A characterization and sensitivity analysis of the benthic biotopes around Scottish salmon farms with a focus on the sea pen Pennatula phosphorea L

Abstract

Man's impact on biodiversity depends on both the type of intervention and the nature of the receiving environment. In the context of salmon farming, environmental impacts occur largely through the flux of farm-derived organic detritus to the seabed. In order to assess these potential impacts this research aimed (a) to identify sensitive benthic habitats, particularly those categorized as Biodiversity Action Plan (BAP) habitats, in relation to salmon farming; (b) to identify and classify the nature of the benthic habitats that occur around Scottish salmon farms and assess the overlap between sensitive habitats and salmon farms; and (c) to conduct a pilot investigation into the abundance of the sea pen Pennatula phosphorea around a single salmon farm. The benthic habitat occurring underneath Scottish salmon farms consisted, predominantly, of muds (58%) and shelly sands (27%) with the bed-type rock, also frequently occurring in close proximity (though not directly underneath) salmon farms. Most farms were located in water between 20 and 50 m deep. These findings are commensurate with the Scottish salmon-farming industry being located in relatively sheltered sea lochs, in close proximity to the shore. Salmon farming was considered, on the basis of spatial overlap and habitat sensitivity, to pose a high risk to maerl and beds of Modiolus modiolus and, on the basis of a lack of information, sheltered muddy gravels and the megafauna associated with the mud in deep water BAP. Sea pen abundance was highly variable but was reduced in close proximity to the salmon farm. While abundance increased to a maximum of 10 per transect at intermediate distances, at the farm peripheries numerous transects were found, again, to contain no sea pens. This may have been as a consequence of the protection offered by the physical presence of the salmon cages against trawling that occurs in the vicinity of the farm.