Survival of nests of the terecay turtle (Podocnemis unifilis) in the Nichare-Tawadu Rivers, Venezuela

Abstract

Mortality factors and hatching success of 422 terecay turtle (Podocnemis unifilis) nests on seven beaches in the Nichare and Tawadu Rivers, Venezuela, were studied following the oviposition phase. Beaches (3.4-28.4 km²), predominantly sandy, and bordered by riparian vegetation, were searched to record nest numbers and their distribution. Elevation and surrounding plant cover were measured for each nest. Most nests were found close to the vegetation's edge (0-11 m), at a significant distance from the water's edge (21-80 m), in more open ground, and along the highest points of the beach (1-2.5 m above the water's edge). Nest density was positively correlated with beach elevation but not with beach dimensions. Most nests were concentrated in less than 10% of beach area, along the upper 20% of the beach. Average clutch size was 20.1±1.7 eggs, but larger clutches were found significantly further away from vegetation. No significant correlation between hatching success and clutch size was found.

A large proportion of nests were subject to animal and human predation but environmental factors (especially flooding) affected some. The major cause of egg loss was human predation. Most clutches collected by humans were found away from vegetation, on the upper 10-20% of beaches, in exposed unvegetated sites. Animals preyed upon those nests along the vegetation's margin (1-2 m) in the upper 0-9% of beaches, in sites of high plant cover (75-100%). Nests reaching the incubation phase were mainly located in the open areas, which are more prone to human predation. About three-quarters of nests showed high hatching rates (91-100%). However, hatchability was highest furthest away from the vegetation. Our results indicate that humans are collecting eggs mostly from sites in which nests have the larger clutches and the higher potential hatching success. Because of this relationship between nest viability and location, sustainable yield programmes must consider where harvesting can take place and must avoid the application of standard harvests per nest.