Organic enrichment of sediments from salmon farming in Norway: environmental factors, management practices, and monitoring techniques

Abstract

Environmental impacts of salmon cage aquaculture resulting from deposition of organic-rich particulate matter to the sea bottom have been thought to be a function of the local environmental conditions and management practices. However, testing of these suppositions have been limited by (1) widely varying monitoring methods employed, and (2) lack of data comparability resulting from the absence of standardized national monitoring schemes. In order to determine the sensitivity of different monitoring methods in detecting benthic environmental effects, a comparative analysis was undertaken of four methods commonly employed in Norway that vary in cost and expertise required: (1) visual diver surveys, (2) faunal analysis, (3) sediment chemistry, and (4) Sediment Profile Imagery (SPI). Results indicate that all methods agreed in the common “impact zone” under and immediately next to the cages. However, each of the methods differed in their sensitivity in detecting more subtle effects at greater distances from the cages. Data from 168 environmental survey samples located at various distances from working Norwegian salmon cage farms collected using similar methodology between 1996 and 1998 were analyzed to determine the relationship between environmental variables, management regimes, and levels of environmental impact. Total organic carbon (TOC) levels in sediments were significantly higher immediately adjacent to cages compared to reference sites, and approximately 32% of the samples under the cages showed significant degradation. At intermediate distances (50–100 m), influences from fish farming are not clearly detected by TOC analysis. Further, while neither depth nor current speed alone are good predictors for environmental management, the results suggest recovery of sites by periodic abandonment (or fallowing) is one of the best management tools for sustainable salmon farming in cold-water environments.