Distribución del pez sierra de dientes grandes (Pristis pristis) en Costa Rica: reconstrucción histórica y actual mediante métodos tradicionales y ADN ambiental

Abstract

The Critically Endangered largetooth sawfish Pristis pristis is one of the most threatened elasmobranch species and is currently thought to be locally extinct in at least 27 countries. Although largetooth sawfish information in Central America is scarce, recent records show that this species is still present in Costa Rica, yet its distribution and current status remain unclear. This study investigated the spatial and temporal distribution of the largetooth sawfish in Costa Rica and identified local threats affecting the populations. We conducted 275 structured interviews in coastal and riverine communities across the country, which resulted in 134 confirmed records in the Pacific, 1 in the Caribbean and 51 in the northern region. Historical and recent records suggest the largetooth sawfish has undergone significant reductions in abundance and distribution from coastal and riverine areas, mainly due to interaction with fishing gear such as gill nets and hook and line. Most sawfish captured by gill nets were reported in the Central Pacific region, whereas hook and line records were more common in the northern region and the South Pacific. Although largetooth sawfish populations in Costa Rica have followed the global decline trend, we found 2 main hotspots where recent sightings and captures appear to be more common, suggesting there is still hope for the species to recover in Costa Rica and possibly in the region. Moreover, Costa Rica recently became the 17th country to ratify national legal protection for sawfishes, which may strengthen conservation efforts to protect populations locally and in the Central American region.

The use of environmental DNA (eDNA) has proven to be an effective approach in detecting rare and threatened species in a wide range of aquatic environments, and may become a valuable conservation tool in ecosystems with high suspended sediment content. Therefore, eDNA can be useful to identify critical habitats for threatened organisms such as the largetooth sawfish (Pristis pristis), an euryhaline-generalist species, once common in Central America. In Costa Rica, knowledge of P. pristis distribution relies mainly on local ecological knowledge, which may not represent the species actual distribution due to their low abundance and cryptic habits in turbid environments. This study assessed the application of eDNA for the detection of P. pristis in Costa Rica through a country-wide survey. Environmental DNA samples were collected in situ by filtering water through filter membranes using a portable filtration device. Laboratory analysis was carried following the protocols used by the Global Sawfish Search project, developed at the James Cook University, Australia. A total of 579 samples were collected at 93 sampling stations in 18 sites within six main regions of the country. P. pristis eDNA was detected from 16 samples collected from the Northern region and Northern Caribbean region, along the San Juan-Colorado River. This study provided the first direct evidence of P. pristis in Central America using eDNA, further demonstrating that eDNA can detect rare species in fast flowing waters with high suspended sediment content. Moreover, detections of P. pristis combined with recent catch/release records from the San Juan-Colorado River suggest their numbers are slowly increasing. Urgent cooperative research is needed to evaluate their population in this freshwater system. An ecosystem approach to manage the system is suggested to secure their populations, by decreasing threats and allow for a stable recovery of a historical core population of their Western Atlantic subpopulation.