Chemical fluxes and mass balances in a marine fish cage farm. II Phosphorus

Phosphorus fluxes were measured in a marine fish cage farm in the Gullmar Fjord, western Sweden.The measured fluxes included fish food, juveniles, harvest, fish loss (dead fish and escapers), sedimentation from the cages and benthic solute release (in situ).Two different types of mass balances were constructed.The flux method was based on seasonal input of fish food and juveniles, sedimentation and removal by harvest, fish loss and benthic fluxes Mass balances according to the flux method were constructed for each of 2 consecutive growing seasons The accumulat~on method was based on total input of phosphorus through fish food and juveniles, removal by harvest and fish loss, and net accumulation in the s e d m e n t over 7 growing seasons.Both types of mass balances gave similar results.Of the total phosphorus input to the farm, 17 to 19 % was recovered in harvest, fish loss constituted 1 to 4 %, and 78 to 82 % was lost to the environment.The environmental loss of phosphorus for each ton of fish produced was 22.4 k g (1985), 19.6 kg (1986) and 21.9 k g (1980 to 1986).Of the loss to the environment, 34 to 41 O/ O was in dissolved form and 59 to 66 % was accumulated in the sediment.O n a seasonal basis, the benthic flux transferred 4 to 8 % of the sedimented phosphorus back to the overlying water This constituted about 1 O/o of the total phosphorus content in the sediment derived from the fish farm.