Acoustic detection of organic enrichment in sediments at a salmon farm is confirmed by independent groundtruthing methods

Abstract

Acoustic backscatter contrast in depositional sediments under salmon farm cages in the Bay of Fundy, Canada, was correlated with localized changes in (unknown) sediment geotechnical properties, as indicated by 4 independent measures of organic enrichment. Sediment total sulfides and redox potentials, enzyme hydrolyzable amino acids, sediment profile imaging and macrofaunal samples, taken at mid-cage positions, each rejected the null hypothesis that salmon cage footprints, defined acoustically as high backscatter areas, were indistinguishable from nearby reference areas. Acoustic backscatter imaging appears capable of mapping organic enrichment in depositional sediments caused by excessive inputs of salmon farm wastes associated with intensive aquaculture.

Empirical relationships describing benthic impacts of salmon aquaculture

“…2). Eh NHE potentials observed in the same area in 2002 (data from Wildish et al 2004) and those from the nomogram (Table 2) generally fall above this regression line when S is < 750 µM and below the line at higher S concentrations (Fig. 1a).…”

“…The transition from oxic to hypoxic sediments also matches the inflection point observed in BQI and other indices reflecting decreased biodiversity under increasingly hypoxic and anoxic conditions. Wildish et al (2004) observed high values of S (2500 to 30 000 µM) in sediment under a salmon farm in southwestern Bay of Fundy associated with a median BHQ value of 0 where Beggiatoa bacterial mats were extensive. Bacterial mats were absent at non-farm locations, S was lower (350 to 2500 µM) and median BHQ values were 8 to 9 (range: 6 to 10).…”

“…1b). The calculated curve describes the relationship between the variables observed in surface sediments from salmon farm sites in southwestern New Brunswick (Wildish et al 2004) and in Norway (Schaanning & Hansen 2005). The median pH in surface sediments from under net-pens in Norwegian salmon farms was lower (7.22) than in non-farm reference areas (7.85).…”

“…However, regression analysis showed that for both data in Table 2 and median values of CS reported by Chamberlain & Stucchi (2007) S was linearly related to CS over the range 0.01 to Table 2. s: data from Letang Inlet, Bay of Fundy in 2002 described in Wildish et al (2004). n: median, minimum and maximum pH values measured below occupied and abandoned farms and non-farm reference areas of coastal Norway from Schaanning & Hansen (2005).…”

“…Studies over the past decade have shown that geochemical methods can also be used as cost-effective measures of impacts of sedimentation of aquaculture-derived waste on surface sediments (Wildish et al 1999, 2004, Brooks 2001, Brooks & Mahnken 2003, Holmer et al 2005. Hargrave et al (1997) compared different variables measured in surface sediments around salmon farms in the southwestern Bay of Fundy and found that S and sediment-water fluxes of O 2 and CO 2 were more sensitive to benthic organic enrichment than biological indicators such as macrofauna biomass or the proportion of deposit and suspension feeders.…”