西北太平洋地區兩百萬年來大黃魚（Larimichthys crocea）的年齡結構、成長及棲地改變

Abstract

大黃魚（Larimichthys crocea）在西北太平洋地區已遭到過度捕撈近半世紀，野外的族群近乎消失。由於過度漁撈則可能造成大黃魚族群生活史改變，包括提早性成熟、生長速率改變等等，因此重建大黃魚群聚構造並研究其成長模式是保育及管理此魚種的重要課題。本研究使用現生、考古遺址、更新世時期的耳石樣本，比較大黃魚在不同漁撈壓力下的年齡成長模式、年齡結構與棲地利用。本研究以大黃魚耳石判定魚齡，並且以耳石穩定氧同位素（δ18O）驗證年齡判定的可靠性。結果顯示，相較於考古遺址及更新世時期，現生的大黃魚具有最高的成長速率（k 值）與最小的極限體長，且平均年齡最低。大黃魚年齡結構的改變可能是由於族群中大型個體的大量縮減，造成剩餘的個體趨向較小的體長，同時為了增加入添量維持族群，因而具有較快的初期成長速率，但此現象也可能是競爭減少、資源增加所導致。考古遺址與更新世時期的樣本，其夏季平均最低穩定氧同位素值皆低於-3‰，已超過最高海水溫度所能造成的最低δ18O 值（約為-2.5‰），此結果顯示考古遺址與更新世時期之大黃魚於夏季會棲息於河口等鹽度較低的環境。此外，於遺址發現的大黃魚耳石碳同位素（δ13O）值與氧同位素值呈現高度正線性關係性，說明當時的大黃魚可能位於潟湖或是較為封閉的河口環境，而現今的大黃魚，其棲地利用卻僅限於海洋，此現象顯示過渡漁撈不僅大幅減少大黃魚的族群量，也改變了大黃魚生活史特性。

Overfishing can lead to significant changes in fish life history, including growth rate and habitat usage. The large yellow croakers (Larimichthys crocea) in the Northwest Pacific have been severely exploited to the brink of ecological extinction for nearly half a century. Reconstructing the baseline of growth pattern and habitat use is crucial to biological conservation and fishery management. This study analyzed the otoliths of L. crocea collected from the early Pleistocene fossil site in Nibu, Chia-Yi County, Tainan Science Park archaeological sites, and fishing ports, each representing three different geological time periods (early Pleistocene 1.90-1.35 Ma, archaeological midden 5000 BP, modern) that correspond to different levels of human disturbance. We aged the otoliths and verified our age analysis using stable isotope (δ18O) analysis. The age classes of modern large yellow croaker are younger (0.9 years) compared to the samples collected from fossil and archaeological sites (5.3 and 2.8 years, respectively). Our findings indicate that the modern otoliths exhibit a higher growth factor (k = 1.16) than the archaeological (k = 0.30) and early Pleistocene otoliths (k = 0.32). The average summer minimum oxygen isotope value of the modern otoliths is higher (-2.2‰) and agrees with the condition for a complete life cycle in the oceanic environment, while the archaeological and early Pleistocene otoliths have the values lower than -3‰. These results suggest that the large yellow croakers living in prehistorical times may inhabit in the estuaries or lagoons. Our study shows that overfishing can act as a selective pressure, resulting in increased growth rates and shifts in habitat use.