2007 SOUTHEASTERN NATURALIST 6(2):365–369
Summer Abundance Estimates of Caretta caretta
(Loggerhead Turtles) in Core Sound, NC
Christopher R. Sasso1,*, Joanne Braun-McNeill2, Larisa Avens2,
and Sheryan P. Epperly1
Abstract - We estimated summer abundance of juvenile Loggerhead Turtles at our
study site in Core Sound, NC with a Horvitz-Thompson type estimator, which uses
count data and recapture probability to estimate abundance. Abundance ranged from
192 (95% CI = 88–1047) to 633 (95% CI = 219–1047) turtles over the six years of
this study. These results provide preliminary estimates of juvenile Loggerhead Turtle
abundance during the summer in Core Sound.
Introduction
Monitoring of sea turtle population abundance often focuses on nesting
beaches where it can be more easily assessed than on foraging grounds
(Bjorndal et al. 2005), but estimates of population abundance from other
habitats and life stages are needed to assist with assessing the effects of
management actions for sea turtles (Chaloupka and Musick 1997, Heppell
et al. 2003).
Heppell et al. (2003) noted that more information is needed about
juvenile Caretta caretta L. (Loggerhead Turtles, hereafter Loggerheads) in
foraging habitats in order to improve the understanding of the status of this
species. Along the US Atlantic coast, juvenile Loggerheads range from 50
to 80 cm straight carapace length and occupy foraging areas in both inshore
and nearshore waters (Musick and Limpus 1997). In North Carolina, sea
turtles are seasonally present in the inshore sounds when the water
temperatures are suitable, but migrate to warmer waters when water temperatures
decline (Epperly et al. 1995a, b). To date, there are no estimates
of abundance for juvenile Loggerheads, the most abundant sea turtle species
inhabiting the North Carolina sounds (Epperly et al. 1995b). The
incidental capture of these turtles in commercial fishing gear in the North
Carolina sounds offers an opportunity to assess population size for this life
stage in foraging habitats.
The National Marine Fisheries Service (NMFS) has conducted a study to
monitor sea turtles inhabiting North Carolina’s Core Sound since 1998.
During their residence in the Sound, turtles are often incidentally captured in
pound nets set inshore of the barrier islands. Pound nets are a stationary
fishing gear (Higgins and Pearson 1928) that allow incidentally captured
1NOAA Fisheries, SEFSC, 75 Virginia Beach Drive Miami, FL 33149. 2NOAA
Fisheries, 101 Pivers Island Road, Beaufort, NC 28516. *Corresponding author -
chris.sasso@noaa.gov.
366 Southeastern Naturalist Vol. 6, No. 2
turtles to surface, breathe, and be released alive. Herein we present estimates
of summer abundance in our study area.
Materials and Methods
Monitoring
We sampled 5 to 8 pound nets twice per week, June through August, from
1998 to 2004. The study site encompassed an area of 18.68 km2 in central Core
Sound, NC. We double-tagged turtles with Inconel Style 681 tags (National
Band and Tag Company, Newport, KY) applied to the trailing edge of each
rear flipper. In addition, we tagged all turtles with 125 kHz unencrypted
Passive Integrated Transponder (PIT) tags (Destron-Fearing Corp. South, St.
Paul, MN) injected subcutaneously above the second most proximal scale of
the trailing margin of the left front flipper to ensure identification of the turtle
in the event that both Inconel tags were shed. Application of the three tags in
this manner resulted in negligible (0.005%) total tag loss (Braun-McNeill et
al. 2003). A unique capture history was created for each individual each year,
resulting in seven annual trapping periods.
Abundance Estimates
Using recapture probabilities from the mark-recapture data (Sasso et. al
2006), we combined recapture probabilities in a Horvitz-Thompson type
estimator to estimate annual abundance in the summer (Seber 1982). This
estimator, whose merits and details for use with sea turtle populations have
been discussed in Bjorndal et al. (2005), Chaloupka (2000), and Chaloupka
and Limpus (2001), uses turtle-count data divided by recapture probability
to estimate abundance:
Ni = (ni / pi),
where Ni is the number of turtles in the sampling population in year i, ni is
the number of turtles captured in the ith year, and pi is the recapture
probability in the ith year. We determined approximate 95% confidence
intervals (CI) of Ni as Ni ± 1.96SE(Ni), calculating the standard error of Ni
(Loery et al. 1997) as
SE(Ni) = {(ni / pi)2[(var(pi) / pi)2]}0.5,
where var(pi) is the variance of the recapture probability in the ith year.
Results and Discussion
We tagged 693 Loggerheads from 42.3 cm to 102.0 cm in straight
carapace length (mean = 63.6 ± 7.4 cm), with 63 individuals recaptured at
least once (Sasso et al. 2006).
Sasso et al. (2006) found the best model of the data had time-independent
apparent survival and proportion of residents with time-dependent capture
probabilities. They estimated survival was estimated to be 0.81 (95% CI =
2007 C.R. Sasso, J. Braun-McNeill, L. Avens, and S.P. Epperly 367
0.69–0.92), with approximately 75% (95% CI = 67–83) of newly captured
individuals estimated to be transients (Sasso et al. 2006). Recapture probabilities
(Table 1) ranged from 0.15 (95% CI = 0.05–0.25) to 0.64 (95% CI =
0.42–0.86) (Sasso et al. 2006).
Summer population estimates of juvenile Loggerheads for our study site
in Core Sound ranged from 192 (95% CI = 127–257) to 633 (95% CI = 219–
1047) turtles (Table 1). While providing a reasonable range of the number of
Loggerheads at our study site, the point estimates should not be interpreted
literally, as there is uncertainty around these estimates and no real differences
given the confidence intervals (Table 1). Our highest estimate of
population size was the year with the lowest recapture probability, while our
lowest population estimate was in the year with the highest recapture probability
(Table 1). The large variability in recapture probability among years
precludes using these estimates for any predictive trend in Loggerhead
abundance in Core Sound, especially given the relationship between recapture
rates and population size estimates. Increasing the recapture probability
would yield more accurate and precise estimates, but may not be possible
given the transient behavior of juvenile Loggerheads in North Carolina
(Sasso et al. 2006).
Even though our estimates likely represent the population from an area
larger than the 18.68 km2 study site as turtles are likely to use an area larger
than the study site, they may not be an estimate for all of Core Sound.
Furthermore, our estimates are certainly not reflective of the juvenile Loggerhead
population likely to be present in the entire sound system of North
Carolina because our study site is relatively small in comparison to the entire
sound system in which turtles are present. However, our study does provide
preliminary estimates of summer abundance in central Core Sound and
indicates that large numbers of juvenile turtles are likely to utilize the sounds
of North Carolina when water temperatures are suitable.
An increase in the spatial scale of monitoring efforts within the sounds
could provide total abundance estimates for North Carolina’s sounds and
would likely maximize recapture probabilities. The establishment of a network
of monitoring sites along the US Atlantic coast may make it possible to
estimate Loggerhead population size across the entire range of the species
and for other age classes as well.
Table 1. Summer population estimates for Loggerhead Sea Turtles in Core Sound, NC. 95%
confidence intervals are in parentheses.
Year Turtles captured Recapture probability Abundance estimate
1999 94 0.35 (0.11–0.58) 269 (88–449)
2000 123 0.64 (0.42–0.86) 192 (127–257)
2001 118 0.38 (0.22–0.54) 311 (182–438)
2002 121 0.25 (0.11–0.39) 484 (218–749)
2003 147 0.30 (0.14–0.46) 490 (233–746)
2004 95 0.15 (0.05–0.25) 633 (219–1047)
368 Southeastern Naturalist Vol. 6, No. 2
Acknowledgments
We wish to thank all those that helped us in the collection of this data: Joel Bell,
Ruth Boettcher, Bonnie Brown, Wendy Cluse, Tim Ellis, Matthew Godfrey, Rebecca
Goldman, April Goodman, Lisa Goshe, Kristen Hart, Jennifer Keller, Matt Kimball,
Roger Mays, Catherine McClellan, Glen Montgomery, Erica Nichols, Zoe Pettitt,
Melissa Snover, Sabrina Varnam, and Jerald Weaver. We also wish to thank all the
commercial pound-net fishermen that cooperated with us, specifically Leonard
Goodwin. Paul Richards and Liz Brooks provided comments on an earlier draft of
this manuscript.
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