Table 2 continued.

Direction of movement
in degrees
relative to

Turtle Time Mean rate North Net current Net wind
period of movement direction direction
(km/day )
7 6/20-6/23 8.0 195 172 334
7 6/23-6/24 16.0 163 256 295
7 6/24-6/26 12.5 171 NA 304
7 6/26-6/28 5.6 253 NA 049
8 6/6-6/9 5.6 325 305 123
8 6/9-6/11 14.7 304 066 114
& 6/11-6/13 25.9 342 001 269
8 6/13-6/16 14.8 307 028 265
8 6/16-6/19 27.1 131 030 265
9 6/6-6/9 6.5 030 010 272
9 6/9-6/11 5.6 150 272 320
10 6/6-6/9 4.8 194 174 352
10 6/9-6/13 3.2 091 165 319
10 6/13-6/17 6.3 214 321 142
10 6/17-6/19 7.2 109 008 219
10 6/19-6/20 15.0 239 106 010
10 6/20-6/23 3.9 030 357 169
10 6/23-6/24 11.1 282 015 051
10 6/14-6/26 1.1 245 NA 018
10 6/26-6/28 3.9 063 NA 219

16


Each turtle was initially placed into the releasing device and allowed
to acclimate to the lagoon water for a two minute period. During that time,
the observer would move back onto shore and situate himself behind the tall
reeds (Phragmites australis) that line the shore of the lagoon. This way the
observer would camouflage himself while observing the turtles. After the two
minutes, the observer pulled the string attached to the pin and released the
turtle. He then recorded the section of the scoring circle that the turtle
moved through and the time required by the turtle to reach the scoring
circle. Using this procedure, an estimate of the direction of movement and
an estimate of the rate of movement were recorded for each turtle used in
this study.

During the arena experiments, the turtles were exposed to wind generated
currents in the lagoon. In order to estimate the effects of these currents
on the movements of the turtles in the arena, a float was made that
approximated the general size and density of turtles used in this study. By
using the turtle releasing device to release the float, its direction of
movement and rate of movement were recorded in the arena immediately after
approximately every four turtles tested in the arena during the study.

Light intensity appears to be an important factor affecting the
sea-finding orientation mechanism(s) of L. kempi. To determine if L. kempi.
continue to use light intensity as an orientation cue when they are in the
water, estimates of the horizontal light field surrounding the arena were
taken during this study. Light readings were taken from the center of each
of the 32 sections of the arena using the same procedures described in
section three. An estimate of the horizontal light field was recorded
immediately before and after approximately every eight turtles tested in the

arena.

37


Figure 6.

Ship Channet

East Lagoon

Gult of Mexico

Location of East Lagoon.

33


of these four groups of headings. The predicted direction of movement that
was chosen for testing the groups of headings that were analyzed with respect
to the current was a direction 180 degrees from the current direction. This
direction was chosen since the L. kempi studied in chapter one apparently
exhibited a tendency to swim against the current. The V tests indicated that
the headings of the turtles' movements (relative to the current direction)
were significantly clustered in a direction opposite the direction of the
current in both arena locations (P < 0.01). The predicted direction of
movement that was chosen, when testing the two groups of headings that were
analyzed with respect to the direction of greatest light intensity, was the
direction of greatest light intensity. This predicted direction was chosen
since hatchling sea turtles are reputed to move in the brightest direction
after they emerge from the nest (see section three). The Y tests indicated
that the headings of the turtles' movements (relative to the brightest
direction) were significantly clustered around the brightest direction in the
west arena (P < 0.01), but they were not significantly clustered around the
brightest direction in the east arena (P > 0.05).

The preceding analysis examined the turtles' directions of movement
relative to three different parameters in an attempt to characterize the
orientation of these young L. kempi in the water. However, the recorded
movement of each turtle was a combination of the turtle's swimming and the
movement of the current. This prevents the Preceding analysis from examining
the actual swimming orientation of these turtles. To overcome this problem,
an estimate of each turtle's direction of swimming was trigonometrically
determined using the resultant movement of each turtle in the arena and the

coincidental movement of the current in the arena.

42


Furthermore, the clear and shallow water together with the abundance of
crustaceans should make this area an ideal habitat for L. kempi. The first
release occurred at 28 47' N lat., 82 52' W long. and the second at 28 40' N
Jat., 82 45' W long. One current drogue was released midway through each
release.

The airplane was then used at one to four day intervals, depending on
weather conditions, to record the locations of the transmitters during the 27
day study. A boat was used on seven occasions to locate the transmitters for
visual verification of their attachment to the turtles.

Average wind velocities during the study were obtained through the
National Weather Service, from an automatic recording station located in the

study area on Cedar Key.

10


INTRODUCTION

The ability of hatchling sea turtles to find the sea from the nest where
they hatch is of interest for two reasons. First, because it is a phenomenon
in its own right and second, because the orientation mechanism(s) responsible
for this behavior may aid in explaining the long distance migrations of sea
turtles (Carr and Ogren, 1960). This has been the rationale for the many
previous studies of sea turtle sea-finding behavior (for a review see
Mrosovsky, 1972). However, these studies have never investigated this
behavior in the Kemp's ridley sea turtle, L. kempi. Considering that
interspecific differences in the sea-finding behavior of sea turtles are
known to occur (Mrosovsky and Shettleworth, 1975), a study of L. kempi
sea-finding behavior would be of interest from a comparative viewpoint.
Additionally, this subject is of special interest because of the following
unique aspects of L. kempi sea-finding behavior: 1) Unlike the hatchlings of
other sea turtle species, which primarily move from the nest to the sea
nocturnally, L. kempi hatchlings primarily move from the nest to the sea
during early daylight hours. 2) Virtually all L. kempi nesting occurs on a
single nesting beach (near Rancho Nuevo in the state of Tamaulipas, Mexico).
Therefore, unlike hatchlings of other sea turtle species, virtually all L.
kempi hatchlings must move in the same general direction from the nest in
order to reach the sea.

Previous sea-finding studies using sea turtle species other than L.
kempi present valuable information concerning the sea-finding behavior of sea
turtles. Experiments conducted by Ehrenfeld and Carr (1967) indicated that
sea-finding orientation was primarily a visual process. Additionally, their

study indicated that hatchlings were obtaining orientation cues primarily

52


effects of the current should not be present. Therefore the swimming
directions obtained from the east arena data should provide accurate and
specific information on the swimming orientation of these turtles.

The. headings of the turtles swimming in the east arena appeared to be
randomly distributed when referenced to north and the brightest direction
(Table 6). However, when the headings were referenced to the current
direction, they were nonrandomly distributed (Table 6). Furthermore, V tests
showed that the direction opposite the current direction was the only one of
the three predicted directions that had had a significant number of headings
clustered around it. These results indicate that the turtles in the east
arena were swimming nonrandomly against the current. Therefore these results
agree with the results of the first chapter in suggesting that young L. kempi.
show a tendency to swim against the current. It is beyond the scope of this
study to determine the advantages of such a behavior, however, if the turtles
were reacting to the inertial effects of the current. This behavior offers a
logical explanation of how hatchling sea turtles could move in an oriented
fashion away from the nesting beach. The waves moving toward shore could act
as a stimulus that the hatchlings swim against. In that way, the hatchlings
would always swim in a direction away from shore.

In conclusion, the results of this study have suggested that young L.
kempi exhibit a tendency to swim against the current. Additionally, this
study has shown that an orientation arena positioned in natural environment

is a viable tool for the study of sea turtle movements and swimming.

50


period the wire enclosure was removed by an observer who quickly moved out of
the arena and behind the dunes. The hatchlings were then allowed to move
freely within the arena for a maximum of 10 minutes or until all hatchlings
had fallen into the trench. Following that time period, the number of
hatchlings in each section of the trench was recorded. If a hatchling had
not reached the trench at the end of 10 minutes, it was excluded from the
results.

During this study, two series of experiments were conducted. In the
first series the experimental procedure described above was conducted
repeatedly during the morning and afternoon to compare hatchling behavior
when the sun was in different positions. On 10 July 1980 and 14 July 1980
these experiments were conducted between 9:44 A.M. and 10:24 A.M. and between
1:50 P.M. and 2:45 P.M. (CTS, daylight savings time).

During the second series of experiments, an estimate of the light field
surrounding the arena was recorded immediately following the arena
experiments in order to determine if the hatchlings were moving in the
brightest direction. On 24 July 1980 this procedure was carried out both in
the morning (between 9:43 A.M. and 10:30 A.M. CST, daylight savings time) and
in the afternoon (between 2:10 P.M. and 3:07 P.M. CST, daylight savings
time). Estimates of the light intensity from each section of the arena were
taken from the center of the arena using a Lightmate/Spotmate photometer
system (Photo Research, Burbank, California) which was sequentially directed
toward the center of each of the 16 sections of the arena. The photometer
measured the light intensity from a circular area one degree in diameter.
Although no study has investigated the spectral sensitivity of L. kempi,
research conducted on C. mydas suggests that the use of a photometer was

justified. A two degree angle of elevation from the beach surface was used

56


Figure 13. As in the first series of experiments, there was a significant
difference between the directions of the hatchlings movements in the morning
and those in the afternoon (Chi-square tests, P < 0.05).. Figure 13 shows
that the brightest region of the light field during the morning experiments
was considerably more intense than the brightest region of the light field
during the afternoon experiments. If this were true during the first series
of experiments, it could be responsible for the more consistent movements of
the hatchlings during the morning experiments. Figure 13 also shows that
brightest region of the light field surrounding the arena shifts during the
day. However, the brightest regions of the surrounding light field did not
coincide with the sections of the trench into which the majority of
hatchlings fell. In fact, no hatchlings fell into the trench sections that
coincided with the brightest regions of the surrounding light field. But, if
one were to divide the arena in half using a perpendicular line to the sea,
the results of the second series of arena experiments show that significantly
more hatchlings moved into the trench sections of the brightest half of the
arena (Chi squared, P > 0.05). These results suggest that a phototropotactic
reaction may be influencing the hatchlings, but their sea-finding behavior
cannot be explained solely in terms of this reaction.

In both series of arena experiments the majority of hatchlings always
fell into at least one of the two trench sections bordering the perpendicular
line to the sea, yet the light measurements taken during the second series of
arena experiments indicate that these sections were not the brightest regions
of the light field surrounding the arena. This suggests that the movements
of these hatchlings were also influenced by an orientation mechanism (or

mechanisms) that did not rely on brightness as a cue, but in some way

60


DISCUSSION

The headings of the turtles’ movements in the west arena were
nonrandomly distributed relative to all three reference directions (Table 5).
Additionally, these headings were significantly clustered around all three
predicted directions of movements used in the V tests. This indicates that
these turtles were moving nonrandomly and that the turtles could have been
moving in any one or all three of the predicted directions. These findings
could have resulted from the similarity of the predicted direction of
movement. The current direction and the brightest direction were relatively
consistent during all of the experiments in the west arena. Furthermore, the
brightest direction was approximately opposite the current-direction (x = 185
degrees, s = + 41 degrees) and was relatively close to the direction of the
most direct path away from the shore (x = 58 degrees, s = + 31 degrees).

Thus all three predicted directions of movement were relatively close to one
another in the west arena. Therefore, the significant clustering of headings
around all three predicted directions of movement could have resulted from a
significant number of turtles moving in only one of the predicted directions.

The current directions in the east arena experiments were variable
because of seasonal changes in wind direction. This variability in the
current direction decreased the similarity of the predicted directions of
movement in the east arena. Nevertheless the headings of the turtles
movements in the east arena were also distributed nonrandomly relative to all
three reference directions (Table 5). The V tests, however, indicated that
the headings of the turtles movements were significantly clustered around

only two of the three predicted directions of movement (Table 5). Those two

48


Table 3 continued.

Direction of swimming
in degrees
relative to

Turtle Time Mean rate North Net current Net wind
period of swimming direction direction
(km/day)
10 6/9-6/13 5.0 096 170 324
10 6/13-6/17 4.5 191 298 119
10 6/17-6/19 2.9 267 166 017
10 6/19-6/20 28.6 283 150 054
10 6/20-6/23 0.9 211 178 350
10 6/23-6/24 6.9 056 149 185

20


As stated previously, the orientation arena was positioned at two
different locations during the study. At each of these locations, the
experimental procedures described above were conducted from 9:30 A.M. to 4:30
P.M. CST. The times and dates of individual experimental sessions varied
because of weather and tidal conditions. While the arena was located on the
west side of the lagoon, the movements of 92 turtles were recorded using the
procedures described above. This initial portion of the study was conducted
between 11 September 1980 and 28 October 1980. The orientation arena was
then disassembled, moved to the east side of the lagoon and reassembled. The
movements of 72 L. kempi were then recorded in the orientation arena
according to the procedures described above. This second portion of the

study was conducted between 13 December 1980 and 29 January 1981.

38


INTRODUCTION

The Kemp's ridley sea turtle, Lepidochelys kempi (Garman), is Presently
the most endangered of all sea turtles. Although almost all sea turtle
species have been heavily exploited by man in the Past 40 years, the L. kempi
Population is the only one that is close to extinction. The primary reason
for this is that, unlike any other sea turtle, L. kempi has only one major
nesting beach, where virtually the entire Population nests (Carr, 1963;
Hildebrand, 1963). Therefore, when the nesting females and eggs on that
beach were heavily exploited by man, almost the entire Population of L. kempi
was extirpated. As many as 40,000 nesting females were recorded on that
beach during a single day in 1947 (Carr, 1963; Hildebrand, 1963), but today
less than 600 females nest there during an entire nesting season.

As a result of the drastic decline in the number of nesting females, L.
kempi is presently the subject of intense conservation. Unfortunately, since
little is known about the life history of this turtle, the conservational
effort is primarily limited to the Protection of nesting females and eggs on
the beach. However, there is now a conservation project that is attempting
to imprint L. kempi hatchlings to the Padre Island National Seashore and is
then rearing them for 11 months in order to circumvent their high mortality
in the wild (Klima and McVey, 1982). But the lack of knowledge concerning
the life history. of L. kempi, makes most aspects of the project experimental
and speculative.

One important aspect to the captive-rearing project is the determination
of an optimal location to release immature L. kempi that have been reared in
captivity. No standard migration patterns have ever been determined for

young L. kempi. Immature individuals have been found throughout the coastal


that they were moving and swimming randomly relative to geographical
direction and wind direction, but were moving and swimming nonrandomly
relative to current direction. Although this radio-tracking technique proved
to be an effective means of monitoring sea turtle movements in open ocean,
the logistics and cost of the technique permitted the monitoring of only ten
turtles.

General characteristics of the movements of young sea turtles in open
ocean have been suggested from the above studies; however, the small numbers
of turtles tracked in each study limit the certainty of those suggested
characteristics. To obtain an accurate knowledge of their movements, larger
numbers of sea turtles need to be studied. Therefore, in order to study the
movements of young L. kempi in the water, I constructed an orientation arena
in a lagoon on Galveston Island, Texas. The arena provided the means of
recording and analyzing the movements of relatively large numbers of young L.
kempi that were being captive-reared by the Galveston Laboratory. This

section reports and discusses the results of that study.

31


Effect of current

| eet ~
\.. o
. :
On 3
£) —€*
%N
% N o
ty \ 2
yoy =
é. -
2% \ 2

Current’s movement

Figure 4, Example of how a swimming vector can be graphically generated from
the movement of a turtle and the movement of the current.

17


These tests revealed that of the 48 groups of headings tested (eight turtles,
six groups of headings per turtle), the headings of two groups were
distributed nonrandomly (P < 0.05). Those two groups were turtle number
eight's directions of movement relative to current direction and turtle
number ten's directions of swimming relative to current direction. However,
when analyzing 48 groups to a significance level of 0.05, one would expect
2.4 groups to be significantly nonrandom because of chance. Therefore, when
analyzing the movements and swimming of individual turtles, we must assume
that they are moving and swimming randomly.

A problem encountered when analyzing their paths was the relatively
small number of segments per path (13 or less). The small sample sizes
decreased my ability to detect nonrandom movements and/or nonrandom swimming
by the turtles. In an attempt to overcome this problem, the directions of
movement and the directions of swimming of al] the turtles were pooled to
form two groups. The two groups represented the directions of movement and
swimming of the entire group. Testing both groups with the Rayleigh test
relative to the three reference directions mentioned above (total of six
groups tested) revealed that the headings of two of the six groups tested
were distributed nonrandomly (P < 0.05). These two groups were the headings
of the turtles' movements relative to the current direction and the headings
of the turtles’ swimming relative to the current direction. Both of these
groups of headings are illustrated in Figure 5.

A modified form of the Rayleigh test known as the V test (Batschlet,
1972) was also performed on these two significantly nonrandom groups of
headings. This test leads to significance only if the headings are
sufficiently clustered around a predicted direction. A movement with the

current was chosen as the predicted direction when testing the net directions

23


Table 3. Estimates of the directions and rates of the turtles’ net swimming.

Direction of swimming

in degrees
relative to
Turtle Time Mean rate North Net current ~ Net wind
period of swimming direction direction
(km/day)
1 6/6-6/9 5.7 292 272 090
1 6/9-6/11 4.6 174 296 344
1 6/11-6/13 11.6 133 152 060
1 6/13-6/17 7.3 183 290 111
1 6/17-6/19 22.1 187 086 287
2 6/6-6/9 2.6 275 255 073
2 6/9-6/11 4.2 129 251 299
2 6/11-6/13 12.7 208 227 135
2 6/13-6/16 13.3 067 148 005
2 6/16-6/17 24.5 218 064 352
2 6/17-6/19 8.6 259 158 009
2 6/19-6/20 8.1 285 152 056
2 6/20-6/23 3.2 065 032 204
2 6/23-6/24 4.7 041 134 170
4 6/6-6/9 2.3 240 220 038
4 6/9-6/11 1.1 160 282 330
4 6/11-6/13 4.4 121 140 048
4 6/13-6/16 4.6 087 168 025
4 6/16-6/17 12.5 025 231 159
4 6/17-6/19 20.0 240 139 350
5 6/6-6/9 3.2 105 085 263
5 6/9-6/13 4.1 177 251 045
5 6/13-6/17 4.7 244 351 172
5 6/17-6/19 6.8 110 009 220
5 6/19-6/20 22.2 279 146 050
5 6/20-6/23 4.2 194 161 333
5 6/23-6/24 13.1 104 197 233
7 6/6-6/9 5.8 255 235 053
7 6/9-6/13 4.9 089 163 317
7 6/13-6/17 3.9 324 071 252
7 6/17-6/19 23.0 318 217 068
7 6/19-6/20 9.5 071 298 202
7 6/20-6/23 12.6 201 168 340
7 6/23-6/24 25.6 122 215 251
8 6/6-6/9 4.6 279 259 077
8 6/9-6/11 14,3 316 078 126
8 6/11-6/13 23.0 343 002 270
8 6/13-6/16 11.1 318 039 256
8 6/16-6/19 19.5 138 037 272
9 6/6-6/9 2.6 198 178 356
9 6/9-6/11 10.0 132 254 302
10 6/6-6/9 9.6 197 177 355

19


METHODS AND MATERIALS

An orientation arena was constructed in the East Lagoon on the
northeastern portion of Galveston Island, (Figure 6) to record the movements
of young L. kempi in a natural environment. This lagoon is approximately 2
km long and varies in width from 150 m to 250 m. To determine if the
direction of L. kempi movement changes as the relative position of certain
environmental factors (such as the position of the shore and the position of
the area of greatest light intensity) change, the arena was used at two
different locations during the study. Initially it was positioned near the
western shore of the widest region of the lagoon. Then after the first half
of the study was completed, it was moved across the lagoon and positioned
near the eastern shore.

The orientation arena is depicted in Figure 7. Since the arena was
always located near shore, the water's depth in the arena ranged from 0.5 m
to 1.0 m. To prevent. any of the turtles from escaping into the lagoon, a net
wall was positioned around the perimeter of the the arena at a distance of 7
m from the arena's center. The net was suspended from a PVC pipe, 1.9 cm in
diameter, that encircled the arena. When experiments were being conducted,
this pipe was suspended approximately 5 cm to 10 cm above the water's surface
by eight poles, made from PVC pipe 5 cm in diameter, that were securely
inserted into the sandy bottom of the lagoon. These eight poles were
symmetrically positioned around the perimeter of the arena and projected
approximately 0.3 m out of the water. At a radius of 5 m from the arena's
center, 32 poles, made from PVC pipe 1.9 cm in diameter, were symmetrically

positioned around the arena. These poles formed a 32 section scoring circle

32


directions and magnitudes of each turtle's net movements are listed in Table
2.

The movements of these turtles resulted primarily from the effects of
two factors: the current and swimming. The effects of these factors on the
movements of the turtles can be estimated from the paths shown in Figure 3.
An estimate of the current's movements can be obtained from the current
monitor's path. However, the current monitor was not continually close to
all of the turtles. Nevertheless, its movements offer a reasonable estimate
of the current's movements in the study area, since the currents in this
region of the Gulf of Mexico are primarily the results of tides, which are
similar throughout the study area (Mofjeld, 1974). Thus, each segment of the
current monitor's path provides an estimate of the current's movement that
affected the corresponding segment of each turtle's path.

The movements of a passively floating turtle should approximate the
movements of the current monitor. Therefore, if a difference exists between
the movement of the current monitor and the movement of a turtle, this
difference represents an estimate of the effect of the turtle's swimming.
This effect can be quantified for each segment of a turtle's path by
subtracting from each segment the corresponding segment of the current
monitor's path (Figure 4). This procedure generates vectors which represent
the effect of swimming. The direction of each vector estimates the net
direction of swimming and the length of each vector estimates the magnitude
of the swimming effect. This magnitude represents the distance of movement a
turtle's swimming would produce in the absence of current. Daily swimming
rates were estimated from the swimming vector magnitudes. These values
ranged from 0.9 to 28.6 km per day with mean values for individual turtles

ranging from 5.2 to 13.9 km per day. Comparisons of each turtle's

14


RESULTS AND DISCUSSION

The results of the first series of arena experiments are shown in Figure
12. The movements of the hatchlings during the morning experiments were very
consistent and no significant differences were detected between the results
of the individual experiments (Chi-square tests, P > 0.05). The movements of
the hatchlings in the afternoon experiments were not as consistent as those
of the hatchlings in the morning experiments. Unlike the morning
experiments, hatchlings were recorded in as many as four different trench
segments following the afternoon experiments. Furthermore, the movements of
the hatchlings during experiment number seven were significantly different
than those of the hatchlings during experiment number twelve (Chi-square
test, P < 0.05). However, no other significant differences were detected
when the results of the afternoon experiments were compared to one another
(Chi-square tests, P > 0.05). Comparisons of the results of the morning
experiments to those in the afternoon revealed that the hatchling's movements
in the morning were significantly different than their directions of
movements in the afternoon (Chi-square tests, P < 0.05). These results are
consistent with the current theory that sea turtles use primarily a
phototropotactic reaction to find the sea. That is, if the hatchlings were
reacting tropotactically to light, one would expect their direction of
movement to shift as the sun (and thus the light field surrounding the arena)
shifts. Therefore the results of the first series of experiments suggest
that the L. kempi hatchlings were reacting tropotactically to the light field
surrounding them. Mrosovsky (1970) recorded similar results with C. mydas.

The results of the second series of arena experiments together with the

diagrams representing the light field surrounding the arenas are shown in

58


METHODS AND MATERIALS

Twelve transmitters were purchased from Wildlife Materials Inc.
(Carbondale, Illinois). The 12 g units could transmit an intermittant pulse
at least 24 km by air and 3 km over the water's surface on the 165 MHz to 166
MHz band. Each transmitter possessed a unique frequency to allow for
individual identification.

Ten of the transmitters were sealed in housings designed to trail behind
sea turtles. The housings, which resembled keeled sailboat hulls (Figure 1),
consisted of plastic resin keels connected to balsa floats coated with a
protective layer of epoxy paint. The transmitter was imbedded in the keel
with its antenna projecting up and through the balsa float. External magnets
operated the transmitter's magnetic on/off switches through the keels. Each
housing was attached to a turtle's carapace with a 10 cm segment of
monofilament line. The lines were securely anchored to the bow of the
housing and through a hole drilled in a postcentral scute of the turtle.

The housings were effective for several reasons. They offered little
hydrodynamic drag and had less than 0.5 cm of freeboard to minimize affects
of the wind. To facilitate diving, the positive buoyancy of each housing was
slight. The housings remained vertical in the water, even when the turtles
were actively swimming. This kept the antennae upright, and was necessary
for signal transmission at the water's surface.

The remaining two transmitters were sealed in housings designed to float
freely as current monitors. The housings were constructed similarly to the
towable housings, but were cylindrically shaped to minimize the effect of the

wind.


SECTION II
THE ORIENTATION OF IMMATURE
LEPIDOCHELYS KEMPI.
IN A LAGOON ORIENTATION ARENA

29


Mrosovsky, N. 1972. The water-finding ability of sea turtles. Behavioral
studies and physiological speculations. Brain Behav. Evol. 5:
202-225.

Mrosovsky, N. and Carr, A. 1967. Preference for light of short wavelengths
in hatchling sea turtles, Chelonia mydas, tested on their natural
nesting beaches. Behaviour 28: 217-231.

Mrosovsky, N., Granda, A.M. and Hay, T. 1979. Seaward orientation of
hatchling turtles: Turning systems in the optic tectum. Brain Behav.
Evol. 16: 203-221.

Mrosovsky, N. and Shettleworth, S.J. 1968. Wavelength preferences and
brightness cues in the water-finding behaviour of sea turtles.
Behaviour 32: 211-257.

Mrosovsky, N. and Shettleworth, S.J. 1974. Further studies of the
sea-finding mechanism in green turtle hatchlings. Behaviour Sl:
195-208.

Parker, G.H. 1922. The crawling of young loggerhead turtles toward the sea.
J. Exp. Zool. 6: 323-331.

Pritchard, P.C.H. and Marquez, R. 1973. Kemp's ridley turtle or Atlantic
ridley, Lepidochelys kempi. IUCN Monograph 2. Marine Turtle Series.
Morges, Switzerland.

Van Rhijn, F.A. 1979. Optic orientation in hatchlings of the sea turtle,
Chelonia mydas. I. Brightness: Not the only optic cue in sea-finding
orientation. Mar. Behav. Physiol. 6: 105-121.

Verheijen, F.J. and Wildshcut, J.T. 1973. The photic orientation of
hatchling sea turtles during water-finding behaviour. Neth. J. Sea Res.

7: 53-67.

65


ACKNOWLEDGEMENTS

Alfred R. Loeblich, III, Robert D. LeBoeuf, Steven A. McConmas, and
Nathan Howe provided me with assistance and inspiration throughout my
research. James P. McVey provided me the opportunity to study the Kemp's
ridley and enthusiastically supported my efforts. Ray Wheeler provided
extensive help during the radio tracking study and was a source of
encouragement through all this research. The U.S. Fish and Wildlife Service
at the Chassahowitzki Wildlife Refuge also provided extensive help in the
radio tracking study. Edwin Bryant, Luther Franklin, and Daniel Sheer
Provided me with their expertise as members of my graduate committee.

Additionally, I would like to acknowledge the conservational efforts of
the Instituto Nacional de la Pesca, the U.S. Fish and Wildlife Service, the
National Marine Fisheries Service and the National Park Service in helping

the Kemp's ridley survive.

67


DISCUSSION

The results of this study indicate that these yearling L. kempi were
moving randomly relative to geographical direction and wind direction.
However, the turtles did not appear to move randomly relative to the current
direction. Their net directions of movement relative to current direction
(Figure 5) suggest that they were significantly displaced by the current.

Ocean currents have long been suspected of being the primary factor
affecting the movements of young sea turtles (Carr, 1955, 1967; Witham, 1976,
1980). There is a paucity of data pertaining to this subject, and little
data exists on the life history of young sea turtles in general. This stems
from the difficulty in obtaining data on young sea turtles in open ocean and
has lead to the term "lost year" being applied to the posthatchling stage of
sea turtles. It is believed that during this stage sea turtles are too small
to overcome the effects of ocean currents and must therefore assume a
planktonic existence (Carr, 1980). However, most aspects of the lost year
are speculative, including the time period associated with this stage and the
size that a sea turtle must attain in order to overcome the effects of ocean
currents.

One might suspect that the current was the primary factor affecting the
movements of the turtles in this study, since the current apparently
displaced them. However, this does not appear to be the case. Comparisons
of current magnitude estimates with the turtles' swimming vector magnitude
estimates revealed no significant differences. We must therefore assume that
the current and the turtles' swimming contributed equally to the movements of
the turtles. This indicates that the L. kempi in this study appeared to be

at a stage in their life history corresponding to the end of the lost year.

26


SECTION I
THE ORIENTATION OF IMMATURE
LEPIDOCHELYS KEMPI
‘FOLLOWING THEIR RELEASE INTO THE GULF OF MEXICO


areas of the Gulf of Mexico and along the Atlantic coasts of North America
and Europe (Pritchard and Marquez, 1973), but the actual paths traveled by
these turtles and the causal basis for their movements from the nesting beach
are unknown. The release area could be vital to the success of this
conservation project, and the movements of the turtles following their
release could be important to their survival and future breeding. For
example, immature L. kempi that move into the Atlantic could be permanently
separated from the breeding population by the Gulf Stream (Carr, 1980;
Hendrickson, 1980).

I have conducted three separate studies of the movements of immature L.
kempi, reported in the three sections of this report. The first study
involved the recording and analysis of the movements of ten yearling L. kempi
in the Gulf of Mexico. In the second study, the movements of immature L.
kempi were recorded and analyzed through the use of an orientation arena that
I constructed in a lagoon on Galveston Island, Texas. The third study
consisted of the recording and analysis of the sea-finding behavior of
hatchling L. kempi on the beach. These studies were conducted under Mexican
permit ABC-IV-0751 Number 276/1-8786 and U.S. Fish and Wildlife permit PRT
2-4481.


Table 2. Directions and rates of the turtles’ net movements.

Direction of movement

in degrees
relative to
Turtle Time Mean rate North Net current Net wind
period of movement direction direction
(km/day )
1 6/4-6/6 13.1 191 NA 269
1 6/6-6/9 7.2 327 307 125
1 6/9-6/11 6.4 200 322 010
1 6/11-6/13 9.0 125 144 073
1 6/13-6/17 8.6 201 308 129
1 6/17-6/19 25.3 163 062 273
2 6/4-6/6 9.5 199 NA 290
2 6/6-6/9 4.3 341 321 139
2 6/9-6/11 4.4 172 294 342
2 6/11-6/13 10.8 220 239 147
2 6/13-6/16 10.0 053 134 351
2 6/16-6/17 27.2 207 053 341
2 6/17-6/19 1.7 162 061 272
2 6/19-6/20 13.9 153 120 284
2 6/20-6/23 7.8 046 013 185 .
2 6/23-6/24 13.9 278 011 147
2 6/24-6/26 15.0 002 NA 135
2 6/26-6/28 11.4 020 NA 176
2 6/28-6/30 7.2 191 NA 325
4 6/4-6/6 10.0 184 NA 275
4 6/6-6/9 2.6 344 324 142
4 6/9-6/11 3.1 218 340 028
4 6/11-6/13 3.1 080 099 007
4 6/13-6/16 1.1 015 096 313
4 6/16-6/17 4.7 054 260 188
4 6/17-6/19 14.2 212 111 322
5 6/4-6/6 8.1 196 NA 287
5 6/6-6/9 5.2 055 035 213
5 6/9-6/11 3.9 202 276 070
5 6/13-6/17 7.4 246 353 174
5 6/17-6/19 17.0 106 005 216
5 6/19-6/20 12.8 213 080 344
5 6/20-6/23 1.7 090 057 229
5 6/23-6/24 6.1 224 317 353
5 6/24-6/26 5.8 320 NA 267
5 6/26-6/28 6.4 028 NA 184
7 6/6-6/9 4.8 298 278 096
7 6/9-6/13 3.2 080 154 308
7 6/13-6/17 5.4 294 042 222
7 6/17-6/19 6.4 339 238 089
7 6/19-6/20 26.1 116 343 247

15


possibility of a home range. Tagging studies conducted by the SEFC in 1979
resulted in the recapture of 54 turtles. They were captive-reared L. kempi
that were released in various areas of the Gulf of Mexico. Turtles released
in the Bay of Florida were recaptured only along the Atlantic coast of the
U.S., suggesting that the Gulf Stream played a major role in their movements.
Turtles released in calmer waters near Homosassa, Florida exhibited a variety
of movements, suggesting that 0.5 to 1.0 kg L. kempi may be capable of
Overcoming the effects of the current in relatively calm waters. Although
the previous tagging studies have recorded the movements of immature L.
kempi, they have not addressed the causal factors.

In the present study I address the two Primary factors that affect the
movements of sea turtles: the current and the turtle's swimming. By using a
radio tracking technique, the movements of ten immature L. kempi and the
movements of two current monitors were recorded during a 27 day study. These
data provided the means of quantifying the effects of the current and the
effects of the turtle's swimming on the turtle's movements. The magnitudes
of the two factors were compared to determine if either had a significantly
greater effect on the turtle's movement. Additionally, the directions of
movement and directions of swimming were analyzed relative to geographical,

current and wind directions to determine if nonrandom patterns were present.


The most obvious reference direction would be the geographical direction
north. However, if a turtle was constantly moving relative to a stimulus
that was independent of a geographical direction, the turtle might appear to
be moving randomly when analyzed relative to north. For example, if a turtle
was continually floating with the current, it would be moving nonrandomly
relative to the current. But, if the current direction shifted periodically
and independently of geographical direction, the turtle May appear to be
moving randomly relative to north. It is therefore advantageous to conduct
multiple testings of a turtle's directions of movement and direction of
swimming using numerous reference directions that are independent of one
another. For this reason, several reference directions were used to analyze
their movements and swimming. North was used to determine if they were
moving and/or swimming randomly relative to geographical direction.
Additionally, current direction and wind direction were used as reference
directions to determine if the turtles were moving and/or swimming randomly
relative to these two stimuli. Estimates of the net current direction
corresponding to each segment of their paths were obtained from the movements
of the current monitor, and estimates of net wind direction corresponding to
the segments of each turtle's path were calculated from.data recorded by the
National Weather Service's recording station on Cedar Key. Estimates of the
net current direction and net wind direction for the time periods
corresponding to the segments of the turtles’ paths are listed in Table 4.
Estimates of the net directions of their movements and swimming are listed in
Tables 2 and 3.

Rayleigh tests were performed on the directions of movement and swimming
of each turtle using north, net current direction and net wind direction as

the reference directions (i.e. six groups of headings tested per turtle).

21


NOAA Technical Memorandum NMFS-SEFC-131

Technical Memorandums are used for documentation and
timely communication of preliminary results, interim
reports, or special-purpose information, and have not
received complete formal review, editorial control, or
detailed editing

ORIENTATION CHARACTERISTICS OF IMMATURE
KEMP'S RIDLEY SEA TURTLES
LEPIDOCHELYS KEMPI

Thane R. Wibbels

January 1984

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldrige, Secretary

National Oceanic and Atmospheric Administration

John V. Byrne, Administrator

National Marine Fisheries Service

William G. Gordon, Assistant Administrator for Fisheries


West Arena East Arena

BRIGHTEST BRIGHTEST

A

Figure 10. Headings of the turtles' swimming in the lagoon orientation
arena. Each dot represents a heading swam by a turtle.

44


Equipment for locating and determining the geographical location of each
transmitter was installed in a Piper Seneca II airplane. Transmitter
locating equipment consisted of a Falcon Five variable frequency receiver
(purchased from Wildlife Materials Inc.) and three antennae. The receiver
could be adjusted with an accuracy of 1 KHz to frequencies from 165 MHz to
166 MHz, enabling us to identify each transmitter's pulse. A three-element,
directional antenna was mounted on the underside of each wing, and an
omnidirectional whip antenna was mounted under the center of the fuselage.
The omnidirectional antenna was used for initial detection of a transmitter's
signal, then the amplitudes of the signals received through both wing
antennae were compared to deduce the heading of the transmitter. By flying
in-that direction and continually adjusting our heading relative to signal
amplitudes through the wind antennae, Transmitter locations could be
determined with a maximum estimated error of 1 km. The geographical location
was then recorded using an Internav 101 Loran-C navigation unit, which was
precise to 61 m.

A second Falcon Five receiver, coupled to a directional, handheld
antenna was used from a boat on seven occasions during the study. The
heading of any transmitter in range was deduced by manually changing the
orientation of the antenna to a position of maximal signal strength.

The ten transmitters in towable housings were attached to 11 month old
L. kempi, which had been captive-reared by the SEFC. The weights and lengths
of these turtles are listed in Table 1. Five of these turtles were released
intermittantly among approximately 665 other L. kempi on each of the mornings
of 3 June 1980 and 5 June 1980. The area is historically known for an

abundance of juvenile L. kempi (Carr, 1955; Carr and Caldwell, 1956).


Figure 1. Transmitter and transmitter-housing unit.


when directing the photometer since the study conducted by Ehrenfeld and Carr
(1967) indicated that hatchling C. mydas obtain orientation cues primarily

from an area within four degrees above the beach surface.

57


West Arena East Arena

BRIGHTEST BRIGHTEST

4 A

Figure 9. Headings of the turtles' directions of movement in the lagoon
orientation arena. Each dot represents the heading of a turtle's movement.

40


The results of this study not only provide information about the
swimming and movements of yearling L. kempi, but also can be used to evaluate
certain facets of the captive-rearing program, specifically the release area
and the length of captive-rearing. In one respect, the results suggest that
the study area was an acceptable place to release the turtles, since the
current was not strong enough to displace these turtles from this propitious
environment. However, since the turtles appear to move randomly relative to
geographical directions, the location of this release area (versus areas in
the western Gulf of Mexico) may cause greater percentages of L. kempi to move
into the Atlantic. This type of movement increases the turtles distance from
the nesting beach and therefore may increase the difficulty of future nesting
migrations. In reference to the length of captive-rearing, the results
indicate that an eleven month period provides sufficient growth to allow the
L. kempi to exhibit swimming abilities that equal the effects of the current
in areas with currents similar to the study area.

In conclusion, the radio-tracking technique used in this study has
provided valuable information on the movements and swimming of yearling L.
kempi. Additionally, this study has provided information that is of
importance in the design and evaluation of a comprehensive captive-rearing

program for sea turtle conservation.

28


analyzed with respect to the current direction were significantly clustered
in both arena locations around.a heading 180 degrees from the current
direction (P < 0.01), but the headings analyzed with respect to the brightest
direction were significantly clustered around the brightest direction only in
the west arena (P < 0.01).

The results of the statistical analyses described in this section are

summarized in Tables 5 and 6.

45


Furthermore, it suggests that the time period that is implicitly indicated in
the term "lost year" is correct for these yearling L. kempi, at least in this
area of the Gulf of mexico.

A unique facet of this study was the ability to estimate the turtles’
swimming vectors from the data collected. The preceding paragraph already
demonstrated the usefulness of these swimming vectors in analyzing sea turtle
movements. Additionally, swimming vectors provide a means of gaining insight
as to the swimming abilities and the swimming orientation of sea turtles in
the wild.

Estimates of each turtle's net rates of swimming were calculated from
their swimming vectors, and the mean values for individual turtles ranged
from 5.2 to 13.9 km per day. The rates indicate that the turtles, which were
reared for eleven months in relatively small containers (Klima and McVey,
1980), were active swimmers following their release into the Gulf of Mexico.
No significant differences could be detected when comparing each turtle's net
swimming rates to those of the other turtles. However, the greatest net
swimming rate (28.6 km per day) and the greatest mean net swimming rate (13.9
km per day) were estimated from the swimming vectors of the largest turtle
(turtle number ten).

Analysis of the turtles' directions of swimming indicates that these L.
kempi were swimming randomly relative to geographical direction and wind
direction. But, they appeared to swim nonrandomly relative to the current
direction. The turtles' net directions of swimming relative to the current
direction (Figure 5) suggest that they were swimming against the current.
However, it is beyond the scope of this study to determine the orientation

mechanism(s) responsible for the observed behavior.

27


Ehrenfeld, D.W. and Carr, A. 1967. The role of vision in the sea-finding
orientation of the green sea turtle (Chelonia mydas). Anim. Behav. 15:
25-36.

Frick, J.A. 1976. Orientation and behavior of hatchling green turtles
(Chelonia mydas) in the sea. Anim. Behav. 24: 849-857.

Hendrickson, J.R. 1980. The ecological strategies of sea turtles. Amer.
Zool. 20: 597-608.

Hildebrand, H.H. 1963. Hallazgo del area de anidacion de la tortuga marina
"lora", Lepidochelys kempi (Garman), en 1a costa occidental del Golfo de
Mexico (Rept., Chel.) Ciencia, Mexico 22: 105-112.

Ireland, L.C., Frick, J.A. and Windgate, D.B. 1978. Nighttime orientation
of hatchling green turtles (Chelonia mydas) in open ocean In: Animal
Migration, Navigation, and Homing. Schmidt-Koenig, K. and Keeton, W.T.
(eds), pp. 420-429. Springer Verlag, New York.

Klima, E.F. and McVey, J.P. 1982. Headstarting the Kemp's ridley sea
turtle, Lepidochelys kempi. In: Biology and Conservation of Sea
Turtles. Karen Bjorndal (ed), Smithsonian Institution, Washington, D.C.

Limpus, C.J. 1971. Sea turtle ocean finding behaviour. Search 2: 385-387.

Mofjeld, H.0. 1974. Tidal currents of the west Florida shelf. Proceedings
of the Conference/Workshop on Marine Envieonrmental Implications of
Offshore Drilling in hte eastern Gulf of Mexico, Jan. 31-Feb. 2, 1974,
St. Petersburg, Fla., pp. 127-130.

Mrosovsky, N. 1967. How turtles find the sea. Science J. 3: 52-57.

Mrosovsky, N. 1970. The influence of the sun's position and elevated cues
on the orientation of hatchling sea turtles. Anim. Behav. 18:

648-651.

64


INTRODUCTION

Factors that initiate and control the movements of immature Kemp's
ridley sea turtles need to be defined. This species has only one major
nesting beach, located near Rancho Nuevo in the state of Tamaulipas, Mexico
(Carr, 1963; Hildebrand, 1963). From this beach virtually all hatchlings
enter the Gulf of Mexico. Despite this fact, immature L. kempi have been
reported throughout the coastal areas of the Gulf of Mexico and along the
Atlantic coasts of North America and Europe (Carr, 1956, 1957, 1980; Carr and
Caldwell, 1958; Pritchard and Marquez, 1973). This indicates that immature
L. kempi are capable of traveling long distances. Furthermore, immature L.
kempi may exhibit migratory movements as suggested by their seasonal
abundance in the Cedar Keys, Florida area (Carr and Caldwell, 1956).
However, the causal basis for these movements of immature L. kempi has never
been studied.

This subject is particularly important to the L. kempi captive-rearing
program conducted by the National Marine Fisheries Service's Southeast
Fisheries Center (SEFC), where approximately 2000 turtles per year are reared
in captivity during the first eleven months of their lives. The turtles are
then released into the Gulf of Mexico. Information on the movements of the
turtles after release could be of vital importance to their survival and
their subsequent breeding.

Previous research on the movements of immature L. kempi consisted of
tagging studies by Carr and Caldwell (1956) and by the SEFC. Carr and
Caldwell's study resulted in the recapture of two out of twenty-five tagged
L. kempi. The two turtles were recaptured in the approximate location as

their initial capture after periods of 43 and 91 days, suggesting the


swimming vector magnitudes with those of the other turtles revealed no
significant differences (Kruskal Wallis, P > 0.05). The magnitudes and
directions of each turtle's swimming vectors are listed in Table 3.

The magnitudes of each turtle's swimming vectors were compared to the
tiagnitudes of the corresponding current movements to determine if the current
or the turtle's swimming contributed significantly more than the other to
each turtle's movements. These comparisons revealed no significant
differences between the magnitudes of these two factors: (Sign tests, P >
0.05).

In order to analyze the turtles’ directions of swimming and directions
of movements, circular statistical tests were performed on each turtle's path
and on each turtle's swimming vectors. However, current statistical methods
cannot prove that paths of this sort are either random or nonrandom, because
of a problem with the independence among the data points (Batschelet, 1972).
However, Batschelet suggests a modest approach of using a test formulated by
Hodges, while assuming that the animals we are tracking are occasionally
choosing new headings that are independant of their previous headings.
Because of the limited number of data points associated with each turtle
path, I used a Rayleigh test (Batschelet, 1965) as an alternative to the
Hodge's test. The Rayleigh test analyzes a group of headings for randomness
and can thus be used to gain insight about the directions of movement and the
directions of swimming of each turtle. Before the Rayleigh test can be
applied, a reference direction must be chosen. Then the headings of the
segments of each path and the headings of the swimming vectors can be

measured relative to the reference direction.

18


Sf SR

Sd
$

Galveston Bay

Galveston Island

STUDY SITE

Gulf of Mexico

North

t

Figure 11.

Location of the study site for the sea-finding experiments

55



indicated the probable direction to the sea. Unfortunately, it is beyond the
scope of this study to suggest the additional mechanism(s) responsible for
"the observed results.

In conclusion, this study indicates that the sea-finding behavior of L.
kempi hatchlings possesses not only characteristics of a phototropotactic
reaction but also characteristics of an orientation mechanism that does not

rely on light intensity as a cue.

62


for quantifying the movements of turtles that were released from the center
of the arena. Each section (the area between two adjacent poles) comprised
approximately 11.25 degrees of the arena's perimeter. Each pole was securely
inserted into the sandy bottom of the lagoon and projected out of the water.
The length of each pole was adjusted so that approximately 2 to 5 cm of pipe
would project out of the water.

The releasing device depicted in Figure 8 was positioned at the center
of the arena. This device provided a method of releasing turtles from the
center of the arena without requiring the presence of someone in the arena.
The device consisted of a turtle holding chamber mounted on two telescoping
PVC poles. The lower pole was securely inserted into the sandy bottom of the
lagoon. The upper pole was prevented from sliding down onto the lower pole
by a release pin that was inserted through holes in both of the poles. The
turtle holding chamber of the device was 40 cm in diameter with a 15 cm high
Plastic wall on its perimeter. The floor of this chamber was plastic and
contained numerous holes to allow for the free flow of water. By adjusting
the amount of the lower PVC pole that was inserted into the bottom of the
lagoon, the height of the releasing device could be adjusted. During the
experiments, the height of the releasing device was adjusted so that 3/4 of
the turtle holding chamber was filled with water (Figure 8). After a turtle
was placed into the chamber, it could be released into the arena by the
removal of the release pin, which caused the chamber to slowly sink below the
surface of the water. A 20 m string attached to the release pin allowed an
observer, who was positioned onshore, to remove the pin.

The L. kempi used in this study were maintained at the Galveston

Laboratory. They ranged from three months to seven months of age.

35


Water Level

4

Turtle Holding Chamber

Holes

String
a4

\

Release Pin

Sandy Bottom

Figure 8. Diagram of turtle releasing device.

36


SECTION III
THE SEA-FINDING ORIENTATION OF HATCHLING
LEPIDOCHELYS KEMPI

51


Directions of Movement using Current Directions of Swimming using Current
as the Reference Direction asthe Reference Direction

CURRENT DIRECTION

CURRENT DIRECTION

Figure 5. Directions of the turtles' movements and swimming relative to
current direction. Each dot represents a heading that corresponds to a
segment of a turtle's path or to a turtle's swimming vector.

24


METHODS AND MATERIALS

The L. kempi hatchlings used in this study were obtained from the
Galveston Laboratory's captive-rearing program. The turtles were
transported, in portable holding tanks, by motor vehicle, to the study site
located on a Galveston Island beach (Figure 11).

In order to quantify the direction of the hatchlings movement on the
beach, a circular arena, similar to that used by Mrosovsky and Carr (1966),
was constructed at the study site. It was positioned above the intertidal
zone and its center was approximately 18 m from the dunes bordering the
island side of the beach. The beach surface within the arena was clear of
vegetation and the sea was not visible at turtle eye level from anywhere
within the arena. The arena was 12 m in diameter and a 0.3 m by 0.3 m trench
was dug around its perimeter. The trench's walls were steeply dug to prevent
any hatchlings from escaping. The trench was divided into 16 sections (each
comprising 22.5 degrees of the arena's perimeter) using wood partitions. The
sections of the trench were positioned so that if a line was drawn from the
center of the arena through the partition between sections 16 and 1, it would
extend north. Additionally, if a line was drawn from the center of the arena
through the partition between sections 6 and 7, it would approximate a
perpendicular line to the sea.

Throughout this study, the following experimental procedure was
repeatedly conducted using groups of 15 hatchlings. All hatchlings were used
only once and the maximum age of the hatchlings was 16 days. Prior to each
experiment, 15 hatchlings were placed in a wire mesh enclosure (0.5 m in

diameter) located at the center of the arena. After a 2 minute acclimation

54


LITERATURE CITED

Batschelet, E. 1965. Statistical Methods for the Analysis of Problems in
Animal Orientation and Certain Biological Rhythms. Am. Inst. Biol.
Sci., Wash. D.C.

Batschelet, E. 1972. Recent statistical methods for orientation data. In:
Animal Orientation and Navigation. Galler, S., Schmidt-Koenig, K.,
Jacobs, G., Bellville, R. (eds) pp. 61-91. N.A.S.A., Wash. D.C.

Carr, A.F. 1956. The windward Road. Alfred A. Knopf, New York.

Carr, A.F. 1957. Notes on the zoogeography of the Atlantic sea turtles of
the genus Lepidochelys. Rev. Biol. Trop. 5: 45-61.

Carr, A.F. 1963. Panspecific reproductive convergence in Lepidochelys
kempi. Ergebn. Biol. 26: 298-303.

Carr, A.F. 1969. So Excellent a Fishe. Natural History Press, New York.

Carr, A.F. 1980. Some problems of sea turtle ecology. Amer. Zool. 20:
489-498.

Carr, A.F. and Caldwell, D.K. 1956. The ecology and migration of sea
turtles, 1. Results of field work in Florida, 1955. Amer. Mus.
Novitates 1793: 1-23.

Carr, A.F. and Caldwell, D.K. 1958. The Problem of the Atlantic ridley
turtle (Lepidochelys kempi) in 1958. Rev. Biol. Trop. 6: 245-262.

Carr, A. and Ogren, L. 1960. The ecology and migrations of sea turtles.
IV. The green sea turtle in the Caribbean Sea. Am. Mus. Nat. Hist.
Bull. 121: 1-48.

Ehrenfeld, D.W. 1968. The role of vision in the sea-finding orientation of
the green sea turtle (Chelonia mydas).. II. Orientation mechanism and

range of spectral sensitivity. Anim. Behav. 16: 281-287.

63


TABLE OF CONTENTS

ABSTRACT... 2. cc cscececcecccrecece cee ee cece scenester eees senate seeeeeeeeee iii
1. INTRODUCTION... ....ceecececee cece cece eee ee eee e eee eneeeeeeeeeeesenenees 1
II. THE ORIENTATION OF IMMATURE LEPIDOCHELYS KEMPI FOLLOWING THEIR
RELEASE INTO THE GULF OF MEXICO
A. INTRODUCTION. ..... cece cece cece cnc eeeeceeeretettesereseeeeeeseess 4
B. METHODS AND MATERIALS.........cccece cece ereecec eres eeeeeeeeseeees 6
C. RESULTS... eee c cece cece cece e cece erent eee n teen eee ee cesses eeeeeeees ll
D. DISCUSSION. ..... cece cece ccc e cence ene e nee eee ceeeeeeeseeee 00 26
IIT. THE ORIENTATION OF IMMATURE LEPIDOCHELYS KEMPI IN A LAGOON ORIENTATION
ARENA
A. INTRODUCTION. «0... cc cece cece cence eee e eee neeeeeeeeeeseeeceseeenes 30
B. METHODS AND MATERIALS........ cece cece cece e cece een c eee eeeeneneenees 32
C. RESULTS... eee cece cece cece cece eee n cnet een e eee ee reese eeeeeeeeees 39
D. DISCUSSION... .. cece cece cece eee c cece eee tence tence essen eee eeeees 48
IV. THE SEA-FINDING ORIENTATION OF HATCHLING LEPIDOCHELYS KEMPI
A, INTRODUCTION... ... eee e cece cece tect eee e teen teen eeeeeeeeneenees 52
B. METHODS AND MATERIALS.......cecececeecececeeeeceeeeteceeeeeesnsens 54
C. RESULTS AND DISCUSSION. ........ cece eee e cece eee e teen eee eeeeeeeeee 58
V. LITERATURE CITED. 0... cc cece cece cece eee e cence tence cece ee eeeeeeeeeeens 63
VI. ACKNOWLEDGMENTS... ..... ce eeece sec ee cece ones beeen nevececeaneeneccenes 67


Figure 10 (A and B) illustrates the turtles' directions of swimming
relative to north in each of the arena locations. Rayleigh tests were
performed on these two groups of headings and these tests indicated that the
headings in the west arena were nonrandomly distributed (P < 0.01), but the
headings in the east arena appeared to be distributed randomly (P > 0.05). V
tests were performed on these two groups of headings using the direction that
approximates the most direct path away from shore as the predicted direction
of swimming. These tests indicated that there was a significant clustering
of headings around the predicted direction in the west arena (P < 0.01), but
there was not a significant clustering around the predicted direction in the
east arena (P > 0.05).

To determine if the current in the arena or the light field surrounding
the arena was affecting the swimming orientation of the turtles, the current
direction and the brightest direction were used as reference in generating
two new headings for each turtle's direction of swimming. Figure 10 (C,D,E,
and F) illustrates the headings of the turtles' swimming directions in both
arena locations relative to the current and brightest direction. Rayleigh
tests indicated that when the swimming directions were analyzed with respect
to the current direction, the headings were nonrandomly distributed in both
arena locations (P < 0.01). However, when the swimming directions were
analyzed with respect to the brightest direction, the headings were
nonrandomly distributed in only the west arena (P < 0.01). V tests were
performed on these four groups of headings, using a direction 180 degrees
from the current direction as the predicted direction relative to the
current, and using the brightest direction as the predicted direction

relative to the light field. These tests indicated that the headings

43


determine the randomness of the headings. These tests indicated that the
directions of these turtles’ movements were nonrandomly distributed relative
to north. To determine if these turtles, like the hatchling C. mydas
followed by Frick (1976) and Ireland et al. (1978), were moving in directions
that approximated right angles to the shoreline, V tests (Batschelet, 1972)
were performed on these two groups of headings. As reported in chapter one,
the V test leads to significance only if there is a sufficient clustering of
headings around a predicted direction. In this situation the predicted
direction chosen in each arena was the heading that approximated the most
direct path away from shore (90 degrees relative to north in the west arena
and 270 degrees relative to north in the east arena). The V tests indicated
a significant clustering of headings around the predicted direction in both
arenas (P < 0.01).

As discussed in chapter one, it is advantageous to use a variety of
reference directions when analyzing the randomness of headings. To determine
if the current in the arena or the light field surrounding the arena affected
the orientation of these turtles, estimates of these two parameters were
recorded throughout the study. Thus for each turtle movement recorded during
the study there is also a coincidental estimate of each of these two
parameters. As a result, the movements of each turtle could be analyzed with
respect to the current direction and the direction of greatest light
intensity (brightest direction). Figure 9 (B,C,D, and E) shows the turtles’
directions of movement in each arena relative to the two reference parameters
mentioned above. Rayleigh tests were performed on each of these four groups
of headings and these tests indicated that the headings in all four groups

were nonrandomly distributed (P < 0.01). A V test was then performed on each

41


Witham, P.R. 1976. Evidence for ocean-current mediated dispersal in young
green turtles, Chelonia mydas (Linnaeus). Master's Thesis, University
of Oklahoma.

Witham, P.R. 1980. The "lost year" question in young sea turtles. -Amer.
Zool. 20: 523-530.

66


RESULTS

The release areas and the final positions of the turtles during the
study are shown in Figure 2. The dates associated with the final recorded
position of the turtles varied because of premature loss of contact with
certain transmitters. Loss of radio contact with two transmitters (numbers
three and six) occurred very early in the study before their attachment to
their respective turtle could be visually verified using a-boat. Therefore,
these two turtles are. not included in Figure 2 or in any of the analyses that
follow. Radio contact with one of the current monitors was also lost early
in the study. Thus the movements of the other current monitor were used in
the subsequent analysis.

Estimates of the paths comprising the net movements of the turtles and
of the current monitor are shown in Figure 3. The initial point of each path
represents the release location. However, the release location was not used
in the analysis of these paths since the boat used for the releases did not
have a Loran-C unit, thus preventing the precise recording of where the
releases occurred. Each path consists of lines connecting all of the
Positions where an individual transmitter was located during the study.
Therefore each segment of a path represents a net movement of a turtle or the
current monitor during a certain portion of the study. The daily net
movements of each turtle were estimated from these paths and their magnitudes
ranged from 1.1 to 27.2 km per day with mean values for individual turtles
ranging from 5.1 to 17.2 km per day. The magnitudes of each turtle's daily
net movements were compared to those of each of ‘the other turtles and no

significant differences were detected (Kruskal: Wallis, P > 0.05). The

11


from an area within four degrees above the surface of the beach. Many
authors have proposed that the principal mechanism by which hatchling sea
turtles find the sea is by a tropotactic reaction to light (Mrosovsky, 1967,
1972; Ehrenfeld, 1968; Mrosovsky and Shettleworth, 1968, 1974; Verheijhen and
Widlschut, 1973; Mrosovsky, et al., 1979). That is, a hatchling apparently
compares the intensities of light that are received through each eye and then
turns its body until both eyes receive equal intensities of light. The
hatchling then moves forward in that direction.

Although a phototropotactic reaction has been suggested as the principal
mechanism in sea-finding orientation, Parker (1922) and Limpus (1971) have
suggested that the distribution of silhouette patterns along the horizon may
also be a major orientation cue. Furthermore, following a variety of
experiments with hatchling Chelonia mydas, Van Rhijn (1979) indicated that
their sea-finding behavior could not be solely explained in terms of a
phototropotactic reaction and that this behavior is Probably a result of a
“multiple input unit system".

The above information demonstrates the need for further studies of the
sea-finding behavior of sea turtles and in particular for studies of the
sea-finding behavior of L. kempi. Previous studies of L. kempi sea-finding
behavior have been prevented by this species’ small population size and by
the remoteness of its only major nesting beach. However, a cooperative
conservation program involving Mexico and the United States has recently
provided a source of hatchling L. kempi. This source of hatchlings
facilitated a study of the sea-finding behavior of L. kempi during the summer
of 1980. The findings of that study are reported in this section.

53


Morning Experiments

7N4awo

1007 am

Sea

710/80

1005 Am

Afternoon Experiments

Figure 12.

Directions of the hatchlings sea-finding movements. Each number

represents the number of hatchlings that fell into the individual trench

sections.

59


quem at TURTLE #4

TURTE a8

TURTLE 29

10 Rlometere,

Figure 3. Estimates of

the paths taken by the turtles during the radio
tracking study.

13


ABSTRACT

Orientation characteristics of immature Lepidochelys kempi were
determined during three separate studies. In the first study, a radio
tracking technique was used to record the movements of ten yearling L. kempi
for a 27 day period following their release into the Gulf of Mexico. The
results of the study indicate that the turtles were moving and swimming
randomly, relative to geographical and wind directions. However, the turtles
exhibited nonrandom movements and nonrandom swimming to current direction.
Although they were displaced by the current, the turtles tended to swim
against the current. Additionally, the current and the turtle's swimming
exerted approximately equal effects on their movements. In the second study,
the movements of 3 to 7 month old L. kempi were analyzed using a lagoon
orientation arena. Results of the study indicate that the turtles were
moving and swimming randomly, relative to the shore position and relative to
the brightest direction, but they tended to swim against the current. During
the third study, the sea-finding behavior of hatchling L. kempi was analyzed.
Findings indicate that the hatchlings possessed not only a phototropotactic
orientation mechanism, but also an orientation mechanism(s) that does not

rely on light intensity as a cue.

Vii


7
GATE POSTS

5m. RADIUS
"TURTLE RELEASER

Figure 7. Diagram of the lagoon orientation arena.

34


Table 4. Current and wind directions for time periods corresponding to the
segments of the turtles’ paths.

Time period Mean current Mean wind
direction direction
(in degrees relative to north)

6/4-6/6 NA 269
6/6-6/9 020 202
6/9-6/11 238 190
6/9-6/13 286 132
6/11-6/13 341 073
6/13-6/16 279 062
6/13-6/17 253 072
6/16-6/17 154 226
6/17-6/19 101 250
6/19-6/20 133 229
6/20-6/23 033 221
6/23-6/25 267 231
6/24-6/26 NA 227
6/26-6/28 NA 204
6/28-6/30 NA 226

22


DISCLAIMER

This document is adapted from a Thesis, presented to the Faculty of the
Department of Biology, University of Houston, in Partial Fulfillment of the
Requirements for the Degree of Master of Science, July 1982. It has been
reviewed by the National Marine Fisheries Service, National Oceanic and
Atmospheric Administration, and approved for printing. Such approval does
not signify that the contents necessarily reflect the views and policies of
the U.S. Department of Commerce, NOAA or NMFS. Mention of trade names or
commercial products herein does not constitute endorsement or recommendation

for use.

ii


Table 5. Analyses of the turtles’ directions of movement. (S = significant,
N = nonsignificant)

West arena East arena

Headings Rayleigh V test Rayleigh V test
relative to test test

North Ss Ss Ss Ss
Current Ss Ss Ss Ss
direction
Brightest Ss Ss Ss N
direction

46


INTRODUCTION

After leaving the nest, hatchling sea turtles move rapidly down the
beach and into the water. Although the sea-finding behavior of sea turtles
has been the subject of many studies, the behavior of young sea turtles after
entering the water has received only scant attention. This is primarily the
result of the small size of hatchlings which has prevented an effective
tagging technique from being developed. Therefore when young sea turtles are
captured, their nesting beach (and more importantly their net movement from
the nesting beach) cannot be determined.

The movements of a smal] number of hatchling sea turtles have been
recorded after they enter the water. Frick (1976) visually tracked Chelonia
mydas hatchlings after they entered the water by swimming behind them or by
following close behind them with a boat. Ireland et al. (1978) attached
sonic transmitters and chemical lights to hatchlings and then followed them
at a distance in a boat. The results of these two studies suggested that
after entering the water, hatchling C. mydas swim in a direction that
approximates a right angle to the beach. However, the logistical aspects of
tracking sea turtles in open ocean permitted only a small number of
hatchlings (21 or less) to be tracked for short time periods (4 h or less)
during each of these studies. Furthermore, the logistical aspects of
tracking young sea turtles in open ocean together with the difficulty of
tagging hatchling sea turtles have resulted in a total lack of knowledge
concerning the long term movements of young sea turtles during their first
year of life (Carr, 1967). However, the radio-tracking study (Section I) has
analyzed the movements of eleven month old captive-reared L. kempi following

their release into the Gulf of Mexico. The results of that study suggested

30


RESULTS

Upon release, most of the turtles swam at a moderate rate near the
water's surface. However, some of the turtles appeared to exhibit an escape
behavior and would swim rapidly in whatever direction they were facing.
Other turtles exhibited low activity levels, using primarily their back
flippers while floating with the current.

Turtles which exhibited either an escape behavior or a low activity
level could confound the results of this study. Therefore, an objective
method was developed in an effort to remove these turtles' movements from the
results. This method was based on the swimming rates of the turtles. The
experimental design of this study provided the means of trigonometrically
generating a swimming vector for each turtle by subtracting the estimated
movement of the current in the arena from the movement of each turtle in the
arena. The swimming rate was then calculated by dividing the magnitude of
the swimming vector by the time taken by the turtle to reach the scoring
circle. The average swimming rate of the 170 turtles used in this study was
0.15 m/s with a standard deviation of +0.08 m/s. To rid the results of the
Possible confounding movements that were mentioned above, only turtles with
swimming rates that were plus or minus one standard deviation unit from the
mean were analyzed (rates greater than 0.07 m/s and less than 0.23 m/s).
Therefore the following results reflect the movements of 119 of the 170
turtles used in this study.

The turtles' directions of movement in both arena locations are
illustrated in Figure 9 (A and B) relative to north. Rayleigh tests

(Batschelet, 1965) were performed on these two groups of headings to

39


of the turtles' movements relative to current direction. This predicted
direction was chosen because it is logical to assume that the current should
displace the turtles. The V test indicated that there was a significant
clustering of headings in the general direction of the current (P < 0.05).
When the V test was performed on the directions of the turtles’ swimming
relative to the current direction, a predicted direction of swimming against
the current was chosen. This predicted direction was chosen for two reasons.
First, if a turtle exhibited territoriality, it would have to swim against
the current to remain in the same general area relative to bottom topography.
Secondly, if a turtle fed on stationary organisms, it would have to swim
against the current while obtaining its Prey so as not to be swept
downcurrent from the prey. The V test indicated that there was a significant

Clustering of headings in a general direction opposite the current direction.

25


Table 1. Tag number, weight, length, and release date for each turtle used
in this study.

Turtle Tag Weight (g) Length (cm) Release
number number date
T NNN643 1170 18.7 6-3-80
2 NNNO87 1270 19.1 6-3-80
3 NNNO21 990 17.8 6-3-80
4 NNNO68 1210 19.4 6-3-80
5 NNN530 1260 19.8 6-3-80
6 NNN134 1700 17.5 6-5-80
7 NNN120 1590 16.4 6-5-80
8 NNN108 1640 17.0 6-5-80
9 NNNO8S 1500 18.6 6-5-80
10 NNN125 1850 17.8 6-5-80



3 4 3 3"
FA 5. 9:43 A.M. FA 5. 9:64 A.M
€ 3
z z |
Section of Trench Section of Trench vn
id 3" 3 3 10
2 5 . 2:18 A.M. 2 5 . 2:32 A.M.
2 2
pia 1
2B. 8000]
8 $4000)
et
= 5 2000
3 & 2000
| | e LI {1 mn | | | |
@ wo ttat 1616 + « Oe wit oy
Section of Trench Section of Trench

Figure 13. Directions of the hatchlings sea-finding movements together with
the estimate of the light field surrounding the arena.

61


Table 6. Analyses of the turtles' directions of swimming. (S = significant,
N = nonsignificant)

West arena East arena

Headings Rayleigh V test Rayleigh V test
relative to test test

North Ss S N N
Current Ss Ss Ss S.
direction
Brightest Ss Ss N N
direction

47


predicted directions were the direction approximating the most direct path
away from shore and the direction opposite the current direction.

The above results indicate that the turtles in the east arena were also
moving nonrandomly. They also indicate that the turtles were not moving in
the brightest direction, but could have been moving away from shore or
against the current. However, if the turtles were moving in. one of those two
directions, their movements were not consistent enough to provide an accurate
indication of the specific direction of movement. This could be the result
of the effect of the current on the turtles movement. For example, the
turtles could be swimming in only one general direction, but the variable
strength and direction of the current could effectively result in a broad
range of movements. In an effort to circumvent the possible confounding
effect of the current, the swimming directions of each turtle were determined
trigonometrically and then analyzed.

The swimming directions of the turtles in the west arena were
nonrandomly distributed relative to all three reference directions (Table 6).
Additionally, the swimming directions were significantly clustered around the
three predicted directions of swimming used in the V tests (Table 6). These
results indicate that the turtles in the west arena were swimming
nonrandomly, but they do not indicate a specific direction of swimming.
Unfortunately the similarity of the predicted directions of swimming in the
west arena may have prevented the determination of a specific direction of
swimming. The east arena, however, should not have this ‘problem. The
variability of the current directions during the experiments in the east
arena decreased the similarity of the predicted directions of swimming used
in the V tests (Table 6). Furthermore, by using the headings of the turtles

swimming rather than the headings of their movement, the possible confounding

49


Turtle #8
6/19

Turtle #2
6/30

6/3/80 Release
Area —»>

6/5/80 Release >
Area

Florida

Gulf of Mexico

Figure 2. Release areas and final recorded positions of the turtles during
the 27 day study. Squares indicate turtles released on 3 June 1980 and dots
indicate turtles released on 5 June 1980.