| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 107 === Cephalopods are among the most important fishery resources in Taiwan. Previous studies have largely focused on the biology or stock assessment of commercially important species; however, little is known about the biodiversity and community structure of demersal cephalopods, as well as the environmental factors controlling their distributions. Therefore, we analyzed 131 samples, covering depth ranges from 10 to 800 m from the research project, “Investigation, Evaluation and Database Establishment of Bottom-Trawl Fishery Resources in Taiwan”. The sampling design was to separate the coastal and offshore waters around Taiwan into four regions (the Northeast, Southwest, and Northwest off Taiwan and around the Penghu Islands) with surveys covering one region each year during 2000–2004. The comprehensive database generated by the project encompasses a total of 15 families, 36 genera and 71 species of cephalopods. In this study, we compared the biodiversity at different spatial scales (i.e., α, β and γ- diversity), and examine the relationship between community structure and environmental factors using the multi-dimensional scale (MDS) and redundancy analysis (RDA). The results showed that the cephalopods around Taiwan can be divided into northeastern (NE) - southwestern (SW), northwestern (NW) - Penghu Islands (PW), and mix-region assemblages (SIMPROF test, p < 0.05). Cephalopod species composition was significantly different among the four geographic regions (PERMANOVA test, region, p < 0.05). On the continental shelf (shallower than 100 m), the compositional variation of cephalopods from NW to PW is related to the decline of primary productivity. The high γ diversity in the NW may be caused by the high dissimilarity among common and dominant species (β- diversity). On the upper slope (deeper than 100 m), water depth and topographic roughness are the main factors causing the compositional difference of the cephalopods between NE and SW region. The more complex topography in the SW comparing to the NE region probably provides more ecological niches, allowing different species to coexist. Species replacement dominated the beta-diversity across regions and depth strata, which can be attributed to the uneven distribution of food sources, resulting in strong environmental filtering, making the poorly adapted species being replaced by species with better fitness. Overall, the community structure of cephalopods was mainly affected by depth, seafloor roughness, and primary productivity. The unexplained variability in the RDA model may represent the effects of other factors such as water mass, biological traits, or biological interactions. Our study provides a historical baseline of demersal cephalopod assemblages against future ecosystem changes and a reference for the fishery resource management and conservation planning around Taiwan’s EEZ.
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