Diving behaviour in two distinct populations of gravid Flatback turtlesNatator depressus

2010·2.6 MB·English

by  Jannie Sperling

Diving behaviour in two distinct populations of
gravid Flatback turtles Natator depressus
Jannie B. Sperling1, Gordon C. Grigg1 and Colin J. Limpus2
1School of Integrative Biology, University of Queensland, Brisbane, Australia 4072
2Environmental Protection Agency, PO Box 15155, City East (Brisbane), Qld, Australia 4002
This paper presents the first data on the diving behaviour of Flatback turtles, Natator depressus, between
nesting events. Dive profiles were recorded in turtles from breeding populations at Curtis Island in
Queensland and at Bare Sand Island in the Northern Territory, using Time-Depth Recorders (TDRs).
Both populations displayed dive types typical of those described in other internesting sea turtles. Dives
spent on the seabed were most prevalent, accounting for 57% of the time at depth. While on the sea
bed, the turtles apparently remained inactive, cyclical changes in depth reflecting the tidal cycle. These
inactive dives were long compared to those typical of other large sea turtles such as C. mydas and C.
T caretta, up to 98 min (mean 80±12 min), and with a mean and median of 50 and 52 min, respectively.
C In both populations, these dives occurred more commonly by day, by an average of 14%, and were
most prevalent in the middle third of the internesting interval in most turtles, while the eggs were
A
maturing. The turtles spent only 10% of their time at or near the surface and surface intervals rarely
R exceeded a few minutes, insufficient to be attributed to a need for either rest or thermoregulation.
T The Bare Sand Island population showed longer dive durations and proportionally more dives with
a gradual ascent phase, a phase presumed to be a method of compensating for buoyancy loss and a
S
way for turtles to conserve energy in mid-water while still moving around. Maximum dive depths of
B
29 m and 44 m were recorded at Curtis Island and Bare Sand Island respectively, probably reflecting
A differences in bathymetry.
Key words: diving, flatback, Natator depressus, internesting, dive duration
Introduction
The availability of Time-Depth Recorders (TDRs) to metabolism (Minamikawa et al. 2000). It has been
offers opportunities to infer information about specific suggested that these dives, described extensively in
behaviours of turtles over long periods. Sea turtles sea snakes (Rubinoff et al. 1986; Graham et al. 1987),
normally nest more than once within the same nesting are favoured during periods of travelling as a way of
season (Hirth 1980) and the period between nesting conserving energy (Hochscheid et al. 1999; Houghton
events, the internesting interval (Limpus 1985), is et al. 2002) and it is known that these dive types are
spent primarily preparing the next clutch of eggs for used by Green turtles, Chelonia mydas, undertaking
laying (Miller 1985). Sea turtles commonly do not oceanic migrations to their nesting beach (Hays et al.
feed at this time (Carr et al. 1974; Bjorndal 1985; 2001). Also, a recent study measuring turtle activity
Tanaka et al. 1995; Limpus et al. 2001; Hays et al. revealed that in most of these dives C. mydas swam at
2002a) and, therefore, behaviours which conserve least some horizontal distance while at the same time
energy may be at a premium. Indeed, many sea slowly ascending (Hays et al. 2004).
turtles spend the majority of the internesting interval
In addition to either sitting on the sea floor or
apparently inactive on the seabed, sometimes referred
slowly ascending in mid-water, a third possibility for
to as resting (van Dam and Diez 1997), surfacing only
conserving energy during the internesting interval is
briefly to breathe (Starbird et al. 1999; Hays et al.
for turtles to stay inactive at the surface (Sapsford
2000a, 2002b; Houghton et al. 2002).
and van der Riet 1979). This behaviour has been
Dives with a slow gradual ascent phase, which may suggested to be a form of surface ‘basking’ (Sapsford
be reptilian-specific (Hochscheid et al. 1999), are also and van der Riet 1979), where a possible increase in
often believed to occur in conjunction with energy- body temperature due to solar radiation may in turn
conserving behaviour and have been recognised in increase the rate of egg production (Whittow and
some species (Minamikawa et al. 1997; Hochscheid et Balazs 1982; Limpus et al. 1994). However, there is
al. 1999; Houghton et al. 2002). During the gradual no good evidence that sea turtles warm themselves
ascent phase the turtles have been found to be at or by ‘basking’ at the surface Sato et al. 1995; Read et
near neutral buoyancy (Minamikawa et al. 1997; 2000), al. 1996) so time spent at the surface, if it occurs,
at which time they are believed to be compensating could be associated with resting. There are some
for the loss of buoyancy. When slowly ascending, the observations that ‘basking’ could be more common
remaining lung air expands, increasing buoyancy, and in N. depressus than in other sea turtles (Cogger and
in so doing compensates for the buoyancy loss due Lindner 1969; Bustard 1972).
Theme edition of Australian Zoologist “Ecology meets Physiology”, a Gordon Grigg festschrift, edited by Lyn Beard,
Daniel Lunney, Hamish McCallum and Craig Franklin. Australian Zoologist Vol 35 (2) 2010.
Sperling et al.
N. depressus differs from other sea turtles in that it is one internesting interval, with one turtle tracked for
restricted to the continental shelf of Australia and two consecutive intervals. The internesting interval
southern New Guinea (Walker and Parmenter 1990). is the time between the end of one successful nesting
They breed only on Australian beaches (Limpus et al. event and the following attempt, whether successful
1988) and lack the post-hatchling oceanic dispersal or not (Limpus 1985). Curved Carapace Length, CCL,
phase seen in other sea turtles (Walker and Parmenter was measured on all turtles at each encounter (Table
1990). The present study describes the normal dive 1). The turtles studied at Curtis Island were 95.6
behaviour between nesting events for N. depressus. An ±2.2 cm long, which is significantly bigger (P<0.001,
important aspect of the study was to determine typical t-test) than the turtles at Bare Sand Island (87.1 ±2.8
dive depth and durations, as such information is lacking cm). The two study sites are 2700 km apart. Curtis
for N. depressus. The species is seldom known to dive Island is a 45 km long island outside Gladstone, with
deeper than 40-45 m (Walker 1991; Poiner and Harris a 5 km nesting beach at the south-eastern end. Bare
1996; Robins and Mayer 1998), while dive durations Sand Island, an uninhabited sandy island with a total
are unknown. Data suggest that N. depressus survive perimeter of 1.8 km, is situated 50 km west of Darwin
forced submersion better than other sea turtle species in Fog Bay. The two populations differ in nesting
caught accidentally by trawlers (Poiner et al. 1990). For season, with the Queensland population breeding
instance, N. depressus has a mortality rate of 11% in the between October and January (Limpus et al. 1984) and
northern Australian prawn fishery in comparison to 22% the Northern Territory population nesting between
for Loggerhead turtles, Caretta caretta (Poiner and Harris March and November (Guinea 1994). The study was
1996). We therefore hypothesized that N. depressus conducted during the nesting seasons of 2000/01 and
would show comparatively longer dive durations to most 2001/02 at Curtis Island and during the nesting season
other sea turtles, which are commonly less than 30-40 of 2001 at Bare Sand Island. Nightly patrols on the
min, and rarely more than an hour (Dermochelys coriacea: nesting beach made sure that all experimental turtles
Eckert et al. 1986; Wallace et al. 2005; Lepidochelys were intercepted.
kempii: Mendonca and Pritchard 1986; Caretta caretta:
Recording of dive profiles
Sakamoto et al. 1990a; 1990b; Lutcavage and Lutz 1991;
Houghton et al. 2002; C. mydas: Brill et al. 1995; Hays Two types of Time-Depth Recorders (TDRs) were
et al. 2000; 2001), reflecting a respiratory physiology used to record dive profiles, both of which record and
particularly suited to sustain prolonged shallow dives store in situ depth and temperature information. Lotek
(Sperling et al. 2007). LTD_100 loggers (Lotek Wildlife Monitoring, Canada)
were used together with a specially made housing (Les
Dive durations may further be influenced by body size,
Fletcher, University of Queensland) and logged the
with maximum durations being positively correlated with
depth every 6 s with an accuracy and resolution of
body size. While this relationship is well documented in
1.1 m and 0.09 m, respectively. The second TDR
endotherms, the same interspecific relationship fails to
used was Minilog 8-Bit TDR (Vemco Limited, Nova
show up in ectotherms but may still be evident within a
Scotia, Canada) which logged depth every 45 s,
species (Brischoux et al. 2008). In the present study, data
with an accuracy and resolution of 1.0 m and 0.2
are presented on the diving behaviour of N. depressus
m, respectively. At Curtis Island, five deployments
from two different breeding populations. One population
were made with Lotek loggers and five with the
is part of the eastern, central Queensland breeding stock,
Vemco loggers, and all but two of the deployments,
which is genetically and physiologically discrete (Limpus
the two experiencing battery failure prior to interval
et al. 1993; Dutton et al. 2002) from the other that breeds
completion, resulted in data collected from complete
in the Northern Territory. The turtles at Curtis Island are
internesting intervals. At Bare Sand Island eight
larger, with an average curved carapace length of 92 cm
deployments were made with Lotek loggers and three
compared to 86 cm at Bare Sand Island (Limpus 1971;
with Vemco loggers. Two deployments were made on
Whiting and Guinea 2006). We therefore hypothesize
the same individual, and all deployments resulted in
that Curtis Island turtles dive for longer to similar depths.
data collected from complete internesting intervals.
The two populations are also likely to encounter different
bathymetry, with the area used by turtles nesting at For all except five turtles the TDRs were attached under
Curtis Island being considerably shallower than around the rear marginal scutes of the carapace using a stainless
Bare Sand Island (Sperling 2007). We hypothesized that steel bolt, washers and nylon threaded nut, following the
dives to the seafloor, as in dive type 1, may be favoured in procedure designed originally by Sally Hopkins-Murphy
shallow water, while mid-water dives may be favoured in (South Carolina Department of Natural Resources). The
deeper water where the seafloor may be less accessible. hole was drilled while the turtle was on its back, using
a battery-operated drill. The attachment method was
Material and methods trialled on a captive turtle (Sea World, Gold Coast) before
any field work, and no contraindications emerged. The
Study sites and animals used
last five turtles, two at Curtis Island and three at Bare
The present study is based on ten individuals at Sand Island, had a satellite transmitter attached as well.
Curtis Island, Queensland (23°45’S, 151°17’E) and ten In these cases, both instruments were bolted to a plate
individuals at Bare Sand Island, Northern Territory which was secured to the turtle’s carapace using a special
(12°32’ S, 130°25’ E). Each individual was tracked for six-strap harness (Sperling and Guinea 2004).
Australian
292 Zoologist volume 35 (2) 2010
Diving behaviour of Flatback turtles
dard min) x 3 3 5 3 1 3 4 1 0 1 2 2 9 8 5 5 8 5 3 3 7 9 6
tan t ( ma 44. 25. 61. 38. 32. 47. 41. 31. 48. 63. 43. 62. 54. 48. 62. 58. 72. 49. 64. 49. 48. 50. 56.
s n
a e
Length’, dual asc edian 20.3 9.6 27.8 13.5 10.6 12.8 14.0 15.8 15.8 33.2 17.3 42.3 15.6 18.0 26.7 27.4 40.5 22.8 35.6 19.7 34.1 21.6 27.7
e ra m
c g
pa n
d Cara Duratio mean 21.7 10.5 28.3 15.1 11.4 12.6 14.9 16.5 16.2 31.3 17.9 35.9 18.6 19.3 27.3 29.9 39.5 23.3 34.0 19.6 30.0 23.2 27.3
e
v
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u
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CL is ‘ min) max 64.5 60.1 87.0 74.3 89.4 95.3 84.0 64.3 63.8 98.4 78.1 80.3 81.7 62.3 79.6 72.8 92.3 80.1 86.0 80.7 95.9 95.7 82.5
). C 1a (
s4 e
3, 4, b4, tion typ median 39.8 41.0 57.4 48.0 51.3 50.3 34.9 38.5 39.8 58.8 45.9 59.2 54.4 48.0 51.1 45.0 62.3 57.5 62.2 56.4 72.9 64.1 57.7
( a
phase Dur ean 9.6 0.0 6.8 5.4 1.7 7.4 5.0 8.8 7.8 7.1 5.0 6.0 2.5 6.5 6.3 3.5 7.9 3.6 7.8 4.1 9.0 1.6 4.4
t m 3 4 5 4 5 4 3 3 3 5 4 5 5 4 4 4 5 5 5 5 6 6 5
n
e
c
s
a
a gradual Unident. 11.2% 7.8% 13.2% 13.1% 5.4% 22.5% 15.5% 1.3% 8.8% 14.6% 11.3% 13.7% 6.7% 36.7% 8.7% 7.0% 3.9% 3.5% 4.5% 16.8% 6.6% 11.7% 10.9%
h
t
wi
es % % % % % % % % % % % % % % % % % % % % % % %
v 5 2 4 1 1 2 6 6 0 2 1 3 8 8 6 5 2 7 4 2 8 3 4 2
di 0. 0. 0. 0. 0. 0. 1. 0. 0. 0. 0. 0. 0. 1. 1. 2. 0. 0. 1. 0. 2. 0. 1.
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an ) % % % % % % % % % % % % % % % % % % % % % % %
pe 1a e-time s4 0.4 0.8 4.4 1.5 1.3 1.3 1.7 0.2 0.8 4.7 1.7 8.5 5.5 8.1 6.3 3.8 5.9 1.1 13.8 4.3 6.8 8.7 6.6
tion of dive ty ype (% of div 4d4 3.2%1.4% 1.4%0.4% 3.5%2.2% 4.1%1.3% 2.4%1.1% 2.7%1.3% 3.4%3.9% 0.7%2.6% 2.8%1.5% 3.9%3.7% 2.8%1.9% 14.3%6.9% 9.9%4.6% 19.5%3.7% 16.2%5.0% 9.1%3.9% 15.6%8.7% 5.4%3.2% 2.8%4.7% 5.6%2.9% 2.9%2.5% 5.6%5.1% 9.7%4.6%
a t
dur ve % % % % % % % % % % % % % % % % % % % % % % %
he Di 3 3.4 0.1 0.2 0.4 0.3 1.3 0.5 0.1 2.1 1.7 1.0 1.0 0.5 0.9 3.3 0.7 1.0 0.6 0.0 0.2 0.0 0.0 0.8
t
d
n % % % % % % % % % % % % % % % % % % % % % % %
a 2 9 2 7 2 6 9 6 5 2 4 8 2 7 6 8 8 4 4 0 0 1 2 7
arious dive types of the carapace. 1a1b 4.7%14.7%0. 0.2%17.6%1. 6.8%18.9%0. 7.0%21.2%1. 2.0%16.7%0. 2.1%26.3%1. 0.1%22.7%0. 3.7%10.9%0. 3.9%19.7%0. 9.2%21.8%0. 1.0%19.1%0. 6.5%8.1%0. 8.4%12.9%0. 2.2%4.8%2. 4.1%14.1%0. 4.7%6.9%1. 8.3%5.5%0. 7.9%7.6%0. 7.2%15.9%0. 0.8%17.5%1. 7.1%11.7%0. 7.0%21.3%0. 4.0%11.5%0.
of the vmidline ce %6 %7 %5 %5 %7 %4 %5 %8 %6 %4 %6 %4 %5 %2 %4 %6 %5 %7 %5 %5 %6 %4 %5
rence g the At surfa 6.1 7.6 7.5 11.0 6.5 9.7 15.9 7.9 6.3 6.8 8.5 11.1 15.2 16.6 18.1 13.1 9.6 7.6 6.2 11.1 10.9 9.1 11.7
rn
Table 1. The occumeasurement alo TagCCL #(cm) Curtis Island K1855192.7 K1990195.1 K2017298.2 T1586592.7 T2090697.6 T7418097.1 T7420396.6 T9715595 T9720395.2 T9721392.2 Mean95.2 Bare Sand Island K 1921587.4 K 1927885.4 K 2630288.5 K 2900187.8 K 4583782.4 K 45918-187.9 K 45918-2- K 4597887 K 796090.8 T 9422283.3 T 9433390.7 Mean87.1
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2010 Zoologist volume 35 (2) 293
Sperling et al.
Categorisation of dive profiles dive. The remaining dives were then classified into
dive types 1a, 1b, 2, 3, 4, s4, d4, 5 or ‘unidentified’, as
Data from the TDRs were downloaded to a computer
follows (Fig. 1):
for analysis. Depth corrections of up to 0.5 m were
made to account for drift in sensitivity, determined Dives of type 1a, described by Minamikawa et al. (1997)
by calibration, i.e. each TDR was lowered into the and Houghton et al. (2002) and similar to the ‘U-dives’
sea to various known depths before and after each described by Hochscheid et al. (1999), Hays et al. (2000;
deployment. Initial analyses was made using a custom- 2002b) and Reina et al. (2005) show a rapid descent,
written dive-analysis program (M. Gordos, Visual Basic, a long bottom time and a rapid ascent (Fig. 1). They
Microsoft Corporation©). The program produced a list frequently show a tidal pattern. The dives are assumed
of dives; each dive defined as a series of consecutive to be dives in which the turtles spend time inactive on
recordings below a certain threshold depth. Choice of the sea floor (Hays et al. 2004) and they have therefore
threshold depth took into consideration the possibility been referred to also as ‘resting dives’ (van Dam and
of turtles breathing at the surface in a fully vertical Diez 1997) although C. mydas nesting on Cyprus have
position, together with the resolution and accuracy been found to forage actively during ‘U-dives’. In the
of the loggers. Because attachment was at the rear present study, only dives with the described time-depth
of the turtle, approximately 1 meter from the head, profile longer than 10 min and with at least 75% of the
depths recorded below 2.0 meters for deployments of bottom phase following the expected tidal pattern were
both Vemco and Lotek TDRs were judged to indicate classified as dive type 1a.
a dive. For turtles with harnesses, the loggers were Dives of type 1b are similar to 1a in profile but, instead of
situated about 0.5 m further up their carapace, and having a period of apparent inactivity at the bottom, the
for these turtles depths below 1.5 m were considered bottom phase shows some changes in depth indicating
indicative of diving. activity. Houghton et al. (2002) made a similar distinction
Despite great variability in the dive patterns seen, many between dives with a flat bottom phase (1a) and a more
of the individual dives could be sorted into categories erratic (variable depth) bottom phase (1b).
based on the shape, depth and duration, most of which Dives of type 2, described by Minamikawa et al. (1997)
matched dive-types defined already in the literature and Houghton et al. (2002), and similar to the ‘V-dives’
(Fig. 1) (Minamikawa et al. 1997; Hochscheid et al. described by Hochscheid et al. (1999), Hays et al. (2004)
1999; Hays et al. 2000; 2002b; Houghton et al. 2002). and Reina et al. (2005), show a steep descent followed
These were not necessarily mutually exclusive, but by an immediate steep ascent (Fig. 1). These dives have
were useful to allow interpretation because they appear been suggested to be exploratory in nature (Hochscheid
to represent different types of behaviour. Each dive, as et al. 1999).
defined by the custom-written dive-analysis program,
Dives of type 3, as described by Minamikawa et al.
was examined and then classified subjectively into a
(1997) and described as dive type 1 by Hays et al. (2001)
dive type based on profile shape and the depth and
resemble the dives described as ‘shallow travelling dives’ by
duration of the dive. To prevent any bias, data sets
Hochscheid et al. (1999). They consist of a quick descent
were looked at in random order without any prior
followed by a gradual ascent phase and then a quick return
knowledge of patterns at the corresponding study site.
to the surface (Fig. 1). The slow gradual ascent phase is
The differences in spatial and temporal scales for the
thought to be the result of the turtle compensating for
two TDRs did not appear to influence the resulting
loss of buoyancy as lung volume decreases (Minamikawa
dive categories. However, more of the data attained
et al. 2000). These dives may be favoured during periods
with Vemco loggers had to be excluded from further
of travelling as a way of resting in mid-water (Hochscheid
analysis because of their poorer depth resolution.
et al. 1999; Houghton et al. 2002).
Before classifying the dives, some patterns identified
Dives of type 4, as described by Minamikawa et al.
erroneously as dives by the dive-analyses program were
(1997) and described as dive type 2 by Hays et al.
subtracted from the total number of dives for each
(2001) are similar to those described as an ‘S-dive’
turtle. As the program would count measurements
by Hochscheid et al. (1999). They consist of a quick
below the threshold depth of 2.0 or 1.5 meters as
descent followed by a quick first ascent; the turtle
a dive, ‘false’ dives would easily be recorded when
then apparently stops at a certain depth, sometimes
turtles stayed around this depth, resulting in a series
referred to as a ‘dive stop’ (Hochscheid et al. 1999)
of apparent dives and surface events measured slightly
where it ascends gradually for a while before returning
below and above the threshold depth, respectively.
to the surface in a final quick ascent (Fig. 1). As in
Also, the shallow threshold depth could lead to
dive type 3 the slow gradual ascent phase is thought
subsurface swimming with periodic breaths being
to be the turtle compensating for loss of buoyancy,
counted as single dives. However, it is believed that
where the ‘dive stop’ is dependent on the lung volume,
error from this source was minimised by excluding
so as to make the turtle at or near neutral buoyancy
short dives (12 s or less for Lotek loggers or 45 s for
(Minamikawa et al. 1997; 2000).
Vemco loggers) which were shallower than 3 meters,
and longer dives when the depth difference between Dive type s4 is a newly described dive type from this study
the pre-dive surfacing event and the dive was less than (Sperling 2008). These dives resemble dive type 4, but
1 meter unless the profile confirmed a single definite have, in addition, ‘spikes’ in the gradual ascent phase, i.e.
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294 Zoologist volume 35 (2) 2010
Diving behaviour of Flatback turtles
Figure 1. The dive types as described here shown schematically. Terminology commonly used for dives with a gradual
ascent phase includes first ascent (i), gradual ascent or dive stop (ii) and final ascent (iii).
additional trips to deeper and/or shallower water. They are and Bare Sand Island, respectively. Only one turtle
suggested to be relatively active compared to dive type 4, (T15865) spent considerable time at the surface on
and may indicate a turtle spending time actively exploring two consecutive days, with depth measurements staying
the water column (Sperling 2008). consistently around 0 m for several hours in the middle
of each day. The turtles at Curtis Island (n=10) averaged
Dive type d4 is also a newly described dive type from
1.9 ±0.4 min at the surface between dives, significantly
this study (Sperling 2008). These dives resemble dive
(P=0.008, power of test 0.772, t-test) less than the
type 4 but the initial descent is followed by some time
2.4 ±0.5 min for turtles at Bare Sand Island (n=11).
spent at the deepest part of the dive. They are suggested
Although turtles carrying Vemco loggers frequently
to stem from the turtle staying on the seabed as in dive
showed a longer surface duration, probably due to the
type 1a, before initiating a gradual ascent phase, and may
greater sampling interval, the choice of TDR did not
therefore be relatively inactive. The vertical distance
affect the average surface duration (P=0.289, t-test).
travelled from the seabed to the point where the gradual
ascent phase is initiated may be indicative of the change Each turtle showed a variety of profiles, with dive
in lung volume from oxygen consumed during the type 1a dominant in all individuals (Table 1). While
bottom phase (Sperling 2008). dives of type 2, 3 and 5 occurred negligibly in
both populations, the turtles at Curtis Island spent
Dives of type 5 resemble type 5 dives described by
significantly more dive-time engaged in dive type
Houghton et al. (2002) and the ‘subsurface dives’ described
1b (P=0.016, t-test) with 19.1% compared to 11.5%
by Hochscheid et al. (1999) which are thought to indicate
near-surface activity. In the present study these dives for the Bare Sand Island population. In contrast, the
are no deeper than 3 meters, last longer than 12 or 45 turtles at Bare Sand Island spent more time engaged
s (for dives recorded with Lotek and Vemco loggers, in the various type 4 dives (4, s4 & d4) with 21.0%
respectively), and do not fit any of the above categories. compared to 6.5% of the dive-time for the Curtis
Island turtles (P<0.001, t-test). A full profile is shown
Statistical analysis for turtle T97213 (Fig. 2).
Mean is given for most variables to allow for comparisons
Variations in dive depth and duration
with other studies, and median values are given also
for some variables where these are thought to give The maximum dive depth for turtles was 29 meters at
a more informative representation of the prevalent Curtis Island and 44 meters at Bare Sand Island, with
behaviour. When mean values are given, they are dives of 20 m or less being most common at both study
presented with 1 SD. Comparisons between the two sites. Generally dives were as expected deeper for Bare
populations were made using Student’s t-test on means. Sand Island turtles (Fig. 3). In most cases the deeper dives
Correlations within individual turtles for dive durations, were dive type 1a, with some dive types 2 and 4 reaching
depths, surface durations etc. were explored using linear similar depths. Most of the turtles showed only a few dives
regressions. Partial regressions were used to test for around the maximum depth and, in addition, have one or
the effect of turtle size on dive duration. SigmaStat™ two modal depths for dive type 1a, resulting in a variety of
Statistical Software (v2.03) and SPSS© for Windows depth distributions (Fig. 4).
(10.0.1) were used for analyses.
N. depressus showed some long dives, the longest being
98 min. However, most of the dives were short. Dive
Results
duration varied with the type of dive, where type 1a was
usually also the longest of the dive types. Dive type 1a was
Time underwater, at the surface, and relative
on average about 10 min longer (P=0.011, power of test
occurrence of different dive types
0.713, t-test) for Bare Sand Island turtles compared to
All the turtles spent most of their time at depth, with Curtis Island turtles, while the maximum dive durations
little time (10 ±4%) at or near the surface (Table 1). were similar (Table 1). The descent and ascent part of
Average time spent diving, i.e. below the threshold these dives usually lasted only between 1 and 3 min, even
depth (1.5 or 2.0 m) and referred hereafter to as ‘dive- for the deepest dives. Dive durations for dive types 1b, 2
time’, was 91.5 ±3.0% and 88.3 ±3.7% for Curtis Island and 5 were similar for the two populations, while turtles
Australian
2010 Zoologist volume 35 (2) 295
Sperling et al.
Figure 2.
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296 Zoologist volume 35 (2) 2010
Diving behaviour of Flatback turtles
Figure 2. continued
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2010 Zoologist volume 35 (2) 297
Sperling et al.
Figure 2 continued. The full dive profile of turtle T97213 from Curtis Island. The bar along the bottom signifies day and
night, with the open bar being from sunrise to sunset, and the black bar from sunset to sunrise.
at Bare Sand Island had longer gradual ascent dives (Table Patterns of behaviour throughout the
1), i.e. dive types 3, 4, s4 and d4 (P=0.007, power of test internesting interval
0.792, t-test). Dives of type 4 were especially long in the
Early and late in the internesting interval there were short
Bare Sand Island turtles (P=0.003, t-test), where the
and presumably active dives, while dives of type 1a were
depth and duration were similar to dive type 1a in any
more prevalent in the approximate middle third of the
one individual.
interval (Fig. 7). Also, the diel pattern described above was
Turtles at Bare Sand Island showed a stronger more common at this time; e.g. turtle K26302 showed a
correlation between the mean dive depth and duration strong diel pattern on days 4–9, but not on other days. The
of dives of type 1a (Fig. 5 a and b) than turtles at Curtis turtles at Curtis Island engaged dominantly in dives of type
Island, with correlations varying extensively within 1a from day 5 and prior to the 3 days before next nesting.
both populations. For both populations the correlations Similarly, at Bare Sand Island the turtles predominantly
between mean depth and dive durations were greater engaged in dives of type 1a from day 6 and prior to 4 days
for dives of type 3, 4, s4 and d4 (Fig. 5 c and d). Average before the next nesting event. For the latter population this
dive duration for dive type 1 was not correlated to the meant that dive type 1a was significantly more frequent
size, i.e. CCL, of the turtles, after removing the effect during egg maturation (day 3 to 9) and least frequent
during the first 36 hours, i.e. prior to ovulation, of the
of depth (partial regression).
internesting interval (relative occurrence of dive type 1a
Only some turtles showed a correlation between the
between 0-36 h and 72-216 h into the internesting interval,
dive duration of type 1a dives and the surface duration
P<0.001, t-test). None of the turtles had more than a few
before and/or after the dive (maximum r2 were 0.35 and
short dives of type 1a the day before the next nesting event.
0.42 for pre- and post-dive surface duration, respectively, Instead, the last three or four days in both populations were
P<0.001, least square linear regressions) (Fig. 6). spent close to the surface (Fig. 7), consistent with turtles
staying in shallow water close to the nesting beach. Dives
Diel patterns of behaviour
of type 2 and 5, which were otherwise uncommon, occurred
Most turtles showed some preference for dive type 1a more frequently at the end of the interval.
during the day, and gradual ascent dives, in particular
The depth to which turtles dived also changed during
dive type 4, during the night. The dives were divided
the course of the internesting interval (Fig. 3). Six of
into those starting at night and those starting during
the ten Curtis Island turtles quickly reached deep water
the day, where night and day was determined by the
after leaving the beach, then after three days appeared to
official sunset and sunrise times for the location of the
stay in shallower water until a few days prior to the next
nesting beach for each day. Turtles at both sites spent
nesting event. They then dived deeper again until shortly
on average 14% more time engaged in dive type 1a
before next nesting event when they apparently returned
during the day compared to the night. On average, 6% closer to land. A different depth pattern was evident in
of the time was spent engaged in dives of either type 3, most Bare Sand Island turtles, where the first and last few
4, s4 or d4 during the day, and 15% at night, with great days were spent at much shallower depths than for the rest
variations between individuals. of the internesting interval (Fig. 3 b).
Australian
298 Zoologist volume 35 (2) 2010
Diving behaviour of Flatback turtles
Figure 3. The mean depth with one standard deviation throughout the internesting interval for N. depressus, from Curtis
Island (left) and Bare Sand Island (right). Day 0 indicates the time prior to midnight on the night recording started and
hence only includes turtles that nested prior to midnight. Day 5 indicates five days after last nesting, while -5 indicates
5 days before next nesting event.
Figure 4. The depth distribution in three N. depressus, from Curtis Island (a, c & e) and three from Bare Sand Island (b,
d & f), emphasizing the range within the species.
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2010 Zoologist volume 35 (2) 299
Sperling et al.
Figure 5. The mean dive depth of a single dive was related to the dive duration of the dive for both dive type 1a (a and
b) and all dive types 3, 4, s4 and d4 (c and d) for turtles nesting at both Curtis Island (a and c) and Bare Sand Island (b
and d). All turtles from each site are included, and the lines indicate linear regressions (least squares).
Figure 6. The time spent at the surface before and after dive type 1a was in some individuals correlated to the duration
of the dive. Two examples are shown: T97155 had the median correlation coefficient for the post-dive surfacing events
(b), and the next highest of any turtle for pre-dive surfacing events (a). K45978 had the maximum correlation found in
any individual turtle for both pre- and post-dive surfacing events (c and d).
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300 Zoologist volume 35 (2) 2010