seaturtle.org : MTN : ARCHIVES : Sign In

Turtle Beach accounts for the majority of leatherback nesting events on Tobago’s index beaches from 2005-2009 (average: 66.2%); it has a steep beach profile, no corals immediately offshore and is the longest of the three (1.76 km). Grafton Beach (15.7% of total average leatherback nesting events from 2005-2009) has large rock formations along the length of the beach (1km), has a gentle slope and the tide often creates sandbanks of approximately 30-80 cm. Back Bay (18.1% of total average leatherback nesting events from 2005-2009) also has rock formations throughout the length of the beach (1 km) and is inundated when high spring tides occur.

The terrestrial environment can be physically difficult for sea turtles to move on and presents hazards such as egg predators and poachers, so nesting turtles should try to minimise exposure to unfavourable environmental conditions by assessing conditions whilst still at sea (Pike 2008). Leatherback nesting processes will be affected by the environment and cues (e.g. oceanic or atmospheric) may help to reduce the energetic and physiological stress of nesting (Pike 2008). Lunar, solar and tidal patterns are strongly linked and interact with each other. When the tide generating forces from the Sun and Moon are parallel or opposite to each other the tidal range is large; these spring tides (higher and lower than average) occur when the Moon is full or new. When the tide generating forces of the Sun and Moon are out of phase i.e. the Sun and Moon are at right angles to each other, the tidal range is below average, known as neap tides, occurring during the first and last quarter of the lunar phase (Wright et al. 1989). Tobago experiences mixed semidiurnal tides due to its position near the equator; two high and two low waters each day, or one tidal cycle per day depending on the Moon’s inclination to the Earth. Tidal ranges within the Caribbean are generally small (about one meter).

The lunar cycle causes environmental changes that may be perceived by animals, e.g. change in the brightness of lunar light, gravitational changes and geomagnetic fields. Solar, lunar and tidal cycles are believed to influence leatherback nesting activities because the turtles generally nest above the high tide line (Kamel & Mrosovsky 2004), so emerging when tides are at their highest will minimise the distance and duration of crawls. The greater the vertical distance between high and low tides, the greater the advantage of emerging at high tides, although beach profile will affect this potential advantage (Frazer 1983). Emergence when the tide is low is particularly difficult for leatherbacks as they are much larger than any other species of sea turtle and as a result their terrestrial movement is slow and metabolically costly (Wallace & Jones 2008). Fretey & Girondot (1989) observed peak leatherback nesting at and around the nightly high tide on certain beaches in French Guiana and suggested that the carrier effect of the rising tide could facilitate the arrival of the turtles.

The lunar phase may also affect leatherback nesting visually. On clear nights when the Moon is full, visibility may be greater and the presence of tourists and egg predators may discourage turtles from emerging. Alternatively on clear nights when the Moon is not bright (e.g. new moon), artificial lights and dark silhouettes may be more apparent and discourage nesting.

This study analysed the nesting events of leatherback turtles on Tobago’s three index beaches from 2005-2009 to test whether environmental factors influence nesting. The following questions were asked: i) How many leatherbacks are nesting on Tobago’s index beaches each year? ii) Do the numbers of nesting leatherbacks vary between lunar phases? iii) Is nesting leatherback emergence time correlated with high tide? iv) Is nesting leatherback emergence influenced by tidal stage? v) Does time at night influence nesting leatherback emergence?

Nesting data for leatherback turtles were collected from three index beaches on the south-west Caribbean coast of Tobago from 2005-2009. During each nesting season (March-August), nightly patrols were conducted between 20:00 and 04:00 h by SOS Tobago head patrollers and volunteers. When turtles were encountered, they recorded the beach, zone, date, time, species and activity. Turtles that successfully dug a nest chamber and laid their eggs were measured (cm, using a flexible tape), checked for physical damage or distinct markings, any flipper tags read and recorded, and Passive-Integrated Transponder (PIT) tags scanned and recorded from each turtle’s shoulder area using a Biomark Pocket Reader (125 kHz). In the absence of tags, rear flipper tags or PIT tags were fitted in either shoulder. Monitors remained with the turtle and recorded the nesting event outcome. The numbers of tourists and locals were recorded and it was noted whether or not the turtle was disturbed by the presence of tourists/locals or beachfront lighting.

Each nesting event was categorised by the eventual outcome: confirmed nest (confirmed successful oviposition); false crawl (the turtle emerged from the surf and returned to the sea without digging a nest chamber); false crawl with body pit (after emergence from the surf and an attempt at digging a nest the turtle did not successfully complete a nest chamber); estimated nest (assumed but unconfirmed nest).

To test how nesting events are distributed between lunar quarters, months during the nesting season were divided into lunar quarters: plus and minus three days from the date of the first quarter, full moon, last quarter or new moon, giving seven days per quarter. Nests laid outside these ranges were not included in the analysis. False crawls and confirmed nests from all years were used in the first analysis of the effect of lunar phase. As the data were binomially distributed they were analysed using a Kruskal-Wallis test with multiple comparisons in Minitab v. 15. The second analysis examined the effect of lunar patterns on confirmed nests only within nesting years also using a Kruskal-Wallis test with multiple comparisons. Other nesting activities were not individually analysed with lunar phase as the numbers were low.

Nesting related to high tides: We recorded the time when we first encountered the turtle if she was initially observed on approach, body pitting and digging. Observed time was used rather than emergence time as turtles were often first found digging or body pitting and these events normally within the first twenty minutes of the nesting process (Miller 1996). Data were analysed using a Pearson’s Correlation.

Nesting related to tidal stage: Data used in this analysis included only turtles that were first seen on approach, body pitting or digging with the time seen recorded. Tides were divided into eight categories/ phases: 1= low tide; 2= low tide rising; 3= mean sea level rising; 4= rising to high tide; 5= high tide; 6= high tide falling; 7= mean sea level falling; 8= falling to low tide. The data for each beach were analysed using a chi-squared test on the combined 2005-2009 dataset.

Nesting related to night phase: Nightly beach patrols (8pm- 4am) were divided into eight equal phases: 1= 20:00–20:59; 2= 21:00–21:59; 3= 22:00–22:59; 4= 23:00–23:59; 5= 00:00–00:59; 6= 01:00–01:59; 7= 02:00–02:59; 8= 03:00–03:59. Only data where the leatherback was seen on approach, body pitting or digging were used in this analysis. Differences in leatherback observed time were analysed using a chi-squared test using the combined 2005-2009 dataset. Where the nesting event outcome was estimated, the data were not used in the analyses. Confirmed nests and both types of false crawls are referred to as nesting events in all graphs.

Table 1. Leatherback nesting outcomes from 2005-2009 for Tobago’s three index beaches.

Total leatherback nesting event outcomes recorded from 2005- 2009 varied year to year (Table 1). The number of individual nesting leatherbacks indicated by tags (returns or newly tagged individuals) ranged from 60-100 per nesting year.

Figure 1. The number of nesting events per night of female leatherbacks for each lunar phase for (A) 2005, (B) 2006, (C) 2007, (D) 2008, (E) 2009 and (F) all years combined. Key: FM=full moon; FQ=first quarter; LQ=last quarter; NM=new moon. Each plotted box displays the interquartile range (box containing 50% of the nesting frequency data); box whiskers show frequency data range; the median is the black line in each box and data outliers from individual nights are shown by asterisk symbols.

The frequency of nesting events (Figure 1) did not significantly differ between lunar phases for 2005, 2006, 2008 and 2009. In 2007 (Figure 1C) there was a significantly higher frequency of nesting events during the full moon phase compared to the first quarter (P = 0.0038) and new moon lunar phases (P = 0.0031). For all years combined (Figure 1F) the frequency of nesting events during the first lunar quarter was significantly less than the median from the full moon (P = 0.0277) and last quarter lunar phases (P = 0.01). When the frequencies of confirmed nests were examined in relation to lunar phase, the numbers of confirmed nests were evenly distributed between lunar quarters. There were no significant differences between the frequencies of confirmed nests across lunar phases.

During 2005-2009 the relationship between high tide and leatherback observed time was not significantly correlated for Turtle Beach (r = 0.036, p = 0.412), Grafton Beach (r = -0.04, p = 0.725) or Back Bay (r = 0.127, p = 0.187). For all beaches there was low nesting frequency during tidal stages 2 and 3 (low tide rising and mean sea level rising) with most of the nesting events occurring at and after high tide and also with high frequencies of nesting activity during low tide. Frequency of nesting for leatherbacks was significantly different between tidal stages for Turtle Beach (Figure 2A) in years 2005-2009 (X² = 51.7, DF = 7, P < 0.001). Nesting for Grafton Beach (Figure 4B) peaked at stages 5 and 6 (high tide and falling high tide); however these frequencies did not significantly differ from expected values (X² = 11.8, DF = 7, P > 0.2). The greatest frequency of nesting activity occurred during tidal stage 7 for Back Bay (Figure 2C), but was not significantly different from expected nesting values (X² = 5.8, DF = 7, P > 0.7).

Figure 2. Nesting events against tidal stage from 2005-2009 for (A) Turtle Beach, (B) Grafton Beach and (C) Back Bay. The expected number of nesting events per tidal stage, if nesting is evenly distributed across stages, is shown by the black line. See text for definitions of tidal stage.

Most leatherback nest were laid during phases 3 – 6 (22:00 – 02:00); and nesting frequencies for Back Bay were below expected values after stage 4 (Figure 3). For all three beaches, most nests were laid after stage 4; Turtle Beach (60.9%), Grafton Beach (67.4%) and Back Bay (67.8%). During 2005-2009 the frequency of nesting events was significantly different from expected for Turtle Beach (X² = 52.3, DF = 7, P < 0.001), Grafton Beach (X² = 23, DF = 7, P < 0.01) and Back Bay (X² = 42.5, DF = 7, P < 0.001).

Figure 3. Frequency of nesting events in relation to night phase from 2005-2009 for (A) Turtle Beach, (B) Grafton Beach and (C) Back Bay. The expected number of nests per night phase, if nesting is evenly distributed across phases, is shown by the black line. See text for definitions of night stage.

We expected there would be a link between tidal stage and nesting frequency as it has been reported in other studies that turtles may emerge in high frequencies when the tide is high (Fraser 1983), and therefore a preference for nesting at the highest tides of the month may also be present.

Tidal range varies during the lunar cycle, with greatest values during full and new Moon stages and lowest values during the first and third quarter stages. Thus, if tidal range is positively linked to nesting frequency, it may be expected that more nests are laid during full and new Moon stages. In Tobago, there was a significant difference between the number of nests laid at full Moon compared to the first quarter and new Moon in 2007. The median values of frequency of nesting activity of the full Moon, first quarter and new Moon phases equalled the lower quartile values for (Figure 1C) and the numerical difference between the medians of full Moon and new Moon was one [nesting event]. Although this difference is statistically significant it is very unlikely to be biologically significant as the numbers of leatherbacks nesting in 2007 were less than the other nesting years. However, in 2007, there was no significant difference between any lunar phase and the number of confirmed nests, suggesting no link between lunar phase and nesting behaviour. For combined nesting data from all years (Figure 1F) the full Moon and last quarter phases had a significantly greater frequency of nesting events than the first quarter lunar phase. As there were no observed differences in frequencies of nesting activity when confirmed nests were analysed, it is possible that the number of other nesting events i.e. false crawls and false crawls with body pits may have influenced the statistical test outcome.

Witt et al. (2009) observed an increase in leatherback nesting on neap days (first and last quarter) for monitored nesting beaches in Gabon. However Ya:lima:po Beach in French Guiana displayed peaks of leatherback nesting every 15 days during spring tides (full and new Moon) (Girondot & Fretey 1996). The influence of lunar phase on sea turtle nesting patterns appear to differ between regions and the influence may depend on local beach topography, tidal patterns (e.g. diurnal, semi-diurnal or mixed tides) and weather.

Per lunar month gravitational pulls peaks twice (full and new Moon) whereas lunar illumination only peaks once (full Moon). There were no differences between frequencies of nesting events (excluding 2007) or confirmed nests for any year between the full and new Moon. Lunar illumination does not appear to be having a discernible effect on leatherback nesting or nesting outcome on Tobago’s index beaches.

Lunar patterns and tides are intrinsically linked; there were no clear relationships between the number of nesting events and lunar phase and for each beach there were no significant correlations between the time of high tide and leatherback observed time. The geography, location and tidal pattern of the nesting beach will greatly influence the difference in the vertical and horizontal distance of high tides. Little Cumberland Island, Georgia has a tidal range of 2 m with a horizontal distance of ~63 m between low and high tide lines, a slope of 1.71 degrees (gentle sloping) and shows a high frequency of turtle emergences at high tidal stages (Frazer 1983). On Costa Rica’s Caribbean beach of Tortuguero, vertical tidal distances are only around one m and there is no relationship apparent between leatherback emergence and tidal stage (Leslie et al. 1996). Tobago also has a tidal range of around one m with the horizontal distance between high and low tide no greater than 20-30 m for all three studied beaches. There may be no correlation between high tide time and leatherback emergence time in Tobago because there is no major benefit in emerging at high tide rather than low tide i.e. the increased horizontal distance that leatherbacks face on Tobago’s beaches at low tide does not deter them from emerging to nest. A correlation between emergence time and high tide time may also not be present due to Tobago’s tidal patterns, as during diurnal tides there may be about 3 days where high tides occur during the day yet leatherbacks still emerge at night regardless.

Nesting frequencies for each tidal stage were variable per beach apart from stages 2 and 3, where nesting frequencies were consistently low for all beaches; it is possible that the tidal velocity may be influencing this trend. Gravid nesting female leatherbacks can weigh up to 435kg (range = 250-435kg, mean = 346.8kg) (Leslie et al. 1996), but a proportion of their weight will be supported when in the marine environment and so tidal velocities may affect them. Emerging at high and low tides could be when tidal velocity is at its lowest i.e. slack water occurs when the current changes (zero water velocity), and therefore the nesting leatherbacks are timing their emergence to low velocity currents in order to reduce energy expended on the approach to the beach. It is possible that on-shore currents are strongest when the tide is rising and this may deter leatherback nesting at tidal stages 2 and 3 as sea turtles prefer to nest where across-shore currents are low (Watanabe et al. 2004). There must be no advantageous carrier effect of approaching when the tide coming in as the observed numbers of leatherbacks emerging at these stages (2, 3 and 4) were consistently lower or similar to expected values for all beaches.

There were no significant differences observed between the frequency of leatherback nesting and tidal stage for Grafton Beach and Back Bay. This may reflect the differences in beach profile as a result of strong on-shore currents. Nesting beaches commonly have steep sloping banks and shelves created by strong on-shore currents (Lamont & Carthy 2007). Turtle Beach has the steepest beach slope compared to Grafton Beach and Back Bay possibly due to stronger on-shore currents and this could also explain the low nesting frequencies at stages 2 and 3 for Turtle Beach (Figure 2A).

Leatherbacks do not generally emerge to nest during the day due to potentially lethal temperatures and so regardless of high tides during the day they suppress emergence till night (Reina et al. 2002). The nesting process takes around 1.5 hours to complete so emerging between 22:00 and 02:00h reduces the probability that leatherbacks will be exposed to higher sand and air temperatures and therefore heat gain will not be a serious problem. Welsh & Tucker (2009) observed similar results for loggerhead peak emergence between 22:00 and 02:00h. Nesting leatherback turtles may be using the time at night as a cue to signal when to commence the nesting process. Intensive monitoring effort is often considered desirable when patrolling marine turtle nesting beaches (Jackson et al. 2008), but if long-term, time intensive monitoring programmes are not possible patrolling between these hours would encounter the majority of nesting events. However, monitoring may also be valuable during other times in order to reduce potential tourist, poacher and egg predator disturbance.

The impact of a conservation effort on the health i.e. numbers and physical wellbeing, of a population is difficult to judge in the short term, especially with long-lived, slow maturing species such as sea turtles. Information on nesting ecology and behaviour for a nesting region is useful in order to direct conservation effort and therefore more efficient coordination of field conservation and data collection. Due to the geographic location, structure of each beach and amplitude of the tide, Tobago’s beaches are not heavily influenced by environmental processes, and these processes do not appear to affect leatherback nesting. Peak leatherback emergence and nesting activity is most closely associated with the time at night. Further work may include accurately profiling Tobago’s three index beaches i.e. angle (slope of the beach), tidal amplitude, the difference between high and low tides per lunar phase, how tidal velocities change within the tidal cycle and whether there is habitat preference across the beaches as a result of potential spatial variation. Several nesting leatherbacks in Tobago are previously tagged in Trinidad and Grenada (personal observation) whereas some individuals return to nest many times within and between seasons; it would be interesting if the observed nesting trends in Tobago occur throughout nesting beaches in the Caribbean.

Our conclusions for leatherback nesting in Tobago are that a) the number of nesting events did not vary significantly between lunar phases; b) emergence time was not correlated with high tides; c) leatherbacks displayed a trend of nesting at and after high tide, with high nesting frequencies continuing to low tide; d) leatherbacks did not nest in high numbers when the tide was low rising to high tide; e) the highest frequency of nesting events took place between 22:00 and 02:00.

Acknowledgements: We wish to thank all the head patrollers and volunteers, both local and international, who collected the data analysed here. The University of Glasgow assisted with the costs of AL’s fieldwork in Tobago. We also thank Nick Kamenos for his help with data analysis.

CLOVIS, T. 2004. The status of marine turtle conservation in Tobago. Living World, Journal of the Trinidad and Tobago Field Naturalists’ Club, Supplement: 30-31.

CLOVIS, T. 2005. Sea Turtle Manual for Nesting Beach Hotels, Staff, Security and Tour Guides. Developed by SOS Tobago with assistance from WIDECAST. Scarborough, Tobago. 35pp.

DOW, W., K.L. ECKERT, M. PALMER & P. KRAMER. 2007. An Atlas of Sea Turtle Nesting Habitat for the Wider Caribbean Region. The Wider Caribbean Sea Turtle Conservation Network and The Nature Conservancy. WIDECAST Technical Report No. 6. Beaufort, North Carolina. 267 p ().

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

FRETEY, J. & M. GIRONDOT. 1989. Hydrodynamic factors involved in choice of nesting site and time of arrivals of leatherbacks in French Guiana. In: S.A. Eckert, K.L. Eckert & T.H. Richardson (Comps.). Proceedings of the 9th Annual Workshop on Sea Turtle Conservation and Biology. NOAA Technical Memorandum NMFS-SEFC-232.

GIRONDOT, M. & J. FRETEY. 1996. Leatherback turtles, Dermochelys coriacea, nesting in French Guiana, 1978-1995. Chelonian Conservation & Biology 2: 204-208.

GODLEY, B.J., A.C. BRODERICK, S. BLACKWOOD, L. COLLINS, K. GLOVER, C. MCALDOWIE, D. MCCULLOCH & J. MCLEOD. 1993. 1991 survey of marine turtles nesting in Trinidad and Tobago. Marine Turtle Newsletter 61:15-18.

JACKSON, A.J., A.C. BRODERICK, W.J. FULLER, F. GLEN, G.D. RUXTON & B.J. GODLEY. 2009. Sampling design and its effect on population monitoring: How much monitoring do turtles really need? Biological Conservation 141: 2932-2941.

KAMEL, S.J. & N. MROSOVSKY. 2004. Nest site selection in leatherbacks, Dermochelys coriacea: individual patterns and their consequences. Animal Behaviour 68: 357-366.

LALSINGH, G. 2008. Summary of Marine Turtle Nesting Activity, Save Our Sea Turtles. 21 pp.

LAMONT, M.M. & R.R. CARTHY. 2007. Response of nesting sea turtles to barrier island dynamics. Chelonian Conservation & Biology 6: 206-212.

LESLIE, A.J., D.N. PENICK, J.R. SPOTILA & F.V. PALADINO. 1996. Leatherback turtle, Dermochelys coriacea, nesting and nest success at Tortuguero, Costa Rica, in 1990-1991. Chelonian Conservation & Biology 2: 159-168.

MILLER, J.D. 1996. Reproduction in sea turtles. In: P.L. Lutz & J.A. Musick (Eds.). The Biology of Sea Turtles. CRC Press, Boca Raton, FL. pp. 51-82.

PIKE, D.A. 2008. Environmental correlates of nesting in loggerhead turtles, Caretta caretta. Animal Behaviour 76: 603-610.

REINA, R.D., P.A. MAYOR, J.R. SPOTILA, R. PIEDRA & F.V. PALDINO. 2002. Nesting ecology of the leatherback turtle, Dermochelys coriacea, at Parque Nacional Marino Las Baulas, Costa Rica: 1988-1989 to 1999-2000. Copeia 2002: 653-664.

WALLACE, B.P. & T.T. JONES. 2008. What makes marine turtles go: a review of metabolic rates and their consequences. Journal of Experimental Marine Biology & Ecology 356: 8-24.

WATANABE, K., S. SEINO & T. UDA. 2004. Changes in emergence location of loggerhead sea turtles resulting from construction of offshore breakwaters. In: Mast, R.B., B.J. Hutchinson, & A.H. Hutchinson (Comps.). Proceedings of the 24th Annual Symposium on Sea Turtle Biology and Conservation. NOAA Tech. Memo. NMFS-SEFSC-567.

WELSH, R. & A.D. TUCKER. 2009. Shifting patterns of nocturnal emergence events of nesting loggerhead turtles (Caretta caretta). Marine Turtle Newsletter 125:10-12.

WITT, M.J., B. BAERT, A.C. BRODERICK, A. FORMIA, J. FRETEY, A. GIBUDI , G.A.M. MOUNGUENGUI , C. MOUSSOUNDA , S. NGOUESSONO, R.J. PARNELL, D. ROUMET, G.P.SOUNGUET, B. VERHAGE , A. ZOGO & B.J. GODLEY. 2009. Aerial surveying of the world’s largest leatherback turtle rookery: a more effective methodology for large-scale monitoring. Biological Conservation 142: 1719-1727.

WRIGHT, J., A. COLLING & D. PARK 1989. Waves, tides and shallow-water processes: prepared by an Open University course team. Oxford: Butterworth/Heinemann in association with the Open University.