Habitat point records from 1986 OPRU Shetland fish farm infaunal sediment survey

This report describes the macrofauna present in benthic sediment samples taken at three salmon farm cage sites in August 1986. Samples were obtained in cores taken along a 50m transect line stretched out across the seabed from the edge of the cages. At two of the sites, fish cages had been operating for approximately 15 months whilst the third site had only been subjected to fish farm inputs for 4 months. At one of the older cage sites, situated in a very sheltered voe, the muddy sediments supported a low diversity community of which the main components were Capitella capitata and unidentified nematodes. Although apparently stressed, no gradients in community structure attributable to fish farm inputs were discernable. This may have been due to the transect line used not being long enough to include the whole of the gradient from polluted to clean conditions, because diver observations certainly suggested the existence of farm input-related gradients in other more readily visible parameters. At the other two sites, sediments were coarser and more mixed although the seabed at one of these was clearly more exposed to tidal currents than the other. They both supported a diverse infauna; that from the more exposed site numerically dominated by Corophium crassicorne, and that from the more sheltered site by Capitomastus minimus and unidentified oligachaetes. At the more exposed (and most recent) of these sites, the infauna within approximately 5m of the cages had been modified by fish farm inputs, and consisted predominantly of dense populations of Capitella capitata. Similar changes had occurred at the more sheltered of these two farm sites although here they extended out to approximately 15m from the cages.