Performance of biochemical tools as fish waste particle tracers in a dispersive marine system

Abstract

As finfish aquaculture expands to more dispersive coastal environments, inputs of aquaculture-derived organic matter become more diffuse, but widespread. Significant amounts of organic material are likely to be exported beyond the near-farm area, where it may encounter sensitive habitats. Biochemical tracers of organic matter, such as stable isotopes and fatty acids may be useful tools for detecting diffuse waste deposition. We used numerical modelling of direct and resuspended farm waste deposition and empirical environmental measurements to evaluate a range of biochemical indicators (major nutrients, bulk stable isotopes, fatty acids, minor and trace elements) for their efficacy as particle tracers of fish waste around a shallow water (∼30 m) salmon farm, in a dispersive coastal farming area. We also compared their efficacy across a range of sampling approaches (in sediment trap material, sediment and Mytilus edulis as a bio-indicator). Elemental carbon, nitrogen and zinc indicators could accurately reflect predicted waste deposition close to the farm, but only terrestrial fatty acid indicators were able to reflect diffuse wastes farther afield suggesting they are more sensitive tracers of fish waste. Their performance as tracers also differed between sediment trap and sediment samples, with most performing better in sediment trap samples. Although translocated mussels showed evidence of fish waste assimilation in areas of high waste deposition, tissue concentrations of fish waste tracers correlated relatively poorly with modelled waste deposition. Our results highlight the sensitivity of terrestrial fatty acids as fish waste tracers for detecting diffuse waste deposition, and demonstrate that concentrations of a number of elements can be used for mapping acute waste deposition. Our findings can inform selection of biochemical tracers for improved monitoring and management of farm wastes in dispersive marine systems.