The relationships between hydrodynamics, the concentration of hydrogen sulphide produced by polluted sediments and fish health at several marine cage farms in Scotland and Ireland

Waste products from marine cage salmon fanning can cause organic enrichment of the benthos in the immediate area of the farm with potentially deleterious consequences to fish health. The relationships between benthic enrichment and hydrodynamic parameters - mean current speed, maximum current speed and incidence of low currents - were examined. Benthic impact was assessed on the basis of SCUBA diver observation and measurements of hydrogen sulphide in the water immediately (within 30 cm) overlying the sediment at slack water. Other factors such as maximum site biomass and the age of the site were also considered. At 7 of the 8 salmon farms studied there was a good correlation between mean current speed (r = -0.877) and incidence of low currents (r = 0.943) with log mean hydrogen sulphide concentration. The lack of correlation for the remaining site was thought to be due to a combination of high levels of terrigenous organic input to the site and overfeeding. The relationship between fish growth and mortality with hydrogen sulphide concentration was also examined. Good correlation between both fish growth (r = -0.819) and cumulative percentage mortality (r = 0.941), and log mean hydrogen sulphide concentration were found when the data from one exceptional site were excluded.