US - In the past two years there has been a great deal of discussion about net-pen aquaculture in the Michigan waters of the Great Lakes. Much of the concerns around Great Lakes net-pen aquaculture is the generation of large quantities of fish waste from these fish production operations. The main issue with fish waste is the release of phosphorus which is the growth-limiting nutrient for primary production in freshwater ecosystems.
Although some phosphorus is necessary to drive the freshwater food chain, the concern arises when excess amounts of phosphorus are available which can result in significant algal blooms and other aquatic plant growth.
To help address the issue of excess phosphorus discharge from freshwater net-pen aquaculture Fisheries and Oceans Canada completed a study on Freshwater Cage Aquaculture: Ecosystems Impacts from Dissolved and Particulate Waste Phosphorus.
The Canadians have had net-pen aquaculture operations in their northern waters of Lake Huron since 1982. Fish digest and absorb phosphorus to sustain their life processes and form body tissue. Fish receiving digestible phosphorus in specific amounts to meet their growth requirements excrete only small amounts of dissolved phosphorus. Dissolved phosphorus is most often the form of concern in impaired waters.
The other form of phosphorus excreted from fish is particulate phosphorus which settles to the bottom sediments. The particulate phosphorus accounts for the majority of the waste from net-pen aquaculture operations which is transported to the bottom sediments and is not immediately available for uptake into the ecosystem. In sediments it can be taken up by the benthic community and enter the aquatic food chain. Both dissolved and particulate phosphorus wastes produced by fish are the results of the diets they consume. The development of low phosphorus high efficient diets has been a tool to help minimize phosphorus waste by aquaculture operations.
The Fisheries and Oceans Canada study found that based on 2006 net-pen aquaculture production in northern Lake Huron and watershed phosphorus loading values that these operations are contributing about 5% of the annual total phosphorus loads to the North Channel.
The study concluded that the likelihood of phosphorus additions to the environment from net-pen aquaculture operations resulting in eutrophication to Canadian freshwater environments under the current level of fish production can generally be characterized as “low.”
The greatest concerns for phosphorus are in the nearshore areas where excess aquatic plant growth can foul the shorelines. In contrast, offshore phosphorus loading is of less concern and higher phosphorus loads may be considered advisable as a means to help mitigate concerns regarding low levels of forage fish and the poor condition of sport and commercial fish species.
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