High predation of marine turtle hatchlings near a coastal jetty

Highlights

• Predation rates were high, irrespective of the presence or absence of artificial light.

• Tagged hatchlings were ingested by predators, allowing us to track the predators movements.

• Predators used the jetty as a daytime refuge and moved along the nearshore at night.

• Nearshore waters adjacent to high density nesting was a focus for predator activity.

• Predation was 7× higher near the jetty compared to an unmodified section of coast.

Abstract

Growing human populations are driving the development of coastal infrastructure such as port facilities. Here, we used passive acoustic telemetry to examine the effects of a jetty and artificial light on the rates of predation of flatback turtle (Natator depressus) hatchlings as they disperse through nearshore waters. When released near a jetty, around 70% of the tagged hatchlings were predated before they could transit the nearshore, irrespective of the presence or absence of artificial light. Only 3 to 23% of hatchlings encountered predators at a second study site nearby where there was no jetty and a similar amount of nesting activity. Evidence for predation was provided by rapid tag detachment due to prey handling by a predator or the extensive movement of the tags within the receiver array suggesting that the tag (and hatchling) was inside the stomach of a predator. We found that 70% of the fish predators that consumed tags used the jetty as a refuge during the day and expanded their range along nearshore waters at night, predating on hatchlings in areas adjacent to the jetty with the highest nesting density. Sampling of potential predators including lutjanid reef fishes under the jetty revealed the presence of turtle hatchlings in their gut contents. By providing daytime refuges for predators, nearshore structures such as jetties have the potential to concentrate predators and they may pose a significant threat to populations of vulnerable species. Such effects must be taken into consideration when assessing the environmental impacts associated with these structures.

Introduction

In all ecosystems, predation is a key process that drives the dynamics of populations and structures communities (Estes, 1996; Preisser et al., 2005). Similar to many marine organisms, the life history stage in marine turtles immediately following hatching is likely to be the most vulnerable to predators (Heithaus, 2013). After successfully traversing the nesting beach, hatchling turtles enter nearshore waters where rates of predation within the first hour are thought to be high (Stancyk, 1982). However, relatively few studies have measured predation during this time and those that have done so have typically been confounded by the presence of an observer. In the past, the size of available tags relative to the small size of hatchlings has prevented passive tracking, so that most studies have used active tracking techniques where hatchlings have been followed by a snorkeler or observer on a small vessel (dinghy or kayak) (Frick, 1976; Witherington and Salmon, 1992; Gyuris, 1994; Pilcher et al., 2000; Stewart and Wyneken, 2004; Whelan and Wyneken, 2007). Most of these studies have reported levels of mortality below 10%, with some exceptions (e.g. Gyuris, 1994; Pilcher et al., 2000 and Reising et al., 2015). However, the presence of an observer (and a vessel) is likely to influence both the process of predation (Frick, 1976; Nowak et al., 2014) and also the behaviour of the turtle (Hendrickson, 1958). Given their relative sizes, predators such as reef fishes are likely to be very wary of the boat or snorkeler following the hatchling (Milinski, 1986). Indeed, some species have been reported to retreat or drop hatchlings when approached by a snorkeler (Frick, 1976). For these reasons, it is possible that active tracking may underestimate levels of hatchling predation in the nearshore.

The recent development of small acoustic tags combined with passive acoustic receiver arrays has now enabled the remote study of the movement patterns of turtle hatchlings as they disperse through the nearshore (Thums et al., 2013; Thums et al., 2016; Wilson et al., 2018). Research using this technology has shown that within 300 m of the shoreline, turtle hatchlings take a fast, directed path offshore, transiting these waters in about 10 to 15 min. Importantly, tracks that do not follow these characteristic offshore routes, but in contrast linger and frequently change in direction, speed and tortuosity of movement, can provide evidence of consumption of tagged hatchlings by predators (Thums et al., 2016). Additionally, tags that cease movement can also represent predation where the tag is removed during prey handling by a predator (Khan et al., 2016). Thus, tracking hatchlings in a receiver array can monitor the movement of both hatchlings and their predators and allow for the calculation of predation rates, which are key questions in the movement ecology of marine megafauna (Hays et al., 2016).

Earlier studies have shown that hatchlings are predated more frequently in the nearshore when close to, or crossing, reef habitat (Frick, 1976; Witherington and Salmon, 1992; Gyuris, 1994; Pilcher et al., 2000), whereas predation tends to be lower when hatchlings cross areas of sand (Stewart and Wyneken, 2004; Whelan and Wyneken, 2007). This implies that rates of predation may be highest where benthic habitats provide refuges for fish. Given that man-made structures such as jetties, wharves, offshore platforms and pipelines are also known to support and attract large numbers of fish (Bohnsack, 1989; Rilov and Benayahu, 2000; Claisse et al., 2014; McLean et al., 2017), it seems possible that these could also increase predation rates on turtle hatchlings. Additionally, for navigation and/or operations during the night, these structures are often required to be lit, which can also attract large-bodied predatory fish (Becker et al., 2013). This could further increase predation rates on hatchlings as they are also attracted towards light sources at night (Thums et al., 2016; Wilson et al., 2018).

Here, we investigate the impacts of a jetty with artificial lights on the predation and nearshore movements of hatchlings of the endemic flatback turtle, Natator depressus, in northern Australia. Light pollution and the development of coastal infrastructure such as jetties and port facilities are recognised as primary threats to marine turtles around the world (Wallace et al., 2011). Several large rookeries of the flatback turtle are located close to industrial developments (Kamrowski et al., 2014) where large jetties have been built for the shipping of mineral and petroleum resources (Drenth, 2007; Department of Environment and Energy, 2017). Consequently, the possibility that these structures are attracting hatchlings (due to lighting) and fish (due to cover) and increasing hatchling predation rates, remains an unresolved and concerning question for the global management of turtle populations. We used acoustic tags and passive receiver arrays to document in-water predation on turtle hatchlings at differing distances from a jetty and in the presence and absence of artificial light. We hypothesised that predation rates would be higher closer to the jetty as it would provide cover for fish predators and that the attraction of hatchlings to lights on the jetty might increase this phenomenon. We also hypothesised that the attraction of hatchlings to light might decline with distance from the light source.