Seven year remediation study at the Carrie Bay Atlantic salmon ( Salmo salar) farm in the Broughton Archipelago, British Columbia, Canada

Abstract

Numerous studies describing the short-term recovery of benthic habitats impacted by organic wastes from commercial salmon farms have been published. A few of these studies have continued for up to 2 years. The Carrie Bay salmon farm in the Broughton Archipelago of British Columbia began operation in 1990 and produced four crops of Atlantic salmon. It was fallowed in 1998 after producing 2587 mt of Atlantic salmon in the preceding production cycle. Significant accumulations of organic waste were observed near the netpens in 1997 when the farm was close to maximum salmon biomass. Bottom sediments were blackened with iron sulfides and gas ebullition together with extensive Beggiatoa mats were observed to a distance of 145 m on the southern transect in 1997. Patchy Beggia remained near the netpens in 1999. Twelve benthic surveys have been completed in Carrie Bay over the last 7 years. The database reported herein describes slow, but steady, remediation of this site. Chemical remediation, including reductions in total volatile solids (TVS), free sulfides, redox potential and zinc were nearly complete at distances >80 m in 2002 and rapid and significant additional chemical remediation was observed throughout the area in 2003. Sediment concentrations of zinc have declined throughout this study at all distances and were approaching background concentrations in 2002 and 2003. Macrofaunal communities at Carrie Bay's reference stations were dominated by mollusks with significantly fewer annelids and very few crustaceans. This community did not respond to organic enrichment in the classic way described by Pearson and Rosenberg [Oceanogr. Mar. Boil. Annu. Rev. 16 (1978) 229], which has accurately predicted the macrobenthic response at many intensive aquaculture facilities. None of the eight opportunistic taxa frequently found proliferating in enriched sediments near B.C. salmon farms were a large component of the Carrie Bay macrobenthos at any place or at any time. Rather, a single annelid ( Nephtys cornuta) was found in moderate abundance and several bivalve mollusks that dominated local reference communities have slowly recruited into the remediating sediments. Biological remediation was nearing completion at distances ≥80 m from the netpens, but was not complete under and at shorter distances in 2002 following 4 years of fallow. Based on regression analysis, it is predicted that chemical remediation (sulfides ≤447 μM) sufficient to support half of the common taxa observed at the local reference station will be complete in 2004 following approximately 65 months in fallow. Unlike four other farms in the Broughton Archipelago, where sulfide concentrations exceeded 20,000 μM, and where chemical remediation has been complete in less than 6 months, sulfide concentrations at Carrie Bay have remained <9410 μM throughout this study. These relatively lower sulfide concentrations; the unique pattern of biological remediation; and the relatively long chemical and biological remediation times observed at Carrie Bay point out the need for a better understanding of the diagenesis of labile carbon compounds in sediments underlying intensive aquaculture operations. A better understanding of the underlying causes of the biochemical processes observed in this study will significantly aid farm operators and regulators in selecting benthic habitats with optimal sustainability for modern salmon farming. In addition to describing the transport of waste, computer programs designed to predict the environmental response to intensive aquaculture must incorporate modules assessing the biogeochemical and biological response to these inputs before their output can predict the unusual responses observed in Carrie Bay.