(A) Mean clearance rate versus the radius of the perception volume r for tracers (, red), dead copepods passively transported by turbulence (, black), and living copepods in turbulence (, green) and in calm water (, blue). is directly obtained from measured for different values of the pairwise separation distance r. We verify the accuracy of our flow field measurements by plotting the theoretical prediction for the clearance rate of tracers, based on the (for the inertial subrange) and (for the viscous subrange) Kolmogorov scaling of the velocity differences (Saffman and Turner, 1956), using proportionality constants found empirically (Pécseli et al., 2014) (dashed line, black). The prediction agrees very well with our experimental data. We show that a very accurate estimation of down to the radius of hydrodynamic interactions can be achieved by considering the separate contributions of turbulent advection and the self-locomotion of two independent organisms. We compute for living copepods in turbulence, using where is randomly drawn from the probability density function of the pairwise radial relative velocity of tracers at a given separation distance r, and is the pairwise radial relative velocity of the copepods. is computed using randomly sampled values of the organism velocity with respect to the flow ub to isolate the behavioral part of their motion. agrees very well with the measurements for mm (dashed line, magenta). It deviates for shorter r because of behavioral interactions within the radius of hydrodynamic interactions: the ratio is approximately two at mm and one at mm. These interactions are studied in the next section. (B) Ratios of mean clearance rates. Active motion plays a preponderant role in enhancing encounter rates in turbulence, as evidenced by a ratio (green) larger than one, especially at short r comparable to or below the radius of hydrodynamic interactions. results from turbulent velocity differences, organism motion, and inertia, while results from turbulent velocity differences and inertia only. The contribution of inertia to is lower than that of self-locomotion but not negligible: the ratio (black), where is the mean clearance rate of small, neutrally buoyant flow tracers that have negligible inertia, is larger than . This indicates that the combination of turbulent velocity differences and effects due to inertia leads to a larger than turbulent velocity differences alone. We also note that while being passively transported by turbulence leads to a larger at large r compared to motility in still water, as evidenced by the ratios (red) and (blue) above one for large r, it provides less mating opportunities than self-locomotion in calm hydrodynamic conditions at shorter r. This indicates that being passively transported by the flow is not an efficient mechanism to encounter many mates within the perception radius. It requires active swimming in turbulence for copepods to achieve an encounter rate that is comparable or even larger than in calm hydrodynamic conditions (ratio , magenta).