290
The Biology of Sea Turtles, Vol. II
it is necessary to estimate the ages of individuals, and growth curves such as von
Bertalanffy curves are used to estimate age from size. This introduces a new source
of error because the true relationship of size and age in sea turtles is not known and
there is likely to be a great deal of variability in this parameter.
One of the most reliable means of estimating survival rates in juveniles is to
apply Cormack-Jolly-Seber statistics to data obtained from the repeat observation
of the same animals in a population, also known as mark–recapture or capture–mark–recapture (Cormack, 1964; Jolly, 1965; Seber, 1965). Unfortunately, this
technique requires either a high recapture probability or a large number of captures
over a long period of time. Few such data sets exist, and the studies are expensive
to run. However, if we are to gain a better understanding of juvenile survival rates,
resources must be applied to these studies.
11.2.6 R EPRODUCTION
Because sea turtles cannot provide parental care to offspring, a potential benefit
conveyed by delaying reproduction is the ability to lay larger clutches with larger
eggs. This is evidenced by a positive relationship between size of adult females
and egg size in sea turtles (Hirth, 1980; Van Buskirk and Crowder, 1994). There
is likely a great deal of variability in hatchling survival from year to year resulting
from factors such as variable nest predation and environmental factors such as
hurricanes. However, the overall survivorship of hatchlings to their first birthday
is likely very low. To compensate for this, sea turtles produce large quantities of
eggs in a nesting year and reproduce many times in a lifetime. There is evidence
that females nesting for the first time may produce fewer nests with fewer eggs
and have a longer period before nesting for a second time (Brooke and Garnett,
1983; Frazer and Richardson, 1985; Miller, 1997; Chaloupka, 2001). This may
affect our estimates of population size and trends on nesting beaches where the
number of females and hatchlings is extrapolated from nest counts.
Few sea turtles nest annually. The length of time between successive nesting
migrations can vary among populations and individuals within a population.
Evidence from tagging studies suggests that remigration intervals are not fixed
and may be in response to environmental conditions such as ENSO events or
ocean cycles of longer periodicity (Chaloupka, 2001). Good ocean conditions
may influence survival and growth as well as nesting frequency, so periods of
favorable ocean conditions may mask population declines caused by anthropogenic factors (Chaloupka, 2001). Green turtles, in particular, show marked periodicity in nest abundance, which may be due to productivity in seagrass beds
(Bjorndal et al., 1999; Chaloupka and Limpus, 2001). Response to ocean conditions has also been suggested for black turtles ( Chelonia agassizi , Fuentes et al.,
2000). Broderick et al. (2001) speculated that green turtle nesting was more
variable than loggerhead nesting at Ascension Island because fluctuating ocean
conditions had a large impact on seagrass, the primary food source for herbivorous
green turtles, but less impact on the invertebrate prey of carnivorous loggerheads.
The sex of sea turtle hatchlings is environmentally determined by a restricted range
of nest incubation temperatures (Mrosovsky and Pieau, 1991). Pivotal and transitional
1123 book.book Page 290 Tuesday, November 12, 2002 7:43 AM
The Biology of Sea Turtles, Vol. II
it is necessary to estimate the ages of individuals, and growth curves such as von
Bertalanffy curves are used to estimate age from size. This introduces a new source
of error because the true relationship of size and age in sea turtles is not known and
there is likely to be a great deal of variability in this parameter.
One of the most reliable means of estimating survival rates in juveniles is to
apply Cormack-Jolly-Seber statistics to data obtained from the repeat observation
of the same animals in a population, also known as mark–recapture or capture–mark–recapture (Cormack, 1964; Jolly, 1965; Seber, 1965). Unfortunately, this
technique requires either a high recapture probability or a large number of captures
over a long period of time. Few such data sets exist, and the studies are expensive
to run. However, if we are to gain a better understanding of juvenile survival rates,
resources must be applied to these studies.
11.2.6 R EPRODUCTION
Because sea turtles cannot provide parental care to offspring, a potential benefit
conveyed by delaying reproduction is the ability to lay larger clutches with larger
eggs. This is evidenced by a positive relationship between size of adult females
and egg size in sea turtles (Hirth, 1980; Van Buskirk and Crowder, 1994). There
is likely a great deal of variability in hatchling survival from year to year resulting
from factors such as variable nest predation and environmental factors such as
hurricanes. However, the overall survivorship of hatchlings to their first birthday
is likely very low. To compensate for this, sea turtles produce large quantities of
eggs in a nesting year and reproduce many times in a lifetime. There is evidence
that females nesting for the first time may produce fewer nests with fewer eggs
and have a longer period before nesting for a second time (Brooke and Garnett,
1983; Frazer and Richardson, 1985; Miller, 1997; Chaloupka, 2001). This may
affect our estimates of population size and trends on nesting beaches where the
number of females and hatchlings is extrapolated from nest counts.
Few sea turtles nest annually. The length of time between successive nesting
migrations can vary among populations and individuals within a population.
Evidence from tagging studies suggests that remigration intervals are not fixed
and may be in response to environmental conditions such as ENSO events or
ocean cycles of longer periodicity (Chaloupka, 2001). Good ocean conditions
may influence survival and growth as well as nesting frequency, so periods of
favorable ocean conditions may mask population declines caused by anthropogenic factors (Chaloupka, 2001). Green turtles, in particular, show marked periodicity in nest abundance, which may be due to productivity in seagrass beds
(Bjorndal et al., 1999; Chaloupka and Limpus, 2001). Response to ocean conditions has also been suggested for black turtles ( Chelonia agassizi , Fuentes et al.,
2000). Broderick et al. (2001) speculated that green turtle nesting was more
variable than loggerhead nesting at Ascension Island because fluctuating ocean
conditions had a large impact on seagrass, the primary food source for herbivorous
green turtles, but less impact on the invertebrate prey of carnivorous loggerheads.
The sex of sea turtle hatchlings is environmentally determined by a restricted range
of nest incubation temperatures (Mrosovsky and Pieau, 1991). Pivotal and transitional
1123 book.book Page 290 Tuesday, November 12, 2002 7:43 AM