Spatial and temporal variability in length of Japanese glass eel, Anguilla japonica

日本鰻為東北亞國家的重要養殖魚類，但所需之鰻苗皆來自於野外採捕。由於近年來日本鰻的資源下降劇烈，導致鰻苗產量減少，使得鰻魚養殖業面臨嚴重困境。因此，瞭解日本鰻加入動態，可提供鰻苗資源的利用、保育與管理之參考。本研究目的為探討日本鰻鰻苗到達河口時的體長之時空變化。本研究所使用的標本是自1983年起28年間從台灣、中國大陸、北韓及日本所收集到的19,059尾日本鰻玻璃鰻樣本。測量其體長全長與色素階段後，分析結果發現日本鰻玻璃鰻的全長從VA開始逐漸減少至VIA2為止，從VIA3階段則開始增加。雖然日本鰻玻璃鰻之平均體長在月份間變化或是年間變化很大，但長期趨勢卻不明顯。日本鰻玻璃鰻平均體長的年間波動變化似乎與聖嬰及反聖嬰現象有關聯。一般而言，日本鰻玻璃鰻的全長在聖嬰現象時大多位在趨勢線的上方，在反聖嬰現象時則大多位在趨勢線的下方。換言之，反聖嬰年的日本鰻玻璃鰻全長較其他年分為短，可能與柳葉鰻期較短有關。而日本鰻玻璃鰻的空間分析結果發現，全長隨緯度增高而增加，可能與北方鰻苗之柳葉鰻期較長有關。總結來說，日本鰻資源量在過去30年雖有顯著下降，並受全球暖化現象影響，但並無影響鰻苗之體型趨勢。而聖嬰現象與輸送速度會影響柳葉鰻期長短乃至鰻苗到達河口時的體型。The Japanese eel Anguilla japonica is an important aquaculture species in Northeast Asia countries, however, all of the eel fry needed for aquaculture depends on the wild catch. That the Japanese eel stock has declined rapidly in recent decades has resulted in the decreasing glass eel catch and the eel aquaculture industry is facing serious problems. Therefore, understanding the recruitment dynamics of the Japanese eel can propose references for the utilization, preservation and management of the glass eels. The aim of this study is to investigate the spatial and temporal variation of the total length (TL) of the estuarine Japanese glass eels. This study uses a total of 19,059 Japanese glass eel specimens which had been collected from Taiwan, mainland China, North Korea and Japan for 28 years since 1983. After measuring TL and observing pigmentation stages, the results revealed that mean TL of the glass eels decreases gradually from the stage VA to stage VIA2, but from stage VIA3, there is an abruptly increase. Although both the monthly and annual mean TL varies considerably, the long term trend is not significant. The fluctuation of mean TL oscillation seems has some relationship with El Niño and La Niña events. In general, the TL of Japanese glass eel during El Niño period is above the regression line. Besides, the TL of Japanese glass eel during the La Niña period is below the regression line. In other words, the mean TL of Japanese glass eel in La Niña year was shorter than the other years; this may be due to the shorter larval duration. The spatial analysis of TL of Japanese glass eel reveals that the TL of most Japanese glass eels increases with latitude and this phenomenon may have some relationship with longer larval duration for northern glass eels. In conclusion, the resource of Japanese eel has been decreasing significantly for past 30 years and suffered from global warming. However, there is no significant impact on the TL trend of Japanese glass eels. The ENSO events and the transportation velocity will influence the larval duration, thus influence the size of the estuarine glass eels