Science Advisory Report  2015/051

Freshwater cage aquaculture: ecosystems impacts from dissolved and particulate waste phosphorus

Summary

• Phosphorus (P) is biologically significant and both naturally and anthropogenically sourced. Naturally, P is found in low but variable concentrations in water bodies and tends to be the growth-limiting nutrient for primary production. P concentration in lakes is significantly correlated with changes in lake productivity and water quality. Anthropogenic change to P loading to a lake ecosystem therefore should be considered in the process of managing activities that can contribute to total P loading.
• To accurately predict and manage potential effects of P additions to lake ecosystems from aquaculture, it will be necessary to develop an understanding of the factors that can influence P cycling on an ecosystem-specific basis. In particular, further work needs to be done to understand the factors influencing release/retention of P from sedimented waste material and how food web structure may influence P cycling between the water column and the sediments.
• Introduction of invasive species and changes to landscape use have the potential to alter P cycling within a lake ecosystem. A change in landscape use can significantly alter not only total P loading but also the ratio of dissolved to particulate P loading. Therefore, continued monitoring of catchment P loading and alterations to trophic structure within the water body are needed in ecosystems that are being managed to accommodate aquaculture in order to understand the total P loads.
• P excretion by aquaculture fish is directly linked to amount and digestibility of P in the diet.
• P loading from aquaculture can be minimized through the use of low content, highly digestible-P in feeds.
• Nutritional modelling of feed and husbandry data provides an accurate estimate of the contribution of P that is not quantifiable by sampling the receiving water because of rapid dispersion, settling, and transformations that occur to waste P once it enters the environment.
• Management of the potential effects of aquaculture-related P inputs to lake ecosystems would be improved by employing nutritional modelling to characterize the amount, form, and timing of P loading.
• Total farm P output is not an accurate measure of potential eutrophication risk of P addition to the ecosystem. The timing of delivery and the fate of the P loading can significantly modify that risk.
• Ecosystem-specific differences in limnology, geochemistry, and trophic structure can modify P cycling, therefore further science is needed to determine the fate of aquaculture P in ecosystems supporting commercial cage culture.
• The low level of monitoring of tributary P inputs is a significant source of uncertainty in the estimation of the total P loads to the North Channel of Lake Huron. There is a need to significantly improve spatial monitoring of P sources to ecosystems that are being managed to accommodate for aquaculture. The accurate estimation of P loading is necessary to determine P budgets.
• Natural variability in annual precipitation can significantly affect watershed P loading to lake ecosystems. It would be advisable to manage all anthropogenic P loading such that there is room to accommodate the natural range of variation in watershed loading and still prevent deterioration of water quality.
• Although the aquaculture industry is currently only contributing approximately 5% of the P load to the North Channel of Lake Huron, the biological importance of P to lake productivity and the need to meet International Joint Commission (IJC) P load targets suggest that P loading needs to be considered in the management of future industry expansion or other activities known to contribute to P loading.
• Development of future P load targets would be improved by consideration of nearshore versus offshore responses to P loading and consideration of particulate versus dissolved forms of loading. Development of separate load limits for nearshore versus offshore and for dissolved P versus particulate P would improve management of the North Channel ecosystem.
• A watershed management strategy that considers not only aquaculture but also other activities within the watershed and their potential contribution to P loads would be of benefit to all stakeholders.
• A significant portion of the P waste (dissolved and particulate) added to a temperate lake by aquaculture operations is sequestered to the sediments, as would be the case for ‘natural’ P coming from tributaries. Therefore, the total P loading is not necessarily an accurate predictor of the eutrophication potential of the P loading. It is the amount of P that remains in the water column and that which is released from sediments that needs to be considered.
• Data, however limited, exists which indicates that the forms of P found in sediments under and around fish cages differs in relative quantities from those found in sediment at reference sites.
• The form of P, as well as the quantity, needs to be understood to properly characterize the potential for remobilization and thereby the development of internal loading. Internal P loading releases P that was sequestered in the sediments and makes it bioavailable. This release is undesirable as it contributes to eutrophication potential.
• Additional sampling for P fractionation at commercial cage aquaculture sites and reference sites, as well as measures of P release from the sediments (i.e., becomes soluble), would improve the ability to characterize the risk that this P accumulation in sediments poses to eutrophication potential.
• The ability to manage the cumulative impacts of freshwater aquaculture in Canada may be improved by using European models that have specifically incorporated aquaculture in predicting the response of whole lake ecosystems to P loads.
• These models need to be verified in Canadian ecosystems because the structure of the foodweb and sediment chemistry may vary.
• Nutritional strategies can be a direct and effective means to manage P release from fish culture.
• Development of feed that is more efficiently utilized by fish could be a means to reduce feed usage and waste outputs.
• In Ontario, licensing requirements that incorporate the use of feed quotas and the requirement for low P feed use have proven effective at mitigating the eutrophication potential of P waste from fish culture operations.
• Cage siting requirements that minimize the potential to develop internal P loading (e.g., sites with sufficient flushing) and routine environmental monitoring are currently used to mitigate the risks of P loading.
• Additional refinements in siting criteria such as fallowing or placement in offshore areas could be developed to further mitigate risks of aquaculture P loading in nearshore freshwater areas. Fallowing is not currently carried out for freshwater cage aquaculture. Movement of operations to the offshore could take advantage of higher P loading targets and would also benefit from greater water depths resulting in greater waste dispersion.
• The likelihood of P inputs from the cage aquaculture industry resulting in eutrophication of Canadian freshwater environments, under the current level of industry production and practices, can generally be characterized as ‘low’.
• Future growth of the industry or expansion of any other anthropogenic activity that contributes to the P loading requires careful consideration of the P input from all sources.

This Science Advisory Report is from the June 17-19, 2014 regional peer review of Freshwater Cage Aquaculture: Ecosystems Impacts from Dissolved and Particulate Waste Phosphorus. Additional publications from this meeting will be posted on the DFO Science Advisory Schedule as they become available.

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