Development and testing of vibration-sensing tags for assessing fish and invertebrate vocalizations

During the last decade, we have realized that many fish and invertebrates rely on acoustic communication as an important part of their ecology, for functions ranging from finding mates and synchronizing spawning events to territory formation and aggressive interactions. Knowledge of the structure and function of vocalizations from sonic taxa may help monitor species distribution and enable identification of critical behaviors such as spawning using acoustic monitoring. However, existing methods for identifying acoustic signals used by wild fishes and invertebrates typically rely on combined cameras and hydrophone arrays for localizing and identifying vocal fish. These approaches do not scale well and they provide a potential bias towards diurnally active species. Here we present our recent efforts to develop new tools for identifying acoustic vocalizations used by fish and invertebrate species. We have developed two new miniature tags incorporating high sample rate tri-axial accelerometers to detect vibrations from sound production of tagged animals. One tag saves raw accelerometer data as well as depth, orientation and temperature directly to a micro-SD card. This tag is useful for designing and validating on-board detection algorithms and for understanding the structure and context of vocalizations. However, the tag needs to be physically retrieved, and thus is limited to more controlled contexts or highly territorial fish that are relatively easy to recapture. The second tag design is uses on-board energy detection algorithms to detect periods with likely vibrations and trigger an ultrasonic acoustic signal with an identity code specific for each tag. This active acoustic tag is implantable and allows identification of acoustic signals produced by tagged animals by recording both the animal sound and active acoustic signal with a broad-band passive acoustic recorder. We demonstrate how these tags can be used to identify acoustic signals using data from toadfish and sea robins and compare advantages and disadvantages of the two different approaches. Finally, we discuss complications including how tags may respond to the particle motion component of other high intensity sounds in the environment, and how on-board detection algorithms may be designed to minimize this problem.