Stable isotopic evidence for uptake of fish farming induced pollutants by filter-feeding mussels (Perna viridis) in a polyculture system

Gao, Qin-Feng1; Lin, Guang-Hui2, 3; Chen, Shi-Ping3; Cheung, Siu-Gin1; Shin, K. –S. Paul1

1Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China2Department of Global Ecology, Carnegie Institute of Washington, 260 Panama Street, Stanford, CA 94305, USA

3Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of Sciences, Beijing 100093, China

IntroductionFish farming activities can result in environmental pollution due to the release of organic and inorganic wastes from uneaten feed, feces and dissolved excretory products. In Hong Kong, trash fish (mainly Thryssa hamiltonii) is the major feed for fish culture. Compared with modern pelletised fishmeal, trash fish is inherently wasteful as a consequence of their lower digestibility and tendency to break up and shed small unconsumed particles during feeding (Leung et al, 1999). Filter-feeding bivalves can take up particulate matter in the water column in a considerable efficiency owing to the nature of their high filtration rate and large population density (Dame, 1996). In an polyculture system combining fish and filter-feeding mussels, mussels can utilize the organic wastes from the culture cages as food sources and function as biofilters to remove the cultural wastes. In present study, we used multiple stable isotope (carbon and nitrogen) to trace the respective contribution of uneaten fish feed and egested fish feces to the food of the mussels cultured in fish cages so that the assimilation efficiency of cultural wastes by mussels was evaluated.

Materials and Methods

The fish culture zone, Kau Sai Bay, is a semi-closed embayment located at the east part of Hong Kong with culture area of 4.6 ha. Water depth ranges from 11-16m and current speed 1.5-22.7 cm s-1. The average stock density is 4.5 kg m-3. Cultured species mainly include grouper, snapper and bream (figure 1).

The study site

3. From culture and reference sites, ~30L seawater was collected and filtered. Sediment was also collected with a van Veen grab (Figure 3). Inorganic carbonate content of the particulate matter and sediment was removed with 1.2N HCl prior to stable isotope measurement.

4. Carbon and nitrogen stable isotope ratios of all samples were measured with an isotope ratio mass spectrometer interfaced with an element analyzer (AE-IRMS, Finnigan MAT DELTAplusXP). Results were expressed as

δ(‰) = (Rsample/Rstandard-1)X1000

5. The respective contributions of potential food sources were computed with the linear isotope mixing model (McClelland and Valiela, 1998).

Figure 1 The fish culture site

Figure 2. The green mussel, Perna viridis

Figure 3. A modified van Veen grab for sediment sampling

1. Green mussels (Perna viridis, figure 2) from a single population were transplanted to a fish culture cage and a reference site 1 km away from the central culture site without effects of fish farming activities.

2. After 3-month acclimation, ~10 mussel individuals (50-60 mm shell length) were collected, pooled and homogenized.

References

Dame R. F., 1996. Ecology of marine bivalves: an ecosystem approach. CRC Press, Boca Raton.

Leung, K. M. Y., J. C. W. Chu and R. S. S. Wu, 1999. Nitrogen budgets for the areolated grouper, Epinephelus areolatus, cultured under laboratory conditions and in open-sea cages. Marine Ecology Progress Series 186: 271-281.

McClelland, J. W. and I. Valiela, 1998. Changes in food web structure under the influence of increased anthropogenic nitrogen inputs to estuaries. Marine Ecology Progress Series 168: 259-271.

Results and discussion

Figure 4: Dual isotope plot showing the food source of mussels cultured in fish cages. Based on linear isotope mixing models, the contributions of particulate organic matter, fish feed and fish feces to mussel food were 56.7%, 34.2% and 9.2%, respectively.

Figure 5: Mussel tissue living inside the fish cages were significantly 13C- and 15N-enriched relative to those at the reference site (t4=7.02, p<0.01), indicating the uptake and assimilation of isotopically heavier fish feed and fish feces.

Isotopic evidence showed that filter-feeding bivalves in polyculture systems can be used as biofilters to reduce the organic pollutants derived from fish farming activities.