Demonstration of environmental effects of marine fish-farms: Quantification of nutrient- and particle-outputs
as a potential food-resource in
integrated seaweed- and mussel-farming

Distributions of dissolved nutrients and particulate matter downstream from a ma- rine fish farm at the Norwegian coast were examined. The samples were taken on transects from 25 m to 215 m distance to a specific fish cage. The samplings were carried out repeatedly while there was fish in that cage and once, after the fish were taken out. The outflow from a fish cage was estimated and a time series of current velocities was recorded downstream from a fish farm. The concentrations of nitrate, nitrite and phosphate were at very low levels and did not reveal any dependence on the distance to the fish cages. An influence of the fish farm on nutrient levels was only visible in the ammonia concentrations, which ranged around 15 μg NH4-N · l−1 on average and showed heavy fluctuations along the transects. Seaweed profits from higher ammonia concentrations in general, but enhanced growth in the study area would be limited by phosphate. On average, the concentrations of total particulate matter and particulate organic matter were at low levels, but showed an increase from 20.04.2005 to 02.06.2005 and a decrease after the fish were taken out of the net cage. This may have reasons other than the clearance of the fish cage, as an effect of the discharge from the cage on the concentrations of particulate matter within more than 50-60 m distance is highly unlikely. The fraction of organic matter was on high levels around 80% throughout the whole period of the study and did not show any dependence on the distance to the fish farm. Structures were found in the wake of a fish farm, that indicate the existence of eddies or swirls in the flow. It is very likely, that a vortex street develops downstream of a net cage, which would be associated with a recirculation area close to the cage. Such a wake characteristic might suppress the horizontal spreading of particles leaving the cage. A net outflow out of a fish cage was found from 3-23 m depth, which indicates the existence of some internal force. This might well be generated by fish swimming in circles. Fish behaviour, therefore, might play a role in the spreading of particles, as already small changes in the strength of the outflow might change the characteristics of the wake flow.