The main direct threats to successful clutch development identified during the monitoring season 2010/11 were Ghost crab disturbance and predation as well as tidal erosion and inundation due to cyclones.
A total of 310/426 nests (73%) within the study area were observed to be disturbed or predated by crabs. Crab disturbance (crab burrows in nests) was observed in 35% of nests, while crab predation (scattered egg shell remnants in burrows or crabs killing hatchlings) occurred in 38% of nests laid in the study area.
The Golden ghost crab (Ocypode convexa) was predominant throughout the study area and was observed to be a predator of both sea turtle eggs and hatchlings. Running ghost crab (Ocypode ceratophthalma) was also witnessed to predate on sea turtle hatchlings. There are at least 4 species of crabs present within the Gnaraloo Bay Rookery, including Golden ghost crabs and Running ghost crabs, and further investigation is required to determine whether any of the other crab also impact on turtle nests and hatchlings.
Adverse weather conditions associated with cyclones brought about unusually large swells and high tides on several occasions during the monitoring season. This created significant sand movement along the beach and impacted a significant number of turtle nests. Inundation and erosion has also been identified as significant factors affecting turtle nests during previous years. Areas with the highest nesting density in the study area occur along narrow stretches of beach, which means that nests present closer to the shore receive significant exposure to high tides, which may in turn arrest egg development as eggs need constant gas exchange with the external environment in order to survive.
Given the prevalent crab activity noted in the Gnaraloo Bay Rookery since monitoring commenced in November 2010, GTCP researchers carried out additional crab burrow surveys during 12 December 2010 – 7 February 2011 to determine density and spatial distribution in the study area. This involved counting crab burrows in 33 quadrats spaced 200m apart along the entire study area, from south to north, on 5 separate occasions. Results showed that crab burrows were present all along the rookery and most prevalent within the northern part of the study area, correlating with areas of highest turtle nest density.
The crab burrow surveys also looked at the distribution of burrows within 3 zones on the beach, namely Inter-tidal Zone, High Tide Zone and the Dune Zone. Results indicated 71% of burrows in the Inter-tidal Zone closest to the shore, 26% in the High Tide Zone and 3% in the Dune Zone. No source of explanation concerning this could be found and only assumptions can be made at this stage in order to explain it, including:
- proximity to the water in order to hide or escape from predators;
- proximity to the water in order to keep gills moist and extract oxygen from the air;
- sand moisture in the Inter-tidal zone allowing a better and easier burrowing process; and/or
- higher presence of plants such as sea weed or micro-organisms in the Inter-tidal zone used as a source of food.
However, the presence of the majority of crab burrows in the Inter-tidal Zone (as opposed to the High Tide Zone and Dune Zone) is clearly not an impediment to easy access for crabs to turtle nests on the beach as the number of crab burrows within turtle nests recorded by GTCP researchers during 2010/11 ranged from 1 – 48 per nest.
Crab research undertaken during 2010/11 was preliminary only, and as of yet no conclusions can be made from these results. It is hoped to carry out additional and more detailed research in future years in order to investigate the relationship between the crabs and the turtles more closely.
The Turtles and Mama