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Title: Use of artificial reefs for removal of nutrients from marine fish farms in Hong Kong
Other Titles: Xianggang ren gong yu jiao dui lai zi hai yu yang zhi zhi ying yang de qu chu zuo yong
Authors: Tsang, Tsui Yun (曾翠茵)
Department: Department of Biology and Chemistry
Degree: Master of Philosophy
Issue Date: 2010
Publisher: City University of Hong Kong
Subjects: Marine eutrophication -- China -- Hong Kong.
Restoration ecology -- China -- Hong Kong.
Artificial reefs -- China -- Hong Kong.
Fish culture -- Environmental aspects -- China -- Hong Kong.
Notes: CityU Call Number: QH91.8.E87 T75 2010
xvi, 203 leaves : ill. 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2010.
Includes bibliographical references (leaves 176-196)
Type: thesis
Abstract: Marine fish farming activities release waste, including uneaten fish feed, faecal pellets and dissolved excretory products, into the surrounding waters. The increased nutrient levels in water not only stimulate the occurrence of nuisance and algal blooms, but also impact the benthic habitat. Deploying artificial reefs (ARs) close to fish rafts in soft bottom habitats is one of the practical ways to remove excess nutrients, as the reef surface can provide a hard substratum for colonization by epifaunal organisms. Most of these organisms are suspension or filter feeders, which can capture and assimilate particulate organic matter in the water column efficiently. To remedy the eutrophic effect of marine fish farming activities, ARs were deployed in the Sham Wan fish culture zone (FCZ), Sai Kung and Lo Tik Wan FCZ, Lamma Island in Hong Kong, in March 2007 and April 2008, respectively. Sham Wan FCZ is a relatively clean site, with no river, sewage or industrial discharges from the nearby catchment. By contrast, the water in Lo Tik Wan is more turbid and has a lower salinity in summer due to the effect of the Pearl River outflows. This culture zone is also located near the East Lamma Channel, where vessel traffic is intense and near the urban area in Aberdeen where coastal sewage outfalls may further deteriorate the water quality. The objective of the present study is to evaluate the effectiveness of ARs to remove excessive nutrients derived from fish farming activities. Investigations on the epifaunal colonization on the AR surface, the water quality, sediment physico-chemical characteristics, macrobenthic community structures and nutrient dynamics in the AR - fish farm system were conducted. In both study areas, three AR stations, two FCZ stations away from ARs and two control stations outside the culture zone boundary were chosen for study. In Sham Wan, six post-deployment samplings were conducted from November 2007 to February 2009, at three-month intervals. In Lo Tik Wan, a baseline sampling was carried out in April 2008 before AR deployment, followed by four post-deployment samplings from July 2008 to May 2009, also at three-month intervals. The AR surface was colonized by a variety of epifaunal species in both Sham Wan and Lo Tik Wan. Most of them were suspension or filter feeders, including barnacles, bivalves, bryozoans, sponges, tubeworms and tunicates. In Sham Wan, strong temporal variations in epifaunal coverage on AR surface were found, with the highest coverage (60%) in May 2008 and the lowest coverage (26%) in August 2008. In Lo Tik Wan, rapid epifaunal colonization was noted, in which the epifaunal coverage area reached 30% in the first three months of the AR deployment and stabilized at around 50% throughout the later sampling periods. The distinct temporal variations in epifaunal coverage in Sham Wan were caused by a large decrease in dissolved oxygen level, from about 7 mg l-1 on the surface to 1 mg l-1 in the bottom waters, as a result of vertical stratification during the summer. The hypoxic bottom waters might kill off the epifauna on the AR surface. In July 2008, which was only three months after deploying ARs in Lo Tik Wan, the phaeo-pigment level in the water column of the AR stations was lower than that of the FCZ stations. For the benthic sediments, no specific spatial patterns in total organic carbon (TOC), total Kjeldahl nitrogen (TKN) and total orthophosphate (TP) levels in the sediment were detected among the control, FCZ or AR stations after the deployment of ARs in Sham Wan (approximately one year). In Lo Tik Wan, however, a substantial decrease in TOC, TKN and TP levels was noted, particularly at one of the AR stations. The results suggested that ARs are able to remove nutrients released from fish farms to the surrounding waters and to reduce the deposition of nutrients at the fish raft bottom. Despite of the improvements in sediment characteristics in both study areas, the macrobenthic communities at the FCZ and AR stations were still less diverse than that at the control stations, which may be due to the slower recovery of macrobenthic communities, compared to the abiotic environment. Sediments in Sham Wan were found to be very silty, but coarser sediments appeared at the AR stations in the latter part of the monitoring period, probably owing to the accumulation of shell fragments from dead epifauna on the AR surface. The higher heterogeneity of the sediment can provide a more diverse habitat for benthic organisms, which may lead to increase in benthic diversity. By contrast, sediments in Lo Tik Wan were highly heterogeneous in particle size distribution. A large amount of fish scales, fish bones, animal shells and rubbish fallen from the fish rafts contributed to the coarser nature of sediments within the FCZ. Approximately one year after AR deployment, samples of suspension or filter feeders, particulate organic matter (POM), fish feeds and fish faeces were collected for the measurement of fatty acid profiles, as well as carbon and nitrogen isotope signatures. In Sham Wan, the pattern of fatty acid profiles of the tunicate Styela plicata and barnacle Balanus spp. tended to be closer to that of trash fish (as fish feed) and POM rather than to that of fish faeces. Based on the carbon isotope mixing model, the contribution of POM and the trash fish - fish faeces group to the carbon source of tunicates were 62.1% and 37.9%, respectively, and their contribution to the carbon source of barnacles were 5.7% and 94.3%, respectively. In Lo Tik Wan, the fatty acid profiles and stable isotope signatures of five suspension or filter feeders were determined, including the barnacle Balanus spp., tunicate Styela plicata, bivalves Perna viridis and Isognomon perna and sponge Mycale adhaerens, and the IsoSource mixing model was used to estimate the feasible range of the relative contribution of different food sources to the epifauna. The results showed that barnacles, fanshells and tunicates had a large feasible range of food combinations, while mussels depended more heavily on trash fish and sponges preferentially dieted on pellet food and faeces of cultured fish that were fed with pellet food. Fatty acid profiles further confirmed the above findings, which unique fatty acid signatures were shared between mussel and trash fish, and between sponges and faeces of cultured fish fed with pellet food. The results also showed that all these suspension or filter feeders were able to consume a mixture of phytoplanktonic matter and animal tissues. In both Sham Wan and Lo Tik Wan, low δ15N values related to their potential food sources were found in the epifaunal tissues. This could have been caused by the consumption of dissolved organic matter with relatively low δ15N values. To conclude, ARs can serve as biofilters to remedy the eutrophic effect of marine fish farming activities. The reef surface can attract a variety of suspension or filter feeders in a very short period of time, which are shown to be effective in removing excessive nutrients through the uptake of fish feed, fish faeces and particulate organic matter in the water column. The present study also demonstrated that ARs can be applied effectively to different fish culture environments in Hong Kong. The system has a high application potential in mariculture industries, which are important economic activities in the coastal areas of the Asia-Pacific region.
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