Estimation of waste outputs by a rainbow trout cage farm using a nutritional approach and monitoring of lake water quality
Introduction
The environmental impact of waste from the fish culture industry, notably from cage culture operations, is increasingly a matter of close scrutiny by the public and various levels of government in Canada and elsewhere around the world. The main concern is the release of solid and dissolved phosphorus (P) and nitrogen (N) wastes since these wastes may lead to environmental degradation. Environmental compliance of cage culture operations is generally assessed through measurements of nutrient concentrations in receiving waters to assure that certain target water quality criteria are not exceeded (Ontario Ministry of the Environment, 1993, Boyd et al., 2001). For example, in Ontario, cage farms are required to take depth-integrated (over depth of cages) water samples for measurement of total P, 30 m from each side of the cages and at two remote reference stations eleven times during the ice-free period (Boyd et al., 2001). There is reason to believe, however, that these measures might not be particularly sensitive to cage outputs.
Net-cage farm sites very often are located in a dynamic, multi-user environment where it is difficult to differentiate farm-loaded nutrients from natural and other anthropogenic sources, and where the effects from separate point sources such as cage farms will not be easily detected (Reid et al., 2006). The rate of horizontal mixing in epilimnetic waters is very rapid (Quay et al., 1979), and dissolved phosphorus, the primary nutrient of interest, is taken up rapidly (within minutes) by bacteria and phytoplankton (Levine et al., 1986) and is removed to sediments through the processes of consumption, senescence and settling (Schindler et al., 1971). Cage rainbow trout production results in the release of approximately 7.3 kg of P to the environment for every tonne of fish produced (Bureau et al., 2003). However, the majority of the waste P (> 60%) is particulate and has a settling rate of ≥ 6 cm/s (Reid et al., 2009), meaning that within a few minutes, most of the faecal material that is generated above the epilimnion will pass to the sediments. Only a portion of the particulate (faecal) P transported to the hypolimnion will be bioavailable (Hua and Bureau, 2006), and once in the hypolimnion, slow diffusion across the thermocline density gradient will largely prevent the movement of P back up into the epilimnion. Thus, P becomes largely unavailable for the production of nuisance algae or detection by epilimnetic water monitoring programs until thermal mixing occurs. As thermal mixing occurs only two times a year in a dimictic system such as Lake Huron, there can be a time lag of up to 6 months between the deposit of P into the water and its availability for sampling over the depth of the cages. The horizontal mixing rates of epilimnetic waters, the dynamics of P loading from fish farms, and the behavior of aquaculture-origin P cycling in the dimictic lake environment all combine to make detection of waste release by monitoring water quality challenging.
Several nutritional-based models have been proposed to estimate the waste outputs from aquaculture operations (Cho and Bureau, 1998, Stigebrandt, 1999, Davies, 2000, Bureau et al., 2003). The models of Stigebrandt, 1999, Davies, 2000 are currently used as environmental management tools for the salmon cage farming industry in Scotland and Norway, respectively. The application of a modified version of the Fish-PrFEQ model as an environment management tool has been adopted by the land-based industry in France (Papatryphon et al., 2005) and has been proposed for the cage aquaculture industry in Canada (e.g., Bureau et al., 2003, Reid, 2004).
The Fish-PrFEQ model is based on feed quality, quantity of feed used, biomass of fish produced and composition of that biomass. Because of its nutritional and bioenergetic nature, provided that accurate measures of the feed (nutrients) ingested and retained by the fish are known, the same degree of accuracy would be expected of waste estimations. Furthermore, the bioenergetic nature of the Fish-PrFEQ model allows incorporation of the effects of water temperature on feed intake, growth, metabolic and excretion rates. This is particularly important when modeling excretions by fish growing under less controlled conditions, such as cages in lakes, where fish may be exposed to seasonally and spatially (i.e., over depth) varying water temperatures.
The present study is part of multidisciplinary, whole-lake experiment to investigate the effects of cage aquaculture on the freshwater environment. This component of the project is concerned with both the quantification of waste loading from the farm and the investigation of its effects. The first objective was the quantification of the farm nutrient loading. For this purpose, the Fish-PrFEQ model was calibrated with actual (measured) feed composition, nutrient digestibility, quantity of feed used, water temperature, growth rate and biomass of fish produced at a trout cage and the fish farm waste loading estimated. The second objective of this study was to examine the utility of water chemistry measures in detecting nutrient loading of cage origin. The relationship between predicted waste outputs and measured water chemistry parameters was investigated to test the suitability of these parameters as tools for environmental assessment of fish cage farms.
Section snippets
Location of experimental area and cage settings
Two production trials were carried out between May and October 2003 and 2004 in Lake 375, an oligotrophic double basin (north and south basins) lake located at the Experimental Lakes Area (ELA), northwestern Ontario, Canada (49°44′43.61″N, 93°47′15.56″W). This lake was the object of frequent monitoring over two decades and long term pre-cage (1982–2002) mean TP and TN values (mean ± SD, n = 314) of 6.3 ± 4.5 μg L− 1 and 245.2 ± 92.0 μg L− 1, respectively were available. The lake has a surface area of 23.2 ha
Model inputs — feed, fish growth and water temperature
Measured nutrient contents (Table 1) and nutrient digestibility (Table 2) of the commercial diet used during the 2003 and 2004 production trials were used as model inputs to calculate nutrient intake and digestible nutrient intake, respectively. The nitrogen content of the diet (%CP/6.25, as is basis) was 7.1% and the phosphorus content was 1.1% (as is basis) resulting in a dietary N:P ratio of 6.4.
Measured initial body weight, weight gain and growth rate (expressed as TGC) (Table 3) were used
Farm waste loading estimations
The present study is part of an ongoing, multidisciplinary, whole-lake experiment to examine the ecological effects of freshwater cage aquaculture. This component of the experiment was concerned with testing our ability to quantify farm nutrient loading and our ability to detect this loading by monitoring of water quality. Some observations from this project of the effects of waste loading have been reported elsewhere (e.g., Bristow et al., 2008, Findlay et al., 2009, Kullman et al., 2009,
Acknowledgements
This project was supported by the Aquaculture Collaborative Research and Development Program (ACRDP) of the Department of Fisheries and Oceans, Canada and by the Northern Ontario Aquaculture Association, Ontario, Canada. Martin Mills, Inc. provided the fish feed for this project. We thank Dr. Michael Turner, Kelly Hille and Adam McFee for constructive comments on this manuscript. We thank Dr. Patrick Buat, Adam McFee, Kelsey Fetterly, Rebekah Rooney, Adrianne Lewis, Erin Forster and Jesse Hamel
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