Detection of critically endangered marine species with dwindling populations in the wild using eDNA gives hope for sawfishes

The detection of threatened species, specially where they have been depleted, is key to their conservation but poses severe logistical challenges precisely because of their very low abundances. Sawfishes (family Pristidae) are among the most threatened marine taxa with globally-reported severe reductions in population size and disappearance from large parts of their former distribution range. This study presents the first report on the use of environmental DNA (eDNA) to evaluate the presence of Critically Endangered sawfishes in parts of their former range, where no recent information about their presence exists. During 2016 and 2017 gillnet and eDNA surveys were carried out at Tecolutla Estuary, Veracruz, and Laguna de Terminos and Los Petenes Biological Reserve, Campeche, in Mexico, where Pristis pectinata and Pristis pristis were abundant 40–50 years ago. Our study aimed at determining: (a) if eDNA was a suitable tool for assessing the presence of sawfishes at suspected very low abundances, and (b) if sawfishes still occur in Mexico. We found evidence of sawfish presence through eDNA samples in all three sites, at very low frequencies, but no evidence through gillnetting. Species resolution was difficulted by DNA degradation, but 2/12 samples were identified as Pristis pectinata. Our findings confirm that eDNA is a suitable tool to rapidly assess the presence of critically endangered sawfishes in the wild, stress the urgent need for eDNA surveys throughout the former range of sawfishes globally and highlight the need for swift measures to protect the remaining individuals still living in Mexico.