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2010 SOUTHEASTERN NATURALIST 9(2):191–200
Time, Temperature, and Depth Profiles for a
Loggerhead Sea Turtle (Caretta caretta) Captured With a
Mark A. Grace1,*, John Watson1, and Dan Foster1
Abstract - During a pelagic longline pilot study conducted by the National Oceanic
and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries
Science Center, Mississippi Laboratories along the US Atlantic Ocean coast
(NOAA Ship OREGON II OT-06-02-269), a Caretta caretta (Loggerhead Sea Turtle)
was captured with longline gear equipped with time-temperature-depth recorders
attached in proximity to hooks. Time-temperature-depth data documented changes
in hook depth and water temperature, and reflected behavior of the Loggerhead Sea
Turtle (rates of descent and ascent, time at depth, time near surface). Sea turtle mortality
mitigation recommendations for pelagic longline gear proved effective for this
Loggerhead Sea Turtle capture since there were successive ascents to surface, and
the viability status was good after landing.
Use of pelagic longline gear is common for several commercial fisheries
operating along the coastal United States (i.e., tuna and swordfish).
Pelagic longline gear is passive, and captures are generally hooked by
mouth or on occasion tangled by gear, at times resulting in mortality of
incidentally captured fish or sea turtles (Lewison et al. 2004, Witzell
1999, Witzell et al. 2001). Assessing how pelagic longline deployment
potentially affects sea turtle mortality is important when developing
mitigation measures for sea turtles, which are often the focus of conservation
efforts (Watson et al. 2005) due to their threatened or protected status
(Plotkin 1995). Even though an emergency bottom longline fishery closure
by the National Oceanic and Atmospheric Administration, National
Marine Fisheries Service (NOAA/NMFS), does not directly pertain to pelagic
longline fisheries (Fisheries of the Caribbean, Gulf of Mexico, and
South Atlantic; Reef Fish Fishery of the Gulf of Mexico; Gulf Reef Fish
Longline Restriction, Federal Register/Vol. 74, No. 83), the emergency
closure reflects the importance of longline gear and sea turtle interactions
and subsequent regulations.
During a 2006 NOAA/NMFS Southeast Fisheries Science Center
Mississippi Laboratories (MSL) pilot study to determine the feasibility
1US Department of Commerce/NOAA/NMFS/SEFSC/Mississippi Laboratories, PO
Drawer 1207, Pascagoula, MS 39568-1207. *Corresponding author - Mark.A.Grace@
192 Southeastern Naturalist Vol. 9, No. 2
of using pelagic longline gear to assess the relative abundance and distribution
of pelagic sharks and finfish (NOAA Ship OREGON II survey
OT-06-02-269; Fig. 1), a Caretta caretta (L.) (Loggerhead Sea Turtle)
was captured, tagged, and released. Since the pelagic longline gear was
equipped with time-depth-temperature recorders (TDRs), the Loggerhead
Sea Turtle capture provided a unique opportunity to collect and examine
associated TDR data that documented the Loggerhead Sea Turtle behavior
by recording changes in depth and time sequences for descent, ascent, atdepth,
and at-surface events (there is no previous record of a capture with
a TDR-equipped pelagic longline).
Materials and Methods
Pelagic longline gear components were 9.3 km (the distance between
the first and last radar reflector hyflyer) of 454-kg-test monofilament
Figure 1. Pelagic longline locations for MSL Pelagic Longline Pilot Study (NOAA
Ship OREGON II OT-06-02-269). Location coordinates are degrees latitude north
(N), and degrees longitude west (W).
2010 M.A. Grace, J. Watson, and D. Foster 193
mainline (4 mm diameter), 3 radar reflector hyflyers (strobe light
equipped) to mark distal mainline ends and mid-set, bullet floats (50),
and 2 gangions attached between bullet floats (100 gangions per set).
Gangions (22 m length) were constructed with #18/0 non-offset steel
circle hooks with a 0.5 m length of multistrand stainless steel fishing wire
(364 kg breaking strength) attached between the hook and a 60-g weighted
swivel. The 21.5-m length of gangion monofilament (2 mm diameter,
179 kg test) connected the weighted swivel with the gangion mainline
clip. All gangions were equipped with hook timers attached to the gangion
mainline clip. Time-temperature-depth recorders (TDRs) were attached to
each gangion at the weighted swivel; TDRs logged data at 256 reports
per hr, pressure accuracy was ±1% of the depth scale (pressure was used
to calculate depth), temperature resolution was 0.2 oC, and temperature
accuracy was ±0.3 °C. Gangions were sequentially numbered with numeric
tags attached to hook timers and TDRs; hook timers were used in
conjunction with TDRs to provide an independent measure of capture activity.
The hook fishing depth beneath the sea surface was a minimum of
40 m, but could be deeper depending on line catenary, activity of hooked
catch, or sea conditions. The pelagic longline gear was deployed and retrieved
from the NOAA Ship OREGON II (51.8 m length).
Pelagic longline gear configuration followed current NOAA recommendations
for mitigating sea turtle captures (NOAA 2001); most
importantly, gangion length was at least 110% greater than floatline
length so captured turtles could surface to breathe if hooked, and use of a
#18 non-offset steel circle hook to minimize hook swallowing. The 18-m
floatline length determined mainline depth and was established to allow
most deep draft vessels to pass over gear without entanglement. Bait was
frozen Scomber scombrus L. (Atlantic Mackerel, 400g–600g weight) used
whole, cut into halves or thirds depending on bait size; most bait was
Pelagic longline soak time was 3 hr, determined from the time the
last radar reflector hyflyer was deployed at the end of the gear set, until
the first radar reflector hyflyer was retrieved at the beginning of the gear
haulback. In order to collect gear set, gear haulback, and catch data in
real time, weather-resistant laptop computers were hard wired via deck
network ports to the ship’s scientific computer system (SCS), and were
operated with software designed for the fisheries science computer system
(FSCS) to record gear events, catch, and biological data. Utilizing the
ship’s SCS data allowed Greenwich Mean Time (GMT) time/date stamps
and the corresponding position (latitude/longitude) to be associated with
set and haulback events (e.g., radar reflector hyflyer, buoy, and gangion
deployments and retrieval, and hook status). TDRs were downloaded if
their associated gangion had a capture, the hook timer had been activated,
or once weekly to clear memory. TDR files were named with vessel code,
194 Southeastern Naturalist Vol. 9, No. 2
cruise number, station number, and hook number to facilitate association of
TDR information to SCS and FSCS events.
During the pelagic longline pilot study (54 longline sets completed
over 42 sea days), a Loggerhead Sea Turtle was captured (Table 1) and
brought aboard with a turtle dip net, then measured, photographed,
tagged, and released after the hook was removed. Even though the weight
of the turtle was 25 kg, it did not pull the gangion with enough force to
activate the hook timer (4 kg–6 kg force needed). The Loggerhead Sea
Turtle was the only capture for the associated longline set (no elasmobranchs
or teleosts captured), and its viability status at release was good
(as determined by active responses to handling and alert behavior). TDR
recorded seawater temperatures that ranged from 22.1–23.3 °C (mean =
22.9 °C , SD = 0.1143). The Loggerhead Sea Turtle was on hook (mouth
hooked) for 223 min (including 21 min to haul to the ship) as determined
by a pronounced depth change at hooking reflected by the TDR data
(Fig. 2); the total time of the hook deployment was 263 min (determined
by the ship’s SCS/FSCS data collection system). At approximately 25
min after hook deployment, there were a series of small depth spikes below
the stabilized hook fishing depth (41 m), and that may have been the
first gear encounter by the Loggerhead Sea Turtle (Fig. 2). After 40 min,
the TDR depth change was pronounced and the Loggerhead Sea Turtle
ascended to sea surface (time of ascent was 2106 GMT or local time
1506 Eastern Standard Time). Seventy-two percent of the hooked time
was spent within 10 m of sea surface, and 28% of the hooked time was
spent below 10.0 m depth. Using TDR depth data to determine surfacing
events (depth less than the 0.5 m length of the steel leader between the
hook and TDR), it appears the Loggerhead Sea Turtle surfaced at least
22 times while on hook; some TDR depth reports from <0.5 m could be
from the TDR aligned along the top of the carapace (sometimes creating a
negative value) or from lift created by forward swimming at sea surface.
The most TDR reports from <0.5 m (10) followed the first ascent after
hooking (Fig. 2).
Table 1. Biological and associated data for a loggerhead sea turtle captured during the 2006
MSL Pelagic Longline Pilot Study (NOAA Ship OREGON II OT-06-02-269).
Hook Bottom Surface Temperature
Latitude/longitude GMT date depth depth temperature at hook depth
32 58.21oN/75 57.16oW 14/02/2006 41 m 2379 m 22.20 °C 22.72 °C
Carapace length Carapace width Weight (kg) Flipper tags Pit tag
660.0 mm 660.0 mm 25.0 kg RRM157 (right rear) 43305E6506
RRM158 (left rear)
2010 M.A. Grace, J. Watson, and D. Foster 195
For the purposes of defining descent/ascent events, a descent/ascent
event was designated when the descent from sea surface exceeded 10.0 m;
according to Boyle’s Law (Joiner 2001), air volume is compressed by 50% or
two atmospheres absolute (2 ata) at 10.1 m depth. Including the initial ascent
to surface after hooking, there were 8 descent/ascent events (Table 2). Descents
to maximum dive depths were direct and without stops at intermediate
depths, and final ascents to sea surface had few or no stops at intermediate
depths. Behavior (as interpreted by depth changes) between descents and ascents
were characterized by periods of uniform depth and by depth changes
spanning a range of several meters (Fig. 3). Except for the fourth descent/
ascent event that reached a maximum depth of 10.4 m, the descent rate
exceeded the ascent rate and was often more than doubled (descent/ascent
events 2, 3, 6 and 7; Table 2). Time between descent/ascent events (time at
surface) ranged from 8 min 9 sec (between descent/ascent number 1 and 2)
to 37 min 30 sec (between descent/ascent number 3 and 4). Descent/ascent
event 7 was the deepest descent event at 38.4 m and the longest duration
descent/ascent event at 11 min. 41 sec.
Loggerhead Sea Turtles achieve neutral buoyancy with lung air and can
remain at a constant depth without swimming (Minamikawa et. al. 1997,
2000), therefore, sequential TDR report groups from uniform depths may be
an indication of neutral buoyancy below 14 m depth (30 TDR report groups;
Fig. 3) since lung volume positively affects buoyancy within a 14-m range
of sea surface (Minamikawa et. al. 1997, 2000). During ascents from maximum
dive depths, 63% of sequential TDR report groups were from depths
>20 m (3 ata), and 87% were from depths >14 m. The deepest and the longest
duration ascent event (ascent event 7; Fig. 3) also had the most sequential
Figure 2. TDR depth and minutes elapsed for a Loggerhead Sea Turtle capture from
MSL Pelagic Longline Pilot Study (NOAA Ship OREGON II OT-06-02-269). Minutes
elapsed does not include the final 21 min haul to the vessel.
196 Southeastern Naturalist Vol. 9, No. 2
TDR report groups at a uniform depth (11 report groups ranged from depths
>23 m); free-swimming Loggerhead Sea Turtles reportedly show a positive
correlation between residence depth and dive duration (Minamikawa
et. al. 1997, 2000). Descent/ascent 7 also had the maximum time at a fixed
depth or at a uniform swimming depth at 32.5 m for 2 min 11 sec. Sea water
Figure 3. Ascent events for a Loggerhead Sea Turtle captured during the 2006 MSL
Pelagic Longline Pilot Study (NOAA Ship OREGON II OT-06-02-26).
Table 2. TDR descent/ascent events for a Loggerhead Sea Turtle capture from MSL Pelagic
Longline Pilot Study (NOAA Ship OREGON II OT-06-02-269). Events are descent/ascent
(D = descent >10 m below surface, A = ascent to surface), and time between descents (TBD =
minutes at surface between descent/ascent events). Event 1A is the initial ascent after hooking
at 41.0 m depth.
Elapsed time Max.depth ascent rate Max. depth TBD
Event (min.sec) (m) (m/sec) temp. (°C) (min.sec)
1A 4.55 41.0 0.139 23.08 8.90
2D 1.30 31.0 0.344 22.90
2A 6.48 0.075 18.20
3D 1.52 26.6 0.223 22.72
3A 3.59 0.105 37.30
4D 1.39 10.4 0.105 22.90
4A 1.10 0.149 12.00
5D 1.59 25.1 0.210 22.90
5A 3.16 0.128 20.00
6D 2.06 26.6 0.211 22.72
6A 9.05 0.049 32.00
7D 2.01 38.4 0.317 22.72
7A 9.40 0.055 20.00
8D 1.25 16.3 0.188 22.90
8A 2.21 0.113
2010 M.A. Grace, J. Watson, and D. Foster 197
temperature potentially affects the neutral buoyancy depth for Loggerhead
Sea Turtles since ambient temperature affects air volume (Charles’s Law;
Joiner 2001); however, since the sea surface to maximum dive depth water
temperatures only varied by 1.27 °C (22.08–23.35 °C), the effects of
water temperature over the relatively short duration descent/ascent events
was probably minimal.
Anecdotal observations by NOAA divers testing turtle excluder devices
(D. Foster NOAA/NMFS Mississippi Laboratories, Pascagoula,
MS, pers. observ.), suggest that Loggerhead Sea Turtles released well
below sea surface often complete a direct and relatively uniform final
ascent to sea surface with minimum physical effort. Minamikawa et. al.
(2000) reported a final ascent phase with a final ascent rate greater than
the gradual ascent that begins after the maximum dive depth. Final ascent
behavior was reflected in the TDR data for 7 of the 8 ascent events (descent/
ascent events 1, 2, 3, 5, 6, 7, and 8; Fig. 3); due to the 14-sec report
period and shallow maximum depth coupled with a relatively immediate
ascent from maximum dive depth, descent/ascent event 4 did not have
enough data points to be useful in a final ascent comparison. The inflection
point marking the start of the final ascent was designated when there
was a consistent depth decrease terminating at sea surface (none of the
sequential final ascent TDR report periods had a depth increase between
reports though a few had equal depths). The inflection point for final ascents
was between 25 m–16 m from sea surface (Fig. 3), and R2 values for
the trend line ranged from 0.97–0.99 (y ranged from -0.0994x + 39.259
to -0.1636x + 162.45), as opposed to a R2 range of 0.27–0.79 (y ranged
from 0.0212x + 13.433 to -0.0215x + 26.462) for the portion of ascents in
depths >25.0 m (initial ascent, excluding descent/ascent 1 that occurred
just after hooking; Table 3). During final ascents, the increase in R2 indicates
a more uniform and less variable ascent rate, and the increase in y
indicates a steeper ascent depth gradient.
Table 3. Ascent trend line R2 and slope (y) values for a Loggerhead Sea Turtle captured during
the 2006 MSL Pelagic Longline Pilot Study (NOAA Ship OREGON II OT-06-02-269); D/A =
D/A Initial ascent R2 Initial ascent y Final ascent R2 Final ascent y
1 0.9858 -0.1269x + 39.660 0.9892 -0.1806x + 48.755
2 0.7933 -0.0700x + 28.186 0.9725 -0.0994x + 39.259
3 0.5940 -0.0324x + 25.427 0.9808 -0.1555x + 44.295
5 0.7539 -0.1174x + 23.563 0.9870 -0.1510x + 31.331
6 0.7265 -0.0215x + 26.462 0.9910 -0.1363x + 70.845
7 0.5865 -0.0119x + 34.884 0.9828 -0.1636x + 162.45
8 0.2648 0.0212x + 13.443 0.9867 -0.1258x + 16.371
198 Southeastern Naturalist Vol. 9, No. 2
Considering a minimum of 25 min to a maximum of 40 min transpired
before the Loggerhead Sea Turtle took the baited hook at the fishing depth
(41 m), it is possible that the Loggerhead Sea Turtle did not follow the baited
hook from sea surface during set operations, but may have encountered the
pelagic longline gear while swimming or at neutral buoyancy depth. Since
the hook timer did not activate, it can be inferred that the Loggerhead Sea
Turtle did not struggle against the pelagic longline gear with enough force to
initialize the hook timer (the vessel slowed once the sea turtle was sighted at
surface). If the Loggerhead Sea Turtle was pulled toward the vessel during
the gear haul, the hydrodynamics of the plastron and carapace did not provide
enough resistance to activate the hook timer (wave height was estimated
at 1 m).
The effects of being hooked by longline on the behavior of the
Loggerhead Sea Turtle were difficult to determine since capture with a
TDR-equipped gangion was a rare event and TDR-derived data were confined
by the 14-sec report periods. If the longline-captured Loggerhead
Sea Turtle experienced a forced diving situation due to altered behavior
from being mouth hooked, there are a number of associated physiological
effects that can include rapid consumption of oxygen stores, anaerobic
glycolysis activation, and heart rate decline (Lutcavage and Lutz 1997).
However, for relatively unstressed free-swimming Loggerhead Sea
Turtles not hooked on longlines, physiological stress from submersion is
minimized (due to a number of unique physiological responses and adaptations;
Lutcavage and Lutz 1997), maximum submergence times range
from 60–240 min for pelagic stage juveniles (339–576 mm straight carapace
length; Dellinger and Freitas 2000), and mean submergence times
for Loggerhead Sea Turtles 400–600 mm straight carapace length range
from 17–76 min (Nelson 1996); the maximum submergence time for the
MSL Loggerhead Sea Turtle capture was 11 min. 41 sec (descent/ascent
event 7, Tab. 2). Dellinger and Freitas (2000) also reported a primary
peak in submergence times just below sea surface and a secondary peak
at 10 m–25 m depth; except for the relatively short descent/ascent events
4 and 8 (Tab. 2), most of the time on hook for the MSL Loggerhead Sea
Turtle capture was just below sea surface (Fig. 2), and the secondary submergence
time peak was in depths >15 m (Fig. 2). TDR data (Houghton
et. al. 2001) from free-swimming Loggerhead Sea Turtles during internesting
intervals (TDRs attached to the carapace then retrieved during
nesting) documented descent rates were generally greater than ascent
rates. Satellite-transmitted data from Loggerhead Sea Turtles also recorded
descent rates greater than ascent rates (Nelson 1996). Houghton et. al.
(2001) and Minamikawa et. al. (1997, 2000) described several types of
dive profiles that were similar to those observed from the MSL capture.
2010 M.A. Grace, J. Watson, and D. Foster 199
Thus, it appears that NOAA/NMFS sea turtle mortality mitigation recommendations
were effective for the Loggerhead Sea Turtle capture since
TDR data documented successive ascents/descents to and from surface,
and viability status was good after landing.
Chuck Schroeder, a contract employee with IAP/NMFS MSL, was the primary
software designer responsible for the SCS/FSCS (SELLIT) program for documenting
longline events. Survey planning and participation by MSL Shark Unit members William
Driggers and Lisa Jones was critical to the project’s success. Patrick Rice of the
University of Miami provided guidance for TDR operations and for interpreting and
displaying TDR-derived data. Sheryan Epperly of NMFS, Miami provided advice
for literature searches, and LaGena Fontroy (MSL) assisted with literature searches.
Survey logistics and catch-handling assistance of crew and command of the NOAA
Ship RV OREGON II was greatly appreciated.
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