Collection rates of detached mobile sea lice according to net mesh and body size: A benchtop model

A variety of aquaculture activities, such as crowding and delousing treatments, can lead to mobile parasitic stages of salmon lice (Lepeophtheirus salmonis) and sea lice (Caligus elongatus) being detached from host salmon (Salmo salar) and entering the environment, where they may pose a reinfestation risk for farmed or wild fish. Here, we constructed a benchtop physical model to simulate the retention of detached mobile lice (L. salmonis and C. elongatus) within crowding nets of differing mesh sizes and mesh types: 0.8, 1.6 and 2.0 mm polyester square mesh, and catchLICE™ mesh, a nominal 2.0 mm hexagonal weave. Mesh types and louse classes (species, life stage, sex) were each measured according to standardized methods. The 0.8 mm square mesh retained all mobile life stages of both sexes (pre-adult I to adult L. salmonis, and adult C. elongatus), while the 1.6 mm square mesh retained all pre-adult II female and adult L. salmonis, but not all adult C. elongatus. The 2 mm square mesh and catchLICE™ mesh retained all adult female L. salmonis, with varying loss rates for all other louse classes. Measurements of the cephalothorax length, width and height of each louse stage indicated that size was not the sole determinant of collection success, as lice that might have fitted through the mesh openings were often retained, even after repeated ‘flushing’ flow events. Behavioural observations of lice grasping and crawling on the net surface may partly explain this discrepancy. Overall, finer-meshed crowding nets offer a partial solution to loss of lice during crowding, especially for the largest life stages.