Characterization of suspended particulate matter surrounding a salmonid net-pen in the Broughton Archipelago, British Columbia

Abstract

A field study was carried out on the central coast of British Columbia in March 1999 to determine particle fluxes arising from a salmonid net-pen during feeding. Water samples were collected within, beside, and at two depths relative to a net-pen and analysed for suspended particulate matter (SPM), major and minor elemental abundance, carbon/nitrogen content, and stable carbon isotopes. Sediment traps were also deployed immediately beside the bottom of the net-pen. The highest mean concentration of SPM (0.6 mg 1-1) during the feeding cycle was observed within the central region of the net-pen. Approximately 87% and 30% of the mean SPM were observed at depth and beside the net-pen, respectively, suggesting that transport of suspended particulates was predominantly in the vertical direction. Sediment trap deployments revealed that sedimentation fluxes of total SPM, carbon, and nitrogen were higher below the farm than at the control site located 500 m away. Major and minor elemental analyses of feed pellets and sediment trap contents showed that calcium, phosphorus, sulphur, and strontium were removed within the net-pen system. The feed-specific carbon isotope signature (δ13C= - 21.4 to - 22.0‰) was not evident in the trap samples deployed beside the bottom of the net-pen (δ13C= - 23.4‰), suggesting the relative absence of feed pellet particles, isotopic alteration through fish assimilation and/or dilution of the isotope signature with other carbon sources. However, a feed-signature was evident in samples collected in the upper water column (depth 5 m), suggesting that δ13C might serve as a useful tracer of feed particles.