台灣淡水河流域烏魚的生活史特徵及洄游行為

Abstract

為了瞭解烏魚（Mugil cephalus）在淡水河流域的生活史特徵及洄游環境史，從2008年4月起至2009年12月止按季節採集淡水河下游及其中上游的基隆河和新店溪的烏魚樣本，共設置34個測站，採獲826尾樣本。同時測定各測站水溫及鹽度等物化因子及相關的生物因子包括浮游藻類及底棲生物量等。每尾烏魚皆測量其外部形態，分析耳石的年輪以及測定性腺成熟度（GSI）的月別變化。並利用電子微探儀（EPMA）測定耳石Sr/Ca比的時序列變化，以便了解其洄游環境。此外，為了驗證耳石邊緣Sr/Ca比與環境鹽度之關係，本研究也採集基隆八斗子外海（鹽度33 PSU）的68尾烏魚，進行耳石Sr/Ca比分析。主要發現如下所述： （1）耳石邊緣相對距離（RMD）證實了烏魚耳石的年輪為一年形成一輪，可以用來判讀其年齡。冬季淡水河下游的烏魚以0歲和3歲以上居多，而上游的新店溪及基隆河則以2歲至3歲的烏魚居多。由年齡及體長組成的時空分布顯示仔魚春季入添後會繼續上溯並成長至性成熟為止，冬季時高年齡的烏魚向淡水河下游移動準備產卵。 （2）耳石邊緣的平均Sr/Ca比在海水域、河口域以及淡水域的烏魚之間有顯著的差異性（ANOVA, p < 0.05），而且耳石Sr/Ca比與鹽度呈正相關關係。棲息在33 PSU以上的烏魚，其耳石Sr/Ca比的平均值大於6.383×10-3；棲息在淡水者，其耳石Sr/Ca比平均值小於3.283×10-3；棲息在河口域者，其耳石平均Sr/Ca比則介於兩者之間。因此耳石Sr/Ca比可以用來回推烏魚在淡海水之間的洄游環境史。其次，由耳石原基Sr/Ca比的變化，發現烏魚的產卵或仔魚的棲息環境可分為三個類型，亦即Type 1（海水型）、Type 2（河口型）及Type 3（淡水型）。 （3）由耳石Sr/Ca比的變化與耳石年輪出現位置之關係，發現烏魚有很明顯的季節性移動現象，亦即一歲以後的成魚，冬季時會往鹽度高的外海越冬，夏季時則進入淡水河流域覓食。同時由耳石原基Sr/Ca比的類型及成魚耳石Sr/Ca比的時序列變化也發現成魚的棲地利用會延續仔魚時期所棲息的環境。換言之，河口型和淡水型的烏魚，其仔魚期也幾乎是同樣的類型。 （4）淡水河流域烏魚生殖腺指數（GSI）的季節性變化之最高值出現在10月，但指數的最高值只有8 %遠低於東北海域產卵群GSI最大值的20 %。而且卵細胞之最高成熟度也只達到卵黃發生期（Vitellogenic stage），並未到達排卵的最終成熟階段（Final maturation stage）。因此，推測烏魚在淡水河產卵的可能性極低。 （5）由各測站的烏魚性比、鹽度及營養豐富度三者之間的關係，發現營養豐富的測站雌魚的比例較高，反之則雄魚的比例較高，顯示烏魚的棲地利用有性擇現象存在。 綜合以上結果：烏魚在淡水河流域的分布在不同的鹽度環境，包括中上游的淡水環境，下游的鹹淡水環境以及海水環境。仔魚入添到淡水河流域一般都會成長至性成熟為止才移出至外海，成熟之後很少再回來。GSI值的高低以及卵細胞的發育程度也顯示烏魚在淡水河內產卵的可能性極低。另外，烏魚為了有效繁延下一代，雌魚會利用食物較豐富的區域。本研究認為淡水河流域是烏魚的哺育場也是攝餌場，因此淡水河涵養的資源量將影響外海產卵族群量，所以淡水河環境的維護極其重要。

To understand the life history traits and the migratory behavior of grey mullets in the Tansui River System, 826 grey mullets were collected during different seasons from 34 sampling sites located on the river mouth of Tansui River to the upper streams, Xindian and Keelung Streams, from April, 2008 to December, 2009. At the same time, the environmental factors such as salinity and algal and benthos densities were also measured. The morphological traits and the monthly changes in GSI of each fish were also measured. The otolith (sagitta) was extracted for age determination and temporal changes of Sr/Ca ratio by Electron microprobe analyzer (EPMA) for identifying the migratory environmental history of the fish. In addition to this, 68 mullets were collected off the coast (salinity: 33 PSU) of Badoutzu in Keelung, north of Taiwan, to validate the relationship between otolith marginal Sr/Ca ratio and environmental salinity. The results indicated that: (1) relative marginal distance (RMD) demonstrated that the year ring of the otolith was completely formed in a period of one year. Age determination by the otolith year rings showed that the age structure in the lower reach of the Tanshui River during winter was composed primary of individuals greater than 3 years old with few less than 1 year old. On the other hand, individuals aged 2 to 3 years old were mostly pooled in Xidian and Keelung Streams. This phenomenon indicated that older mullets migrated to the river mouth for spawning during winter, while the larvae or juveniles recruited into the river during spring, to grow in the middle or upper tributaries of the river until maturity. (2) Otolith Sr/Ca ratio had a positive relationship with environmental salinity, hence, otolith Sr/Ca ratio could be used to trace back the migratory environmental history of fish. Otolith Sr/Ca ratio was significantly different among seawater, estuary, and freshwater (ANOVA, p < 0.05). Mean otolith Sr/Ca ratio greater than 6.383×10-3 indicated that the fish inhabited environment whose salinity was more than 33 PSU. On the other hand, mean otolith Sr/Ca ratio less than 3.283×10-3 indicated that the fish inhabited freshwater. Depending on the Sr/Ca ratio of the primordium, the environment in which the larvae inhabited can be classified into Type 1 (Seawater type), Type 2 (Estuary type), and Type 3 (Freshwater type), respectively. (3) Adult fish more than 1-year old migrated to the high salinity environment during winter, but went back to the river for feeding during summer. In addition, the environment in which the larvae inhabited influenced the habitat use of the adult fish, that is, Type 2 (Estuary type) and Type 3 (Freshwater type) individuals preferred the initial environment until sexual maturity. In general, mullets in the Tanshui River System gradually migrated to the middle and lower stream of the river as they grow. (4) Monthly changes of gonadosomatic index (GSI) showed peak value (8 %) during October, but this value was extremely low, compared to the GSI (20 %) of the spawning group in Northeastern Taiwan. The egg cells of the mullets captured during the spawning season was in vitellogenic stage, and not in final maturation stage. Hence, the probability of mullets spawning in the river was low. (5) The relationship between sex ratio, environmental salinity, and the food density showed that there were more female fish in the nutrient-rich environment, whereas in the nutrient-poor, more male fish were found. This phenomenon indicated that the habitat use of the mullets was influenced by sex. In conclusion, the larvae or juveniles of mullets were wildly distributed in the Tansui River System, growing in the river until sexual maturity. As the fish matured, they migrated to the sea for spawning. Evidences from GSI and egg histology indicated that the possibility of mullets spawning in the river was extremely low. In addition to this, female fish preferred to food-rich environment for growth.