# A Collection of Best Practices for the Collection and Analysis of Bioacoustic Data
## 7 Predator-Prey Relationships  
One last application of bioacoustic analyses that we now consider concerns relationships between predators and prey. Some recent progress in this area has built on the integration of acoustic analyses with complementary data types to explore research questions and/or ecosystem dynamics that were previously beyond the scope of what could be studied. Predator-prey relationships have long been a central topic of ecological study, but in many environments these relationships have remained uncharacterized due to the difficulty in observing them with available technology (e.g., deep ocean, aerial, or extremely remote/densely vegetated environments), especially as predator-prey interactions are often rapid and occur in unpredictable times and locations. In many cases, however, acoustic analyses can be combined with other remote sensing technologies, such as satellite tagging or active acoustic monitoring (in which a signal is transmitted and the echo is recorded and analyzed), to allow scientists to monitor predator and prey behavior simultaneously and thus potentially document predation events. In one recent example, [PERSON] et al. [72] applied a combination of passive and active acoustic methods to describe the large-scale distributional relationships between harbor porpoise and their prey in an enclosed sea off the coast of the United Kingdom. Harbor porpoise are difficult to see at the surface of the ocean and are therefore difficult to survey using visual methods [138]. However, they vocalize frequently [136] producing clicks that are well documented and easily distinguished from those of other cetaceans [139,140]. This makes them well-suited for an analysis of their distribution using passive acoustic data. In the passive acoustic portion of the study, [PERSON] et al. [72] surveyed the Clyde Sea using a towed array of two hydrophones. They ensured that their survey design covered the entire ecosystem in a representative way and surveyed only during daylight hours to minimize the effect of diel variability in acoustic behavior or depth of animals. Using two hydrophones allowed them to determine the bearing angle to the sound source, which aided in discriminating among echolocating individuals. Omnidirectional hydrophones were used, which reduced biases related to the direction of the sound source. Hydrophone sensitivity was calibrated to reduce equipment bias, and because harbor porpoise vocalize using only high-frequency echolocation clicks, a high-pass filter was used to reduce bias from noise in the lower frequency bands. A significant challenge in combining passive and active acoustic monitoring techniques is that signals emitted by an active acoustic source will be detected by passive acoustic recorders. In order to address this challenge, [PERSON] et al. [72] developed a classification algorithm using open-source PAMGuard software [141] to distinguish between harbor porpoise clicks and narrow-band 38, 50, 120, and 200 kHz pulses emitted at regular intervals by the active acoustic systems used by the researchers. Signals classified as harbor porpoise clicks were subject to an additional manual review based on their waveforms, power spectra, and wigner plots to ensure they were classified correctly. This two-step classification and review process reduced any potential false detections caused by signals emitted by the active acoustic source or other sources of impulsive sound. For the purpose of this study, the authors generated a method to rate their confidence in the classification of a click train to harbor porpoise, and only used click trains that were rated as \"certain click trains\" to indicate the species actual presence. Individual encounters were defined when more than 90 s passed between click trains, which is the time it took the ship to travel 300 m, the maximum known distance a harbor porpoise echolocation click will travel before it fully attenuates. Encounters were also separated if they occurred within 90 s of each other but at different bearing angles.