臺灣東部海域雨傘旗魚之營養階層結構與攝食生態研究

Abstract

過去漁業資源的利用與評估著重於單一魚種的管理模式，為資源永續利用，FAO提出以生態系為基準之漁業管理方式，其漁業管理方式需考慮物種間的營養階層結構與能量流，有關雨傘旗魚的族群特徵已累積許多研究成果，但對於食物階層結構研究卻是完全闕如。本研究為利用雨傘旗魚肌肉之氮、碳穩定同位素來探討其食物階層結構，經檢測雨傘旗魚肌肉樣本 (n=263) 之氮同位素 (δ15N) 的測量值為7.51‰至14.19‰，平均值為12.05 ± 1.23‰；碳同位素 (δ13C) 的測量值為-22.04‰至-15.48‰，平均值為-17.68 ± 1.15‰。氮同位素的測量值與雨傘旗魚之體長呈正相關；碳同位素的測量值與體長呈負相關。各體長組間的雨傘旗魚之估算營養階層，顯示不同體型之雨傘旗魚分別位於生態系中不同的生態位階，推論影響的因素為餌料食物來源、棲地環境及洄游路徑之不同。 穩定同位素分析是根據消費者穩定同位素比值與其食物來源之同位素比值相近的原理，進而推估餌料生物來源貢獻，可以反映生物長期攝食生態的結果。本研究以雨傘旗魚及其餌料生物之碳、氮穩定同位素，本研究應用Stable isotope mixing model，來推算雨傘旗魚的餌料生物來源組成，依先前生殖研究的結果，將雨傘樣本分為幼魚(<140 cm, LJFL )、成熟中(141-180 cm, LJFL)及成魚(>181 cm, LJFL)三組。研究結果顯示，雨傘旗魚幼魚攝食以烏魴科(15.90%)為最多、其次為眼眶魚(11.04%)與鯡科(9.62%)；而雨傘旗魚成魚攝食以白帶魚(32.63%)為最多、其次為正鰹(15.82%)與紅目鰱(11.30%)。雨傘旗魚的食物來源組成種類多樣，包含有：頭足類、沿岸底棲魚類、表水層魚類、大洋中層魚類，以穩定同位素分析結果，雨傘旗魚食物來源主要為表水層魚類為佔多數。 魚類食物網是海洋生態系中物質循環與能量流動的重要途徑，營養結構則是顯示能量流動過程中，消費者與被捕食者之間的營養關係。本研究根據胃內容物與穩定同位素分析推估雨傘旗魚的攝食組成，參考漁獲統計資料，應用Ecopath with Ecosim 建構台灣東部海域生態系統營養結構模型，並經由模型分析系統能量流動和總體特徵。結果顯示台灣東部海域生態系統各功能群的營養階層範圍為1至4.52，哺乳動物及沙魚佔據了最高的營養階層。利用生態網路分析結果，系統總吞吐量為3054.43 t/km2/yr；系統總生產量、總消費量及總呼吸量分別為1787.29、824.89、528.62 t/km2/yr；總初級生產量/總呼吸量為3.13。系統連接指數 (Connectance Index, CI) 和系統雜食指數 (System Omnivory Index, SOI) 分別為0..47 和0.25。本研究初步建立台灣東部大洋海域生態系統模型，探討生態系的結構與功能，提供未來建置以生態系為基礎的漁業管理相關科學的資訊。經由Ecosim模式預測各功能組的未來50年後生物量變動，其結果顯示，如維持目前的漁業規模，對於海洋哺乳類、鮪魚、旗魚、沙魚等大型掠食性魚種，將會造成資源量的減少。本研究結果與過去漁獲量統計變動的趨勢相符合，為持續小幅度下滑降低；顯示目前台灣東部海域漁獲努力量仍偏高，建議應進一步檢討降低。

Fishery stock assessment and management used to focus on single species method. FAO promoted the ecosystem-based fishery management (EBFM) for the sustainability of living aquatic resources. EBFM need to be taken into account with trophic structure and flows of biomass through species interactions. In present study, nitrogen and carbon stable isotopes analyses were applied to investigate the trophic structure of sailfish. A total of 263 sailfish muscle samples were examined. The δ15N values for sailfish ranged from 7.51 to 14.19 ‰ (mean=12.06‰ ± 1.16) and the δ13C values ranged from -22.04 to -15.48 ‰ (mean=-17.62‰ ± 1.10). The δ15N values were positively related to body length, while δ13C were negatively related to body length. The results revealed the different size class of sailfish occupied different trophic position of pelagic ecosystem. We postulate that the factors which influence on trophic position of sailfish between young and adult were food sources, habitats and migratory pattern. Stable isotope ratios of a consumer are related to those of its prey. Stable isotope mixing model was used to estimate diet composition of consumer by mass balance equation. In this study, stable isotope mixing model was applied by carbon and nitrogen stable isotopes of sailfish to estimate the food sources of their prey. The sailfish were segregated by juveniles (<140cm, LJFL), maturing stage (141-180cm, LJFL) and adults (>181cm, LJFL) from previous research. The most important preys for juvenile sailfish were Bramidae (15.90%), Mene maculata (11.04%) and Clupeidae (9.62%). Therefore, adult sailfish feed on Trichiurus lepturus (32.63%), followed by Katsuwonus pelamis (15.82%) and Priacanthus macracanthus (11.30%). In spite of the apparent high prey diversity of diet composition for sailfish, they feed mainly on epipelagic species in the waters off eastern Taiwan. Food web is an important element in pelagic ecosystem to transport nutrients and energy flow between consumers and prey. In order to investigate the role of sailfish and to explore the function and structure of pelagic ecosystem in eastern Taiwan waters, we applied Ecopath with Ecosim model inferred from stable isotope analysis, stomach content analysis and fisheries statistical data to perform pelagic trophic model of eastern Taiwan. The results reveal that the trophic level of the functional groups varied from 1.0 to 4.52. Marine mammals and shark are occupied top trophic level. Total ecosystem throughput is 3054.43 t/km2/yr. Sum of all production, sum of all consumption and sum of all respiratory flows are 1787.29, 824.89 and 528.62 t/km2/yr, respectively. Total primary production to total respiration ratio is 3.13. Connectance Index (CI) and System Omnivory Index (SOI) are 0.47 and 0.25, respectively. In this study, we preliminary established pelagic trophic model in eastern Taiwan and the results can provide scientific references for ecosystem-based fisheries management. Ecosim model was applied to predict biomass trajectories of each functional groups in next 50 years. The predicted biomass trends indicate that if we maintain current fishing efforts in eastern Taiwan will reduce most of biomass of top predators in the future (e.g. Marine mammals, tuna, billfish and shark). The results show current fishing efforts in eastern Taiwan is still high, we suggest the fish stock need to be monitored and fishing efforts should be further reduced.