A flume study to evaluate the processes governing retention of sea lice therapeutants using skirts in the treatment of sea lice infestation

Results are presented from laboratory model investigations into the processes governing the retention of therapeutant added to skirted fish cages during treatment for sea lice infection. The results demonstrate the importance of turbulent entrainment and shear at the edges of the skirt in reducing the concentration of therapeutant within the cage. The effect of cross currents in establishing cellular circulations within the cage is identified as potentially playing a key role in the entrainment process, with the consequential finding that the rate of retention of therapeutant depends upon not only the current speed beneath the cage but also the aspect ratios of the cage and skirt attachment ( h/H) respectively. Entrainment of water into the skirt and subsequent reduction in dye concentration increased by a factor of 2 when h/H = 0.24, compared against h/H = 1. Scaling arguments, together with measured values of entrainment velocities for geometrically-similar flows, are used to estimate typical e-folding retention times of therapeutant within the skirted cage. E-folding retention for an illustrative values of U = 0.076 m.s − 1 and 0.003 m.s − 1 the corresponding mean values for T (dye retention time) were 1.46 h and 37 h respectively. Such time scales are in excess of the current treatment times in the field, albeit subject a number of assumptions that needed to be tested in the field more fully.