Effect of substrate type and pellet age on the resuspension of Atlantic salmon faecal material

The influence of substrate type and particle age on the remobilization of settled Atlantic salmon Salmo salarfaecal material was studied through a set of controlled experiments in horizontal flow flumes, simulating different bottom conditions present in fish-farming locations along the coast of Norway. There was no significant effect of pellet age on remobilization for up to 1 week old faecal material, but critical shear stresses (tau(c)) and velocities required for resuspending faecal pellets were strongly dependent on substrate type. Smooth substrates such as mud and rock slate required lower stresses for the onset of faeces resuspension (tau(c) approximate to 0.06 Pa) than rougher surfaces such as sand (tau(c) approximate to 0.12 Pa) or fragmented rock (tau(c) approximate to 0.32 Pa), where bedforms and large fractures shield the particles from the direct influence of the drag forces. These newly determined substrate-dependent r c resuspension thresholds will contribute to the construction of more accurate numerical models that include bottom type as a parameter regulating the extent of particle spreading, in contrast to the constant-value approach that has been used to date.