Research Document 2017/059

Freshwater Cage Aquaculture: Ecosystems Impacts from Dissolved and Particulate Waste Phosphorus

By Otu, M.K., Bureau, D.P., and Podemski, C.L.

Abstract

Open net-pen finfish aquaculture operations in freshwaters release waste directly into aquatic environments. These wastes contain phosphorus (P) in the form of particulate P (e.g., faeces and uneaten feed) or dissolved P (e.g., soluble P released from feed, faeces, and metabolic excretions of the fish). Phosphorus is the nutrient most often limiting primary production (i.e., photosynthetic rates and biomass) in freshwater and, thus, anthropogenic additions of P raise concerns for potential impacts on lake productivity. Assessing and mitigating the adverse effects of P loading is critical to proper management of freshwater resources.

This Canadian Science Advisory Secretariat (CSAS) review does not constitute a formal risk assessment of P inputs from cage aquaculture operations in Canadian freshwaters, but instead, synthesizes the current state of science with regards to two key components necessary to understanding the potential for effects of P waste from aquaculture: the estimated P loads per tonne of Rainbow Trout (Oncorhynchus mykiss Walbaum) produced and the lake response to such P loads. Estimation of P release from cage aquaculture has been best accomplished through bioenergetics models and one such model has been designed for and tested in commercial Rainbow Trout cage culture operations within Canada. The complex physical, chemical, and biological interactions that may occur within the P cycle mean that impacts of P loading from aquaculture or other anthropogenic sources are challenging to predict. The complexities of the P cycle are particularly well documented in the Laurentian Great Lakes, and here we have drawn on this literature. Attempts to better predict the effect of anthropogenic P loads have prompted the development of water quality models to predict P concentrations; P fractionation techniques to assess bioavailable P fractions in particulate form; whole lake mass balances to quantify the P loss to sediments; and total P loading estimates from the catchment to contextualize P loads in an area of interest.

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