280
The Biology of Sea Turtles, Vol. II
pelagic stage, juveniles of most species recruit to nearshore habitats and switch to
feeding on benthic organisms. For juveniles found in temperate regions, there are
usually migrations between summer and winter habitats, whereas migrations are not
as extensive for more tropical species (Musick and Limpus, 1997).
A dramatic shift in habitat and diet occurs at least once in the life cycle of
most juvenile sea turtles. This happens when juvenile sea turtles switch from an
oceanic, pelagic habitat and epipelagic feeding to a neritic habitat and benthic
feeding. For the green sea turtle, this habitat switch is accompanied by a shift
from omnivory to herbivory (Bjorndal et al., 1995; 2000a). Most of the species of
sea turtles appear to spend little time as pelagic juveniles, as they are first seen
in coastal habitat at small sizes. The loggerhead is an exception to this; they do
not recruit to nearshore habitats until they are 40–50 cm straight carapace length
(SCL) for the southeastern U.S. population or > 70 cm SCL in Australia (Limpus
et al., 1994; Bjorndal et al., 2000b; 2001). Exceptions to the benthic habitat shift
by juveniles occur in leatherback and, possibly, olive ridley sea turtles. Leatherbacks have been observed foraging in coastal waters; however, they are considered
to remain pelagic throughout their lives (Eckert et al., 1989). Little information
exists for olive ridleys, but they appear to remain epipelagic until they are adults,
when they have been observed using both nearshore and pelagic habitats (Reichart,
1993; Plotkin et al., 1996). An additional exception to the general life cycle is the
flatback sea turtle, which remains neritic throughout its life (Chaloupka and
Musick, 1997).
11.2.2 G ROWTH R ATES AND S TAGE L ENGTHS
Age at sexual maturation (ASM) and growth rates are not definitively known for
any species. Typically, age-based growth functions are applied to mark–recapture,
length frequency, or skeletochronological data to infer ASM on the basis of size at
maturity. We have summarized growth rate studies for sea turtles with the authors’
estimates for ASM or the portion of the life cycle that they studied (Table 11.1).
Each habitat that sea turtles use over their ontogeny has different environmental
parameters, such as food availability and temperature, that will influence growth
rates. Little information is available on sea turtle growth rates in the pelagic environment (although see Zug et al., 1995; and Bjorndal et al., 2000b). In a skeletochronology study, Snover et al. (in review) found evidence of a shift in growth rates
that corresponds to the ontogenetic shift from pelagic to benthic feeding. The shift
is typified by a surge in growth followed by declining growth rates. Chaloupka and
Limpus (1997) and Limpus and Chaloupka (1997) noted increasing growth rates
after settlement in the hawksbill and green sea turtles. Growth rates increased until
50–60 cm curved carapace length (CCL) for the hawksbill and 60–63 cm CCL for
the green. After these peaks, growth rates declined monotonically to adulthood in
both species. Of course, it cannot be determined if the increasing growth rates
postsettlement were continuing from the pelagic stages or if they were surges in
growth after settlement. However, the results of these studies on sea turtle growth
highlight the likelihood that sea turtle growth rates are compartmentalized and that
shifts occur in conjunction with ontogenetic habitat shifts.
1123 book.book Page 280 Tuesday, November 12, 2002 7:43 AM
The Biology of Sea Turtles, Vol. II
pelagic stage, juveniles of most species recruit to nearshore habitats and switch to
feeding on benthic organisms. For juveniles found in temperate regions, there are
usually migrations between summer and winter habitats, whereas migrations are not
as extensive for more tropical species (Musick and Limpus, 1997).
A dramatic shift in habitat and diet occurs at least once in the life cycle of
most juvenile sea turtles. This happens when juvenile sea turtles switch from an
oceanic, pelagic habitat and epipelagic feeding to a neritic habitat and benthic
feeding. For the green sea turtle, this habitat switch is accompanied by a shift
from omnivory to herbivory (Bjorndal et al., 1995; 2000a). Most of the species of
sea turtles appear to spend little time as pelagic juveniles, as they are first seen
in coastal habitat at small sizes. The loggerhead is an exception to this; they do
not recruit to nearshore habitats until they are 40–50 cm straight carapace length
(SCL) for the southeastern U.S. population or > 70 cm SCL in Australia (Limpus
et al., 1994; Bjorndal et al., 2000b; 2001). Exceptions to the benthic habitat shift
by juveniles occur in leatherback and, possibly, olive ridley sea turtles. Leatherbacks have been observed foraging in coastal waters; however, they are considered
to remain pelagic throughout their lives (Eckert et al., 1989). Little information
exists for olive ridleys, but they appear to remain epipelagic until they are adults,
when they have been observed using both nearshore and pelagic habitats (Reichart,
1993; Plotkin et al., 1996). An additional exception to the general life cycle is the
flatback sea turtle, which remains neritic throughout its life (Chaloupka and
Musick, 1997).
11.2.2 G ROWTH R ATES AND S TAGE L ENGTHS
Age at sexual maturation (ASM) and growth rates are not definitively known for
any species. Typically, age-based growth functions are applied to mark–recapture,
length frequency, or skeletochronological data to infer ASM on the basis of size at
maturity. We have summarized growth rate studies for sea turtles with the authors’
estimates for ASM or the portion of the life cycle that they studied (Table 11.1).
Each habitat that sea turtles use over their ontogeny has different environmental
parameters, such as food availability and temperature, that will influence growth
rates. Little information is available on sea turtle growth rates in the pelagic environment (although see Zug et al., 1995; and Bjorndal et al., 2000b). In a skeletochronology study, Snover et al. (in review) found evidence of a shift in growth rates
that corresponds to the ontogenetic shift from pelagic to benthic feeding. The shift
is typified by a surge in growth followed by declining growth rates. Chaloupka and
Limpus (1997) and Limpus and Chaloupka (1997) noted increasing growth rates
after settlement in the hawksbill and green sea turtles. Growth rates increased until
50–60 cm curved carapace length (CCL) for the hawksbill and 60–63 cm CCL for
the green. After these peaks, growth rates declined monotonically to adulthood in
both species. Of course, it cannot be determined if the increasing growth rates
postsettlement were continuing from the pelagic stages or if they were surges in
growth after settlement. However, the results of these studies on sea turtle growth
highlight the likelihood that sea turtle growth rates are compartmentalized and that
shifts occur in conjunction with ontogenetic habitat shifts.
1123 book.book Page 280 Tuesday, November 12, 2002 7:43 AM