Freshwater rainbow trout (Oncorhynchus mykiss) farming affects sediment and pore-water chemistry

Abstract

Marine aquaculture has come under scrutiny, whereas little is known about the nature and extent of the effects of cage aquaculture in freshwater. We describe the development of changes in sediment and pore-water chemistry caused by an experimental Oncorhynchus mykiss farm located in freshwater Lake 375 (Experimental Lakes Area, north-western Ontario, Canada) during its first two production cycles along a distance transect from the cage. Significant changes in sediment chemistry were quick to develop; pore-water ammonia was elevated under the cage after 1 month and sediment nutrients were elevated 1 month later. The effects on the benthic environment were spatially localised, although variables responded to different extents. Within 16 months, nutrient concentrations in surface sediment reached an asymptote, whereas concentrations of metals and ammonia continued to increase. Copper (Cu) and zinc (Zn) concentrations under the cage reached levels that exceed sediment quality guidelines and may cause adverse biological impacts. An anti-fouling coating applied to the net pen was the major source of Cu, whereas Zn originated also from fish feed. Ammonia and pH are recommended for inclusion in monitoring programs because they were sensitive to fish farming and are biologically relevant. We also suggest inclusion of Cu and Zn in monitoring programs at farms with treated nets.