Physical characteristics of commercial pelleted Atlantic salmon feeds and consideration of implications for modeling of waste dispersion through sedimentation

Models used to employ waste dispersion from sea cages rarely take into acco unt the physical characteristics of fish feed and faeces. In this study we determine the hardness, friability, water absorption properties and settlin g velocity of a range of commercial Atlantic salmon (Salmo salar L.) feeds, under defined laboratory conditions. Hardness of large, high energy pellets (greater than or equal to 8.5 mm) wa s generally greater than that of 6 mm high energy pellets. With the excepti on of 100 mm pellets, the friability of large pellets was also greater than that of the smaller pellets. Settling velocities were significantly greate r at 20 psu than at 33 psu and significantly higher for most pellet types a t 10 degrees C than at 20 degrees C. Settling velocities for unsoaked pelle ts were found to increase with pellet size, from a mean of 5.6 cm s(-1) for the smallest pellet (2 mm) to 13.9 cm s(-1) for the 10 mm standard (20 to 24% fat) pellets. The smallest feed pellets (2 mm) had the greatest percent age wet weight increase on immersion, irrespective of salinity. Settling ve locities of pellets were not significantly affected by immersion time (0-15 min). Given the water depths at fish cage sites and the settling times involved, it is concluded that it is unnecessary to take account of changes in food p ellet settling velocity as a result of immersion. Effects of pellet size an d salinity on settling velocity are best incorporated into waste dispersion models through the use of 'look-up' tables.