292
The Biology of Sea Turtles, Vol. II
aggregations brought about by male-mediated gene flow. Studies have indicated that
the genetic structuring based on nuclear DNA (nDNA) is not as strong as that of mtDNA,
indicating that there may be male-mediated gene flow between proximate nesting
regions (Karl et al., 1992; Francisco, 2001). If this is the case, the status of one nesting
aggregation may not be independent of the status of an adjacent aggregation.
For example, consider temperature-dependent sex determination, which occurs
in all species of sea turtles. Pivotal and transitional ranges of temperatures determine
whether the nest will produce males, females, or both, with cooler temperatures
producing males and warmer temperatures producing females (Mrosovsky and
Pieau, 1991). Studies recording nest chamber temperatures near Cape Canaveral,
FL, indicate that these nests produce nearly all females (Mrosovsky and Provancha,
1989; Hanson et al., 1998). The smaller nesting aggregation adjacent to the south
Florida aggregation extends northward to North Carolina, and presumably produces
a greater number of males. Although females from these two regions will return to
nest in their region of birth, it is possible that males mate with females from both
regions. We do not understand the mating structure of loggerhead sea turtles, but it
may be that the males produced in the northern nesting areas are important to the
continued health of the very large south Florida nesting aggregation.
Similar genetic structuring has been identified in the green (Bowen et al., 1992;
Allard et al., 1994; Lahanas et al., 1994; Encalada et al., 1996) and hawksbill (Bass
et al., 1996) sea turtles. Leatherbacks show somewhat less genetic structuring,
whereby adjacent nesting regions are indistinguishable, suggesting either that the
nesting areas were recently colonized or that there is less precise natal nest site
fidelity for this species (Dutton et al., 1999).
Juvenile feeding grounds for sea turtles are of mixed stocks and consist of individuals from different nesting regions. An analysis of mtDNA from loggerheads that were
caught in drifting longline fisheries within the Mediterranean demonstrated that 51–53%
of the turtles originated from nesting aggregations within the Mediterranean, 45–47%
originated from the south Florida nesting aggregation, and approximately 2% originated
from the northeast Florida to North Carolina nesting aggregation (Laurent et al., 1998).
Similarly, juvenile feeding grounds off Charleston, S.C., are composed of approximately
50% south Florida and 50% northeast Florida to North Carolina loggerheads (Sears
et al., 1995). For hawksbills, Bowen et al. (1996) found that feeding grounds at Mona
Island, Puerto Rico, were composed of turtles originating from throughout the Caribbean. Juvenile green sea turtles found foraging at Great Inagua, Bahamas, also originated from nesting colonies throughout the Caribbean (Lahanas et al., 1997). Hence,
fisheries that incidentally take sea turtles do not just impact local nesting populations,
but have a much broader influence. The spatial and genetic structure of sea turtles is
complex and requires that we assess population dynamics on a global scale.
11.4 ASSESSING POPULATION CHANGE
THROUGH TIME
Sea turtle populations exhibit both long- and short-term dynamics (Figure 11.1). Yearto-year changes in sea turtle abundance are caused by environmental stochasticity,
1123 book.book Page 292 Tuesday, November 12, 2002 7:43 AM
The Biology of Sea Turtles, Vol. II
aggregations brought about by male-mediated gene flow. Studies have indicated that
the genetic structuring based on nuclear DNA (nDNA) is not as strong as that of mtDNA,
indicating that there may be male-mediated gene flow between proximate nesting
regions (Karl et al., 1992; Francisco, 2001). If this is the case, the status of one nesting
aggregation may not be independent of the status of an adjacent aggregation.
For example, consider temperature-dependent sex determination, which occurs
in all species of sea turtles. Pivotal and transitional ranges of temperatures determine
whether the nest will produce males, females, or both, with cooler temperatures
producing males and warmer temperatures producing females (Mrosovsky and
Pieau, 1991). Studies recording nest chamber temperatures near Cape Canaveral,
FL, indicate that these nests produce nearly all females (Mrosovsky and Provancha,
1989; Hanson et al., 1998). The smaller nesting aggregation adjacent to the south
Florida aggregation extends northward to North Carolina, and presumably produces
a greater number of males. Although females from these two regions will return to
nest in their region of birth, it is possible that males mate with females from both
regions. We do not understand the mating structure of loggerhead sea turtles, but it
may be that the males produced in the northern nesting areas are important to the
continued health of the very large south Florida nesting aggregation.
Similar genetic structuring has been identified in the green (Bowen et al., 1992;
Allard et al., 1994; Lahanas et al., 1994; Encalada et al., 1996) and hawksbill (Bass
et al., 1996) sea turtles. Leatherbacks show somewhat less genetic structuring,
whereby adjacent nesting regions are indistinguishable, suggesting either that the
nesting areas were recently colonized or that there is less precise natal nest site
fidelity for this species (Dutton et al., 1999).
Juvenile feeding grounds for sea turtles are of mixed stocks and consist of individuals from different nesting regions. An analysis of mtDNA from loggerheads that were
caught in drifting longline fisheries within the Mediterranean demonstrated that 51–53%
of the turtles originated from nesting aggregations within the Mediterranean, 45–47%
originated from the south Florida nesting aggregation, and approximately 2% originated
from the northeast Florida to North Carolina nesting aggregation (Laurent et al., 1998).
Similarly, juvenile feeding grounds off Charleston, S.C., are composed of approximately
50% south Florida and 50% northeast Florida to North Carolina loggerheads (Sears
et al., 1995). For hawksbills, Bowen et al. (1996) found that feeding grounds at Mona
Island, Puerto Rico, were composed of turtles originating from throughout the Caribbean. Juvenile green sea turtles found foraging at Great Inagua, Bahamas, also originated from nesting colonies throughout the Caribbean (Lahanas et al., 1997). Hence,
fisheries that incidentally take sea turtles do not just impact local nesting populations,
but have a much broader influence. The spatial and genetic structure of sea turtles is
complex and requires that we assess population dynamics on a global scale.
11.4 ASSESSING POPULATION CHANGE
THROUGH TIME
Sea turtle populations exhibit both long- and short-term dynamics (Figure 11.1). Yearto-year changes in sea turtle abundance are caused by environmental stochasticity,
1123 book.book Page 292 Tuesday, November 12, 2002 7:43 AM