Elsevier

Aquaculture

Volume 187, Issues 3–4, 20 July 2000, Pages 351-366
Aquaculture

The degradation of fish-cage waste in sediments during fallowing

https://doi.org/10.1016/S0044-8486(00)00317-3Get rights and content

Abstract

The composition and rate of degradation of organic waste deposited in sediment underneath and adjacent to fish cages in the Huon Estuary, Tasmania, Australia, were investigated. Sediment samples from two near-adjacent sites, but with different sediment types and depths, were analyzed for total organic matter, lipids (fatty acids and sterols), %C, δ13C, %N, δ15N, and redox potential during a 12-month fallowing period. Additionally, representative samples of fish food and fish faeces were analyzed. It was found that most of the accumulation of organic matter was confined to an area directly underneath the fish cages, but at 30 m from the center of the cage, indicators of fish cage waste (faeces and fish food) were still elevated compared with reference sites. As both fish food and faeces have distinctive fatty acid profiles, the relative proportion of food and/or faeces deposited on the sediment could be determined. After 12 months fallowing, fish-farm-derived organic matter in surface sediment at the center of the cage remained greater than at 30 m distance, even though redox potentials indicated that normal oxic conditions had returned.

Introduction

During salmonid production, substantial quantities of organic matter are applied as food. A substantial quantity of food is deposited to the sediment either directly or as fish faeces. Sedimentary organic matter will accumulate over a period of time if removal by biochemical degradation and physical processes are less than the input from farming activities. Although the rate of organic matter accumulation is influenced by the physical nature of the location, particularly water current velocities, in most cases, organic matter will accumulate under and/or adjacent to fish cages. The input of highly labile organic matter can modify the sediment characteristics and promote chemical processes with products that may be detrimental to farmed fish Weston, 1990, Ye et al., 1991, Findlay et al., 1995, Wu, 1995. For example, the production of methane and hydrogen sulphide is toxic to fish and highly undesirable for fish farm operations. The input of highly labile organic matter can also induce a substantial impact on the benthic communities directly under and adjacent to fish cages Weston, 1990, Ye et al., 1991, Wu, 1995.

In the Huon Estuary, Tasmania, the normal practice is for fish to be removed periodically (by removal of the cage) and the sediments allowed to fallow. In an earlier study, the respiration rates of sediment during a short fallowing period were investigated (Woodward et al., 1992). However, the dynamics of fallowing have not been extensively studied, and the optimal time required for fallowing has not been determined. So that the duration of fallowing required for the sediment to recover and to allow restocking with fish could be better determined, we investigated the distribution of accumulated fish waste, and the rate at which organic deposits degrade in sediment under and adjacent to fish cages. An adequate fallowing period avoids cumulative increases of organic matter above site-specific background levels in sediments directly under fish cages.

Section snippets

Trial site

The trial site was located at the Huon Aquaculture lease at Hideaway Bay in the Huon Estuary, Tasmania, Australia. At the time of this study, the total leased site was 14 ha and annual production was based on 260,000 smolt. Transects were established at two different cage sites. Site 1 was located at a depth of 15 m and the transect extended for 30 m westward (up river). Sampling stations were located at 0, 10, 20, and 30 m along the transect from the center of the cage. Site 2 was located at a

Results

The lipid composition of fish food and trout and salmon faeces were determined (Table 1). The major sterol in fish food was cholesterol with minor amounts of 5α-cholestanol, sitosterol, and 5α-sitostanol. Cholesterol was also the major sterol in faeces, but at substantially reduced concentrations compared with fish food. The microbial degradation products of cholesterol (coprostanol and epicoprostanol) were present in faeces, but at low concentrations; whereas they were not present in the fish

Faecal and food input to the sediment

The organic matter derived from the fish-farm accumulating underneath fish cages will be either fish faeces or fish food that has not been consumed. Differences in the sterol and fatty acid compositions of fish food, and of salmon and trout faeces, indicated that the fatty acid composition can be used to distinguish fish food from faeces. The fatty acid composition of fish food, in particular the relatively high levels of 20:5ω3 (EPA) and 22:6ω3 (DHA) in fish food samples, are consistent with

Acknowledgements

Sally Rozemulder provided technical assistance with the analysis of lipids, Andrew Revill performed the analyses for δ15N, %N, δ13C, and %C. Rhys Leeming, Peter Nichols, and John Volkman provided advice on the analysis and identification of sterols and fatty acids. The research reported here is a component of the Huon Estuary Study. This study is led by Dr. Ed Butler and is funded jointly by CSIRO and the Fisheries Research and Development with additional support from the aquaculture industry.

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