Salmon Aquaculture, Nutrient Fluxes and Ecosystem Processes in Southwestern New Brunswick

Abstract

Salmon aquaculture discharges organic wastes into the marine environment. Salmon metabolism and the waste discharges add nutrients and organic matter to and remove oxygen from both the water column and sediments. The salmon industry in Southwestern New Brunswick (SWNB) is used to illustrate how waste discharges can be estimated in the absence of detailed information on farm operations. A fish growth model and mass balance calculations are used to estimate carbon, nitrogen and phosphorus wastes at the farm scale. Feed nutrition data and environmental measurements are used to partition wastes into fractions, and to estimate the oxygen demand. The predicted demand at the time of maximum discharge is 200 times greater than the oxygen uptake measured in surface sediments at cages, suggesting that most farm wastes are dispersed over wide areas and do not accumulate directly under cages. The number of farmed fish in an inlet is then used to predict total discharges to that inlet. In SWNB, salmon aquaculture is the largest anthropogenic source of organic input to the coastal zone. The significance of the wastes on inlet scales (2–25 km) is evaluated by comparing element fluxes through salmon farms with fluxes due to natural processes: primary production, nutrient regeneration, and community respiration. In intensively farmed bays, fluxes due to salmon farms reach values of 20, 330 and 160% of those due to natural processes for oxygen, nitrogen and carbon, respectively: significant changes to the ecosystem have occurred in these bays. Spatial scales are critical in describing such impacts: effects will be greater close to the farms, and smaller when averaged over larger areas.