Effects of nutrient enrichment from fish farming activities on macrobenthic assemblages in subtropical waters Qin Feng Gao, Siu Gin Cheung, Paul K S Shin Department of Biology and Chemistry, City University of Hong Kong Introduction Environmental impacts of fish farming activities have received increasing research attention in last 2 or 3 decades. Owing to the release of organic and inorganic wastes from the uneaten feed, faeces and dissolved excretory products, one of the most significant impacts of fish farming is the shift of nutrient status in culture areas, in terms of absolute nutrient loadings and relative nutrient ratios. Compared with planktonic and nektonic assemblages, benthic soft-sediment macrobenthos are a more useful indicator in response to environmental changes because of its predominantly sedentary nature and relatively long life span. In the present study, the impacts of fish farming on nutrient status and the response of macrobenthic assemblages to nutrient changes were investigated. The Study Site Figure 1. The study site The fish culture zone in Kau Sai Bay is a semi-closed embayment located on the eastern coast of Hong Kong with culture area of 4.6 ha. 6 points were selected as sampling stations, 2 outside (KS1, KS6 = reference sites), 2 at the boundary (KS2, KS5) and 2 within (KS3, KS4) the culture zone (Fig. 1). Materials and Methods Figure 2. A 0.05 m 2 modified van Veen grab for sediment sampling Sediment samples were collected at the 6 stations with a 0.05 m 2 modified van Veen grab (Fig. 2) bimonthly from August 2001 to June At each station, 5 sediment replicates were collected and sieved through a screen with mesh size of 500  m. Animals retained were preserved, stained and hand-sorted (Fig. 3). All the animals were taxonomically identified to the lowest level as far as practicable and enumerated. At each station, 3 sediment replicates were also dried and digested. Physico-chemical characteristics, including moisture content, silt/clay fraction, total organic carbon (TOC), total Kjeldahl nitrogen (TKN) and total phosphorus (TP), were determined. The delineation of benthic communities and study of relationships between macrobenthos and environmental conditions were analyzed with multivariate, univariate and regression methods. Figure 3. Sieving of animals on board Results and Discussion Non-metric multi-dimensional scaling (MDS) ordination of macrobenthos abundance data showed that the benthic assemblages were classified according to the distance to the fish culture zone (Fig. 4). The diversity index gradually increased from the inside stations (KS3 and KS4) to outside/reference stations (KS1 and KS6), indicating that the benthic community structure was considerably affected by the eutrophic condition derived from fish culture. The concentrations of TOC, TKN and TP of the sediment inside the fish raft area were significantly higher (45.3%, 56.2% and 92.9%, respectively) than the reference sites. This also resulted in a shift in the N:P molar ratio from 8.8 at the reference stations to 1.8 at stations near the fish cages (Fig. 5). Figure 4. MDS plot showing the classification of benthic assemblages according to the distance of sampling stations to fish culture zone. The different bubble size represents Shannon-Wiener diversity index (i.e., the larger the bubble, the higher the index value) The moisture content and silt/clay fraction of the sediment at the 6 stations over the sampling period ranged from 34-66% and 33-98%, respectively. There were no significant changes in these 2 parameters at each station over time (ANOVA, p>0.05). The eutrophic impacts on the benthic community were confirmed by the negative relationships of Shannon-Wiener Index (H’) to the sediment nutrient concentrations: H’= ×TOC (r 2 =0.37, p<0.001) H’= /TKN (r 2 =0.51, p<0.001) H’= ×log(TP) (r 2 =0.45, p<0.001) Figure 5. Spatial and temporal variations in the N:P molar ratio of sediment In temperate mariculture, pelletized fishmeal of relatively better digestibility and lower feed conversion coefficient (ratio of food input to fish production) is widely used, whereas in subtropical Asia-Pacific trash fish is the major feed for fish culture. Such a difference thus results in a higher nutrient loading and nutrient ratio accumulated in the sediment beneath the fish rafts. Consequently, the effects of nutrient enrichment on macrobenthic assemblages are more pronounced in subtropical waters. Urgent attention is thus needed to improve fish farming practices locally, e.g., substitution of trash fish feed by pelletized fishmeal, control of stock density, operation of ‘farming fallow’, rotation of cultured species. In addition, the culture of bivalves as biofilters at fish raft areas may offer potential to reduce localized nutrient enrichment. Such a polyculture practice is being investigated in Hong Kong.