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Analysis of clutch size variation for loggerhead sea turtles ( Caretta caretta ) nesting on Bald Head Island, NC USA. Introduction. Regression Results. ANOVA Results.
Loggerhead sea turtles exhibit variation in the number of clutches laid per season as well as number of eggs per clutch within a single season. The following analyses explore variables that have been shown to have significant effects on clutch size in the literature. A regression analysis explores clutch size (number of eggs per clutch) as a function of year and straight carapace length (SCL). The change in clutch size as the season progressed was also analyzed to determine if clutches 1-3 were significantly different in size. Significant results will help to understand the fluctuation in nesting behavior of loggerhead sea turtles.
The two analyses presented here will be included as parameters in a loggerhead sea turtle population model. The data are significant because Bald Head Island lies at the northern range of the northern nesting subpopulation of loggerhead sea turtles in the USA.
The small number of observations of turtles laying more than 3 nests in a season is not sufficient to a statically robust ANOVA test. For this reason, the analysis was limited to observations of females laying three or fewer nests in a season.
Figure 1 displays the distribution of clutch size for the 342 observations used in the regression. Each observation represents one randomly selected clutch for each tagged female nesting on Bald Head Island from 1991-2005. The normal distribution of eggs indicates a a good fit for use of clutch size as a response variable in the regression.
The ANOVA was limited to 743 observations. All observations for all turtles were included. Outliers result from nests that suffered degradation during incubation, resulting in incomplete egg counts during excavations.
There is no significant difference between clutch size for the first three observed clutches in a season (F1 = 0.12, p = .7333).
The regression of SCL and year to clutch size is significant (F29,312 = 6.53, p<.0001, r2= 0.378). While SCL and year explain 37% of the variation in clutch size, there is still a great deal of unexplained noise found in the data, as can be observed in Figure 2.
The positive trend of increasing clutch size with increasing SCL is clearly evident. Straight carapace length has a significant effect on clutch size (F1,312 = 81.87, p<.0001).
The significant correlation of carapace length with clutch size has also been identified for females nesting on Cyprus (Broderick et al. 2003), Cephalonia, Greece (Hays & Speakman 1991), and Little Cumberland Island, Georgia USA (Frazer & Richardson 1986).
Melissa Hedges1,2 & Jim Berkson2
1 Bald Head Island Conservancy, 2NOAA Fisheries RTR Unit at Virginia Tech
Map of study site, Bald Head Island, North Carolina, USA. The island has 9.5 miles of beach on which sea turtles nest.
Data were collected on Bald Head Island, NC USA from 1991 to 2005 during nightly beach patrols, daily nest monitoring and nest excavations. Individual females are identified by Inconel flipper tags (National Band and Tag Co.). Yearly averages of SCL were taken for females that were measured multiple times a year. The dataset is not a complete census of nesting females, their nests, or egg counts. The average sighting rate of nesting females from 1991-2005 was 78% and lower for nesting data due to nest destruction during incubation, prohibiting an excavation.
Regression of the effect of year and SCL on clutch size
Observations included in the regression analysis are those for which both the straight carapace length (cm) and total number of observed eggs are available. To eliminate female effects, one random observation was chosen for each individual female if she laid more than one clutch within the 15 year period.Clutches of eggs with less then 40 observed eggs were removed from the analysis. All observations with less than 40 observed eggs per clutch suffered a disturbance during incubation that resulted in a loss of eggs; these observations are not representative of the total number of eggs laid.
ANOVA model of observed clutch within year and clutch size
Data used to perform the ANOVA were limited to turtles laying three or fewer nests. There are not enough females laying four or more nests to perform a balanced ANOVA. All observations were used in the ANOVA because it is reasonable to assume observations for individual females nesting in different years are independent. A one-way ANOVA was performed using observed clutch number as the variable.
The preliminary survival rates presented in the abstract were obtained using program MARK. The authors believe that multistate models represent a more biologically sensible model to obtain survival and recapture rates for loggerhead sea turtles. Rivalan et al. (2005) recently applied multistate modeling to leatherback sea turtle capture data and preliminary runs show that the multistate model also fits the capture data from the Bald Head Island study. Survival and recapture rates will be calculated in the near future.
Year has a significant effect on clutch size (F14,312 = 5.96, p<.0001). The abnormal data from 1998 is a result of a high destruction rate of nests and limited data.
The interaction effect of year*SCL was not significant (F14,312 = 1.71, p = .053).
Clutch size variation is a function of numerous factors as can be seen from only a moderate amount of variation explained by the two variables analyzed here. Long time series of data are not particularly common for loggerhead turtles and only a limited number of variables are available for analyses.
Broderick, A.C., F. Glen, B.J. Godley, G.C. Hays. 2003. Variation in reproductive output of marine turtles. Journal of Experimental Marine Biology and Ecology 288:95-109.
Frazer, N.B. and J.I. Richardson. 1986. The relationship of clutch size and frequency to body size in loggerhead turtles, Caretta caretta. Journal of Herpetology 20:81-86.
Hays, G.C. and J.R. Speakman. 1991. Reproductive investment and optimum clutch size of loggerhead sea turtles (Caretta caretta). Journal of Animal Ecology 60:455-462.
Rivalan, P, A. Prevot-Julliard, R. Choquet, R. Pradel, B. Jacquemin, and M. Girondot. 2005. Trade-off between current reproductive effort and delay to next reproduction in leatherback sea turtle. Oecologia 145:564-574.
We would like to thank Penelope Pooler (Virginia Tech) for statistical consulting and data interpretation and committee members Nancy Thompson and Marcella Kelly.
Special thanks to: Penelope Pooler for statistical consulting