Quantifying aquaculture-derived organic matter in the sediment in and around a coastal fish farm using stable carbon and nitrogen isotope ratios
Introduction
Intensive fish farming in coastal waters generates large amounts of particulate organic wastes in the form of waste feed (unconsumed feed) and fecal matter. Such particulate organic wastes settle onto the seabed and produce enriched sediments, which result in deoxygenation of the bottom water, the production of reduced compounds such as ammonium and sulfides and changes in the structure of benthic communities (reviewed by Gowen et al., 1991, Wu, 1995, Findlay and Watling, 1997, Pearson and Black, 2001). Such environmental deterioration often produces negative consequences for farm management.
The degree and extent of the impacts from fish farming have been investigated, and it has been revealed that the impacts on the benthic environments are localized; that is, the effects do not usually extend beyond 25 to 250 m distance (reviewed by Brown et al., 1987, Gowen et al., 1991). In most of these previous investigations, dispersion and deposition of organic wastes have been quantified by the analysis of bulk organic matter in the sediment in terms of carbon and nitrogen elements and gross indicator such as ignition loss and chemical oxygen demand (COD). However, bulk organic matter alone may not provide an adequate estimate of the extent to which organic matter from aquaculture disperses, because the bulk organic matter is composed of various sources. A knowledge of the net accumulation of aquaculture-derived organic matter (AOM) in the sediment is necessary for the objective assessment of the potential environmental impacts arising from fish farming. Metals such as Cu and Zn (Chou et al., 2002) and lipids (Johnsen et al., 1993, Henderson et al., 1997, McGhie et al., 2000) in the sediment have also been used as tracers of aquaculture wastes. However, such attempts provided little insight into the quantitative aspects of aquaculture-derived organic matter. It is necessary to find a reliable and convenient tracer to quantify aquaculture-derived organic matter in the sediment.
Stable isotope analysis has been used successfully not only in determining sources of nutrition for consumers and trophic relationships among organisms but also in assessing the mixing ratio between different sources of organic matter such as terrestrial plants and marine phytoplankton (e.g., Wada et al., 1987). Recently, the stable isotope technique has been used to trace the fluxes of aquaculture wastes entering the food webs of a shrimp pond (Yokoyama et al., 2002), a lake (Grey et al., 2004), a tidal creek (Costanzo et al., 2004) and nearshore waters (Vizzini and Mazzola, 2004), and transformations of particulate matter in a recirculating sea bass rearing system (Franco-Nava et al., 2004). Stable isotope analyses have also been used to determine the influence of waste deposition from mariculture fish farms to sediments (Ye et al., 1991, McGhie et al., 2000, Yamada et al., 2003). Ye et al. (1991) first tried to quantify the contribution of aquaculture-derived organic carbon (AOC) to total organic carbon in the sediment based on the δ13C values for fish feed (− 21.53‰), fish feces (− 20.48‰), settling organic matter at a station under a cage (− 19.25‰) and a reference station (− 13.03‰), and sedimentary organic matter (SOM) under a cage (− 21.76‰). They calculated the δ13C value for AOC as − 24.14‰ and estimated the dispersion and effects of fish-farm wastes on marine sediments. Their methods, however, may not be applicable to other localities, because the δ13C value for the background organic matter was markedly enriched probably due to the contamination of organic matter from seagrass, and the substances in AOC, of which δ13C was more depleted than values for the feed and feces, were not identified. The other isotopic studies provided scarce information about quantitative aspects of AOM.
In the present study, we tried to differentiate waste feed and fecal matter from bulk SOM using the dual isotope technique. The purpose of our research is to: (1) develop a method to quantify waste feed and fecal matter in the sediment using stable carbon and nitrogen isotope ratios, (2) determine the spatial extent of AOM around a fish farm, and (3) examine the relationship between deposition and accumulation of AOM into the sediment and environmental factors.
Section snippets
Study area
Gokasho Bay has a ria style coastline with an area of 22.2 km2 and a mean depth of 12.7 m (Fig. 1). Freshwater flows into the bay mainly from the Iseji River, which drains 39 km2 of forested land covered by evergreen broad-leaved trees (C3 plants) that are almost free of anthropogenic influence. The average flow rate of this river is 1.1 m3/s (Iwasaki et al., 1997). Amplitude of tidal current in the middle layer at the mouth of Gokasho Bay is 20 cm/s, whereas the mean velocity reduces to 2 to 8
Waste feed
Moist pellets were produced by mixing raw fish such as anchovy Engraulis japonicus, chicken grunt Parapristipoma trilineatum and sand lance Ammodytes personatus (mean δ13C = − 18.4‰, mean δ15N = 11.8‰), krill (− 20.9‰, 7.0‰) and mash (− 20.0‰, 10.0‰) (Table 1). The mean (± SD) δ13C and δ15N for the 12 samples of moist pellets used in the Hazama-ura fish farm were − 20.2 ± 0.3‰ (range = − 20.6‰ to − 19.7‰) and 10.2 ± 0.8‰ (range = 8.8‰ to 11.2‰), respectively.
The mean δ13C and δ15N for materials that were used for
Discussion
In this study, we could successfully quantify the aquaculture-derived organic matter in the sediment in and around the fish-farm area based on the isotopic compositions of waste feed, fish feces and SOM reference, which were distinct from each other. This approach is based on the assumption that isotopic ratios are conservative and that physical mixing of organic matter sources determine the isotopic distribution of SOM.
In the study area, the isotopic composition of SOM varied progressively
Acknowledgements
Sincere thanks are due to Kumano-nada Fisheries Cooperative Association, Sakamoto Feed Co. Ltd., Nichimo Co. Ltd. Maruyo Kaisan Co. Ltd. and Ishikawa Syoukou Co. Ltd. for providing us fish feed, and Drs. Takeshi Yamamoto and Tsuyoshi Sugita for instructing us in feces collection and for providing information on fish nutrition, and Dr. Yoshihiro Yamada for his useful suggestions. This research was conducted under the project ‘Multifunctionality of fisheries and fishing villages,’ which was
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