Sea Turtle Population Ecology
285
and Hohn (in review) present evidence of the annual nature of the growth marks for
loggerhead and Kemp’s ridley sea turtles. In addition, Snover and Hohn (in review)
demonstrate a constant proportionality between bone growth and somatic growth
measured as SCL for the loggerhead and Kemp’s ridley. This will allow the use of
growth mark diameters for estimating growth rates in these species, making skeletochronology a very powerful tool in realizing actual sea turtle growth rates and in
understanding how much individual variability influences our perceptions.
11.2.3 A GE AT S EXUAL M ATURATION
All species of sea turtles exhibit delayed maturity. The extent of this delay has only
recently been appreciated by researchers. Initial estimates of ASM for loggerheads
from the southeast U.S. were around 20 years (Frazer and Ehrhart, 1985; Mendonca,
1981) More recently, however, estimates exceed 30 years (Frazer et al., 1994,
National Marine Fisheries Service, 2001). Exactly where the correct age or, more
accurately, age range for reproductive maturity lies for this population or any other
loggerhead population or sea turtle species remains to be definitively determined.
Long-term population growth rates and responses to perturbations are strongly
influenced by ASM (Heppell et al., 2000a). Heppell et al. (2002a) demonstrated the
effect of increasing ASM on the effectiveness of turtle excluder devices (TEDs) for
the southeast U.S. loggerhead population. The longer generation times caused by
increased ASM result in slower long-term population growth.
There is a great deal of variability in ASM among those species that have been
studied. For the hard-shell turtles, Kemp’s ridleys mature earliest at 7–12 years
(Table 11.1). Kemp’s also have the smallest adult female size of 64.6 cm SCL (Van
Buskirk and Crowder, 1994). For loggerheads and greens, the high-end estimates
are similar, with values in the 30–35 year range (Table 11.1). Average adult female
body sizes in these two species are similar (87.0 cm SCL for the loggerhead and
99.1 cm SCL for the green; Van Buskirk and Crowder, 1994). Given that postpelagic
loggerheads are primarily carnivorous, whereas postpelagic greens are herbivorous,
it is possible that greens have a longer benthic juvenile stage than do loggerheads
simply because of the lower conversion potential of plant material.
The only other sea turtle for which ASM has been estimated from growth rate
studies is the leatherback. Zug and Parham (1996) estimated an ASM of 13–14 years
for Pacific leatherbacks using skeletochronology. Leatherbacks are the largest species
of sea turtle (148.7 cm SCL for the adult female; Van Buskirk and Crowder, 1994),
but this may not indicate that they should have the longest ASM. They have been
shown to be distinct from the hard-shell sea turtles using different means of classification. They show a distinct phylogenetic separation from the rest of the sea turtle
species (Bowen and Karl, 1997) and a distinct phenotypic separation based on
reproductive traits (Van Buskirk and Crowder, 1994). In addition, leatherbacks are
warm-blooded and might be expected to have a faster growth rate than poikilothermic
species of similar mass (Musick, 1999). It is possible that growth rates in juvenile
leatherbacks are extremely high in comparison to the hard-shell sea turtles, and that
they do have a comparatively low ASM, but much more information is needed on
this species.
1123 book.book Page 285 Tuesday, November 12, 2002 7:43 AM
285
and Hohn (in review) present evidence of the annual nature of the growth marks for
loggerhead and Kemp’s ridley sea turtles. In addition, Snover and Hohn (in review)
demonstrate a constant proportionality between bone growth and somatic growth
measured as SCL for the loggerhead and Kemp’s ridley. This will allow the use of
growth mark diameters for estimating growth rates in these species, making skeletochronology a very powerful tool in realizing actual sea turtle growth rates and in
understanding how much individual variability influences our perceptions.
11.2.3 A GE AT S EXUAL M ATURATION
All species of sea turtles exhibit delayed maturity. The extent of this delay has only
recently been appreciated by researchers. Initial estimates of ASM for loggerheads
from the southeast U.S. were around 20 years (Frazer and Ehrhart, 1985; Mendonca,
1981) More recently, however, estimates exceed 30 years (Frazer et al., 1994,
National Marine Fisheries Service, 2001). Exactly where the correct age or, more
accurately, age range for reproductive maturity lies for this population or any other
loggerhead population or sea turtle species remains to be definitively determined.
Long-term population growth rates and responses to perturbations are strongly
influenced by ASM (Heppell et al., 2000a). Heppell et al. (2002a) demonstrated the
effect of increasing ASM on the effectiveness of turtle excluder devices (TEDs) for
the southeast U.S. loggerhead population. The longer generation times caused by
increased ASM result in slower long-term population growth.
There is a great deal of variability in ASM among those species that have been
studied. For the hard-shell turtles, Kemp’s ridleys mature earliest at 7–12 years
(Table 11.1). Kemp’s also have the smallest adult female size of 64.6 cm SCL (Van
Buskirk and Crowder, 1994). For loggerheads and greens, the high-end estimates
are similar, with values in the 30–35 year range (Table 11.1). Average adult female
body sizes in these two species are similar (87.0 cm SCL for the loggerhead and
99.1 cm SCL for the green; Van Buskirk and Crowder, 1994). Given that postpelagic
loggerheads are primarily carnivorous, whereas postpelagic greens are herbivorous,
it is possible that greens have a longer benthic juvenile stage than do loggerheads
simply because of the lower conversion potential of plant material.
The only other sea turtle for which ASM has been estimated from growth rate
studies is the leatherback. Zug and Parham (1996) estimated an ASM of 13–14 years
for Pacific leatherbacks using skeletochronology. Leatherbacks are the largest species
of sea turtle (148.7 cm SCL for the adult female; Van Buskirk and Crowder, 1994),
but this may not indicate that they should have the longest ASM. They have been
shown to be distinct from the hard-shell sea turtles using different means of classification. They show a distinct phylogenetic separation from the rest of the sea turtle
species (Bowen and Karl, 1997) and a distinct phenotypic separation based on
reproductive traits (Van Buskirk and Crowder, 1994). In addition, leatherbacks are
warm-blooded and might be expected to have a faster growth rate than poikilothermic
species of similar mass (Musick, 1999). It is possible that growth rates in juvenile
leatherbacks are extremely high in comparison to the hard-shell sea turtles, and that
they do have a comparatively low ASM, but much more information is needed on
this species.
1123 book.book Page 285 Tuesday, November 12, 2002 7:43 AM