THE RELATIONSHIPS BETWEEN HYDRODYNAMICS, THE CONCENTRATION OF HYDROGEN-SULFIDE 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 enric hment of the benthos in the immediate area of the farm with potentiall y deleterious consequences to fish health. The relationships between b enthic enrichment and hydrodynamic parameters - mean current speed, ma ximum current speed and incidence of low currents - were examined. Ben thic impact was assessed on the basis of SCUBA diver observation and m easurements of hydrogen sulphide in the water immediately (within 30 c m) overlying the sediment at slack water. Other factors such as maximu m site biomass and the age of the site were also considered. At 7 of t he 8 salmon farms studied there was a good correlation between mean cu rrent speed (r = -0.877) and incidence of low currents (r = 0.943) wit h log mean hydrogen sulphide concentration. The lack of correlation fo r the remaining site was thought to be due to a combination of high le vels of terrigenous organic input to the site and overfeeding. The rel ationship between fish growth and mortality with hydrogen sulphide con centration was also examined. Good correlation between both fish growt h (r = -0.819) and cumulative percentage mortality (r = 0.941), and lo g mean hydrogen sulphide concentration were found when the data from o ne exceptional site were excluded.