台灣西南部七股溪口紅樹林沙洲招潮蟹密度影響因子之研究

Abstract

台灣西南部七股溪口具有紅樹林所組成的沙洲，本區生物資源豐富同時為野鳥重要棲息地，其中招潮蟹更是本區的底棲優勢族群。先前試驗顯示目視計算地面活動個體數法最有效率，且適合七股溪口紅樹林沙洲招潮蟹族群密度的採樣方法；同時以挖掘法捕捉部分招潮蟹進行形質測量。本研究在2008年7月至2009年6月間在七股溪口紅樹林沙洲進行採樣，主要目的為探討紅樹林沙洲招潮蟹密度之影響因子，生物採樣部分採用垂直潮水線的穿越線法目視鑑種並計數；採樣水質因子有pH值、鹽度、水溫、含氧量；底質因子有含水量、有機質含量、粒徑及泥沙比。本區棲息著弧邊招潮蟹、清白招潮蟹、三角招潮蟹及北方凹指招潮蟹大種類，弧邊招潮蟹與清白招潮蟹的甲寬與體重間呈現高度相關，而北方凹指招潮蟹則於甲長與體重有較高的相關。七股溪口紅樹林沙洲可依據水質和底質的不同明顯分為七股溪河道、三股溪河道及小排水溝三大區塊。主成份分析發現七股溪口紅樹林沙洲環境變異，首先來自於底質因子，接著才是水質因子。三股溪河道區為較適弧邊招潮蟹與清白招潮蟹棲息的河道區，且皆於高潮位有最高的密度；此區底質因子粒徑為48.8 ± 5.68 μm；泥沙比為1.7 ± 0.38％；含水量為35.5 ± 4.76 ％；高潮位有機質含量則是3.56 ± 0.93 ％。

The study mangrove area, which was located at southwestern Taiwan has abundant biological resources and is one of important habitat of wild bird. The fiddler crabs were the most dominant species of benthic animals. In previous study, visual survey was the most efficiency and suitable method to estimate fiddler crab’s density in the mangrove estuary of Cigu River. Fiddler crabs were also captured and measured their morphology. The data were sampled monthly from July, 2008 to June, 2009, the purpose of this study was to comprehend density influence factors of fiddler crab in mangrove estuary of Cigu River. The transect sampling was vertical to tidal line. Besides, species discrimination, individual counts and sex judgement were also investigated by visual 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 there were four species, such as Uca arcuata, Uca lactea, Uca triangularis triangularis and Uca vocans borealis. Carapace width of Uca arcuata and Uca lactea lactea had highly relevant with weight, and carapace length of Uca vocans borealis had highly relevant with weight. The mangrove estuary of Cigu River 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 variation of the mangrove estuary of Cigu River was mainly from sediment factors then water factors. Sangu River sector was suitable habitat for Uca arcuata and Uca lactea lactea compared to other two sectors. There were high Uca arcuata and Uca lactea lactea densities in high tidal zone of Sangu River sector. Sangu River sector had sedimental factors as following：grain size 48.8 ± 5.68 μm；mud/sand proportion 1.7 ± 0.38％；organic matter 3.56 ± 0.93 ％；water content 35.5 ± 4.76 ％。