Developing low-cost tags: Assessing the ecological impacts of tethered tag technology on host species

Abstract

Understanding and mitigating potential effects of tags on instrumented animals is a crucial consideration when developing new tracking techniques. Some populations of aquatic megafauna spend the majority of their lives occupying small home ranges, yet conventional finescale tracking methods generally provide a limited number of non-continuous locations, while new technology is cost prohibitive. We developed a low-cost tethered telemetry system ( < 185 USD tag-1) for short-term tracking of marine turtles in nearshore environments that incorporated standard GPS data loggers and VHF transmitters into buoyant tags of 3 different designs. We then estimated the drag of each tethered tag using an instrumented flow tunnel, deployed them on freeliving green turtles along Mexico's Baja California peninsula, and compared movement patterns of turtles equipped with high- and low-drag tags. All tags provided high-resolution tracks that ranged from 5.2 to 184.0 h (mean ± SD = 43.2 ± 37.8 h; n = 26 turtles) for a total of 1122 h. We found that the first 2 tag designs increased drag on large juveniles at typical swimming speeds by approximately 7 to 10%, which is comparable to predicted drag increases incurred by similarly sized green turtles from most commercially available electronic tags. By contrast, the third tag design increased drag by 1% or less. Turtles fitted with the high-drag tags made fewer course changes and exhibited straighter (less tortuous) movements than those fitted with the low-drag tags. Although it is unclear if the observed behavioral differences were due entirely to the tags, our results highlight the importance of evaluating potential ecological impacts of telemetry devices on host species, particularly when developing new technology.