Crocodile night survey: detectability and relationships with environmental variables

PIÑA, CARLOS; PORTELIHNA, THIAGO; CIOCAN, HERNÁN

Santa Fe

Otro; 25th Working Meeting of the Crocodile Specialist Group Santa Fe, Argentina, 7-10 May 2018; 2018

IUCN-SSC Crocodile Specialist Group

Nigtht survey is the most used method for surveying crocodile populations. It is used to confirm the presence of crocodilians in monitored sites, to estimate abundance and also to determine population size structure. However, the results of these counts are usually variables and related to environmental characteristics of the place and at the time of monitoring. These conditions can modify the monitoring results in a significant way. In our research, we monitor monthly for 3 seasons (2015-2018), a Caiman latirostris population located in "El Fisco" reserve (Santa Fe province, Argentina), which has a lake of 300Ha and a nearby canal of 4.5km of length. In the counts, water and air temperature, water depth, wind direction and intensity, and moon phase were measured. In addition, to determine the sensibility of the method, animals were released from the ranching program (Proyecto Yacaré) in successive seasons (25% of the estimated population), counting the population before and after the releases. After 24 nocturnal monitoring events in the lagoon, we obtained very variable results, ranging from 78 to 314 animals sighted per night. No increase in the population was observed and even fewer animals were counted in some counts after the releases. It was observed that the number of counted animals increases, as the temperature and depth of the water fall. The monitoring schedules carried out coincided with those of maximum detection observed in previous telemetry works (20.811.6%) of animals possibly visible between 2200h and 0300h in the lagoon. Our monitoring showed an average of 180.2572.78 animals per night, which could indicate only a small percentage of the population present in this place. However, it is necessary to consider that caimans also use the canal and that a monitoring in this environment could increase the percentage of animals visualized. Therefore, knowing the interference of temperature, water level and monitoring schedule regarding the detectability of caiman population studies and monitoring programs should carefully consider the conclusions regarding the abundance of the animals based on the results of night counts. This variability of the counting data can lead to gross errors when making management and conservation decisions, as well as regarding the environmental impact analysis.