澎湖地區海上箱網養殖漁業對於鄰近海域環境及亞熱帶珊瑚礁生物群聚之影響

Abstract

過去二十年來，由於台灣面臨過度的漁獲捕撈壓力，澎湖地區海上箱網養殖漁業逐漸開始變成台灣重要的漁業經濟活動。然而，養殖污染也會對於鄰近海域環境與生態上產生負面影響，例如水質惡化、優養化、過量有機質堆積海底 、甚至造成底棲生物群聚結構產生相變。由於澎湖箱網養殖為了以防止颱風侵害，多設置於淺水域、半封閉且擁有豐富亞熱帶珊瑚礁群聚的海灣地形。因此，箱網養殖不但對鄰近海域造成影響，甚至會對鄰近珊瑚礁的生態系造成衝擊。此外，澎湖地區秋冬季所盛行的強烈東北季風亦可能改變養殖區內鄰近海域的水文狀況，進一步影響到養殖漁業的汙染與擴散。本研究藉此探討澎湖季節性季風於2006-2007年間對於半封閉海灣：馬公灣內箱網養殖鄰近海域環境水質與底質的影響，並利用底棲生物和亞熱帶珊瑚礁生物組成評估其棲地環境之劣化程度。 在生物棲地環境影響方面，結果顯示養殖生物排遺及未食用飼料所溶解產生高濃度的氨氮以及有機物質堆積為造成水質及底質惡化的主要因子。在東北季風季節裡所影響的水質及底質範圍（分別向養殖區外延伸1000公尺、500公尺）遠高於西南季風季節（僅限養殖區內）。由於馬公灣為一淺水域海灣，強烈東北季風不但可能攪拌水體造成底質的再懸浮作用，甚至會加快灣內水流速將養殖排放物及底質在懸浮有機物質輸送到更遠的距離，但是卻無法將養殖排放物攜帶出此呈現半封閉狀態的馬公灣之外。不過，東北季風所造成的底質的再懸浮作用對於箱網養殖所引起的底質環境惡化情況具有顯著性的降低效果。 在底棲生物群聚組成影響方面，東北季風並不會影響生物群聚的分佈狀況。在養殖區下方軟底質的高耐汙種多毛類（絲鰓蟲科和小頭蟲科）分布密度不但與有機物質含量具有顯著性相關，而且與少數二枚貝、陽燧足、水棲昆蟲及甲殼類等底棲生物共同生活在受影響的養殖區內，這結果說明了食物資源充足供給才是決定底棲生物群聚分布的主要因素。此外， 典型相關分析結果亦顯示大型藻類、菟葵及海綿覆蓋率與礁區水體營養鹽（氨氮、磷酸）呈現顯著性關係。鄰近養殖區內的珊瑚群聚多以高耐污的團塊及次團塊狀珊瑚為主，並且缺乏分枝狀且低耐污的軸孔珊瑚。相對而言，在中間及對照區內，主要的珊瑚群聚則具有較高的形態歧異度及複雜度的珊瑚及分支狀的軸孔珊瑚。 綜合以上結果顯示，馬公灣內箱網養殖漁業已經造成珊瑚礁周遭海域的營養鹽增生以及珊瑚礁生物棲地劣化，並且引發大型藻類、菟葵及海綿的過度生長，進而影響珊瑚群聚的組成。 此外，本研究利用團塊狀微孔珊瑚（Porites）骨骼具有長時間記錄環境水質之特性來做為追蹤過去時間海域環境變遷的指標。結果指出箱網養殖漁業開始後（1990~2007）珊瑚骨骼生長率明顯高於養殖漁業開始前（1977~1989），並說明環境營養鹽增生可能會加速珊瑚骨骼生長。然而，儘管骨骼直線生長速率能替過去缺乏的環境監測資料提供部分的說明，目前仍需結合其他影響珊瑚骨骼生長因子，如骨質密度和鈣化作用以及環境水質參數資料來建構珊瑚生長與養殖漁業環境水質間的關係，並利用珊瑚骨骼生長來加強說明海洋環境變遷之重要性。 本論文為首篇研究針對澎湖地區箱網養殖漁業，並研究與評估其對於鄰近海域的水質、水文狀況、底質環境數據及其軟底質與珊瑚礁生態系之影響。本研究亦說明未來箱網養殖漁業發展方向以及與其他相關環境污染研究方法之結合 。例如長期環境影響評估計畫以及漁業污染擴散預測模式的建立、並根據結果設立漁業法規來規範養殖環境的水質及底質環境標準、甚至可利用分子生物技術及穩定同位素來追蹤漁業污染對於珊瑚疾病、水質狀況及珊瑚骨骼之影響。本論文的結論與建議將能夠提供政府漁業相關單位及水產養殖管理機構發展出適切的環境準則以及管理策略來讓管理單位維持海洋環境原有的功能性。

Due to increases in local fishing pressures, marine cage culture (MCC) in the Penghu Islands of Taiwan has become an important commercial activity over the past two decades. However, MCC also poses the risk of degrading the local environment by ways of water quality deterioration, organic enrichment of the sediment, change in composition of benthic organisms, induction of macroalgal blooms and, occasionally, phase shifts. Natural phenomena, such as the northeasterly (NE) and southwesterly (SW) monsoon winds, also stand to impact marine environments of the Penghu Islands. In this study, the effects of both MCC and the seasonal monsoons on the water and sediment quality and benthic ecology were investigated in a shallow, semi-enclosed embayment, Magongwan, from 2006 to 2007. We also investigated the possibility of the deterioration of suitable habitats for coral reef organisms. Significant changes in sedimentation between monsoon seasons were observed, likely due to resuspension induced by strong winds or wind-driven currents. Moreover, elevated levels of ammonia and organic matter produced from the MCC were the main pollutants found in the water and sediments, respectively, and distinguished the cage-culture and intermediary zones (1000 m and 500 m away from the cages, respectively) from the reference zone during the NE monsoon season. This suggests that changes in the local hydrographic conditions induced by the NE monsoon winds influence the water and sediment quality in Magongwan. Specially, it appears that the resuspension and currents produced by the NE monsoon winds may extend the spread of nutrient-enriched water without necessarily flushing such polluted water outside of the bay. The abundance of stress-tolerant polychaetes, such as those of the Cirratulidae and Capitellidae families, was significantly enriched within 500 m of the cages, and they were found in cohabitation with some small bivalves, Ophiuroidea, insects, and crustaceans. Importantly, the density of these worms was correlated with sediment organic matter content. Hence, food availability was the main factor influencing the distribution of these polychaetes. For the areas dominated by reef-building coral communities, the cover of macroalgae, sponges, and zoanthids was strongly correlated with levels of two nutrients, ammonium and phosphate. Coral communities in the cage-culture zone were mostly composed of stress-tolerant massive and submassive corals, and typically lacked branching acroporids. In contrast, coral communities in the intermediary zones, which were characterized by high habitat complexity and species richness, were dominated by coral species with diverse morphologies, including abundant branching acroporids communities. These results suggest that MCC in Magongwan might have resulted in the deterioration of suitable habitats for certain corals, and may even have shifted the fragile equilibrium between corals and algae. Nevertheless, the intermediate levels of nutrients and particulate organic matter caused by the MCC in the intermediary zone may actually have resulted in its high coral cover and diversity. The average coral skeletal extension rate of poritids sampled at Magongwan was significantly lower than that of corals from the control site, Do-ji Island. Moreover, the temporal variation in the skeletal extension rate was significantly higher in Magongwan during the period after the establishment of the MCC (1990-2007), indicating that the ensuing changes to the local marine environment, particularly nutrient enrichment, may have influenced the coral skeletal extension rate. As such, these poritid corals may serve as valuable environmental recorders that can help to document marine environmental changes in years in which surveys were not undertaken. The remediation of this unfortunate situation at Magongwan will require a number of steps. First, predictive modeling should be used to establish acceptable and sustainable water quality parameter criteria. Secondly, environmental strategies for sustainable development should be developed. Collectively, it is hoped that these suggestions will provide insights for local governments to develop appropriate conservation management strategies, and for fishery authorities and aquaculture planning agencies to maintain a functional marine environment.