以自然標記研究台灣沿岸水域烏魚的族群結構及洄游環境史

Abstract

本研究欲瞭解台灣沿岸水域烏魚的族群結構及其洄游環境史。研究材料是2005 ~ 2008年採自下列地點：1)每年冬至出現在台灣東北與西南海域的生殖洄游群；2)淡水河中游；和3)高屏溪河口域。此外，為了證明年輪，也從雲林縣的兩個養殖場內採集已知年齡的烏魚。所有烏魚樣本皆進行外部形態測量，並取出耳石、鱗片、肌肉組織和生殖腺組織，分別進行年齡判定、成長率估計、外部形態之多變値解析，並利用微衛星DNA和雷射偶合電漿質譜儀瞭解其族群結構及洄游環境史。 結果顯示：1)鱗片與耳石上的年輪與實際年齡一致。2)台灣東北部海域的烏魚在12月中旬到達性成熟，而且體長、生殖腺指數、卵徑以及生殖細胞發育狀態皆與台灣西南海域的產卵洄游群類似，此結果顯示東北海域也是烏魚的產卵場。3)微衛星DNA的分析結果顯示台灣地區的烏魚可以分為三個族群(P1, P2, P3)。東北與西南海域的三個族群比例分別為94% (P1)，1% (P2)以及5% (P3)；高屏溪三個族群的比例分別為8% (P1)，30% (P2)和62% (P3)，其中P1族群就是以往所認知的冬至洄游群。成長率和外部形態在這三個族群之間則皆無顯著差異。4)耳石邊緣的Sr/Ca比值在海水、河口和淡水之間呈現顯著差異(ANOVA, p < 0.05)。當烏魚棲息於淡水環境時，其耳石平均Sr/Ca比值小於3.2 × 10-3；當鹽度大於33 PSU，耳石平均Sr/Ca比值則大於6.0 × 10-3。5)依據耳石核心之Sr/Ca比值可將烏魚分成三種類型。Type I 之Sr/Ca比值高於6 × 10-3，表示初期發育階段是在海水環境；Type II之Sr/Ca比值介於3.2 × 10-3 ~ 6 × 10-3之間，表示初期發育階段是在半淡鹹水環境；Type III的Sr/Ca比值低於3.2 × 10-3，表示這類烏魚的初期發育階段可能是在淡水環境。6)耳石鍶鈣比的時序列變化則顯示烏魚洄游環境史的多樣性，但是三個族群的洄游環境史並沒有明顯的區隔。綜合以上結果，發現除了每年冬季洄游至台灣西南與東北海域產卵的溫帶洄游型的烏魚群(P1)外，還有亞熱帶洄游型(P2)和亞熱帶混合型(P3)兩個族群。這三個烏魚群的族群遺傳結構雖然不一樣，但其外部形態及生活史特徵卻沒有顯著差異，表示不同烏魚族群是共存在同一水域中。

To understand the population structure and migratory environmental history of flathead mullet Mugil cephalus in the costal waters of Taiwan, the allele frequency of 9 microsatellites loci of the mullet was analyzed and the elemental composition in otolith of the mullet was analyzed by LA-ICP-MS. The specimens of the mullet were collected from the spawning grounds in Northeastern (NE) and Southwestern (SW) waters of Taiwan, Tanshui River and Kaoping River estuary in 2005 ~ 2008, respectively. In addition, to validate the annulus in the scale and otolith of the mullet, the known-age cultured mullet were collected from two fish farms in Yunlin prefecture, western Taiwan. After collection, the morphometric characteristics including length and weight of the mullet were measured, and the scale, otolith, muscle tissue and gonad tissue were collected. The results revealed that: 1) Annuli in both scale and otolith of the cultured mullet were consistent with their true ages. 2) The mullet in the NE waters matured and spawned in mid-December as indicated by gonadosomatic index, fecundity, oocyte diameter frequency, and the maturation stages of gonad cell, which were all similar to those of the mullet that migrated to the SW waters at the same time. This indicated that the NE waters was also one of the spawning grounds of M. cephalus in the coastal waters of Taiwan. 3) The microsatellites indicated that there are three different populations (P1, P2, and P3) co-existed in the coastal waters of Taiwan. The contributions of the three populations were different among areas, in both spawning grounds, they were P1: 94%, P2: 1% and P3: 5%. The population genetic structure of the mullet in Kaoping River was different from those in NE and SW waters, which were mainly composed of P2 (30%) and P3 (62%). The growth rate and morphological variables were not significantly different among three populations. This suggested that growth rate and morphological variables can not be used as an indicator to distinguish the population units of M. cephalus in the coastal waters of Taiwan. 4) The Sr/Ca ratios in the otolith edges were significantly different for the mullet collected among sea water, estuary and freshwater (ANOVA, p < 0.05). The mean Sr/Ca ratios in otolith are lower than 3.2 × 10-3 when mullet lived in freshwater environment, larger than 6.0 × 10-3 when mullet lived in sea water environment. 5) Based on the Sr/Ca ratio in the otolith core region, the mullet was divided into three types: Type I, the mean Sr/Ca ratio was greater than 6.0 × 10-3, indicating that the mullet at early stage lived in sea water. Type II, the Sr/Ca ratio between 3.2 × 10-3 ~ 6.0 × 10-3, indicating that mullet at early stage lived in brackish water. Type III, the Sr/Ca ratio was less than 3.2 × 10-3, indicating that the mullet at early stage lived in freshwater environment. 6) The temporal change of Sr/Ca ratios in the otolith indicated that the migratory environmental histories of the mullet are similar among the 3 populations. In conclusion, the population genetic structures analyses indicated that M. cephalus in the coastal waters of Taiwan were not only the well-known migratory population which spawned in NE and SW waters of Taiwan, but also there were two more populations to be found in the Kaoping River estuary. Although these populations seemed to have different genetic structures but they can not be distinguished by morphology and growth rate. The difference in population genetic structure and non-differences in morphologic characteristics and life history strategies might be resulting from coexistence because of inhabitating the same or overlapping geographic areas.