Assessment of crocodile abundance and seasonal effects of salinity on distribution using both boat based and aerial drone surveys
The American crocodile (Crocodylus acutus) is a common resident of Central American waterways, with one of the largest ranges of all crocodilians. It is very salt-tolerant, with only Nile crocodiles and Saltwater crocodiles superseding it in terms of being the most saline tolerant. Globally it has lost 9% of its range, with habitat destruction and fragmentation as the leading threat to this species. Overall there is a great need to understand the habitat uses by this species in order to mitigate consequences of further anthropogenic incursions along its range. Thus, this study aims to investigate seasonal effects of salinity on habitat selection of American crocodiles in a natural mangrove habitat, and to validate use of a commercial drone as a surveying tool for crocodilians in such habitats.
This study found that as the seasons shift from wet to dry there was an associated increase in salinity of the estuarine waterway that resulted in an increase in the halocline of the estuary waters the greater the distance from the ocean. When broken down into the following groups: Hatchlings and juveniles (HJ) vs subadults and adults (SbA) it was found that salinity as well as season both had a correlation with the presence of crocodiles from the HJ group (p<.05) and there was no correlation in regard to SbA crocodiles. Further, HJ size classes predominantly were found in the further reaches of the estuary, regardless of season.
Surveys taken via drone were as efficacious as surveys taken diurnally via boat, with a survey encounter rate of 0.40 km-1, but still less efficacious than nocturnal eyeshine surveys, which had an encounter rate between 1.2 – 2.9 km-1. The drone was able to identify animals that were submerged under water, but was unable to ID animals resting on banks in thick mangroves. However, the drone was able to identify crocodiles as small as 0.7m. Further, the drone showed encounter rates for sharks and rays that were much higher than crocodiles, 0.84km-1 and 0.64km-1 respectively.