Competitive interactions between artificial lighting and natural cues during seafinding by hatchling marine turtles

Abstract

Artificial lighting disrupts the nocturnal orientation of sea turtle hatchlings as they crawl from their nest to the ocean. Laboratory experiments in an arena were used to simultaneously present artificial light (that attracted the turtles toward “land”) and natural cues (a dark silhouette of the dune behind the beach) that promoted “seaward” orientation. Artificial lighting disrupted seaward crawling in the presence of low silhouettes, but not high silhouettes. Low silhouettes provided adequate cues for seaward crawling when the apparent brightness of artificial light was reduced. Based upon these results, we postulate that artificial light disrupts orientation by competing with natural cues. Current restoration practices at nesting beaches emphasize light reduction. However at many sites some lights cannot be modified. Our results suggest that pairing dune restoration (to enhance natural cues) with light reduction (to the extent possible) should significantly improve hatchling orientation, even at nesting beaches where lighting cannot be entirely eliminated.

Introduction

Hatchling marine turtles emerge from underground nests at night and, within minutes, crawl to the sea. This “seafinding” is directed by two visual cues; light intensity and horizon elevation. Light intensity is an orientation cue mediated by a positive phototaxis (Daniel and Smith, 1947; Mrosovsky, 1972; Verheijen and Wildschut, 1973). Light is reflected from the ocean and absorbed by vegetation behind the beach. Hatchlings crawl away from the dimmer landward horizon and crawl toward the brighter seaward horizon (Mrosovsky, 1967; Mrosovsky and Shettleworth, 1968; van Rhijn and van Gorkom, 1983). Horizon elevation is a second cue (Limpus, 1971; Witherington, 1992a; Salmon et al., 1992). Turtles crawl away from a higher dune and its associated shrubbery, and toward the lower, oceanic horizon (Parker, 1915; Mrosovsky and Shettleworth, 1968; van Rhijn and van Gorkom, 1983).

Hatchling orientation is often abnormal when beaches are exposed to artificial lighting (Verheijen, 1985). Instead of moving toward the sea, turtles may crawl on circuitous paths (“disorientation”), or they may crawl landward, apparently attracted to the lights (“misorientation”; Witherington and Martin, 1996). On Florida's nesting beaches, artificial lighting (hereafter, “lighting”) poses a threat to the survival of marine turtles. Thousand of hatchlings that fail to locate the sea perish annually as a consequence of exhaustion, dehydration, or capture by predators (Witherington and Martin, 1996).

Abnormalities in seafinding are positively correlated with luminaire “directivity”: the contrast in irradiance between light sources and background (Verheijen, 1958). In Florida, habitat restoration is accomplished through “light management” that reduces directivity by turning off unnecessary lights, reducing wattage, and lowering and/or shielding luminaires (Witherington and Martin, 1996). Management reduces the incidence of abnormal hatchling orientation and its consequences. However, the procedure is most effective where development is modest and few lights are located near the beach.

At sites where development is more extensive (many lights, some located inland), it is impossible to modify all of the luminaires. An alternative strategy at such sites might be light management where possible, coupled with dune restoration to increase horizon elevation. This strategy assumes that hatchlings will respond favorably to enhanced natural cues, even when substantial lighting remains. That hypothesis has not been formally tested.

On the basis of preliminary observations, we hypothesized that hatchling behavior at illuminated beaches might be the outcome of interactions between lighting and natural orientation cues (the “cue competition” hypothesis). We predicted that the three responses resulting from such a comparison (seafinding, disorientation, or misorientation) might grade, one into another, depending upon different perceived ratios of the “competing” stimuli. If true, then increasing horizon elevation, reducing light directivity, or combining both manipulations might improve seafinding accuracy. To test this hypothesis, we performed experiments in an arena where we could change horizon elevation and lighting directivity while hatchlings were exposed to simulated street lighting.