Despite intensive management, the increasing conflict between humans and saltwater crocodiles in places such as Queensland, Australia, has led to the investigation of alternative techniques to improve public safety. The reliability of digital video surveillance systems (DV) placed above water and multi-beam sonar (sonar) placed under water to detect and monitor saltwater crocodiles was tested in a seminatural freshwater environment over a 2 h period (16:00–18:00 h). A total of 29 crocodiles were detected within the study area using DV, and 28 with sonar. One was obscured by a section of bank, and thus not visible on sonar. Of the 28 crocodiles detected by both methods, sonar recorded both entry and exit for all, while DV recorded both entry and exit for 15 crocodiles. The length of time that crocodiles were detected was longer on average for sonar (4 min 27 s) compared with DV (2 min 50 s). This reflected the time spent above (detected by sonar and DV) or below water (not detected by DV), as only sonar was able to detect crocodiles underwater. The use of sonar may provide a valuable management tool for detecting and monitoring saltwater crocodiles in areas frequented by people (e.g. beaches, boat ramps, upper freshwater areas) where there is a high chance of a negative interaction.
