seaturtle.org : MTN : ARCHIVES : Sign In

This present note on emergence events reviews 21 seasons of nightly beach patrols for nesting loggerheads on the southern 6 km of Casey Key in Sarasota County, Florida (27.13N, -82.47W). The beach is not yet subject to beach nourishment (Rumbold et al. 2001) or ecotourism (Wilson & Tisdell 2001) as factors that might influence turtle nesting. Patrols by ATV occurred hourly from approximately 2000 hours to 0500 hours for the main nesting months of June and July. Time of each emergence event was recorded for 93.3% (2535/2717) of the turtles that were encountered. Each encounter was classed to the following categories: 1) Pre-Oviposition which included the following behaviors: beach approach, body pitting, nest cavity construction; 2) Oviposition; and 3) Post-Oviposition which included the following behaviors: covering/camouflaging, and leaving the nest.

Figure 1. Distribution of loggerhead nesting activities by hourly categories on Casey Key, Florida.

The emergence events had a strongly bimodal pattern with an increase after dusk to a peak at 2300 hr, a drop for the 0000 hour, and a second peak at 0100 hr, then decreasing until dawn (Fig. 1). The timing of emergence events was significantly different from a normal distribution (Kolmogorov-Smirnov goodness of fit, D = 0.818). The behavioral activity when encountered was recorded for 52.6% (1334/2535) of the emergence events (Fig. 2) and the bimodality was found in all three of the defined nesting behaviors.

Figure 2. Categories for loggerhead activities by nocturnal hours on Casey Key, Florida.

Both graphs (Figs. 1, 2) suggested that emergence events were comprised of two events, a first peak arriving around the 2200 and 2300 hours accounting for 44% of the nesting activity and a second smaller peak around the 0100 and 0200 hours representing 29% of the nesting activity. Overall, nesting activities were concentrated between 2200 to 0200 hr., with 81.9% of the defined activities. However, emergence events were still found, although in declining numbers, during the remaining hours of the nocturnal period. Interestingly, 1.4% of turtle encounters were around dusk (1900- 2000 hrs), but there were no recorded observations of turtles arriving to nest during the dawn hours (0600-0700 hrs). These data for 6 km of Casey Key are supported by anecdotal observations for the 56 km of beach monitored daily at dawn by Mote Marine Laboratory staff and volunteers that document about one loggerhead nest per year during the post-dawn hours.

Taking the same data from a different perspective produced a temporal contrast of nesting behavior across years, as we compared the percents of females arriving before and after midnight (Fig. 3). From 1987-2000 the females typically emerged before midnight (2400 hr), which contrasted with a change in 2003 as females shifted to nesting predominantly after midnight.

Figure 3. Percents of all loggerhead turtle emergences before midnight at Casey Key, Florida

Our results for a peak of emergence activity from 2200 to 2300 hr confirm earlier findings from Sanibel Island, Florida (LeBuff 1990). However, a notable difference is a secondary peak of activity on Casey Key that was absent in Sanibel loggerheads. Casey Key has people present on the beach most nights, particularly on weekends near public access areas in contrast with the Sanibel Island studies that were conducted over 30 years ago on a relatively remote beach. One might suspect a difference in patrol coverage if monitoring patrols were concluded earlier on Sanibel and so did not detect a secondary peak, but accounts in Lebuff (1998) appear to rule that out. A more plausible scenario may be of altered turtle behavior with the secondary peak on Casey Key as a possible behavioral artifact, resulting from a non-nesting emergence early in the evening and a postponed return of the same turtle.

Tidal influences can be discounted as a determinant in determining female emergence times, in the manner explored by other papers (Fritts & Hoffman 1982, Frazer 1983, Azanza et al. 2003) because, Casey Key generally has tides of less than 1 meter. It remains unclear what factors may be associated with a behavioral shift that began around 2003 for females emerging later at night than in previous years (Fig. 3). Some factors can be discounted, such as usage of ATVs for night patrols, as this patrol method without lights has occurred throughout the study (J. Foote, pers. comm.).

Nevertheless, there is an unquestionable increase in coastal development and human use on beaches over the time frame of monitoring. Consequently, a shift in emergence times may be associated with more people on the beaches especially during the hours of dusk, in which sunset watching and continued use is a popular activity on the western coast of Florida. This is a pragmatic hypothesis, but at the present time there are no corresponding data on human activity to test that premise. A need exists for further studies to understand if behavioral shifts of emergence timing are related to human activities on the beach.

In conclusion, a peak of nesting female emergence activity occurs in the 2300 hour with sea turtle activity occurring through the night though rarely before dusk or after dawn. For beaches where there are potentials for overlap of human activities and nesting turtles, new studies should evaluate the potential for anthropogenic disturbance to females approaching the nesting beaches (Waayers et. al. 2006). For beaches hosting traffic by foot or vehicle, the form of distribution of nesting emergences may offer new data to test the hypothesis of anthropogenic disturbance. Although we presented no data here on hatchling emergence times (however, see Witherington et. al. 1990), such data may also be critical for better informed management decisions relating to human activity on the beaches (Lamont et al. 2002).

Acknowledgments: We are grateful to all staff, interns, and volunteers who assisted with the nocturnal tagging patrols. ATV storage at Casey Key is facilitated by Sarasota County Parks and Recreation. J. Foote shared essential details concerning the early history of the tagging program on Casey Key.

AZANZA, R.J., M.E. IBARRA, G. ESPINOSA, R. DIAZ & G. GONZALEX-SANSON. 2003. Nesting behaviour of green turtles (Chelonia mydas) in Antonio and Caleta de los Piojos beaches from Guanahacabibes Peninsula, Pinar del Rio, Cuba. Revista de Investigaciones Marinas 24: 231-240.

FRAZER, N. 1983. Effect of tidal cycles on loggerhead sea turtles (Caretta caretta) emerging from the sea. Copeia 2:516-519.

FRITTS, T.H & W. HOFFMAN. 1982. Diurnal nesting of marine turtles in southern Brevard County, Florida. Journal of Herpetology 16:84-86.

LAMONT, M.M., H. F. PERCIVAL & S.V. COLWELL. 2002. Influence of vehicle tracks on loggerhead hatchling seaward movement along a northwest Florida beach. Florida Field Naturalist 30: 77-82.

LEBUFF, Jr., C. 1990. The loggerhead turtle in the eastern Gulf of Mexico. Caretta Research, Inc., Sanibel Island, FL.

LEBUFF, C. 1998. Sanybel Light: an historical autobiography. Amber Publishing, Sanibel, FL.

RUMBOLD, D.G., P.W. DAVIS & C. PERRETTA. 2001. Estimating the effect of beach nourishment on Caretta caretta (loggerhead sea turtle) nesting. Restoration Ecology 9: 304-310.

SPOTILA, J.R. & E.A. STANDORA. 1985. Environmental constraints on the thermal energetics of sea turtles. Copeia 3:694-702.

WAAYERS, D., D. NEWSOME & D. LEE. 2006. Observations of noncompliance behaviour by tourists to a voluntary code of conduct: a pilot study of turtle tourism in the Exmouth region, Western Australia Journal of Ecotourism 5: 211-222.

WILSON, C. & C. TISDELL. 2001. Sea turtles as a non-consumptive tourism resource especially in Australia. Tourism Management 22: 279-288.

WITHERINGTON, B.E, K. A. BJORNDAL & C. M. MCCABE. 1990. Temporal pattern of nocturnal emergence of loggerhead turtle hatchlings from natural nests. Copeia 1990: 1165-1168.