Environmental DNA evidence of the Critically Endangered smalltooth sawfish, Pristis pectinata, in historically occupied US waters

Formerly common in tropical and subtropical waters of the Atlantic Ocean, the Critically Endangered smalltooth sawfish, Pristis pectinata, underwent severe declines over the past century, restricting population(s) to south and south-west Florida in the US, and Bahamian waters. Anecdotal evidence (e.g. encounter reports from the public) suggests that P. pectinata have recently been observed in historically occupied US waters; however, no directed surveys have been conducted to verify their extent of occupancy. Here, environmental DNA (eDNA) surveys were used to investigate the occurrence of P. pectinata in three formerly occupied estuaries in US waters. Water samples were collected in the summer from the Indian River Lagoon and Tampa Bay, Florida, in 2018 and 2019, and from Mississippi Sound, Mississippi, in 2018, and screened for target DNA using a highly sensitive Droplet Digital (TM) polymerase chain reaction assay. Target DNA was detected at four sites in the Indian River Lagoon in 2018 and at one site in 2019 (average concentration: 0.086 copies mu l(-1); SE = 0.004), but was not detected in either year in Tampa Bay. Target DNA was also detected at three sites near Deer Island in Mississippi Sound in 2018 (average concentration: 0.090 copies mu l(-1); SE = 0.005). These surveys provide additional lines of evidence that P. pectinata is re-occurring within two historically occupied estuaries in US waters. More comprehensive eDNA surveys in historically occupied regions, combined with clearly defined post-survey management actions, can direct additional research and outreach initiatives in emerging priority areas, fostering recovery of this Critically Endangered species.