286
The Biology of Sea Turtles, Vol. II
11.2.4 G ROWTH R ATE V ARIABILITY
The ranges in ASM values reported in studies are generally indicative of uncertainty
in size-at-maturity (minimum or average size of nesters) or in the calculated growth
curve (Table 11.1), rather than an attempt to capture the true variability in ages at
which maturity occurs. There can be a great deal of year-to-year variability in growth
rates of loggerheads within the same 10-cm juvenile size-class (Braun-McNeill et al.,
in review). The cumulative variability over nine 10-cm size classes can result in a
wide range of predicted ASMs for individual turtles.
Variability in growth rates may be caused by a number of factors such as genetics,
environmental conditions, and individual health. Limpus and Chaloupka (1997)
found that immature, female green turtles in the southern Great Barrier Reef displayed significantly decreased growth rates at a time that coincided with a strong
ENSO event in the early 1980s. Bjorndal et al. (2000a) measured growth rates of
immature green sea turtles in the southern Bahamas. Over their 18-year study period,
population densities increased by a factor of six, then decreased by a factor of three.
They found a significant negative relationship between the estimated annual population density and the estimated mean annual growth rate, suggesting density-dependent effects on growth rates for this population.
There is also evidence of sex-specific growth rates (Chaloupka and Limpus,
1997; Limpus and Chaloupka, 1997). Female hawksbills from the southern Great
Barrier Reef display faster growth rates at all benthic juvenile sizes than do males
(Chaloupka and Limpus, 1997). In the same region, at sizes greater than 60 cm CCL,
female green sea turtles grow faster than male green sea turtles in the southern Great
Barrier Reef (Limpus and Chaloupka, 1997). This same study found a sexual dimorphism in adult body size for greens, indicating that breeding males are an average
of 7 cm CCL smaller than breeding females. It has been demonstrated for reptiles
that animals with fast growth mature to a larger size than animals with slow growth,
and this may also be the case with sea turtles (Stamps et al., 1998). It is unclear
how much of the variability observed in growth rates of similar-sized individuals
may be attributable to sex-specific growth rates.
Most growth curves of sea turtles have been prepared with small sample sizes
(Table 11.1). If the variability in growth rates for sea turtles is high, it will take large
sample sizes to determine the “average” growth rate. With the possible exception of
the data from Australia, from which large sample sizes are available, we can look
at stage duration and ASM as only rough estimates.
11.2.5 S URVIVAL R ATES
One risk of the delayed maturity representative of the sea turtle life history is the
increased risk of dying before reproducing. ASM is extremely high in sea turtles
(Table 11.1), and hatchling survival is extremely low; therefore, there must be
high survival of juveniles and adults (Congdon et al., 1993). Crouse et al. (1987)
determined that for loggerheads in the southeast U.S., the population intrinsic rate
of increase ( r ) was most sensitive to proportional changes in the survival rate of
large juveniles, which equates to the benthic juveniles. In other turtle species,
1123 book.book Page 286 Tuesday, November 12, 2002 7:43 AM
The Biology of Sea Turtles, Vol. II
11.2.4 G ROWTH R ATE V ARIABILITY
The ranges in ASM values reported in studies are generally indicative of uncertainty
in size-at-maturity (minimum or average size of nesters) or in the calculated growth
curve (Table 11.1), rather than an attempt to capture the true variability in ages at
which maturity occurs. There can be a great deal of year-to-year variability in growth
rates of loggerheads within the same 10-cm juvenile size-class (Braun-McNeill et al.,
in review). The cumulative variability over nine 10-cm size classes can result in a
wide range of predicted ASMs for individual turtles.
Variability in growth rates may be caused by a number of factors such as genetics,
environmental conditions, and individual health. Limpus and Chaloupka (1997)
found that immature, female green turtles in the southern Great Barrier Reef displayed significantly decreased growth rates at a time that coincided with a strong
ENSO event in the early 1980s. Bjorndal et al. (2000a) measured growth rates of
immature green sea turtles in the southern Bahamas. Over their 18-year study period,
population densities increased by a factor of six, then decreased by a factor of three.
They found a significant negative relationship between the estimated annual population density and the estimated mean annual growth rate, suggesting density-dependent effects on growth rates for this population.
There is also evidence of sex-specific growth rates (Chaloupka and Limpus,
1997; Limpus and Chaloupka, 1997). Female hawksbills from the southern Great
Barrier Reef display faster growth rates at all benthic juvenile sizes than do males
(Chaloupka and Limpus, 1997). In the same region, at sizes greater than 60 cm CCL,
female green sea turtles grow faster than male green sea turtles in the southern Great
Barrier Reef (Limpus and Chaloupka, 1997). This same study found a sexual dimorphism in adult body size for greens, indicating that breeding males are an average
of 7 cm CCL smaller than breeding females. It has been demonstrated for reptiles
that animals with fast growth mature to a larger size than animals with slow growth,
and this may also be the case with sea turtles (Stamps et al., 1998). It is unclear
how much of the variability observed in growth rates of similar-sized individuals
may be attributable to sex-specific growth rates.
Most growth curves of sea turtles have been prepared with small sample sizes
(Table 11.1). If the variability in growth rates for sea turtles is high, it will take large
sample sizes to determine the “average” growth rate. With the possible exception of
the data from Australia, from which large sample sizes are available, we can look
at stage duration and ASM as only rough estimates.
11.2.5 S URVIVAL R ATES
One risk of the delayed maturity representative of the sea turtle life history is the
increased risk of dying before reproducing. ASM is extremely high in sea turtles
(Table 11.1), and hatchling survival is extremely low; therefore, there must be
high survival of juveniles and adults (Congdon et al., 1993). Crouse et al. (1987)
determined that for loggerheads in the southeast U.S., the population intrinsic rate
of increase ( r ) was most sensitive to proportional changes in the survival rate of
large juveniles, which equates to the benthic juveniles. In other turtle species,
1123 book.book Page 286 Tuesday, November 12, 2002 7:43 AM