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Sea turtle nest excavation at Gnaraloo

First Nest Excavation at Gnaraloo Bay!

Last week, the 2015/16 GTCP Field team collaborated with the WA Department of Parks and Wildlife (DPaW) in excavating a small number of hatched sea turtle nests at Gnaraloo Bay. As the GTCP only looks at what is happening to our sampled nests (marked nests that are monitored daily throughout the season) from above the surface of the sand, the excavation was a great opportunity for the team to have a first look at the state of the hatched nests from below. Joining us on the beach were Dr. Peter Barnes (Marine Program Coordinator, Exmouth District), who issued the permit for the GTCP to carry out the excavation, and Matt Smith (Senior Marine Ranger).

What is nest excavation and why is it important?

Nest excavation involves digging up hatched sea turtle nests and looking at the contents. It is an important way to determine the success of a nest – both hatching success (percentage of turtles that hatched from their eggs) and emergence success, as well as predation impacts and species identification. Together, these gives us a glimpse into how the nests fared at the incubation site.

Nest excavation at Gnaraloo
GTCP Program Assistant Dr. Jordy Thomson excavating a hatched nest (Photo by Mel Do)

Nest excavation process

Five hatched turtle nests were excavated on 17 February 2016 at Gnaraloo Bay. All nests had either hatched five days before they were excavated to allow hatchlings to emerge naturally, or nests that had been laid more than 90 days ago, which is the maximum incubation period for the sea turtle nests at Gnaraloo. The nest excavation begins with locating the egg chamber then digging into the sand until the top of the chamber is reached – this is usually determined by the first sign of eggshell fragments, dead or alive hatchlings, or unhatched eggs. The entire content of the egg chamber then gets removed, sorted, counted and recorded.

The GTCP Field team and Dr. Barnes from DPaW working together to sort and record the contents of an excavated nest (Photo by Mel Do)
Hatched eggshells
Sorted hatched eggshells (Photo by Mel Do)

What we found

The hatchlings that remained in the excavated nests were all confirmed loggerhead turtles (Caretta caretta), and all nests were impacted by various environmental factors and predation. These ranged from tidal inundation, heightened temperatures in shallow nests, and predation of eggs by golden ghost crabs (Ocypode convexa) – all contributing to low egg counts. Here at Gnaraloo Bay, golden ghost crabs occur in high abundance and are native predators of sea turtle eggs and hatchlings. While the exact rate of crab predation on turtle eggs at Gnaraloo remains unknown, according to Dr. Barnes, crab impacts here appear to be greater than at nesting sites further up the Ningaloo coast.

Golden ghost crab
Golden ghost crab (Ocypode convexa), a native predator of sea turtle eggs and hatchlings at Gnaraloo Bay (Photo by Peter Koch)

Final message

Overall, this was a very important first look into the hatched sea turtle nests at Gnaraloo, especially in terms of native predator impacts. We would like to thank DPaW for collaborating with the GTCP field team in, and issuing the permits for, the nest excavations. It was a pleasure to have Peter and Matt join us on the beach and we hope it won’t be the last!

– Written by Mel Do (GTCP Community Outreach Intern)

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