Protecting loggerhead turtle nests 327
TABLE 1 Logistic regression models used to determine the effective- ness of predator exclusion devices on loggerhead turtle Caretta ca- retta clutches, showing predictor variables and difference in Akaike information criterion values from best-performing model (ΔAIC).
Model
Model 4 Model 3 Model 2 Model 1 Model 5
Variables1
Treatment × Location Treatment
Treatment + Location
Treatment + Location + Days Treatment ×Days
ΔAIC 0.00
0.57 1.22 3.12 4.45
1Days, days since the beginning of the experiment; Location, location of the nests measured by the peg numbers along the beach; Treatment, clutch treatment with a predator exclusion device.
we estimated the total number of hatchlings produced at Wreck Rock beach during the nesting season to be 11,599 with a 95%CI of 10,439–12,865 (Supplementary Fig. 1). Some loggerhead turtle clutches were only partially pre- dated. Although sample sizes were small, themean number of predated eggs per nest was c. 16 ± SE 4.5 (16.4%, n = 18). Undeveloped eggs (unhatched eggs with no obvious em- bryo) also accounted for 16% of the total eggs in nests. The Bayesian equivalent of a two-sampled t test
indicated there was no significant difference in the tim- ing of predation between treatment and control nests (Supplementary Fig. 2). Further, the 95% posterior interval (Gelman et al., 2014) of the contrast coefficient (control vs
treatment effects) included zero (−1.973, 0.724). For both control and treatments, however, predation seemed to occur either in the early or late stages of incubation (Supplementary Fig. 2).
Predation
With individual cameras operating for 51–62 days, the total camera-trapping time was 13,800 hours over a period of 79 days. Species recorded by automatic cameras included hares Lepus sp., kites Milvus sp., cats Felis sp., foxes Vulpes sp., emus Dromaius sp., pied butcherbirds Cracticus nigrogularis and wallabies Macropus sp. The two species of goannas recorded near turtle nests were the yellow spotted or Argus monitor Varanus panoptes and the lace monitor Varanus varius (A. Amey, 2014, pers. comm.). A third goanna Varanus gouldii may be present, but the species identification could not be confirmed. As expected, goanna activity tended to occur during daylight hours (6.00–18.00). Based on tracks, predation at both treatment and control
plots was carried out by goannas, with only one incidence of fox predation recorded, at a treatment plot. On one occa- sion, more than six goannas were observed predating on loggerhead turtle hatchlings as they emerged from a nest. This was the first recorded observation of goanna predation
TABLE 2 Estimated linear model coefficients of the best-performing model (Model 4), with standard deviations and P values. The model was Predation*Intercept + Treatment + Location + Treatment × Location.
Parameter (Intercept)
Treatment Location
Treatment:Location Estimate
−1.40 ± SE 0.38 −0.64 ± SE 0.37 −0.16 ± SE 0.41
0.71 ± SE 0.40 P
0.00 0.08 0.70 0.08
on emerging hatchlings at Wreck Rock beach. On several occasions we observed predation on nests adjacent to control plots (i.e. on nests not included in this study). We recorded 177 goanna visitation events to 12 logger-
head turtle nests monitored by automated cameras. The yellow spotted monitor was found most frequently (53 recorded visits) followed by the lace monitor (37 visits). Of the 12 monitored clutches, 8 were visited multiple times by the same species on 74 occasions (46 times by yellowspotted monitors and 28 times by lace monitors). The remaining four nests were visited once by one goanna species, and at two of these nests the second species continued to visit multiple times after the first species had visited. Ambient air temperature during goanna visits to the
clutches was 20–47 °C. We found differences between the mean temperatures at visits by the two goanna species and other species (birds and mammals; Supplementary Fig. 4) Lace monitors visited turtle nests at a lower ambient tem- perature (mean = 32.7 ± SE 0.44 °C, n = 46) than yellow spotted monitors (36.8 ± SE 0.55 °C, n = 53).
Discussion
As the South Pacific loggerhead turtle subpopulation con- tinues to decline, it requires renewed attention to decrease mortality at all life history stages. Nest predation by terres- trial predators may significantly reduce recruitment and could lead to additional declines in the already depleted population (CMS, 2014). This study provides new infor- mation about goanna depredation and demonstrates that exclusion devices effectively reduce predation. We also pro- vide a recent estimate of the predation rate on loggerhead turtle clutches at a major nesting beach in the South Pacific. The predation rate of loggerhead turtle clutches (45.8%)
at Wreck Rock beach was lower than that reported at other nesting beaches. For example, 52% of flatback turtle clutches were predated at Fog Bay (Blamires, 1999;Blamires & Guinea, 2003), 65–70% of flatback turtle clutches at Pennefather beach (J. Doherty and Cape York Peninsula Development Association, unpubl. data in Whytlaw et al., 2013), 64–89% of green and loggerhead turtle clutches in
Oryx, 2020, 54(3), 323–331 © 2019 Fauna & Flora International doi:10.1017/S0030605318001564
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