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Biologist (2003) 50 (4) 163
In south-eastern Florida, sandy beaches attract tourists
during the day whose principal activities are recreational:
basking, building sand castles, swimming and socialising.
But, at night, a clientele with more utilitarian objectives
frequents the beach. These visitors approach shore from
the ocean rather than from land. Their stay (like tourists’)
is brief and also recorded by tracks on the sand surface.
However, these nocturnal visitors leave behind something
of substance: clutches of about 100 eggs buried deeply in
the sand.
Our nocturnal visitors, marine turtles, are huge reptiles
that have a residency of tens of millions of years on this
planet. But ‘newcomers’ – humans – have catastrophically
reduced their numbers directly (by egg and adult harvest-
ing) and indirectly (incidental capture by fisheries, and
habitat modification and degradation). Here, I describe
another kind of habitat modification: how our use of night
lighting repels females from nesting beaches and causes
the death of many of their hatchlings.
We’ve recognised the severity of this impact for only
about 30 years. Now, there is a growing International
effort to restore darkness to coastal and other wildlife
habitats. It’s a global problem, affecting the survival of
many species of nocturnally active wildlife and the
integrity of the communities in which they play a role.
Solving the lighting problem for sea turtles thus becomes
part of the international effort to modify how we design
and use artificial lighting, with widespread economic,
aesthetic and ecological consequences.
Sea turtles and nesting beaches
To see a marine turtle nest is an inspiring, as well as fasci-
nating, experience (Figure 1). The process is much the
same in all seven species of sea turtles. The patient
observer first sees a huge head, then a glistening carapace
(top shell) emerge from the surf. The turtle then pauses,
perhaps gauging the slope and elevation of the beach,
perhaps scanning for predators, perhaps stunned by feel-
ing its 200–400 kg body weight without the buoyancy
provided by water. If all seems well, she begins to drag her
huge mass up the beach. It is a slow, but deliberate, move-
ment punctuated by frequent pauses. Finally, the female
reaches a location between the dune vegetation and high
tide wrack, and begins to dig a depression (‘body pit’) using
sweeping motions of the fore and hind flippers. Next, and
with delicate movements of her rear flippers, she scoops
out a flask-shaped egg chamber with dimensions deter-
mined entirely by ‘feel’. Finally, the turtle will spread her
rear flippers to each side of the egg chamber and, every few
seconds, drop two to five soft-shelled eggs at a time into the
egg chamber until the cavity is almost filled.
The process is completed by ‘covering’ and ‘hiding’ the
eggs. Covering is also accomplished by feel, using the rear
Artificial night lighting
and sea turtles
Michael Salmon
Florida Atlantic University, USA
Natural transitions between light and darkness influence the biology and behaviour
of many organisms. What happens when humans introduce light into darkness?
Oceanic beaches, where sea turtles nest, provide an example of both the problem and
approaches to its solution.
Artificial night lighting
and sea turtles
Courtesy of J Amos, National Geographic
164 Biologist (2003) 50 (4)
flippers to shovel sand over the open egg chamber, then to
compress sand over the opening. Hiding that follows is
done by scattering surface sand about the whole area with
sweeping movements of the foreflippers. Finally, the turtle
turns toward the sea and crawls, somewhat less labori-
ously, down the beach, leaving behind another track that
terminates at the surf but leads the eye up the beach
toward the wide area of disturbed sand. The whole process
takes anywhere between 30 minutes to over an hour.
It’s been estimated that depending upon species, a female
will reach sexual maturity in 10–50 years. By marking
nesting females with external and internal
tags, we’ve learned that nesting will occur
from two to eight times in a season, at inter-
vals ranging between nine and 14 days.
Using satellite telemetry, we’ve also learned
that females arrive at nesting beaches from
feeding grounds that could be either adja-
cent to the beach or over hundreds of kilome-
ters away. Long journeys are accomplished
through the use of a spatial ‘map’ that
enables turtles to travel on precise, and
often direct, routes between these locations.
Having completed a seasonal nesting
cycle, a female may take from two to five
years to accumulate enough energy at the
feeding grounds to again support both her
migration to the nesting beach (where, typi-
cally, there’s no food) and the production of
hundreds of eggs over a season lasting a few
weeks. Long-term records for individual females show they
have reproductive life spans that can exceed 40 years.
Finally, we know from maternal genetics that females
nesting at specific rookery beaches are descendants of one
or a few ‘founders’ – turtles that were the first to initiate
nesting in the area many generations ago.
After incubation in the moist, sun-warmed sand for
about 50 days, hatchlings break through the eggshell, dig
their way almost to the surface and then wait for the sand
to cool (a signal that it’s night time). Suddenly, the turtles
will break through the surface and emerge, then scamper
en masse (like little wind-up toys) directly from the nest to
the ocean (Figure 2). Carried seaward by a retreating
wave, they will swim non-stop for 24–36 hours on a migra-
tion that takes them to ‘nursery’ areas located many miles
distant in the open sea.
This process of locating the ocean from the nest, or
seafinding, is accomplished visually. A small hatchling
crawling upon an uneven beach surface cannot directly see
the ocean from the nest; it must find its way using reliably
simple, but indirect, cues. Hatchlings instantaneously scan
180˚ wide areas close to the horizon, then crawl away from
scans that contain elevated, darker locations (the light-
absorbing dune and its covering vegetation behind the
beach) and toward scans with lower, flatter and, typically,
brighter locations (the light-reflecting view, seaward). This
response usually depends upon a simultaneous evaluation
of light intensity and detection of object detail, using form
vision. When turtles nest on continental shorelines, hatch-
lings probably use both cues; when nests are placed on
isolated low, flat islands lacking much vertical detail,
intensity cues are probably more important. Equipped
with these rudimentary essentials, the outcome is much
the same: hatchlings’ crawls seldom deviate more than
± 20˚ from a heading directly toward the sea (Figure 2).
Only one of every few thousand hatchlings will have the
unique combination of good genes and good luck to survive
the many decades of growth required to reach sexual matu-
rity. Those that do will join the small proportion of individ-
uals in the population that perpetuate the next generation.
Female survivors will choose nesting beaches geographi-
cally near to sites where, many years before, they emerged
from nests as hatchlings. How such memories are learned
and retained is one of many mysteries of marine turtle biol-
ogy. But what is certain is that this form of rapid learning
early in life (‘imprinting’) makes it possible for a female,
nesting for the first time, to avoid a prolonged search for an
appropriate nesting location. She places her eggs where
her ancestors completed the process successfully. Such an
Artificial night lighting and sea turtles
Figure 1. Anterior and posterior views of a nesting loggerhead.
Note that her body lies in a shallow pit that she dug earlier (upper).
Lower photo shows the egg chamber, with two eggs at the top of the
clutch just visible. Photos courtesy of J Wyneken.
Figure 2. Loggerhead hatchlings emerge (left). Their tracks, visible the morning after,
lead directly from the nest (foreground) to the ocean. Photos, courtesy of J Wyneken
and R Ernest.
Biologist (2003) 50 (4) 165
efficient strategy has probably served
marine turtles well during most of their
history, but it was never designed to antici-
pate how rapidly humans could alter habi-
tats. Within one sea turtle generation, a
once-ideal nesting beach can become a
coastal town, a major port, or even a city.
Thus, by the time a hatchling reaches matu-
rity and performs her first nesting migra-
tion, she may find a natal site that is no
longer ‘safe’ or ‘attractive’.
Artificial lighting and nesting
behaviour
Since the 1950s, the State of Florida has
been keeping records of nesting ‘activity’
along its coastline. Three species routinely
use the beach: the loggerhead (Caretta
caretta), green turtle (Chelonia mydas) and
leatherback (Dermochelys coriacea), with
the loggerhead most common. While nesting
is widespread, most of it is confined to the
south-eastern coast. Furthermore, all three
species nesting in Florida seem to prefer the
same beaches within this area, suggesting
that common selection pressures determine
their choice (Figure 3). These sites have
proximity to the Florida Current (western
portion of the Gulf Stream) in common, so close to shore
that it can be reached within a few days by a hatchling.
Since hatchlings are slow but strong swimmers, proximity
to favourable oceanic currents is one of many factors influ-
encing the choice of a nesting beach by females.
Currently preferred nesting sites are also locations
where coastal development is sparse. Why? There are
numerous possibilities. Less development means fewer
people on the beach at night to disturb wary females.
Alternatively, it could be that the association with human
development is accidental, and that at those sites sand
quality is better for egg development, or predators take
fewer hatchlings. In a simple experiment, one of my
colleagues, Blair Witherington, showed that absence of
artificial lighting was important. He used portable genera-
tors to illuminate a portion of two prime
nesting sites every few days: Melbourne
Beach in Florida, where loggerheads nested,
and Tortuguero in Costa Rica, a location
favoured by green turtles. When the lights
were on, nesting activity declined nearly to
zero; when they were off, the females
returned. Light quality was also important.
‘White’ light (containing both short and long
wavelengths) repelled the turtles, while
yellow light (composed of a single long wave-
length, visible to the turtles) did not.
At locations in Florida where beaches are
exposed to lower levels of artificial lighting,
nesting still occurs, though in lower num-
bers. Thus, the repelling effect is ‘dose
dependent’. We studied the distribution of
nests at such a location: a seven-kilometre-
long beach located in front of a small
Floridian city (Boca Raton). Half of the
beach is backed by tree-filled parks while
the remainder is backed by high rise condo-
miniums, unoccupied (and dark) during the
summer nesting season. To our surprise,
records over the years showed that more nesting occurred
in front of the condominiums than in front of the parks,
though surely the latter more closely resembled a natural
shoreline. We looked for reasons, both on land and under-
water, to explain why condominium sites were preferred.
The only consistent variable was the amount and pattern of
city lighting reaching the beach. At the condominium site,
the darkened buildings acted as light barriers that shaded
the beach, except at the spaces between buildings. The tur-
tles nested selectively in front of the buildings, avoiding the
illuminated gaps. Nest counts revealed that numbers in
front of each building were positively related to building
elevation (Figure 4). At the parks, trees also shadowed the
beach from inland urban lighting but less effectively, per-
haps because of their lower elevation and uneven density.
Artificial night lighting and sea turtles
80
26
28
5
Figure 3. Florida at night, showing areas of brightest irradiance (red). Right plot shows
latitude of the major nesting beaches along the southeastern coastline. Dashed arrow
indicates the approximate western margin of the Florida Current that carries hatch-
lings to their nursery areas. All three species nest at the darkest remaining beaches in
closest proximity to that current. Satellite photo courtesy of C Elvidge (NOAA).
Figure 4. ‘Condominium row’ at Boca Raton, Florida. Top: Elevation (in degrees above
the horizon) of the buildings. Below: Overhead view showing the buildings and distri-
bution of turtle nests (black dots) on the beach. Nests are clustered in front of the build-
ings, with more nests in front of the tallest structures (from Salmon et al., 1995).
166 Biologist (2003) 50 (4)
Even so, nest ‘densities’ per length of city
park beach exceeded those at locations adja-
cent to Boca Raton where the dune had been
flattened and cleared, and where beaches
were directly exposed to lighting.
What can we conclude from these find-
ings? We know from other studies that
many variables are correlated with
preferred rookery sites. Typically, sites are
remote, exposed to relatively low wave ener-
gies (and, therefore, weaker erosion that
could destroy nests), located near
favourable oceanic currents, and charac-
terised as well by the absence of large
terrestrial predators that could take
females. Before humans (and their lighting)
became a presence, all potential beach sites
were dark and so contrasts in ‘darkness’
probably played no role in the selection
process. Now, in what must represent only
‘seconds’ in a long marine turtle history,
coastal lighting has become an important
intruding variable that is likely to compro-
mise site selection based upon cues with
‘proven’ survival value.
At locations like Florida, where the pace
of coastal development has been exponen-
tial, the consequences of this trend can
easily be predicted: more nests will be
concentrated in a decreasing area of dark
beaches. The spatial concentration of nests
is known, elsewhere in the world, to attract
both terrestrial and marine hatchling
predators, and to increase hatchling mortal-
ity rates. Spatial concentration of nests has
other negative effects including: destruction
of previously deposited nests by females
that nest later; microbial blooms in sands
with too many left-over dead eggs; and
increasing probabilities that chance events
(such as local storms or a hurricane land-
fall) will destroy a large proportion of the
annual allotment of nests.
Artificial lighting and seafinding
Hatchlings that emerge from nests exposed
to even a few luminaires often fail to locate
the sea. What happens is documented on
the beach surface by their flipperprints
(Figure 5). Instead of tracks leading directly to the sea,
turtles leave evidence that they crawled for hours on
circuitous paths (‘disorientation’), or on direct paths away
from the ocean and toward lighting (‘misorientation’). In
Florida, thousands of these hatchlings die annually from
exhaustion, encounters with terrestrial predators, entan-
glement in dune vegetation, dehydration after sunrise, or
even crushing by cars as turtles traverse coastal roadways.
We don’t know what physiological changes are respon-
sible for the breakdown in normal orientation behaviour,
either in hatchling sea turtles or in the many other
nocturnally active species whose ‘orientation systems’ are
similarly disrupted by exposure to artificial lighting.
Disorientation may signal either an inability to perceive
natural cues, or a competition between natural cues and
artificial lighting that can’t be resolved. Misorientation
suggests another consequence: directional cues are
received but they represent ‘misinformation’, which
directs organisms toward goals that promote death
rather than survival.
We do, however, now understand how natural and
artificial lighting differ as visual stimuli (Table 1); these
differences form the basis for hypotheses that may
explain why normal orientation behaviour fails with
artificial lighting. The contrasts between natural and
artificial light were effectively highlighted in papers
published by Verheijen (1985). Verheijen was critical of
contemporary attempts to classify visual orientation by
animals. His criticisms were prompted, firstly, by the
simplified visual environments used and, secondly,
because the responses they evoked (‘forced movements’
either toward or away from lamps) were not representa-
tive of those shown by animals in nature, especially those
with image-forming eyes.
For example, in many animals, orientation involves
choosing a biologically appropriate direction away from
Artificial night lighting and sea turtles
Ocean
(1) (2)
(3) (4)
a = 97°
r = 0.96 a = 85°
r = 0.84
a = 74°
r = 0.53
a = 143°
r = 0.07
Centre
Figure 5. Use of ‘arena assays’ to document the effect of artificial lighting on hatchling
orientation. These assays are staged emergences at locations varying in exposure to
artificial lighting. Turtles are collected just prior to a natural emergence (upper left),
then transported to another beach. They are released in the centre of a four-metre-
diameter circle, drawn on the beach surface at a location where nests are typically
deposited. As it crawls away, each turtle leaves a track in the sand. Its orientation is
measured by the angle between the arena centre and the arena boundary exit point
(arrows, upper right).
Line drawings below show tracks at four Boca Raton sites, ranging from darkest (1) to
most lighting-exposed (4). The ocean is East (~ 90˚). Circular statistics are used to
determine ‘a’, the mean angle of orientation for all of the turtles in that group, and ‘r’
(r-vector), a measure of their angular dispersion, which ranges between 0 and 1. Both
indicate whether artificial lighting disrupts orientation. At dark beaches, the mean
angle is typically ± 20˚ of the seaward direction, and the r-vector is ≥0.9. Note that at
sites exposed to more lighting, ‘a’ and ‘r’ depart increasingly from those values. In this
example, artificial lighting causes disorientation (modified from Salmon et al., 1995).
Biologist (2003) 50 (4) 167
one habitat and toward another – for reasons that promote
either short- or long-term survival. The visual cues used to
govern these movements are often located at some angle
relative to the body, not directly ahead or behind. In solar
and lunar orientation, which is commonly used for these
purposes, animals choose a constant direction (away from
habitat A and towards habitat B) and compensate (using
their time sense) for the constantly changing azimuth of
the celestial reference. Thus, argued Verheijen, a proper
understanding of visual orientation could not be gained by
analysis of movements ‘toward or away’. Neither could the
properties of animal perceptual systems (visual receptors
and neural connections to information-analysing brain
areas) that control orientation – in this case relative to
objects that change in elevation, colour, brightness and
even apparent size as they ‘travel’ across the sky.
Perceptual systems, he insisted, were designed by evolu-
tion to process natural distributions of light stimuli. When
presented with experimentally simplified distributions
(typically, a single luminaire in an otherwise dark
surround), the result was pathological behaviour such as
disorientation and misorientation, observed not only in
hatchling sea turtles but many animals under these test-
ing conditions. In 1985, Verheijen proposed the term
‘photopollution’ to describe ‘... degradation of the photic
habitat by artificial light’.
There are several differences between natural and arti-
ficial light (Table 1), but most of them lead to a common
result: excessive ‘directivity’ (greater brightness in one
direction, toward the luminaire, than in all other, back-
ground, directions). If directivity caused abnormal behav-
iour, then an increase in background illumination should
reduce the directivity of luminaries as well as the patho-
logical behaviour that they cause. Verheijen reported that
just such an effect was well documented (but previously
unexplained) in the wildlife literature. Many night-
migrating birds (that fly en route, by the thousands, into
lighted towers, lighthouses, or other illuminated struc-
tures) and countless nocturnal insects (that similarly
aggregate at lights) are injured or killed annually. But
the incidence of injury or death in birds and insects
declines under full moon illumination. Witherington and
I found much the same pattern on Florida beaches.
Reports of hatchling orientation problems state-wide
reached their peak during the days surrounding new
moon, but declined to almost zero during the evenings
when a full moon was present.
Solving the ‘photopollution’ problem
Beaches in Florida are exposed to lighting because, until
recently, nobody realised that improperly designed or
placed lighting fixtures caused a problem. They do.
Instead of focusing light where it is intended (generally,
downward), it is scattered in all directions including
upward, where it serves no useful purpose (except as a
tool to demographers who estimate the growth of cities by
night-satellite photography!). In the US alone, it’s esti-
mated that 30% of all outdoor lighting is wasted by illu-
minating the atmosphere, at an estimated cost of $1.5 bil-
lion in wasted electricity (and six million tons of burned
coal used to generate that energy). Reducing or eliminat-
ing this waste not only benefits nocturnal wildlife, but
human health and treasury.
Strategies required for effective light management
almost anywhere are intuitively obvious. (1) Turn off
unnecessary lights. (2) Reduce luminaire wattage to the
minimum required for function. (3) Redirect and focus
lighting so it only reaches the ground, or those areas (e.g.,
signage, parking lots, streets) where it is intended. Such
control is achieved through the use of properly shielded
fixtures that redirect lighting, or the addition of appropri-
ate shielding to luminaires that scatter lighting. (4)
Eliminate all upward-directed decorative lighting. (5) Use
alternative light sources where possible and practical.
These include luminaires that emit restricted subsets of
(longer) light wavelengths, which are less disruptive to
most wildlife, or those that carry out their function not by
brightening areas, but rather by directing humans or
human traffic in specific directions (‘chains’ of light-emit-
ting diodes in walkways, along trails, or embedded in
roadways). (6) In any new construction, incorporate the
latest light management technology so that continued
growth and expansion leads to no increase in the impact
of artificial lighting. The summed effect of these modifica-
tions is not only energy conservation, but also night light-
ing that is optimally functional for humans. Indeed, the
aim is not to eliminate lighting but rather to reduce its
unintended impact.
Most coastal counties in Florida have passed lighting
ordinances or laws that regulate and restrict lighting prac-
tices adjacent to sea turtle nesting beaches. Enforcement is
stricter in some counties than in others, but the very
passage of these laws indicates a growing public awareness
that marine turtles are exceptional creatures whose
continued existence has intrinsic value, despite the costs.
Many of Florida’s beaches are getting darker, and numbers
of turtles nesting annually are slowly, but significantly, on
the rise. This positive outcome must, however, be tempered
by the knowledge that the threat of artificial lighting is not
going away in Florida, but simply changing. In a few years,
Artificial night lighting and sea turtles
Table 1. Differences between artificial and natural lighting
(modified from Verheijen, 1985).
Source Artificial lighting comes from nearby luminaires;
natural lighting comes from distant celestial objects
(sun, moon, or stars).
Scattering Because luminaires are nearby, there is little scatter-
ing or reflection before their light is detected; natural
light is scattered by the atmosphere before it reaches
an observer.
Reflection Artificial light appears bright because of its proximity,
but fades rapidly with distance, where there is little
energy left to reflect. Natural light is everywhere and
is abundantly reflected by both distant and nearby
objects.
Directivity Artificial sources radiate light from one direction (the
source) but not from other directions (high directivity).
Brightness toward the source greatly exceeds bright-
ness measured from elsewhere. Natural light illumi-
nates and reflects from many objects. Its brightness
differences, as a function of direction, are much less
extreme (low directivity).
Direction Artificial light sources can be positioned anywhere
(above, below, to the side) relative to an observer.
Natural light sources are above, and reliably indicate
downward. Exposed to the former, body orientation of
flying or swimming animals may be abnormal,
whereas, under natural light, orientation is usually
normal.
168 Biologist (2003) 50 (4)
the most important lighting issues affecting turtles may
not be from luminaires at the beach, but from those placed
inland. Light pollution from those sources (such as shop-
ping centres, open-air sports arenas and car dealerships)
produces sky glow, which also disrupts the orientation of
hatchlings. But perhaps, by then, we will have adopted a
national lighting policy so that not only coastal marine life,
but all wildlife, can benefit.
Further Reading
Salmon M, Reiners R, Lavin C and Wyneken J (1995) Behavior of
loggerhead sea turtles on an urban beach. I. Correlates of nest
placement. Journal of Herpetology, 29, 560–567.
Salmon M and Witherington B E (1995) Artificial lighting and
seafinding by loggerhead hatchlings: evidence for lunar modu-
lation. Copeia, 1995, 931–938.
Verheijen F J (1985) Photopollution: Artificial light optic spatial
control systems fail to cope with. Incidents, causations, reme-
dies. Experimental Biology, 44, 1–18.
Witherington B E (1997) The problem of photopollution for sea
turtles and other nocturnal animals. In: Behavioral Approaches
to Conservation in the Wild. Clemmons J R and Buchholz R
(Eds). Cambridge University Press.
Websites
www.urbanwildlands.org/nightlightbiblio.html
This private environmental advocacy group recently sponsored a
meeting of experts entitled Ecological Consequences of Artificial Night
Lighting. See their web site for meeting abstracts; these will soon be
published in book form.
www.darksky.org.html
The International Dark-Sky Association was formed by a few
astronomers concerned about the impact of night lighting on their
ability to view the heavens. Its much larger membership now consists
both of scientists and citizens who actively promote light management
worldwide. It is an excellent source of information about lighting
issues that includes packages for public education, bibliographies,
meeting announcements, legal issues, lighting technology and
legislation.
Michael Salmon is a Professor of Biology with research interests
in animal behaviour and the conservation of sea turtles.
Department of Biological Sciences,
Florida Atlantic University
Boca Raton, Florida 33431-0991, USA
Salmon@fau.edu
Artificial night lighting and sea turtles
