台灣西南部七股溪口沙洲環文蛤族群數量分佈與環境因子關係之研究

Abstract

台灣西南部七股溪口具有紅樹林所組成的沙洲，由於位處感潮河段，使本區周遭擁有著豐富的生物資源，其中俗稱赤嘴仔的環文蛤（Cyclina sinesis）是本區最常見的經濟性二枚貝。本研究在2008年7月至2009年6月間在七股溪口沙洲以穿越線進行採樣，採集環文蛤測量其殼長、殼高、殼寬、體重並同時推算環文蛤年齡及估計沙洲上環文蛤密度；環境因子上測量pH、鹽度、水溫、含氧量、底泥含水量、底泥有機質含量、粒徑、泥沙比。主要目的為探討不同環境因子對沙洲上環文蛤族群分佈數量的影響。結果顯示，七股溪口沙洲的環文蛤只發現分佈在七股溪河道區上，並且主要由殼長3 cm以下及不到2齡的環文蛤族群所構成。環文蛤密度上顯示，七股溪口沙洲的環文蛤主要分佈在低、中潮位，以高潮位的紅樹林氣根區為其分佈上界。每年的3月環文蛤密度因殼長2 cm及1齡的幼貝加入而開始上升，但在六月後由於當地民眾的採集而開始下降。環境上七股溪口沙洲受七股溪河道、小排水溝、三股溪河道環繞，七股溪口沙洲可依據環繞水道的不同分成三塊由不同水質及底質所構成的棲地。主成份分析中發現七股溪口沙洲及七股水溪河道的環境主要變異來自於底質因子。綜合上述結果，七股溪口沙洲的環文蛤，因七股溪口沙洲三塊棲地中的七股河道區有環文蛤偏好的環境，尤其是細砂底質環境使環文蛤稚貝易定著，成貝有較佳的生長率及存活率，使七股溪河道成為七股溪口沙洲環文蛤唯一的棲地。從環文蛤外部形態組成及密度變化上也可以了解，七股溪口沙洲的環文蛤面臨沉重的漁獲壓力。

The study mangrove area, which was located in the estuary of Cigu River in southwestern Taiwan has abundant biological resources. The clam Cyclina sinesis was one of the most common economical bivalve in the estuary. The data sampling was carried out from July, 2008 to June, 2008. The transect sampling was used to collect the clam stock and environmental factors. Clam’s morphology such as shell length, height, width, weight, age and density were investigated in the survey. The environmental factors, such as pH, salinity, temperature, dissolved oxygen, water content, organic matter, grain size and the proportion of mud/sand were also investigated. The results indicated that the clam was mainly concentrated in the Cigu River sector, which consisted of clam less than 3 cm shell length and 2 years old. The clam mainly distributed in the low and mid tidal zone. The aerial root zone of mangrove in high tidal zone became a boundary of the clam distribution. The density of the clam increased in March because of recruitment of juveniles which shell length less than 2 cm and age about 1 year old, and decreased in June because of fishing. The Cigu estuary could be divided into three sectors, such as Cigu River sector, small water channel sector, Sangu River which was composed of different water and sediment factors. PCA also indicated that the main variation of Cigu estuary and Cigu River sector were from sediment factors. In conclusion, Cigu River sector of Cigu estuary became the only habitat of the clam Cyclina sinesis because it possessed the environmental factors they preferred, especially in the fine sand sedimental environment with less mud, which could provide juvenile clam’s successful settlement and better growth and survival rate for adult clam. According to the clam’s morphological composition and density variation, the clam Cyclina sinesis in Cigu estuary had faced heavy fishing pressure.