Impacts of Marine Cage Culture on the Marine Environment and Subtropical Coral Reef Benthic Communities in Penghu Islands, Taiwan

• Main Authors: Yuan-Chao Angelo Huang, 黃元照
• Other Authors: Chaolun Allen Chan
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/29536979472366488982

博士 === 國立臺灣大學 === 海洋研究所 === 99 === 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.