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Sea Turtles

Turtle hatchlings attracted to artificial light like moths to a flame

Article: Wilson P, Thums M, Pattiaratchi C, Meekan M, Pendoley K, Fisher R, Whiting S (2018) Artificial light disrupts the nearshore dispersal of neonate flatback turtles Natator depressus. Mar. Ecol. Prog. Se.r 600:179-192. https://doi.org/10.3354/meps12649

At the beginning of nesting season, a mother sea turtle, joined by hundreds of others, ventures out of the ocean at night to dig a deep hole and lay dozens of eggs on the beach. When she’s done, thoroughly exhausted, she covers the eggs with sand and leaves them to mature. She never returns, trusting that her offspring will thrive without her. Most of these eggs will not make it to infancy. A lucky few will hatch into newborns who instinctively know the drill; they must evade all obstacles to crawl to sea and begin their lives, vulnerable and alone.

Figure 1: Flatback turtle hatchlings crawling towards the ocean. (via YouTube)

The dangers they encounter don’t end once they enter the water. Sea turtle eggs and hatchlings are particularly susceptible to becoming prey of both land and sea animals who know their behavior well. To many of these animals, the frenzy of hatchlings attempting to swim to safety becomes a feeding opportunity.

Conservation efforts

Many sea turtles are essential to the health of marine habitats by maintaining seagrass beds, coral reefs, and dunes. They help to balance the marine food web by their mutual relationship with other marine life. All seven species of sea turtle around the world are considered either vulnerable or endangered – the reason scientists and conservationists closely monitor the conditions that hinder hatchling survival. Aside from natural threats like animal predation and disease, anthropogenic (human-induced) factors such as industrialization, pollution, and climate change are known to affect their livelihood.

Australian flatback hatchlings

A recent study led by Phillipa Wilson at the University of Western Australia focuses on the nesting rituals of the Australian flatback turtle, or Natator depressus – named for its smooth, flat shell (Figure 1). Flatbacks don’t migrate very far, mainly keeping near the Australian coast to nest, and as such have the smallest geographic range of any other sea turtle. This makes them one of the least-studied sea turtles in the world, despite the shallow depths at which they dwell. Their conservation status is unknown due to deficient data, but they are believed to be vulnerable, and their behavioral response to anthropogenic changes may be similar to other more critically endangered sea turtles.

Hatchling movement: like moths to a flame

Figure 2: Green turtle hatchling equipped with a Vemco V5 acoustic transmitter; the current study uses the same device for flatbacks. (Credit: Thums et al. 2016)

Recent studies have looked at sea turtle tracks by satellite, and hatchling movement using acoustics (sound transmission), equipping them with tags that signal their location to nearby sound receivers with a ping (see Figure 2, Thums et al. 2016). The hatchlings can then be tracked as they use local conditions such as tidal currents and waves to navigate away from the beach to safety offshore.

They’re also guided by visual cues such as low, bright horizons that signal where they can swim to deeper water. When they sense light, such as a sunrise, they gravitate towards it like moths to a flame. As development along coastlines grows, this instinct has disoriented them in the presence of artificial light sources like oil rigs and shipping ports. Instead of swimming for safety, they linger near these lights, becoming weak and exposed to predators.

Tracking the flatback hatchlings

During the 2016 peak nesting season, in February, the researchers set up an array of 36 acoustic receivers near the shore of western Australia to collect data on the locations of 91 flatback hatchlings equipped with tags and released on shore. Their release was timed at the start and end of rising tide over the course of three days to differentiate the effects of currents from flood and ebb tides. It followed the setting of the new moon to ensure moonlight changes didn’t affect the results.

Three different light conditions were tested: ambient (no artificial light), amber light, and white light. The two artificial lights, situated on the western side of the array, represented common industrial lighting – the former known to be less attractive to hatchlings, and the latter, more so. Hatchling speed, bearing (direction), and bearing variance (variability in direction) were recorded. These data, along with environmental conditions such as current speed, surface waves, temperature, and depth, were analyzed to find patterns in their behavior.

Confused hatchlings swim against the current

Under ambient conditions, the bearing of hatchling movement was strongly influenced by the tidal current, which became much less important as artificial light was introduced (see Figure 3). While all three conditions resulted in a westward track, artificial light conditions led to increased time spent within the array, reduction in swim speed, and enhanced bearing variance. This variance indicated confusion and disorientation as the hatchlings switched direction often, trying to figure out which way to turn.

Figure 3: Mean track taken by hatchlings for ambient (left), amber (middle), and white (right) lighting. Light location indicated by orange circle. (Credit: Wilson et al. 2018)

Artificial light conditions, especially white light, led to a ‘trapping effect’ where sea turtles  became stuck near the light source, unable to disperse despite the presence of a current. This means that they would use energy to swim against the current just so they could stay near the light. Details of the relationship between hatchling dispersal, their swimming abilities, currents, and the influence of artificial light are still not well known.

Turning out the lights

Figure 4: Estimated time hatchling spent in the acoustic array (left plot) and average hatchling speed (right plot) for ambient (left), amber (middle), and white (right) lighting.

While white light was most disruptive to hatchling dispersal, even amber light – recommended as a safer alternative – significantly affected hatchling behavior (see Figures 3 and 4). These results indicate that coastal light pollution along Australia’s coast is a threat to flatback hatchling survival, and that more effective lighting mitigation strategies – such as light shields, platform-embedded lights, or simply light reduction – must be considered to help them survive in their natural habitats.

Ensuring the survival of these sea turtles is essential to maintaining coastal ecosystems. A world without them could mean an end to seagrass beds, coral reefs, beach dunes, and healthy oceans as a whole.

Additional References

Thums, M., Whiting, S.D., Reisser, J., Pendoley, K.L., Pattiaratchi, C.B., Proietti, M., Hetzel, Y., Fisher, R., and Meekan, M.G. (2016). Artificial light on water attracts turtle hatchlings during their near shore transit. R. Soc. Open Sci. https://doi.org/10.1098/rsos.160142.

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