Chemical and biological remediation of the benthos near Atlantic salmon farms

Salmon farms in British Columbia now typically produce over 2000 mt of fish during each 18- to 24-month production cycle. An 18-month study of Stolt Sea Farms' Arrow Pass farm was undertaken in 1996 to examine the spatial and temporal extents of chemical and biological benthic effects. Microbial metabolism of farm biodeposits in nearfield sediments resulted in increased sulfide concentrations and decreased redox potential. These physicochemical changes modified the macrobenthic community. Species sensitive to enrichment were quickly extinguished from the immediate vicinity of the farm and were replaced by the organic carbon tolerant opportunistic annelids Capitella capitata and Ophryotrocha cf. vivipara. These effects extended to at least 50 m from the net-pen's perimeter on the downcurrent transect. Chemical remediation, defined as a return to reference levels of organic carbon, a reduction in free sediment sulfides to <960 micromolar and an increase in sediment redox potential to reference conditions, began as soon as the harvest commenced and was complete by the end of harvest. Biological remediation began during the harvest and was complete within 6 months of fallow. Annelids and crustaceans recruited first into the remediating sediments and were followed by at least two seres of mollusks. Routine quarterly monitoring of sediment-free sulfides, redox potential and total volatile solids (TVS) at four Stolt Sea Farms' tenures from 1999 through present has demonstrated that sediment sulfides increase rapidly during the early stages of production and decrease equally quickly during harvest. Fallow periods of 4-6 months appeared adequate for complete chemical remediation at the tenures examined. Elevated concentrations of zinc, which is added as a supplement to salmon feeds, have been observed in sediments. However, they are bound by sulfides and are not biologically available. Zinc concentrations declined to background during chemical remediation. No cumulative effects associated with either organic carbon or zinc were observed at any of these farms during successive production cycles.