Monitoring the distribution, abundance and breeding areas of Caiman crocodilus crocodilus and Melanosuchus niger in the Anavilhanas Archipelago, Central Amazonia, Brazil

Abstract

Caiman crocodilus crocodilus and Melanosuchus niger occur throughout most of the Anavilhanas Archipelago, located in the lower Rio Negro, Central Amazonia. The observed densities of these species vary from 0 to 58 per km of shoreline and 0 to 8 per km of shoreline, respectively. Multiple regression analysis indicated that the temperature difference between the water and the air, and water depth affected the observed density of C. crocodilus during spotlight surveys in the Archipelago. Percentage illumination by the moon, cloud cover, frequency of occurrence of grass, and food availability did not significantly affect the observed density of C. crocodilus. Nine hatchling groups of M. niger that hatched in 1992 were found on islands near the north bank of the Rio Negro, and 15 of 20 hatchlings groups of C. crocodilus were found closer to the south bank, indicating a tendency for separation of nesting areas. Discriminant function analysis indicated that water depth and presence of grass were correlated with the occurrence of hatchling groups. Melanosuchus niger hatchling groups occurred in areas with deeper water and more grass than those of C. crocodilus. This study showed that surveying hatchling groups in the Anavilhanas Archipelago is more efficient than surveying for subadults and adults, which, by itself, can give false impressions about the suitability of areas for conservation of breeding populations of these species.