Lower nesting success of flatback turtles caused by disorientation

Nesting behaviour of sea turtles is stereotyped and typified by turtles making their way directly up the beach, making a body pit, digging an egg chamber, laying their eggs, covering the eggs, scattering sand over them, and returning to the sea (detailed process in another species described by Hendrickson 1958). The physical environment causes deviations to this process when debris and man-made barriers cause nesting attempts to be aborted. Several hundred flatback sea turtles (Natator depressus) nest on Bare Sand Island each year. This rookery is about 50 km west of Darwin in the Northern Territory, Australia, and is typical of the lowenergy beaches in the area. The main nesting beach faces west, is composed of fine sand, and rises gently from the low water region to a small primary dune. Wave action is slight but tidal variation reaches 8 metres during spring tides. Winds are a significant influence on beach formation with the southeast monsoon establishing during the dry-season months from May to October, the peak sea turtle nesting period, and the northwest monsoon dominating during the wet season months of November to February. In 2004 and 2005, the normal nesting behaviour of flatback sea turtles at Bare Sand Island was disrupted by the presence of an intertidal ridge and runnel in the middle of the beach that was exposed at neap tides and low and mid spring tides. The ridge was formed parallel to the strand by natural wind and wave action during the preceding wet seasons. The crest of this ridge varied from 0.3 to 1.5 metres in height above the runnel that filled with water during spring high tides. This ridge and runnel behind it confused and disorientated nesting flatback sea turtles and their hatchlings leading to lower nesting success and higher hatchling mortality. Nesting sea turtles were monitored nightly for five weeks in each of 2004 and 2005 and their behaviour was opportunistically observed from the edge of the dune vegetation when sufficient moonlight or daylight allowed. On other occasions, the tracks were followed and paths were measured in daylight to minimise potential observer effects on the nesting turtles. Nesting sea turtles crossed the beach exhibiting typical behaviour similar to green turtles as described by Hendrickson (1958) until they reached the crest of an intertidal ridge. Few turtles would proceed downwards into the runnel before ascending the vegetated dune of the beach (< 5 % during neap and low and mid spring tides), with most turtles zigzagged their way along the crest until ultimately returning to the water without nesting (54.8 % during neap and low and mid spring tides in 2004, n= 31; and 12.2 % in 2005, n= 49). Tidal height and the height of the ridge affected this behaviour. During the neap tides turtles approaching the crest would generally slow their speed noticeably and zigzag along the ridge for as much as 150 m before heading back to the water for a few meters and then turning around to continue their ascent further along the beach. When the spring tide height was greater than 6.6 m water became trapped in the runnel forming a channel along the nesting beach where water remained throughout the day. While there was water in the channel, turtles were not disturbed by the ridge with all turtles having a successful ascent up the beach – exhibiting similar nesting behaviour to that previously observed by flatback turtles at Bare Sand Island during 1996-2003 by nesting on or directly in front of the primary dune. The channelling of water along the beach caused greater erosion that increased the depth of the runnel throughout the season. All successful nests hatching emerged on the landward side of the ridge causing much disorientation of hatchlings on their path to the water. As hatchlings entered the runnel, their view of the low light horizon was disrupted. Many walked parallel to the shore towards the lower light horizon (using sea finding mechanisms described by Limpus 1971) orientating towards the water at the far end of the beach some 500 m away. Many of the hatchling tracks in the trough terminated at either crab or bird tracks with only a few making their way over the ridge to the waters edge. The persistence of water in the channel throughout the day during spring high tides trapped hatchlings in the channel well into the daylight hours, further increasing their chances of predation and reducing their survivorship (Pilcher & Enderby 2001). This is the first observation of the formation of an intertidal ridge at Bare Sand Island that is persistent in the dry-season months. The intertidal ridge was more pronounced in 2004 than in 2005, resulting in greater disturbance to sea turtle nesting activity during the 2004 nesting season. After failed nesting attempts, turtles generally came back on subsequent nights and continued attempting to ascend the beach until a successful ascent past the intertidal ridge was made which would generally result in successful nesting. This process often continued until the following spring high tides, when most sea turtles nested successfully. The impact on individual’s reproductive output for the season due to this additional energy expenditure is unknown. This intertidal ridge has been present for two subsequent nesting seasons. Long-term effects on the nesting population may occur if the intertidal ridge remains in subsequent years, potentially causing turtles to choose other nesting beaches. This paper shows that beach structure or form has major implications to nesting behaviour and should be considered during any development or beach modification including sand mining and artificial beach nourishment.