雀鯛科魚類攝食關係再評估

Abstract

魚類在珊瑚礁生態系統中數量眾多，是重要的消費者。魚類食性的研究可以闡釋礁區生物種間複雜掠食關係，增進對海洋生態系能量傳遞鍵結的瞭解，因此是生態學研究的一個重要課題。雀鯛科魚類 (Damselfishes; Pomacentridae) 是棲息於熱帶珊瑚礁及亞熱帶礁區海域的重要魚類之一，以往雀鯛科魚類的食性資料，都是藉由分析魚類個體的胃內含物而獲得。胃內含物分析 (Stomach content analysis) 所顯示的是生物個體被捕獲前一刻的攝食情況，但對於碎屑或是細小生物類的食物，並無法進一步定性與定量。在另外一方面，由於攝食者身上組織的穩定同位素13C/12C及15N/14N值會分別比其食物項的為高，因此可以應用在攝食生態的研究上。本論文以雀鯛科魚類做為研究的材料，利用胃內含物與穩定性同位素兩種分析方法，重新檢視這些魚類的食性以及食物資源在體內的吸收利用。文中所探討的課題包括：變色雀鯛食性的時序性變動、真雀鯛間的食性比較、太平洋真雀鯛與鳚科魚類間的攝食互動以及人為營養源對雀鯛食性的影響。所收集到的資料經過分析，獲得如下結果：變色雀鯛個體的肝臟有高於白肉組織的同位素替換速率，能快速反映出環境中食物源同位素的改變，而白肉組織的同位素值則較為穩定，屬長期累積的結果。另外，草食性的領域性真雀鯛可能是以棲身在領域內藻床上的多毛類動物為食，其藻耕行為則是為了培養多毛類食物，而太平洋真雀鯛與鳚科魚類之間領域的重疊則可能是一種互利的現象。此外，小琉球箱網養殖所產生的殘餌，會直接被雀鯛所攝食或由浮游生物攝食後再傳送到雀鯛魚類，而不是經由基礎生產者往高階的消費者的這一途徑傳送。由於這些新的發現，我們得以對珊瑚礁生態系中的食物網及魚類的攝食關係提出一些新的觀點，並建議重新檢定珊瑚礁區草食性魚類的營養源，以進一步瞭解魚類的攝食生態。

A sheer number of fishes inhabit coral reefs, where they are also major consumers. Complex feeding interactions occur between fishes and their preys. Since these interactions involve fluxes of nutrients and energy, there is a need to learn their feeding relationships to elucidate the thermodynamics on coral reefs. Damselfishes (Pomacentridae) are common inhabitants on both coral reefs and sandstone reefs. Previously the dietary data of damselfishes was mostly based on conventional stomach content analysis. However, stomach content analysis reflects short-term diets of fishes, and food items that are digested quickly, such as detritus and invertebrates, are generally difficult to identify and enumerate. Because the stable isotopes in tissues of consumers are typically enriched relative to those of their prey, stable isotope analysis was used to study feeding ecology of fishes here. In this study, damselfishes were used as materials. Their dietary components and potential foods were examined by both stomach content and stable isotope analyses. Issues dealt with included temporal dietary changes of neon damsel, dietary comparisons between Stegastes spp., feeding interactions between a territorial damselfish and blennies, and the effect of anthropogenic enrichment on diets of damselfishes. Results revealed that in the neon damsel, the liver bore a higher isotope turnover rate compared with the white muscle, thus might reflect rapidly changes of δ13C and δ15N from the food source. In contrast, isotopic signature of the white muscle was steadier, reflecting a sign of long-term accumulation. The major organic source for algal-farming Stegastes damselfishes was likely to be the polychaetes hidden in the algal lawn. The algae farmed by the fish were possibly to secure polychaetes rather than to use as food source. Moreover, the widespread territory overlap between Pacific gregory and blennies was possibly a sign of mutualism. Fish feeds released by cage fish farms in Siaoliouciou Island were taken by resident damselfishes either by direct ingestion or by way of uptake by zooplankton. Thus the retention of nutrient enrichment had gotten through in a food chain not involving primary producers such as algae. These findings have unraveled insights into food webs on coral reefs and feeding relationships between reef fishes. Overall, it is suggested that the nutrient source of herbivorous fishes on coral reefs be re-examined for a better understanding of the feeding ecology of fishes.