Abstract:
Evaluation of the long-term cumulative impacts of fish farming requires long-term data sets. In this study, we analysed water-quality monitoring data collected monthly from 1993 to 1999 and information on phytoplankton available since 1951. Seventeen parameters were measured in water at the farm and control sites: namely oxygen, nitrite, nitrate, ammonia, total phosphorus (TP), soluble reactive phosphate, chlorophyll a, suspended solids, pH, biochemical oxygen demand, chemical oxygen demand, alkalinity, silicate, total nitrogen (TN), conductivity, colour and temperature. Lake bed sediments and water at the cages were analysed for TP, TN and organic carbon in 1988, 1990 and 1998. The phytoplankton indicate that there have been no changes in the oligotrophic status of the lake since 1951. There were no trends in water quality over the past fifteen years. Except for ammonia, no other significant differences in water parameters between farm and control sites were detected. Chemotherapeutants were not detected. Several parameters were more variable in near-shore water samples than in fish cages and open-water control sites. There was seasonal variation in water parameters and a thermocline in summer. At the fish cages, ammonia was elevated above and oxygen depleted below the thermocline. These levels were not a risk to fish health and were not of environmental concern, nor were they close to permitted levels. In sediments there were no significant correlations between TP and TN and none between lake depth or distance from the cages. Overall, this study shows that salmon culture can be conducted in a freshwater lake for over thirteen years without polluting either the water or lake bed.
