降的趨勢。而在性別比方面，太平洋黑鮪<220 cm多為雌魚；而> 230 cm則以雄性魚隻為主，而臺灣周遭的太平洋黑鮪隨著漁獲年輕化及小型化，雌魚比例也有提升的趨勢。依上述結果推測，因為大幅減少幼魚之捕撈量，確實讓更多幼魚能存活到成魚階段，進而使臺灣附近海域於近5年持續大量出現較為年輕的個體，顯示此項管理措施發揮良好的成效。

Pacific bluefin tuna (Thunnus orientalis) is a large, migratory fish species and an important fishery resource for several countries. Its high economic value has led to intense fishing pressure from various fisheries in Taiwan, Japan, South Korea, the United States, and Mexico. This overfishing prompted the WCPFC and IATTC to implement management strategies. These strategies included quota limits for young Pacific bluefin tuna and catch limits for each country, aiming to rebuild the population. This study aims to investigate the fishery dynamics of Pacific bluefin tuna landed in Taiwan from 2011-2023 to inform fisheries management of this species. The number of Pacific bluefin tuna caught by Taiwanese fisheries gradually increased from about nine hundred individuals in 2011. The catch then rose significantly to about four thousand in 2020 to about ten thousand in 2023, indicating an increase in the spawning population. Age estimation using otoilth annuli and the age-length-key method was employed to estimate the age composition of the Pacific bluefin tuna over the past 13 years. The results indicate that the catch of adult Pacific bluefin tuna has become younger in recent years. In 2011, the population was mostly composed of fish between 14-18 years old. Bimodal distribution emerged in 2014 and 2015. From 2020 to 2023, the fish caught by Taiwanese longliners were mainly consisted of fish aged 8-12 years. The length distribution showed similar results, which gradually decreased in the past 13 years. The linear mixed model indicated that the fork length of Pacific bluefin tuna is related to the year, with the length of all age groups decreasing over times. The analysis of von Bertalanffy growth equation showed that the growth rate of Pacific bluefin tuna also declined in recent years. The fish <220 cm are predominantly female, while the fish > 230 cm are mainly male. As the Pacific bluefin tuna landed in Taiwan has become younger and smaller, there has been an increasing trend in the proportion of female fish. These findings suggest that reduced fishing pressure on young juveniles since 2011 has allowed more juveniles to survive to the adulthood. The continuous presence of the Pacific bluefin tuna near Taiwan waters in the past 5 years indicates the effectiveness of the implemented management measure.