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Chelonian Conservation and Biology, 2006, 5(2): 000–000
Ó 2006 Chelonian Research Foundation
Filtered Streetlights Attract Hatchling Marine Turtles
K
RISTEN NELSON SELLA
1
,MICHAEL SALMON
2
, AND BLAIR E. WITHERINGTON
3
1
Palm Beach County Department of Environmental Resource Management, 3323 Belvedere Road, Bldg 502, West Palm Beach,
Florida 33406 USA [turtlewomyn@juno.com];
2
Department of Biological Sciences, Florida Atlantic University, Boca Raton, Florida 33431 USA [salmon@fau.edu];
3
Florida Fish and Wildlife Conservation Commission, Florida Marine Research Institute, 9700 South A1A,
Melbourne Beach, Florida 32951 USA [witherington@cfl.rr.com]
ABSTRACT. – On many nesting beaches, hatchling marine turtles are exposed to poled street lighting
that disrupts their ability to crawl to the sea.
?1
Experiments were done to determine how hatchlings
responded to street lighting transmitted through 2 filters that excluded the most disruptive
wavelengths (those , 530 nm; those , 570 nm). Filtered lighting, however, also attracted the
turtles though not as strongly as an unfiltered (high-pressure sodium vapor) lighting. Filtering is
therefore of limited utility for light management, especially since other alternatives (such as
lowering, shielding, or turning off unnecessary lighting; use of dimmer lights embedded in
roadways) are more effective.
K
EY WORDS. – Reptilia; Testudines; Cheloniidae; Caretta caretta ; Chelonia mydas; sea turtle;
hatchlings; artificial lighting; light ‘‘ trapping’’ ; orientation; seafinding; Florida; USA
Hatchling sea turtles emerge from their nests at night
(Bustard 1967; Mrosovsky 1968; Witherington et al. 1990)
and crawl toward the sea. This behavior, known as
‘‘ seafinding’’ (Parker 1922; Daniel and Smith 1947; Carr
and Ogren 1960; Ehrenfeld and Carr 1967; Mrosovsky
1972), is based upon 2 orientation cues. Hatchlings crawl
toward the brightest area (typically, the seaward horizon)
using a positive phototaxis (Mrosovsky 1972; Mrosovsky
and Kingsmill 1985). Hatchlings also detect regions
elevated above the horizon (such as a tall dune and its
associated vegetation). Turtles crawl away from the dune
and toward the beach that presents a lower, flatter, horizon
(Salmon et al. 1992; Witherington 1992).
Artificial lighting disrupts seafinding orientation.
Bright luminaires on land attract turtles so that they crawl
toward the lights and away from the ocean (‘‘ misorienta-
tion’’ or ‘‘ light trapping’’ ; Verheijen 1958, 1985). When
sources of artificial light are less attractive, hatchlings may
show ‘‘ disorientation’’, or an inability to maintain a
directional crawl. This response probably occurs when
hatchlings simultaneously respond to natural cues and
artificial lighting, but cannot orient toward either stimulus
(Tuxbury and Salmon 2004).
At night, most marine turtle hatchlings will crawl
toward visible light; the shorter wavelengths (violet, blue)
are especially attractive (Mrosovsky and Carr 1967;
Mrosovsky and Shettleworth 1968). However, this re-
sponse varies with wavelength among the species. With-
erington (1992) and Witherington and Bjorndal (1991a)
used monochromatic lights as stimuli in laboratory
experiments showing that green turtle (Chelonia mydas),
olive ridley (Lepidochelys olivacea), and hawksbill (Eret-
mochelys imbricata) hatchlings were attracted to wave-
lengths between 350 and 600 nm (ultraviolet to yellow).
Loggerheads (Caretta caretta), however, were attracted to
wavelengths between 350 and 500 nm (ultraviolet to green)
and were either indifferent to, or repelled by, wavelengths
between 530 and 700 nm (green-yellow to red).
These results led to the hypothesis that lights containing
only green-yellow to red wavelengths (530–700 nm) would
not attract loggerhead hatchlings or interfere with their
orientation. Assuming this hypothesis was correct, the
Florida Power and Light Company (FPL) installed light
filters (orange acrylic sheets) in hundreds of pole-mounted
streetlights bordering coastal roadways in South Florida.
Without the filters, these lights disrupt seafinding because
they are frequently visible at nest sites, and because their
high-pressure sodium vapor (HPS) luminaires emit wave-
lengths shorter (as well as longer) than 530 nm (With-
erington and Bjorndal 1991b). Orange filters exclude the
shorter but transmit the longer light wavelengths.
Filtered lighting consists of a spectrum of longer
(orange to red) wavelengths. Monochromatic light within
these wavelengths evokes either indifference or aversion
from loggerhead hatchlings tested under laboratory
conditions (Witherington 1992). The present study there-
fore had 2 goals. The first was to determine if loggerheads
showed indifference and/or aversion when presented with
a spectrum of orange or red (filtered HPS) wavelengths.
The second goal was to determine how other species
(represented here by green turtle hatchlings), attracted to
longer monochromatic light wavelengths than logger-
heads, responded to the same stimuli.
METHODS
Hatchlings. — Hatchling loggerhead turtles and green
turtles were obtained during the 2000 nesting season from
previously marked nests at Coral Cove Beach in Palm
Beach County, Florida (26857
0
N, 80805
0
W). These
hatchlings were used in arena experiments conducted
nearby at the Marinelife Center, Juno Beach, Florida.
Hatchlings during the 2001 season were obtained from
nests relocated to a hatchery at the Hillsboro Club,
Broward County, Florida (26818
0
N, 80805
0
W). These
turtles were used in T-maze experiments conducted nearby
at Florida Atlantic University in Boca Raton, Florida. All
turtles were transported from the collection site in
polystyrene foam containers that allowed for air exchange
but were covered with a black cloth to exclude light.
Experiments were done in air-conditioned, dark rooms
rendered lightproof by sealing all windows with black
plastic sheeting.
During both years, hatchlings were collected in the
afternoon of the evening when they were scheduled to
emerge. Date of emergence was estimated by adding 55
days to the egg deposition date. Turtles were used only if
they were captured within 15 cm of the sand surface, and if
their plastron was ‘‘ flat’’ (indicating they were ready to
emerge and developmentally competent to migrate off-
shore). To ensure genetic diversity, hatchlings from at least
2 nests were used each evening, and all tests were repeated
over 2 or more evenings.
Before testing, turtles were held in a dark room for
several hours at ambient temperatures (278–308C), until
dusk. They were then briefly (ca. 15 min) exposed to dim
light and slightly cooler temperatures to stimulate
locomotor activity (Bustard 1967; Mrosovsky 1968). Each
turtle was used in a single trial, then released later that
evening on a nearby dark beach, in accordance with state
guidelines (Florida Department of Environmental Protec-
tion 1996).
Light Measurements. — Light was measured (in
fractions of watts) with a radiometer (Model 351; United
Detector Technology, Baltimore, MD) that had a uniform
response between 400 and 700 nm. An Optec stellar
photometer (Optec, Inc, Lowell, MI; Model 351 with a 168
angle of acceptance; range of 300-1100 nm; peak
sensitivity at 520 nm) was used to determine relative
street light radiance at coastal roadways. Both the
radiometer and stellar photometer were calibrated against
a 500-nm light of known intensity across a 3 decade range
of light amplitudes. Measurements from both instruments
were then converted to a common scale of absolute
radiance (in photonscm
2
s
1
at 500 nm).
Arena Experiments. — The arena was a circular
horizontal platform used to determine how green turtle and
loggerhead hatchlings responded to HPS and to filtered
HPS light (Fig. 1). It was made of rough-textured
plywood, painted light brown (sand) in color. It contained
60 cloth-lined pockets at the periphery sufficiently large to
trap each turtle that had crawled there from the arena
center (where each turtle was released). Crawling vectors
for each turtle were measured by the angle between the
light source, the center of the arena, and the pocket. The
pocket in front of a light was arbitrarily designated as 08.
The light (an attenuated 70-W HPS light enclosed in a
small wooden box) was located 1.87 m from the arena
center and elevated 40 cm above the platform surface (Fig.
1). Light escaped from the box through a small pinhole
opening made in aluminum flashing. This configuration
matched the elevation and radiance of poled streetlights
observed from the location of several nests. Filters were
attached to the box over the pinhole. Two identical light
boxes were placed near the arena about 908 apart, but
during each experiment only 1 light was turned on. The
light beam from each box was aimed down at the arena
center but could be seen from any location within the
arena.
Turtles were exposed to 4 treatments: 1) light turned
off (turtles crawl in complete darkness), 2) HPS light on,
3) HPS light transmitted through a 2422 amber filter
(excludes wavelengths , 530 nm), and 4) HPS light
transmitted through a NLW red filter (excludes wave-
lengths , 570 nm; Fig. 2).
?2
Both filters were made of dyed
plastic sheets manufactured by the General Electric
Lighting Corporation (Lexington, KY).
Equal numbers of green turtles and loggerheads from
several nests were exposed to each treatment until a target
sample size (n ¼ 30 turtles/treatment) was achieved for
Figure 1. The arena: light boxes were 908 apart and designed to
mimic the radiance of a 70-W HPS streetlight. Boxes were
positioned 40 cm (178) above the arena surface and 1.87 m from
the center of the arena.
Figure 2. Transmission characteristics of the General Electric
Lighting Company filters. The orange-colored 2422 filter
excludes wavelengths , 530 nm; the red-colored NLW filter
excludes wavelengths , 570 nm.
CHELONIAN CONSERVATION AND BIOLOGY, Volume 5, Numbe r 2 – 2006
each species. A trial began with the release in the arena
center of 5 hatchlings (to simulate the typically simulta-
neous emergence of several turtles from one nest)
contained inside a black cloth bag and ended after all of
the turtles had fallen into a pocket. Crawling progress was
monitored using a video camera and monitor (suspended
out of sight of the turtles, above the arena). In the ‘‘ no
light’’ treatment, records were videotaped under infrared
illumination.
Because filtering excluded some wavelengths, filtered
radiance was lower than unfiltered (HPS) radiance. The
intensities of the light stimuli used (as measured at the
arena center) were (in photonscm
2
s
1
): HPS,
12.0 3 10
12
; 2422 filter, 7.0 3 10
12
; and NLW, 7.0 3
10
12
. The 2422 intensity fell within the range of values
measured in the field from several filtered 70-W cut-off
fixtures, mounted 60 m distant from nests on 10-m-tall
street poles.
T-Maze Experiments. — This apparatus was used to
study the response of the turtles to lighting in choice
situations (Fig. 3). Turtles crawled down the runway
toward a white plate that reflected light from a single or
paired source. Once it reached the ‘‘ T’’ , the turtle turned
either to the right or left. Twenty-five hatchlings from 2 or
more nests were used in each trial. Because green turtle
hatchlings were less abundant, only loggerheads were used
in these tests.
The same light boxes were used to house each light.
One box was placed on each side of the maze and
positioned so that its illumination was only directly visible
to turtles that had crawled to the end of the runway (Fig.
3). Light intensity (as measured at the position of the ‘‘ T’’ )
of the HPS light was adjusted to match an unfiltered 70-W
HPS streetlight located either 40 or 60 m distant from a
nest.
The turtles were exposed to 4 treatments: 1) a single
HPS light from either the right or left side of the maze; 2) a
single filtered HPS (2422 or NLW) light from either the
right or left side; 3) paired HPS and filtered HPS (2422 or
NLW) lights (as in Fig. 3) presented from opposite sides of
the T-maze; and 4) paired filtered HPS (2422 and NLW)
lights presented from opposite sides of the T-maze. In the
paired light treatments, each treatment was replicated 4
times: with 1 of the 2 luminaires (HPS or the 2422 filtered
HPS light) presented at full intensity, or with their
intensity reduced by 1, 2, or 3 log units using neutral
density filters (made from layered plastic hardware cloth).
In control experiments, hatchlings were exposed to
pairs of lights (HPS or filtered HPS) adjusted to an
identical radiance. These tests were done to confirm that
no variable other than light was responsible for deviations
from an expected 50:50 turning ratio.
Statistics. — Crawl vectors for each turtle in a single
arena treatment were used to calculate a second-order
group mean angle and r-vector (measure of dispersion).
Rayleigh tests (Zar 1999) were employed to determine
whether groups were significantly oriented (p 0.05).
The number of hatchlings that turned right or left was
recorded for each T-maze treatment. The null hypothesis
of a 50:50 turning distribution was rejected when that
distribution resulted in a p 0.05 (by a binomial test;
Sokal and Rohlf 1995).
RESULTS
Arena Experiments. — Neither the loggerheads (Fig.
4) nor the green turtles (Fig. 5) showed significant
orientation when tested in darkness. Turtles exposed to a
HPS or to a filtered (2422 or NLW) HPS light were
strongly attracted to the stimulus.
Figure 3. The T-maze (overhead view). Hatchlings were released
at *, crawled toward the light reflecting barrier at the end of the
runway, then turned left or right toward one of the lights.
Figure 4. Response of loggerheads to a 70-W HPS light at 08
(top of each circle). In different treatments, the light is either
turned off (A) or on (B–D). When on, it passes through a filter in
C and D. Sample size is 30 turtles per treatment. Line length is
proportional to the number of turtles orienting in each direction.
A, group mean angle; r, dispersion; n.s., no significant group
orientation.
SELLA ET AL. — Filtered Streetlights Attract Hatchling Marine Turtles
T-Maze Experiments. — None of the control turtles
showed a distribution of turns that deviated statistically
from an expected 50:50 ratio. Hatchlings were significant-
ly attracted to a single HPS light at the 40- and 60-m
radiance levels (Table 1). Both types of filtered lighting
(orange 2422, red NLW) failed to result in a significant
attraction at the higher (40-m) radiance level. However, at
the lower (60-m) radiance, the turtles were attracted to
each light source. Attraction to filtered lighting was
weaker (76% of the turtles to the 2422; 84% to the
NLW) than attraction to HPS lighting (96%; Table 1).
More turtles turned toward the HPS light when it was
paired with either a 2422 (Fig. 6) or a NLW (Fig. 7)
filtered light. When HPS radiance was reduced by 2 log
units, the number of turtles that turned toward each light
did not differ statistically (Figs. 6 and 7). When HPS
radiance was reduced by 3 log units, more turtles turned
toward the 2422 filtered than the HPS light (Fig. 6). This
trend was also evident when the HPS light was paired with
a NLW light, though the probabilities just missed
significance (Fig. 7).
Turning tendencies shown in response to paired
filtered lights were statistically equal (Fig. 8). When the
2422 source was reduced in radiance by 3 log units, more
turtles turned toward the NLW light (Fig. 8).
DISCUSSION
Response to HPS and Filtered Lighting. — Our arena
experiments showed that in the absence of lighting, green
turtle and loggerhead hatchlings did not show significant
group orientation. However, in the presence of lighting the
turtles of both species crawled toward the source (Figs. 4
and 5). We conclude that both HPS and filtered HPS
lighting attract sea turtle hatchlings.
Our T-maze experiments (Figs. 6 and 7) indicate that
HPS lighting is more attractive to loggerheads than filtered
HPS lighting. Two variables might account for these
results. The first is light intensity because initially (in the
0:0 tests), the HPS light was slightly brighter than the
filtered HPS light with which it was paired. A second
possibility is that the HPS light was more attractive
because its spectral composition included some shorter
light wavelengths. Intensity was eliminated as a factor by
Figure 5. Arena experiments with green turtles. Format and
sample size, as in Fig. 4.
Figure 6. T-Maze experiments that show the percentage of
hatchlings turning toward a HPS light when it is paired with a
filtered (2422) HPS light. The 2 lights are initially presented at
intensities comparable to a street light at a distance of 40 m (top
graph) or 60 m (lower graph) from the turtle (0:0, left side of each
graph). In 3 additional treatments, the HPS light is reduced in
intensity by 1 (1:0), 2 (2.0), or 3 (3:0) log units while the
filtered light remains unchanged in intensity. n ¼ 25 different
hatchlings in each treatment. Points falling on or above the upper,
or on or below the lower dashed lines are significant statistical
departures (at p 0.05 level) from a 50:50 ratio (by a binomial
test).
Table 1. Percentage of hatchlings turning toward a single high-
pressure sodium vapor (HPS), 2422, or NLW light stimulus
presented from one side of the T-maze. Intensities are comparable
to a 70-W streetlight placed 40 or 60 m from the nest. Sample
size, n ¼ 25 hatchlings for each light stimulus. Probabilities (p)
are based upon the outcome of a binomial test.
40-m light 60-m light
Light stimulus % p % p
HPS 100 , 0.001 96 , 0.001
2422 68 n.s. 76 , 0.02
NLW 68 n.s. 84 , 0.002
CHELONIAN CONSERVATION AND BIOLOGY, Volume 5, Numbe r 2 – 2006
repeating the tests after HPS radiance was reduced by 1 or
more log units below the radiance of the filtered light. In
response, the turtles either continued to orient preferen-
tially toward the dimmer HPS source, or showed no
significant orientation toward either light (Figs. 6 and 7).
We conclude that in our experiments, the spectral
composition of the HPS light made that stimulus more
attractive to the hatchlings than filtered lighting.
Amber (2422) and red (NLW) filters were designed
for use with pole-mounted HPS streetlights on roadways
adjacent to nesting beaches. In a field experiment, the 2422
filter proved effective with adult nesting female logger-
heads (Pennell 2000). However, field experiments with
loggerhead hatchlings produced equivocal results (Cowan
and Salmon 1998) because of nightly variation in other
sources of artificial lighting (skyglow from nearby
communities). Because this lighting could not be con-
trolled, it was impossible to distinguish between responses
caused by filtered lighting and responses to changes in
background illumination. These problems led us to do
further testing in a laboratory setting where extraneous
sources of illumination could be excluded.
HPS lighting at both a higher (40-m) and lower (60-
m) intensity attracted the turtles, but the hatchlings were
attracted to filtered lighting only at a lower (60-m)
radiance level (Table 1). In addition, the NLW light at
60 m was apparently more attractive to the turtles than the
2422 filtered light at 60 m (Table 1). Yet the NLW light
excluded a larger proportion of the wavelengths around
530 nm that elicit ‘‘ indifference’’ , while leaving present
those wavelengths (. 570 nm) that elicit ‘‘ aversion’’
(Witherington 1992).
The explanation for these responses may center on
how hatchling loggerheads respond to different intensities,
rather than wavelengths, of light. In previous experiments,
Witherington (1992) found that responses such as
‘‘ attraction’’, ‘‘ indifference’’ , and ‘‘ aversion’’ were elicited
at relatively high (perhaps photopically mediated) light
levels. At lower light levels (perhaps mediated scotopi-
cally), all wavelengths of monochromatic light were
attractive to hatchling loggerheads. We hypothesize that
the 40-m light stimulus was sufficiently intense to permit
wavelength discrimination (and indifference or aversion),
whereas the 60-m light stimulus was not (and therefore
attracted the turtles).
These results reveal some of the complexities
associated with using filtered lighting as a management
tool. Filtered lighting may be unattractive to hatchlings
when they emerge from their nests because the light source
is in close proximity, and therefore more intense. But as
Figure 7. T-Maze experiments that show the percentage of
hatchlings turning toward the HPS light when it is paired with a
filtered (NLW) HPS light. Format as in Fig. 6.
Figure 8. T-Maze experiments that show the percentage of
hatchlings turning toward a 2422 filtered HPS light when it is
paired with a NLW filtered HPS light. Format as in Fig. 6 except
that the 2422 filtered light is reduced in intensity using neutral
density filters while the NLW light is left unchanged in intensity.
SELLA ET AL. — Filtered Streetlights Attract Hatchling Marine Turtles
the turtles crawl away from the light (and toward the sea),
the light source decreases in perceived intensity and could,
as a consequence, become attractive. To properly assess
the impact of filtered lighting on turtles at any location,
then, hatchlings must be exposed to the entire range of
light intensities they encounter as they crawl from their
nests (and the light) toward the sea.
Management Implicatio ns. — Ideally, filtered HPS
lighting should have no effect on the orientation of
hatchling sea turtles. Our results show, however, that
hatchlings can, under some circumstances, be attracted to
filtered lighting. Other problems are also associated with
the use of filtered lighting.
First, responses to filtered lighting probably vary,
depending upon the species. The 2422 and NLW filters
were developed primarily for use near loggerhead
rookeries and are based upon the unique response of
loggerhead hatchlings to light wavelengths (Witherington
1992). The few tests that have been done with leatherbacks
(Dermochelys coriacea) and green turtles (Cowan and
Salmon 1998; Tuxbury 2004) suggest that these species
respond differently even to the longer wavelengths
transmitted by these filters.
?3
Second, the filters currently in use may represent the
best technology that can be used with HPS luminaires,
which for economic reasons are preferred for street
lighting. Excluding any more of the shorter wavelengths
transmitted by HPS luminaires may reduce luminance
levels below levels required for roadway safety (as
mandated by the Florida Department of Transportation
[Scott Stephens, Florida Power and Light Co, pers.
comm.]). These standards were set by engineers to provide
minimum levels of illumination for motorists. However, a
variance from these standards can be obtained if the
roadway custodian accepts liability for accidents and
installs warning signs to notify motorists of poor lighting
conditions (Ecological Associates, Inc 1998). Currently,
roadways in Florida with lighting that affects nesting
beaches are being identified, and new standards are being
determined for lighting roadways. Whether filtered
lighting can meet those standards remains to be deter-
mined.
Third, there are better alternatives for managing
coastal roadway lighting. One promising technology is
the use of light-emitting diodes placed in the pavement
itself (‘‘ embedded’’ roadway lighting). These lights
produce far less illumination than streetlights and confine
that light to the roadway itself (where it is needed). Field
tests were recently done at a coastal roadway where
embedded lighting was installed. Their illumination could
not be detected at the beach either by humans, their
instruments, or by loggerhead hatchlings. Turtles crawled
toward the sea when the embedded lights were on and
when they were turned off. However, when the poled HPS
streetlights were turned on, orientation dispersion (and in
some tests, mean angle) were affected (Bertolotti and
Salmon, in press).
Fourth, filtered lighting is a ‘‘ half-way technology’’
(Frazer 1992) because it fails to eliminate the cause of the
problem (light scatter to the beach); rather, it seeks to alter
the impact of that light by modifying spectral output. The
only proven methods of light management, however, are to
turn off or redirect lighting so that it is no longer visible at
the beach (Witherington and Martin 2000).
On the other hand, there are circumstances where
filtered lighting might be useful. Hatchling loggerheads are
less likely to crawl toward visible lighting if a tall, dark
landward silhouette is present (Witherington et al. 1994;
Tuxbury and Salmon 2004). Because filtered lighting is
less attractive to the turtles, it might be used to illuminate
roadways without affecting seafinding, even if some
lighting escapes to the beach. However, before such a
modification is made permanent, tests must be done at
these sites to confirm that the turtles exposed to both
filtered lighting and tall silhouettes will complete a
seaward crawl.
Filtered lighting may also be beneficial at locations
where the public believes that lights prevent crime and/or
reduce roadway accidents (Witherington and Martin
2000). Filtered lighting at such a site has 3 benefits. It
has a favorable psychological impact on users, and (for the
turtles) reduces light intensity while transmitting less
attractive spectra to the environment.
New technologies must be explored to determine their
potential for reducing the impact of artificial lighting on
wildlife. Initially, light filters were a new technology
promoted by their manufacturer (General Electric Lighting
Corporation) as a simple method for converting harmful,
attractive lights into those that were ‘‘turtle friendly’’ .
These claims; however, were made in the absence of
adequate testing. Having now completed testing, we
conclude that at the present time filtered lighting is
potentially beneficial only under special, and unfortunately
somewhat limited, circumstances.
A
CKNOWLEDGMENTS
This study was completed by K.A.N. as part of the
requirement for a Master’s degree in the biological
sciences at Florida Atlantic University. C. Makowski
assisted in the laboratory experiments. W.P. Irwin, J.A.
Seminoff, A.G.J. Rhodin, and an anonymous referee made
suggestions that improved the manuscript. Financial
support was provided by the Florida Power and Light
Company and the National Save-the-Sea-Turtle Founda-
tion of Fort Lauderdale, Florida.
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Received: 29 September 2004
Revised and Accepted: 13 September 2005
SELLA ET AL. — Filtered Streetlights Attract Hatchling Marine Turtles
