Tracking marine tetrapod carcasses using a low-cost mixed methodology with GPS trackers, passive drifters, and citizen science

Drift experiments are essential to understand stranding patterns and to
estimate the mortality of beached animals. Most studies do not use
telemetry technology due to the high costs of this methodology. The
objective of this paper is to describe the possibilities of tracking
marine tetrapod carcasses with a low-cost and replicable methodology. The
study was conducted in the Southern Subtropical Shelf (~ 28º–34º S), a
highly productive and key ecological region of the southwestern Atlantic
Ocean (SWA).  We designed and tested a low-cost mixed methodology
including Global Positioning System trackers, passive drifters (reused
glass bottles), and Citizen Science (through instant message platform and
email) for tracking carcasses of marine tetrapods. We performed four drift
experiments, during the four seasons of 2019. We released 787 drifters
(600 non-biological and 187 carcasses of seabirds, sea turtles, and
cetaceans) at sea, at five equally separated distances (5–25 km) from the
coast. Beach surveys and Citizen Science were implemented to recover the
beached drifters. We recovered 71.83% of non-biological and 27.27% of
carcasses released. We tracked the movements of 38 carcasses (25 sea
turtles and 13 cetaceans) with 17 GPS devices. The drifting time, until
reaching the beach, ranged from 12h to 17 days for carcasses and 12h to
406 days for bottles. Citizen Science was the most important source of
recoveries of non-biological drifters, representing 66.67% of the total
bottles recovered. For carcasses, active search was the most important
recovery source, representing 64.7% of the total carcasses recovered. Our
study contributes with new findings about drift patterns of marine
tetrapods in the SWA and describes an accessible low-cost mixed
methodology for small and medium-budget projects that can be replicated in
other coastal regions of the world for tracking a wide range of marine
tetrapod species.