Sea-finding Behaviour Research Articles

The Effect of Light Pollution on the Sea Finding Behavior of Green Turtle Hatchlings on Lanyu Island, Taiwan.

octurnal animals use their vision and acute hearing to adapt to the nighttime environment. Light pollution has become a serious problem for nocturnal animals in coastal areas, especially nesting sea turtles and sea turtle hatchlings. Hatchlings use visual clues to find the ocean. However, when the artificial light is stronger than the natural light, hatchlings become either misoriented, disoriented or both. Due to rapid tourism development on Lanyu Island, new sources of light pollution, especially streetlights, pose a serious threat to sea turtle hatchlings. In this study, we used a portable lamp constructed by Liteon Inc. on a circular area of a turtle nesting beach to see how artificial light sources could affect green turtle hatchlings' sea finding behavior. In the experiments, we tested hatchling behavior under different lamp settings (strong or weak light intensity; white or yellow light; lamp shield presence or absence) and moon visibilities (moonlit or moonless). The hatchlings' crawling tracks and locations at the end of the trials were recorded. Results showed that the light intensity had no effect on hatchling sea finding behavior. White light had a stronger impact on hatchling sea finding behavior than yellow light. When the lamp shield was installed on moonlit nights, more hatchings were able to find the sea under both white and yellow lights. Thus, it is recommended that light shields be installed on the streetlights of Lanyu Island in order to protect the sea turtle hatchlings effectively.

Differences in visual perception are correlated with variation in sea-finding behaviour between hatchling leatherback, Dermochelys coriacea, and loggerhead, Caretta caretta, marine turtles

After completing embryonic development, marine turtle hatchlings emerge from their subsurface nest, generally at night, and crawl to the ocean (‘sea finding’). That response depends upon the ability of the turtles to discriminate between the brighter seaward versus a dimmer landward horizon, followed by a positive phototaxis. While the crawls of most marine turtle hatchlings are well oriented and straight, those of leatherback hatchlings are sometimes interrupted by bouts of circling. We conducted experiments comparing the orientation and crawling behaviour of leatherbacks to those of loggerhead hatchlings to determine why those differences occur. The two species did not differ in the light spectra attracting the hatchlings, but leatherback thresholds for detection and for intensity discrimination were significantly higher than those of loggerheads. At the nesting beach, loggerheads under full (bright) or new (darker) moon conditions crawled straight to the ocean; circling rarely occurred. Leatherback crawls under a full moon were indistinguishable from those of loggerheads, but during new moon trials, when horizon brightness differences approached leatherback intensity discrimination thresholds, circling increased significantly. We conclude that circling is probably used by leatherbacks to reinforce orientation decisions when horizon cues become more difficult to discern. Circling could be costly as it lengthens the crawl and increases exposure to terrestrial predators. We hypothesize that those costs persist because other visual adaptations affecting sensitivity enhance the ability of leatherbacks to detect prey, mates or favourable habitats in an open ocean environment. • We compared leatherback and loggerhead hatchlings' phototaxis responses. • Leatherbacks were less sensitive to wavelengths of 340–580 nm than loggerheads. • Under full moon, both species crawled directly from the nest to the sea. • Under new moon, leatherbacks were less accurate; loggerheads remained unchanged. • These responses may reflect sea-finding orientation–foraging efficiency trade-offs.

Light pollution affects nesting behavior of loggerhead turtles and predation risk of nests and hatchlings

The introduction of artificial light into wildlife habitats is a rapidly expanding aspect of global change, which has many negative impacts on a wide range of taxa. In this experimental study, which took place on a beach located on the island of Boa Vista (Cabo Verde), three types of artificial light were tested on nesting loggerhead sea turtles as well as on ghost crabs, which intensively predate on nests and hatchlings, to determine the effects they would produce on the behavior of both species. Over the course of 36days, female loggerheads and ghost crabs were studied under yellow, orange and red lights, with observations also being made on dark nights that served as a control treatment. During this period, the frequencies of nesting attempts, the time taken by turtles to complete each phase of the nesting process, and ghost crab abundance and behaviors were carefully recorded. 1146 loggerhead nesting attempts were observed and recorded during the experiments, and results showed a decrease in nesting attempts of at least 20% when artificial lighting was present. A significant decline in successful attempts was also observed within the central sections of the beach, which corresponded to those that received more light. This artificial lighting significantly increased the time that turtles spent on the nesting process and forced them to do more extensive beach crawls. Despite this, the presence of light had no apparent effect on the final selection of the nesting site. Yellow and orange lights significantly disrupted the sea finding behavior and turtles were often unable to orient themselves seaward under these color lights. Disoriented turtles were observed crawling in circuitous paths in front of the light source for several minutes. In addition, artificial lights had the potential to increase the number of ghost crabs present within the illuminated stretches of the beach. However, only yellow lighting produced a significant change on aggressive and prey searching behaviors. These changes in abundance and behavior could cause a greater predation on loggerhead turtle nests. Red light had no significant impact on the behavior of either species. It should be a priority to enforce preventive measures and light mitigation strategies to ensure the conservation of important loggerhead rookeries.

The effect of on-shore light pollution on sea-turtle hatchlings commencing their off-shore swim

Context Off-shore recruitment impairment of sea-turtle hatchlings because of light pollution is a growing concern to conservation of sea-turtle population throughout the world. Studies have focussed on sea-turtle hatchling sea-finding behaviour, and ignored the possible effect that on-shore lighting might have on hatchlings after they have entered the sea. Aims We experimentally evaluated the effect that on-shore light pollution has on the swimming behaviour of green turtle hatchlings once they have entered the sea and begun swimming off-shore. We also estimated the decrease in off-shore recruitment of hatchlings as a result of light pollution disruption of the off-shore swim. Methods Hatchling misorientation rates were quantified by releasing marked hatchlings to the sea from different land-based locations adjacent to light-polluted beach areas under a variety of environmental conditions. The beach in light-polluted regions was then searched for marked hatchlings returning to shore from the sea. Key results Misorientation rates were highest in trials conducted during moonless nights (66.7% of trials had some hatchlings return to shore) and lowest during trials conducted during moonlit nights (no trials had hatchlings return to shore). Green turtle hatchling off-shore recruitment for the entire 2014–15 nesting season at Heron Island was estimated to decrease 1.0 –2.4% as a result of on-shore lights disrupting hatchling off-shore swimming behaviour. Conclusions On moonless nights, sea-turtle hatchlings after having successfully completed their journey from nest to sea and entered the sea can be lured back to shore again by shore-based light pollution and, this will decrease their off-shore recruitment success. Implications To ensure maximum off-shore recruitment of sea-turtle hatchlings, on-shore light pollution adjacent to nesting beaches needs to be minimised so as to minimise misorientation and disorientation of hatchlings while on the beach and in near-shore waters.

An assessment of ‘turtle-friendly' lights on the sea-finding behaviour of loggerhead turtle hatchlings (Caretta caretta)

Context It is well established that artificial light can disrupt the sea-finding ability of sea turtle hatchlings, and some manufactures are now marketing ‘turtle-friendly’ lights that are supposed to be minimally disruptive to this sea-finding behaviour. However, there have been no studies that have tested whether ‘turtle-friendly’ lights are benign to hatchling sea turtle sea-finding ability. Aims We tested two different types of ‘turtle-friendly’ lights (LED amber-light peak intensity 620 nm and LED red-light peak intensity 640 nm) to see whether they are disruptive to the sea-finding ability of eastern-coast Australian loggerhead turtle hatchlings. Methods Using standard circular-arena experiments, we assessed the directional preference of newly emerged loggerhead turtle hatchlings from the Woongarra Coast of Queensland, Australia, during different moon phases without artificial lighting and in the presence of ‘turtle-friendly’ lights. Key results Contrary to expectations, sea-finding ability of hatchlings was disrupted by the amber lights, particularly in the absence of a moon. The less intense red lights were less disruptive to hatchlings; however, misorientation and disorientation events still occurred when lights were within 4 m of hatchlings. The disruptive impact on sea-finding ability increased with the cumulative impact of multiple lights increasing light intensity. Conclusions The ‘turtle-friendly’ lights we used disrupted the sea-finding ability of eastern-coast Australian loggerhead turtle hatchlings, with the most pronounced disruption occurring under moonless conditions. Implications The use of amber and red LED lights adjacent to the nesting beaches of loggerhead sea turtles should be managed because this lighting has the potential to disrupt the sea-finding ability of sea turtle hatchlings.

Influence of horizon elevation on the sea-finding behaviour of hatchling flatback turtles exposed to artificial light glow

Influence of horizon elevation on the sea-finding behaviour of hatchling flatback turtles exposed to artificial light glow

Influence of industrial light pollution on the sea-finding behaviour of flatback turtle hatchlings

Context Numerous studies show that artificial light disrupts the sea-finding ability of marine turtle hatchlings. Yet very little has been published regarding sea-finding for flatback turtles. Given the current industrialisation of Australia’s coastline, and the large potential for disruption posed by industrial light, this study is a timely investigation into sea-finding behaviour of flatback turtle hatchlings. Aims We investigate sea-finding by flatback turtle hatchlings in relation to ambient light present in areas of planned or ongoing industrial development, and evaluate the fan and arena-based methods that are frequently used for quantifying hatchling dispersion. Methods Using a combination of methods, we assessed the angular range and directional preference of sea-finding hatchlings at two key flatback turtle rookeries, Peak and Curtis Islands, during January–February 2012 and 2013, and at Curtis Island in January 2014. Relative light levels at each site were measured using an Optec SSP-3 stellar photometer, and moon phase, moon stage and cloud cover were also recorded. Key results We found no evidence of impaired hatchling orientation, and observed very low levels of light at Peak Island. However, at Curtis Island, hatchlings displayed reduced sea-finding ability, with light horizons from the direction of nearby industry significantly brighter than from other directions. The sea-finding disruption observed at Curtis Island was less pronounced in the presence of moonlight. Conclusions The reduced sea-finding ability of Curtis Island hatchlings was likely due to both altered light horizons from nearby industry, as well as beach topography. Both methods of assessing hatchling orientation have benefits and limitations. We suggest that fan-based methods, combined with strategically placed arenas, would provide the best data for accurately assessing hatchling sea-finding. Implications Sky glow produced by large-scale industrial development appears detrimental to sea-finding by flatback turtle hatchlings. As development continues around Australia’s coastline, we strongly recommend continued monitoring of lighting impacts at adjacent turtle nesting beaches. We also advise rigorous management of industrial lighting, which considers cumulative light levels in regions of multiple light producers, as well as moon phase, moon-stage, cloud cover and time of hatchling emergence. All these factors affect the likelihood of disrupted hatchling sea-finding behaviour at nesting beaches exposed to artificial light-glow, industrial or otherwise.

Artificial lighting and disrupted sea-finding behaviour in hatchling loggerhead turtles (Caretta caretta) on the Woongarra coast, south-east Queensland, Australia

Coastal development adjacent to sea turtle nesting beaches can result in an increase in exposure to artificial lighting at night. That lighting can repel nesting females and interfere with the orientation of hatchlings from the nest to the sea. Disrupted hatchling orientation is a serious source of turtle mortality, sufficient to reduce recruitment and contribute to a long-term marine turtle population decline. The purpose of this study was to assess whether artificial lighting disrupts hatchling sea-finding behaviour at the largest loggerhead rookery in the South Pacific, the Woongarra coast, south-east Queensland. The crawling tracks of hatchlings that emerged from nests, as well as staged emergences, were used to assess the effect of lighting conditions at several local beaches on hatchling sea-finding behaviour. Disrupted orientation was observed at only a few locations, excluding the majority of the main nesting beach at Mon Repos Conservation Park. At the sites where orientation was disrupted, normal orientation was restored when a full moon was visible, presumably because lunar illumination reduced the perceived brightness of the artificial lights. The controlled use of lights used for guided turtle-viewing tour groups within Mon Repos conservation Park did not interfere with the sea-finding behaviour of hatchling turtles. Further coastal development, especially at the nearby town of Bargara, requires that a light management plan be formulated to ensure that development does not adversely affect the marine turtles that utilise the local nesting beaches.

Ocean-finding in marine turtles: the importance of low horizon elevation as an orientation cue

Sea finding behaviour in hatchling sea turtles is widely believed to be guided by orientation towards the brightest horizon and away from high silhouettes. We propose that the horizon profile at the lowest angle of elevation is a more important cue for ocean finding than light intensity. Examples of hatchling orientation occurring at natural nest sites, at preselected beach release sites, and within a walled wooden arena under various conditions in the field, are presented. We conclude that hatchlings orient as follows: (1) Hatchlings move towards the horizon line at the lowest angle of elevation; (2) Hatchlings move away from high silhouettes, in a direction that remains close to the horizon line at the lowest angle of elevation; (3) Should a conflict exist regarding the direction of lowest horizon elevation, hatchlings move towards the brightest lowest horizon. This refinement to current theory explains numerous instances of natural non-ocean-finding behaviour in marine turtles, which occur both by day and by night, and in the absence of artificial lighting. We propose that the disruption caused to hatchling orientation by artificial lighting close to nesting beaches occurs because bright lights mask the horizon line as a cue by making it impossible to discern, causing the hatchlings to move towards the only visible “horizon” — the artificial lights. Consequently, we recommend that managers of sea turtle rookeries where turtles have an ocean-finding problem should manipulate the beach profile to ensure the direction of the ocean matches the lowest angle of elevation, in combination with light reduction strategies — as the most effective means of preventing disrupted orientation.

Lighting the way: Towards reducing misorientation of olive ridley hatchlings due to artificial lighting at Rushikulya, India

Sea-finding behavior in sea turtle hatchlings is modified by the visual cues provided by artificial beach front lighting. The consequent landward movement of hatchlings in response to coastal electric lighting reduces their survival rates. We assessed the potential impact of coastal lighting at Rushikulya, an important mass nesting site of the olive ridley sea turtle ( Lepidochelys olivacea) in the Indian Ocean region. We examined the response of hatchlings to light characteristics in an experimental setup, as well as to the existing lighting regimes along the beach, using arena trials. Previous studies on other species indicate preferential orientation towards low wavelength and high intensity light. Our study confirms these preferences among hatchlings from the Indian Ocean population of olive ridleys. In addition we also found that wavelength and intensity could have an interactive effect upon hatchling orientation. Hatchlings at the study site respond both to visible point sources of light and to sheer glows of light. Though beach plantations of introduced Casuarina equisetifolia are generally considered to have negative impacts on sea turtle nesting beaches, we found that they acted as an effective light barrier when planted about 50 m away from the high tide line. We developed a model of the expected impact of artificial lighting on hatchling orientation during mass hatching events of previous years, and predict as much as 50% misorientation in some years. We also developed a map representing the misorientation of hatchlings due to artificial lighting based on arena trials in different regions of the beach. The results of the study helped identify focal areas for light management on the beach, which could be critical for the survival of this population.

The sea-finding behavior of hatchling olive ridley sea turtles, Lepidochelys olivacea, at the beach of San Miguel (Costa Rica)

Newly hatched olive ridley sea turtles (Lepidochelys olivacea) were tested for their directional preferences in a sand-filled circular arena in total darkness. Hatchlings that had crawled about 5 m on the beach, toward the sea preferred the southwesterly direction that would have brought them to the water line, whereas hatchlings that had been denied this experience headed eastward, a direction of unclear origin. These data suggest that a short crawl across the natural beach can set the direction in which the young turtles subsequently move. The crawling experience was sufficient to acquire the compass course that they later follow, probably with the help of a magnetic compass, not only in the water, but already while still on land.

Behavior of Loggerhead Sea Turtles on an Urban Beach. II. Hatchling Orientation

Behavior of Loggerhead Sea Turtles on an Urban Beach. II. Hatchling Orientation

Beach vegetation and seafinding orientation of turtle hatchlings

In Suriname, leatherback Dermochelys coriacea and green Chelonia mydas sea turtle eggs laid below the high tide line are routinely relocated to safer ground by local conservation workers. Some of these nests are placed in dense vegetation. The nocturnal seafinding behaviour of leatherback and green sea turtle hatchlings in the vegetation was compared to that in the open beach. Groups of hatchlings of each species were tested in one of two orientation arenas: open sand or dense vegetation. In both species, those tested in the open arenas oriented strongly towards the ocean. Turtles placed in the vegetation either showed no significant orientation or oriented away from the ocean. Animals in the vegetation moved more slowly from the center to the perimeter of the arena. Implications for relocation as a conservation tool are discussed.

Influences of Wavelength and Intensity on Hatchling Sea Turtle Phototaxis: Implications for Sea-Finding Behavior

Visual cues are important to sea turtle hatchlings in determining seaward direction upon emerging from the nest. In this study, we examined the roles that color and intensity play in the sea-finding mechanisms employed by loggerhead (Caretta caretta) and green turtle (Chelonia mydas) hatchlings. We tested hatchling preference for a standard source of constant intensity and color (1.26 x 101 photons s-' m-2 at 520 nm), versus an adjustable light source (one of five monochromatic colors at each of seven photon intensities), using a two-choice apparatus. Both species oriented toward near-ultraviolet (360 nm), violet (400 nm), and blue-green (500 nm) light but chose the standard light source over yellow-orange (600 nm) and red (700 nm) light. There was a positive relationship between intensity and preference with 360, 400, and 500 nm light. We also examined hatchling choice of either a darkened window or a window lighted by one of eight monochromatic colors at each of two intensities. In these experiments, loggerheads oriented toward 360, 400, and 500 nm light but away from light in the green-yellow to yellow-orange range (560, 580, and 600 nm). Loggerheads oriented toward 700 nm light only at high intensity. Green turtles responded insignificantly to 600 or 700 nm light at either intensity. The contrast of green turtle behavioral responses with published electrophysiological data and the aversion to yellow light observed in loggerheads suggest some level of spectral quality assessment in sea finding for both species.

Sea-finding behaviour of loggerhead hatchlings: the time course of transient circling following unilateral and asynchronous bilateral blindfolding.

Marine turtle hatchlings orient seaward by responding to photic cues that guide them toward the brightest horizon. Earlier research has suggested that they have a positive phototropotactic reaction to light but the behaviour must be more complex than this because circling both toward the uncovered and the covered eye has been observed in unilaterally blindfolded turtles. The present work resolved some of these problems by studying circling direction as a function of how long one eye had been covered prior to testing. Different groups of neonate loggerhead turtles (Caretta caretta) were tested from 10 S to 19 h after the application of a unilateral blindfold. The longer this had been one, the less the turtle circled, but beyond 1 h further changes were slight. Initially, circling was predominantly toward the uncovered eye but after the blindfold had been on for 1 h circling toward the covered eye occurred equally often. The time course of circling was also examined in asynchronously, bilaterally blindfolded turtles. One eye was covered 6 h before the other, and tests were made at various times after the second eye had been covered. Asynchronous bilateral blindfolding caused more intense circling and disrupted seaward orientation more severely than unilateral blindfolding. The time course of the decline in circling showed similarities between both treatments, suggesting that they were activating the same mechanism. Dark adaptation was examined as a possible explanation of such transient turning tendencies and previous models of sea-finding were elaborated.

Seaward orientation of hatchling turtles: turning systems in the optic tectum.

This paper reports studies on the mechanisms underlying seaward orientation in hatchling turtles. The particular aim was to investigate whether activity in different regions of the retina and associated tectal areas, as assessed by some comparator mechanism, results in turning in different directions. Hatchling green turtles (Chelonia mydas) were tested for sea-finding ability in a natural situation on the beach following lesions of the optic tectum. Asymmetrical bilateral lesions resulted in a number of turtles making circles in the direction of the tectum with the posterior lesion and in other turtles deviated in this direction. Bilaterally lesioned turtles were also slower and less consistent in their sea-finding behaviour. No major disruptions of sea finding were detected in animals with unilateral tectal lesions. By suspending lights in the nasal visual field of unilaterally blindfolded green and leatherback turtles (Dermochelys coriacea) it was possible to produce circling in the direction of the covered eye; in contrast, with the light suspended in the temporal field, turning was always in the direction of the uncovered eye. The results are consistent with the view that sea finding depends on a complex phototropotactic system with stimulation in different parts of a single retina being associated with turning in opposite directions.

Effects of flashing lights on sea-finding behavior of green turtles

In preference tests between a flashing and a continuous light, batchling green sea turtles ( Chelonia mydas ) were not influenced much by the flashing light unless it entailed a considerable reduction of illumination averaged over time. The rate of flashing had little effect over the ranges tested, 1–14 flashes per second, but the ratio of off-time to on-time made a considerable difference. The greater the amount of off-time, the greater the depression of the preference for the flashing light. The results suggest that turtles integrate brightness information over time in their sea-finding behavior. The possible adaptive value of this mechanism is discussed.

Further studies of the sea-finding mechanism in green turtle hatchlings.

Abstract Further studies were made of the simultaneous comparison mechanism used by hatchling green turtles, Chelonia mydas, in sea-finding behaviour. The aim was to learn if visual inputs from different directions, stimulating different parts of the retina, had the same effects on orientation. To do this goggles were designed that permitted occlusion of parts of the visual field. Wearing goggles did not in itself affect orientation. When the nasal visual field for one eye and the temporal field for the other eye were covered, turtles circled consistently in the direction of the eye with the nasal occlusion. The total numbers of circles made in a given time by such turtles were not significantly different from those made by animals with simple unilateral blindfolds. Covering the temporal field of one eye resulted in some, though not all, turtles turning contralateral to that eye, provided the other eye was totally covered. On the whole the data were consistent with the idea that inputs in the nasal field are associated with contralateral turning and those in the temporal field with ipsilateral turning. Some complications to this interpretation are also discussed. However, the results demonstrate unequivocally that inputs from different directions contribute differently to sea-finding orientation of green turtles.