http://www.swflorida.com/turtletime/lighting/intro.htm

Introduction

     Light from artificial sources differs markedly from other pollutants both in its form -- light is energy rather than substance -- and in its effect on sea turtles. Whereas heavy metal, petroleum, and other chemical pollutants produce predominately physical or physiological effects, the effect that light pollution has on sea turtles is essentially psychological. For sea turtles, artificial light is best described not as a toxic material but as misinformation. With its great potential to disrupt behaviors that rely on correct information, artificial lighting can have profound effects on sea turtle survival. Critical sea turtle behaviors affected by light pollution include the selection of nesting sites by adult turtles and the movement off the beach by hatchlings and adults.

http://www.swflorida.com/turtletime/lighting.htm

Executive Summary

     Although there is a tendency for turtles to prefer dark beaches, many do nest on lighted shores, but in doing so, the lives of their hatchlings are jeopardized. This threat comes from the way that artificial lighting disrupts a critical nocturnal behavior of hatchlings -- crawling from their nest to the sea. On naturally lighted beaches, hatchlings escaping from nests show an immediate and well-directed orientation toward the water. This robust sea-finding behavior is innate and is guided by light cues that include brightness, shape, and in some species, color. On artificially lighted beaches, hatchlings become misdirected by light sources, leaving them unable to find the water and likely to incur high mortality from dehydration and predators. Hatchlings become misdirected because of their tendency to move in the brightest direction, especially when the brightness of one direction is overwhelmingly greater than the brightness of other directions, conditions that are commonly created by artificial light sources. Artificial lighting on beaches is strongly attractive to hatchlings and can cause hatchlings to move in the wrong direction (misorientation) as well as interfere with their ability to orient in a constant direction (disorientation).
     Understanding how sea turtles interpret light cues to choose nesting sites and to locate the sea in a variably lighted world has helped conservationists develop ways to identify and minimize problems caused by light pollution. Part of this understanding is of the complexity of lighting conditions on nesting beaches and of the difficulty of measuring light pollution with instrumentation. Thankfully, accurately quantifying light pollution is not necessary to diagnose a potential problem. We offer this simple rule: if light from an artificial source is visible to a person standing anywhere on a beach, then that light is likely to cause problems for the sea turtles that nest there.

NOTE:  This report was written in 1996, taking instrumental measurements of light levels is what we are trying to work on .... KA Duhring, a consultant for Volusia County Department of Environmental Services, gave us the suggestion to work on a light meter that would help both county officials and hotel owners measure the lighting conditions on the beach .... our concentration is on loggerhead turtles, they make up over 90% of the nesting turtles on our beaches ....

     Because there is no single, measurable level of artificial brightness on nesting beaches that is acceptable for sea turtle conservation, the most effective conservation strategy is to simply use "best available technology" (BAT: a common strategy for reducing other forms of pollution by using the best of the pollution-reduction technologies available) to reduce effects from lighting as much as practicable. Best available technology includes many light-management options that have been used by lighting engineers for decades and others that are unique to protecting sea turtles. To protect sea turtles, light sources can simply be turned off or they can be minimized in number and wattage, repositioned behind structures, shielded, redirected, lowered, or recessed so that their light does not reach the beach. To ensure that lights are on only when needed, timers and motion-detector switches can be installed. Interior lighting can be reduced by moving lamps away from windows, drawing blinds after dark, and tinting windows. To protect sea turtles, artificial lighting need not be prohibited if it can be properly managed. Light is properly managed if it cannot be seen from the beach.
     Best available technology also includes light sources that emit a color of light that has minimal effects on sea turtles. Light sources emitting low levels of short-wavelength light -- sources that appear deep red or yellow -- affect both hatchlings and nesting adults less than do sources emitting higher levels of short-wavelength light -- sources that appear whitish or any color other than deep red or yellow. Low-pressure sodium-vapor luminaires are pure yellow sources that make good substitutes for more disruptive lighting near sea turtle nesting beaches. Yellow-tinted incandescent "bug-light" bulbs are not as pure a yellow source but can be an acceptable substitute.

A Model for Measuring Brightness

     Researchers have learned much about sea turtles' perception of brightness by using a procedure called electroretinography (ERG) to measure the relative electrical potential across retinas of turtles exposed to different wavelengths of light. ERG data show that green turtles are most sensitive to light in the violet to orange region of the visible spectrum, from 400 to 640 nm (Figure 4; Granda and O'Shea, 1972). In daylight, green turtles show a greater spectral sensitivity within the shorter-wavelength (blue) region of the spectrum than humans do.

     Although ERG data provide important physiological information, the most direct way to determine the effects of spectral light on orientation is to conduct behavioral experiments. The earliest studies on hatchlings' responses to light wavelength employed broad-band (multiple-wavelength-transmission) filters to vary the wavelengths that reached orienting hatchlings (Mrosovsky and Carr, 1967; Mrosovsky and Shettleworth, 1968). Although reactions to specific wavelengths could not be determined, it was clear that the green turtle hatchlings studied were more attracted to blue light than to red light.

     In later experiments, researchers used narrow-band (monochromatic) filters to vary the wavelengths reaching loggerhead, green turtle, hawksbill, and olive ridley hatchlings (Witherington and Bjorndal, 1991a; Witherington, 1992b). The use of monochromatic filters allowed a simple measure of light intensity so that researchers could determine the responses of hatchlings to a set number of photons at each of several wavelengths. As in previous experiments, hatchlings showed a preference for short-wavelength light. Green turtles, hawksbills, and olive ridleys were most strongly attracted to light in the near-ultraviolet to yellow region of the spectrum and were weakly attracted or indifferent to orange and red light (Figure 5).

Loggerheads were most strongly attracted to light in the near-ultraviolet to green region and showed an unexpected response to light in the yellow region of the spectrum. At intensities of yellow light comparable to a full moon or a dawn sky, loggerhead hatchlings showed an aversion response to yellow light sources (Figure 5), but at low, nighttime intensities, loggerheads were weakly attracted to yellow light (Figure 6). It may be that the hatchlings cannot discriminate color at low light levels. This is common for animals (such as turtles) that have rod-and-cone retinas (Granda and Dvorak, 1977).

     It should come as no surprise that humans and sea turtle hatchlings see the world differently. For most of their lives, sea turtles see the world through a blue ocean filter (water selectively absorbs reddish, long-wavelength light), so it makes sense that sea turtles would be most sensitive to short-wavelength light.
     Because sea turtle hatchlings respond to light that we cannot see (ultraviolet light) and are only weakly sensitive to light that we see well (red light), instruments that quantify light from a human perspective (such as most light meters) cannot accurately gauge brightness from the perspective of a sea turtle. Humans also cannot assess color exactly as a sea turtle would. Although we can see colors, we cannot tell what assortment of wavelengths may make up those colors. For example, a light source emitting both 525-nm (green) and 645-nm (red) light, a source highly attractive to hatchlings, appears to a human observer to emit yellow light comparable to a 588-nm monochromatic source, which would be only weakly attractive to hatchlings (Rossotti, 1983).
     Directional properties of the brightness detector. -- Just as a hatchling's detector has a sensitivity to specific light wavelengths, it is also sensitive to light direction. The directional properties of a detector determine how much of the world the detector measures at any one instant. These properties are described by a specific "cone of acceptance" or by bidimensional (horizontal and vertical) "angles of acceptance." The height and breadth of a detector's acceptance cone critically influences brightness measurements and the determination of brightest direction (Figure 7). This conceptual acceptance cone may be only a portion of a turtle's complete field of view.

     The horizontal component of the acceptance cone for green turtle and olive ridley hatchlings (Verheijen and Wildschut, 1973) and for loggerhead hatchlings (Witherington, 1992b) has been deduced from the way that hatchlings orient in controlled light fields. In these studies, light fields were artificially controlled so that detectors with different acceptance-cone widths measured different brightest directions. Hatchlings of each species typically oriented in the brightest direction as it would be measured with a wide acceptance cone, approximately 180� horizontally.
     To determine the vertical component of the acceptance cone, the researchers cited above measured the orientation of hatchlings presented light sources that were positioned at various vertical angles. The angular height of this vertical component was approximated to be "a few degrees" for green turtles and olive ridleys (Verheijen and Wildschut, 1973) and between 10� below and 30� above the horizon for loggerheads (Salmon and Wyneken, 1990; Witherington, 1992b). Although the measures are approximate, it is clear that light closest to the horizon plays the greatest role in determining orientation direction.
     The detector model for hatchling orientation predicts that hatchlings measure brightest direction by integrating the light they detect over a broad and flat acceptance cone (Figure 8).

Again, we see that the attributes of this hypothetical detector differ from those of most light meters. The most commonly found light meters, illuminance meters, measure light with an acceptance cone that is less flattened and not as wide as the acceptance cone that hatchlings use. Another type of light meter, a luminance or "spot" meter, measures light with a very narrow acceptance cone. Careful consideration should be given to the directional attributes of a light-measuring instrument if its measurements are to be used in predicting hatchling behavior.

Cathy [e-mail link]