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 account the physical characteristics of fish feed and faeces. In this study we determine the hardness, friability, water absorption properties and settling velocity of a range of commercial Atlantic salmon (Salmo salar L.) feeds, under defined laboratory conditions.Hardness of large, high energy pellets ( 8.5 mm) was generally greater than that of 6 mm high energy pellets. With the exception of 100 mm pellets, the friability of large pellets was also greater than that of the smaller pellets. Settling velocities were significantly greater at 20 psu than at 33 psu and significantly higher for most pellet types at 10 °C than at 20 °C. Settling velocities for unsoaked pellets 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 percentage wet weight increase on immersion, irrespective of salinity. Settling velocities 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 pellet settling velocity as a result of immersion. Effects of pellet size and salinity on settling velocity are best incorporated into waste dispersion models through the use of 'look-up' tables.