Tracking aquatic vertebrates in dense tropical forest using VHF telemetry

Abstract

As part of a broad study of river dolphins and caimans in the Brazilian Amazon rainforest, transmitters in the radio frequency range 173-174 MHz were deployed on 36 animals over a 4 yr period. Tracking was carried out both by hand and using automatic, scanning, directional receiving stations situated above the forest canopy. Results were initially poor, due largely to equipment failure in such a hot, humid environment and inexperience of the scientific personnel in attempting such a study in dense rainforest. However, with modified equipment and greater experience, radio telemetry became a powerful and benign research tool without which the study would have been very substantially weakened. Although performance was poorer than in open habitat, careful design of the receiver network, aided by field-testing of signal range under various conditions, provided knowledge of the whereabouts of most tagged animals for most of the time. After four deployments, expectations of at least nine months tag longevity and receiving stations remaining functional 90% of the time are realistic, but success is critically dependent on adequate manpower for monitoring and data-collection. Although an excellent source of information in its own right, radio telemetry of cetaceans yields the greatest insights when combined with intense observational fieldwork.