Discovery of a suburban wetland refuge for a depleted American crocodile (Crocodylus acutus) population in northwestern Mexico, using a commercial Unmanned Aerial Vehicle

Abstract

Background and Research Aims The Urías Coastal Lagoon (UCL) is a heavily modified and polluted lagoon near Mazatlán, in northwestern Mexico. Once abundant in the UCL, the American crocodile ( Crocodylus acutus) is now rarely seen in the area. A healthy population of apex predators is essential to maintain the trophic balance of ecosystems. Our research aim was to lay the groundwork for studies of American crocodile population ecology in the UCL. Methods Traditional boat surveys (day and night) of crocodiles were undertaken in the UCL in May-July 2022. Two exploratory aerial (∼ 30 m altitude) surveys of a small wetland located close to a habitational complex in the vicinity of the UCL were performed in June and December 2022, using a commercial Unmanned Aerial Vehicle (UAV). We used images obtained with the UAV to estimate the size of the crocodiles detected in the wetland. Results This study confirms the presence of American crocodiles in the UCL and presents the first information on individuals of this species for this area. Aerial surveys revealed the presence of 17 individuals in the wetland; the mean size of nine of those individuals was 183.3 ± 60 cm (range: 130-310 cm). Two yearlings (∼37 cm length), possibly born in June 2022, were captured in the same wetland in September 2022. We also surveyed 50.7 km of UCL coastline by boat, but no crocodiles were sighted. Conclusion The crocodiles seem to avoid the human-impacted UCL and likely use the less impacted surrounding wetlands as a refuge area, which they also use to reproduce. The presence of both yearlings and adults is a good sign for the survival of this vulnerable population. Implications for Conservation We believe that the surveyed wetland should be closely monitored for conservation purposes because it might be one of the last crocodile breeding refuges in the heavily modified and polluted coastal environment of the UCL. We summarize our results with a series of recommendations for local and federal authorities. These recommendations would likely help the conservation of the American crocodile population in the UCL.

Estimating Total Length of Partially Submerged Crocodylians from Drone Imagery

“…Drones have recently been used in several crocodilian studies, mostly to count the number of individuals or nests, but also for individual identification, including for the American alligator Alligator mississippiensis [24,25], American crocodile Crocodylus acutus [26], Yacare caiman Caiman yacare, and broad-snouted caiman Caiman latirostris [27], saltwater crocodile Crocodylus porosus [28][29][30][31], mugger crocodile Crocodylus palustris [32,33], Nile crocodile Crocodylus niloticus [34][35][36], West African crocodile Crocodylus suchus [10], and gharial Gavialis gangeticus [37]. Aubert et al (2021) proposed optimal drone flight parameters to count crocodiles under various field conditions and compared their efficiency to other survey methods.…”