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.

We used a commercial vehicular GPS tracker (©TKSTAR TK905, Figure S2a) to monitor the trajectories of the carcasses of sea turtles and franciscana dolphins. The device works with a GSM network and GPS satellites and can locate and monitor any remote target by Short Message Service (SMS), Application (APP), and the Internet (technical specifications are presented in Supplementary Material S1). In order to extend battery life, the device was connected to a 5000mAh power bank (©XTRAX, Figure S2c).

In 2019, we conducted four drift experiments (summer, autumn, winter, and spring) in the State of Rio Grande do Sul, Southern Brazil, to refine the estimates of at-sea mortality of marine tetrapods, using a commercial vessel (length: 16m, beam: 5.5m; Figure S3). The drifters were released at sea at five points at different distances from the beach (Table S2). Points were numbered from 1 to 5 from the coast to offshore. The standard composition of drifters by release point was the following: 30 bottles at every point and 15 carcasses (11 seabirds, three sea turtles, and one cetacean) at points 1, 3, and 5. However, whenever available, more specimens were included (e.g. sea turtles and franciscana dolphins, Table S1), maximizing sampling effort. Seabird carcasses were not tracked because they have a mean body mass lighter than 2 kg.

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

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Abstract

Methods