326 C. A. Madden Hof et al.
with a predator exclusion device (Treatment; Table 1). We compared models using the Akaike information criterion (AIC; Akaike, 1974). Using the best model from the analysis on the effectiveness of the treatment, we predicted the prob- ability of predation at all observed nest locations along the entire beach. To determine whether the exclusion device delayed pre-
dation, we compared the mean number of days from laying to predation between control and treatment plots using a Bayesian t test. We hypothesized that the mean number of days between laying and predation would be greater for the treatment plots than for the control plots. To estimate the total number of hatchlings on the entire
beach, we used a parametric bootstrap procedure. For all observed clutches at the nesting beach, we estimated the probability of predation using the logistic regression models. The median nest size and median hatching success ratewere used to compute the total number of hatchlings for the unpredated nests. We repeated this process 7,000 times to obtain the uncertainty in the estimate from the predation probability. We conducted all statistical analyses in R 3.1.2 (R Core
Team, 2015) with packages ggplot2 (Wickham, 2009) and gamm4 (Wood & Scheipl, 2014). Bayesian computations were conducted using jags 3.3.0 (Plummer, 2015) through rjags (Plummer, 2015)in R.
Results
During the 2013–2014 nesting season (1 December 2013–12 March 2014) we recorded 78 loggerhead, 30 green and 20 flatback turtles laying one or more clutches at Wreck Rock beach (Supplementary Table 1). Of the loggerhead turtles, 16 were new recruits and 62 were recaptures from previous nesting seasons. During the nesting season, we recorded 218 loggerhead nesting activities, with 195 successful nests and 23 failed attempts. Of the 195 successful nesting events, 148 clutches were laid on the north and 47 on the south beach. We used 57 of the 195 successfully laid nests in our study, deployed exclusion devices on 33 treatment clutches and used 24 as control. During the study, 19 plots (eight treat- ment and 11 controls) were lost to erosion when cyclone Dylan made landfall near Wreck Rock beach on 31 January 2014.
Clutch loss
During the 2013–2014 season, 11 of 24 control clutches were depredated (mean = 45.8%, 95%CI = 26.2–66.8%), an add- itional 11 were lost to erosion. In total, clutch loss to preda- tors and erosion events (exacerbated by a cyclone)was 91.7% (95%CI = 71.5–98.5%). The predation rate was greater prior to the cyclone (41.67%, 10 of 24 control plots depredated;
95%CI = 22.8–63.1%) than after the cyclone (7.7%, 1 of 13 control plots depredated; 95%CI = 0.4–37.9%). The major- ity of predation of control nests occurred on the northern beach (9/14 = 64%; 95%CI = 35.6–86.0%). Exclusion devices appeared to be effective in reducing
clutch predation. With exclusion devices deployed at 33 nests, only one hatchling became entrapped during emer- gence. Compared with the control plots (11 of 24 control nests; 45.8%; 95%CI = 26.2–66.8%), fewer treatment plots were depredated during the nesting season (5 of 33 treatment nests; 15.2%; 95%CI = 5.7–32.7%). The point estimate of pre- dation with exclusion devices was 6%(2/33; 95%CI = 0.7– 20.2%) prior to the cyclone, and 12%(3/25; 95%CI = 3.2– 32.3%) after the cyclone. Although the point estimate was greater for the treatment plots than control (12.0%vs 7.7%), the binomial 95% CI overlapped (3.2–32.3%vs 0.3– 37.9%). Because of the large uncertainties associated with small sample sizes, they were not statistically different. Amongst the models used to determine the effects of the
exclusion device on the predation of loggerhead turtle clutches, Model 4 had the smallest AIC value, although dif- ferences in AIC values between the top 3 models were ,3, indicating that these models were not substantially different (Table 1). In the following analyses, we use the best model, which included treatment, location, and their interactions. The interaction term was significant at α = 0.10 (Table 2),
indicating that the effectiveness of the treatment varied sig- nificantly along the beach. This can be seen in Fig. 2, where the predicted probability of predation decreases with in- creasing peg number (i.e. from north to south along the beach) for the control plots, but increases for the treatment clutches. However, because of the small sample sizes in the area farthest from the field station, on the south beach, confidence intervals are wide in this area. For the area with larger sample sizes, on the north beach, the predicted probability of predation was smaller for the treatment than for the control plots. The main effect of the exclusion device was also signifi-
cant at α = 0.10 (Table 2), indicating that the devices signifi- cantly reduced the predation of loggerhead turtle clutches. This is supported by the percentage of predated clutches in control vs treatment plots (47.8 vs 15.2%, respectively).
Hatching success
The mean clutch size was 113.6 ± SE 7.7 (n = 31; median 118) and the mean hatching success was 0.779 ± SE 0.056 (n = 42;median 0.894). Using the most parsimonious logis- tic regression model (Model 4) from the previous analysis, we predicted the probability of predation at all observed nest locations; i.e. extrapolated to the entire beach (Fig. 3). Using the probability of predation, median hatching success, median clutch size, and a parametric bootstrap analysis,
Oryx, 2020, 54(3), 323–331 © 2019 Fauna & Flora International doi:10.1017/S0030605318001564
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