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The sea turtle rookery at Gnaraloo Bay, Western Australia: using nocturnal observations to validate diurnal track interpretations

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journal contribution
posted on 2016-12-01, 00:00 authored by Jordan Thomson, N Hajnoczky, K Hattingh
Critical gaps remain in our understanding of many sea turtle nesting aggregations in
remote or undeveloped regions. Here, we summarize the first 8 yrs of systematic monitoring of the
rookery at Gnaraloo Bay, Western Australia. Diurnal track surveys on this approximately 7-km
mainland beach were conducted daily during nesting seasons 2008/09 to 2015/16. The total
number of emergences (i.e., nests and failed nesting attempts) recorded per season ranged from
480 to 813 (mean = 679.0, SE = 49.1), whereas the number of nests ranged from 305 to 522
(mean = 376.0, SE = 26.7). Peak nesting activity occurred between mid-December and late
January, with approximately 70 emergences and 35 nests recorded on average per week during
this time. The majority (97%) of emergences and nests were from loggerhead turtles (Caretta
caretta), whereas the remainder (3%) were from green turtles (Chelonia mydas). The number of
loggerhead turtle emergences recorded per season declined significantly over the course of the
study, wheras the number of nests did not, although we suspect that nest detection errors
contributed to the difference between trends. We conducted nocturnal surveys (i.e., direct
observations) during parts of seasons 2010/11 to 2015/16 to validate diurnal track interpretations
and assess potential biases in the diurnal data set. Diurnal nest counts for loggerhead turtles were
underestimates in all seasons but one, with an average nest detection bias of !13.0% (SE = 3.0).
After accounting for this bias, we estimate that approximately 405 nests are dug by 85 female
loggerhead turtles in the Gnaraloo Bay survey area annually. A similar or slightly lower amount
of loggerhead turtle nesting activity occurs at the Cape Farquhar survey area, also located on the
Gnaraloo coast; thus, this region contains previously underreported nesting aggregations of this
species. The Gnaraloo rookeries may play an important role in the dynamics of the southeast
Indian Ocean loggerhead turtle subpopulation and may still be depleted relative to historic levels
due to historical predation by introduced foxes. Monitoring, research, and the protection of
Gnaraloo beaches are, therefore, critical at this juncture.



Chelonian conservation and biology






187 - 196


Chelonian Research Foundation


Lunenberg, Mass.







Publication classification

C Journal article; C1.1 Refereed article in a scholarly journal

Copyright notice

2016, Chelonian Research Foundation