吳郭魚代謝性組織與耳石有機物之穩定性同位素分餾

Abstract

耳石屬於代謝惰性的結構，其穩定性氮同位素分餾程度可能與其他的代謝性組織不同，但欠缺相關研究。本論文以莫三比克種吳郭魚作為實驗對象，分別餵食五種同位素值不同的餌料一年多後，取代謝性組織(肌肉、鰓絲、肝臟、鱗片)，以元素分析儀聯結穩定性同位素質譜儀分析，而耳石有機物(非代謝性組織)則分別以元素分析儀聯結穩定性同位素質譜儀和氣相平台聯結穩定性同位素質譜儀進行分析，後者需先經過脫硝菌將有機氮轉化為氧化亞氮，分析結果指出氣相平台聯結穩定性同位素質譜儀所需的耳石粉末量僅佔元素分析儀聯結穩定性同位素質譜儀的1/20，可提供更高時間解析度。此外，代謝性組織的穩定性碳、氮同位素與餌料間有明顯的同位素分餾，穩定性氮同位素部份，以肌肉、鰓絲的同位素分餾值最大(3.4-5.6‰)，肝臟則最小(2.1-3.5‰)。穩定性碳同位素部分，則以鱗片的同位素分餾值最大(4.7-6.4‰)，肝臟則最低(-0.5-0.9‰)。不同於代謝性組織，耳石有機物的穩定性氮同位素與餌料間無顯著差異，攝食不同餌料的吳郭魚個體間，分餾值範圍為-0.51-0.69‰，且不論是耳石有機物的穩定性同位素值或是與餌料間同位素分餾，均與體重、標準體長間無顯著相關性。耳石中的氮同位素可能是直接來自攝食分解後且未經代謝過的胺基酸。此外，攝食不同餌料的吳郭魚耳石氮百分比，與個體攝入餌料的粗蛋白質百分比極為相似，但與標準體長、體重間，均無顯著相關性(除了餵食鰻魚粉的個體)。此研究提供往後研究的基礎，例如藉由分析耳石核心至邊緣的有機物穩定性同位素，將能追溯魚類在不同階段的攝食類別與食階轉變等生活史特徵。

Otolith isotopic fractionation may differ from other metabolic tissues due to it is metabolically inert calcified tissue. However, lack of knowledge is pronounced for stable isotopic values of otolith organic matrix. In this study tilapia (Oreochromis mossambicus) were fed with five different diets for up to one and half year under controlled conditions, then stable carbon and nitrogen isotope compositions of the metabolic tissues (muscle, liver, gill and scale) and otolith organic matrixes (metabolic inert) were measured by EA-IRMS and GasBench-IRMS, respectively. For the latter, otolith organic matrix had to be converted to N2O by Pseudomonas aureofaciens and P. chlororaphis before the analysis. The results suggested that the minimal mass reqired for the analysis was lower (1/20) for the GasBench-IRMS compared with the conventional EA-IRMS. In addition, isotopic compositions were significantly different between metabolic tissues and the diets. For stable nitrogen isotopic fractionation, muscle and gill had the largest values (3.4-5.6‰), while liver had the lowest values (2.1-3.5‰). For stable carbon isotopic fractionation, scale had the largest values (4.7-6.4‰), while liver had the lowest values (-0.5-0.9‰). Nevertheless, the δ15N compositions were not significantly different between otolith organic matrixes and the diets with the fractionation between -0.5-0.7‰. This result indicated that the source of nitrogen isotope in otolith organic matrix may be directly derived from amino acid without any metabolic transamination. There was no significant correlation among stable isotope values of otolith organic matrixes and somatic weight and standard length. Apart from these, the distribution of N% in otolith also showed similar pattern with crude protein percentage of the dies, except the tilapia fed on eel meal. N% of otolith in other individuals was not correlated to somatic weight and standard length of the fish. We conclude that high temporal resolutions of δ15N profiles analyzed by GasBench-IRMS and N2O can have wide applications, in studying the food sources and trophic shift of the fish at different life stages.