Citation: O'Connor, B., Costelloe, J., Dinneen, P. & Faull, J. 1992 The effect of harrowing and fallowing on sediment quality under a salmon farm on the west coast of Ireland. Report by Aqua-Fact International Services Ltd.. pp. Tully Mountain Salmon Farms, Co. Galway
Keywords: Aquaculture, Benthos, Chemistry, Fish, Impact, Marine, Marine Inlet, Pisces, Pollution, Salmon, Sediment
Abstract: It has been well established that the major area of impact on the marine environment from organic wastes produced by fish farms is the sea floor directly underneath and at some distance away from the cages. It has been suggested that this build up of wastes can be detrimental to fish health due to the release of toxic gases, and disperse organic material. Speeding up the return of more normal sediment quality conditions has been advocated. This paper presents findings from a survey designed to describe pre- and post- harrowing conditions following a fallowing period under a salmon farm off the west coast of Ireland. Such values as depth of the redox layer and types and numbers of benthic animals were used to describe the conditions which pertained both before and after the harrowing took place. A diver-operated Sediment Profile Camera was also used to document sedimentary compositions and standard underwater photographic methods described the sediment surface in general. Results show that before the harrowing took place, the various measures of sediment quality described a heavily impacted sea floor with low redox levels on the sediment surface, high organic carbon values in the sediment and a very poorly developed infauna. Diving observations and underwater photography showed that the surface was blackened and mounded with waste food. A post harrowing survey carried out three months after the event, demonstrated that conditions had significantly improved, with high redox levels being recorded up to 2 cm into the sediment, low sedimentary organic carbon values and a well developed, if reduced, infauna. The paper discusses the benefits of harrowing and fallowing and how, by significantly enhancing sediment quality, production success and environmental quality can be ultimately improved.