Hydrodynamic properties and distribution of bait downstream of a zooplankton trap

The flow regime around a chemically baited trap is crucial for the trapping process and distribution of bait downstream of traps. We measured the flow field downstream of a trap prototype in flume experiments and mapped the distribution of bait using laser induced fluorescence. The trap produced a downstream wake, where flow recirculated towards the trap, allowing organisms slower than the free stream flow to interact with the trap. The chemical tracer revealed an average gradient with increasing concentrations towards the trap. Finally, we evaluated trap performance in field experiments. Traps with internal light caught on average 3.4 times more zooplankton than traps without light in short-term deployments (1 h). Trapping efficiency could be manipulated by chemical stimuli; A piece of fish (Salmo salar) inside traps deterred 79% of the zooplankton compared to traps without fish. We conclude that the flow regime around a cylindrical trap may facilitate trapping and that combined stimuli modalities may allow higher selectivity. The effective radius of the trap will depend on the surrounding flow and will likely be small when flow-rate exceeds swimming speed of targeted organisms. Finally, we propose applications for selective traps in aquaculture and pest management.