Quantifying the impact of predation on sea turtle nests by varanids at Fog Bay
Creators: name: Blamires, Sean John
Publisher: name: Charles Darwin University
Year: 1999

description: The study area at Fog Bay was divided into four beaches, each of which was divided into 100 m sectors and Flatback (Natator depressus) and Olive Ridley (Lepidochelys olivacea) sea turtle nesting was monitored. A spatial database was created using Geographic Information Systems software, where features, such as flora, fauna and dune height and slope and vegetation were entered. Sea turtles nested where the sand was deepest and where access at the spring high water mark was not obstructed by rock. Future monitoring and updating of the database is recommended. Any biotic or environmental influences affecting emergence success of N. depressus hatchling were determined. Fifty-two percent of all nests were raided by the goannas Varanus panoptes. Of the undestroyed nests a 94.7% hatchling emergence success was found. Very few nests were lost to bacteria, fungus or insect infestation. The number of nests failing to emerge was of more concern than the emergence success in hatched nests. High temperatures, lethal to incubating eggs, may have been responsible for nests failing to hatch between September and November. A female biased hatchling sex ratio was predicted, with males only produced between June and August. Unless the number of females produced was over-estimated, the current hatchling sex ratio should allow stable future breeding populations, provided adequate protection was given to adults. Few nests hatched on beach 3 probably because spring high tides inundated nests, although the influence of tidal inundation was of less concern than the level of predation. Comparisons were made between the nesting behaviour of N. depressus at Fog Bay, where the dunes are steep, and Mundabullangana, Western Australia, where the dunes are not as steep. Similar crawl paths were followed at both sites but nesting was predominantly at the dune base at Fog Bay and on the primary dune crest at Mundabullangana. It appeared that nesting was restricted to the dune base at Fog Bay because of the steepness of the dunes. Experiments showed, however, that nests on the dune crest would not offer the eggs more protection from goanna predation. Radio tracking was done on five V. panoptes and between 85 and 100% of locations were made among the dunes or on the beach, with males having larger activity areas than females. The period of tracking (early, middle or late sea turtle nesting season) had no influence on activity area size therefore it appeared males may have been involved in courtship throughout the tracking period. Sea turtle eggs, insects, mammals, reptiles and crustaceans were the dominant prey, with sea turtle eggs consumed in greatest abundance between April and November. Feeding appeared to take place, year round, on the beach and in the grasslands associated with the dunes. Varanus panoptes searching paths, when on the beach, were determined by following and measuring tracks. Because N. depressus eggs have a relatively high caloric content and are readily located much more are consumed than is needed by V. panoptes to replenish energy expended searching. An energetically expensive searching strategy was employed in all months except for September. In September V. panoptes may conserve energy for October; when they are highly active on the beach and search extensively, presumably for hatched nests, to consume unhatched eggs and hatchlings that fail to emerge. Search paths were not directed toward encountering the maximum number of turtle nests at all times of year because V. panoptes often used the beach for other purposes such as; searching for other prey or thermoregulation. Management of the Fog Bay sea turtle rookery must target the period June-August, when most nests hatch and more male hatchlings are produced. Wire-mesh screens appeared the best method to deter Varanus and did not affect emergence success of hatchlings. Relocating eggs to the dune crest would be ineffective at protecting them from goanna predation. Nests laid in March-May or September-November, below the high water mark or on Beach 3, could be relocated to a hatchery or laboratory, left for varanids or made available for indigenous harvest.