Evaluating Silver Carp behaviors with adaptive resolution sonar in response to driven and undriven entanglement gear

Objective Entanglement nets are the primary method used to monitor and mechanically remove invasive Silver Carp Hypophthalmichthys molitrix in the Mississippi River basin, and catch data are widely used to estimate abundance and guide suppression efforts. Silver Carp are notoriously difficult to capture, and turbid water has generally prevented direct observation of behaviors that lead to net avoidance or escape. We quantified the frequency, size structure, and composition of Silver Carp catch by net type and mesh size; determined potential differences in entanglement between driven and undriven nets; and categorized and quantified net encounter behaviors, including the rate of capture (i.e., efficiency), using adaptive resolution imaging sonar. Methods We evaluated gill nets constructed from four mesh types (double density, multistrand, multifilament, and monofilament; 8.9-cm bar mesh) and trammel nets with four mesh sizes (range = 5.1–8.9-cm bar mesh) under driven or undriven deployment approaches. During driven sets, fish were driven toward nets with the noise of the outboard boat motor; undriven nets were left undisturbed. We used adaptive resolution imaging sonar to quantify behaviors in response to net encounter across gill-net and trammel-net configurations fished under driven and undriven conditions in the Missouri River system. Results Silver Carp (42.9%) and Smallmouth Buffalo Ictiobus bubalus (30.5%) were the two most common species captured. Across 80 net sets, we recorded 14,499 net encounters (all fish species) that were categorized into 10 behaviors. Common behaviors were fish encountering the net and reversing course (bounced back; 65%) and parallel swimming to the net (25%). Fish encounters that resulted in entanglement and retention were 0.40% and 0.73% for undriven and driven nets, respectively. Capture efficiency was low across all nets. Conclusions Strong avoidance behavior, likely mediated by sensory detection and rapid swimming speed, constrains mechanical removal efficacy and suggests that conventional entanglement nets may underrepresent Silver Carp abundance. Directly observed behaviors reported here provide insights into the cause of low capture efficiencies, and this knowledge may be used to inform management efforts or the development of innovative capture techniques that maximize net encounters and exploit responses.