Assessment of Sea-Turtle Status and Trends Integrating Demography and Abundance (2010) / Chapter Skim
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5 Demographic Rates
5 Demographic Rates
Pages 73-90
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From page 73... ...
All demographic parameters exhibit variation within and among spe cies and populations and over space and time; some -- such as clutch frequency (i.e., the number of clutches deposited by an individual turtle in a nesting season) , interbreeding intervals, and somatic growth rates -- vary within individuals over time.
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From page 74... ...
For example, nutrition affects age at sexual maturity, clutch frequency, and the number of years between breeding seasons, but it does not affect clutch size (Bjorndal, 1985)
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From page 75... ...
. Within a nesting season, egg output of an individual is the product of the num ber of clutches deposited (clutch frequency)
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From page 76... ...
Many monitoring programs on nesting beaches rely on nest counts to generate estimates of and trends in population abundance with the explicit assumption that clutch frequency is constant, but clutch fre quency requires continual monitoring because it may vary among years (Broderick et al., 2003)
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From page 77... ...
. Unlike clutch frequency, clutch size apparently is not greatly affected by environmental factors (Bjorndal, 1985; Bjorndal and Carr, 1989)
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From page 78... ...
As temperatures increase, pri mary sex ratios may shift toward females. Because many nesting beaches already produce primary sex ratios strongly biased toward females, there is concern that the proportion of males will be insufficient and that fertil ity of eggs could decline (Hawkes et al., 2007; Poloczanska et al., 2009)
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From page 79... ...
but did account for tag loss. Efforts to assess loggerhead status and interpret trends in nests with lifecycle and simulation models have been stymied by the lack of new estimates (Turtle Expert Working Group, 2000; National Marine Fisheries Service Southeast Fisheries Science Center, 2001)
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From page 80... ...
A serious limitation of both approaches, particularly in Atlantic populations in which immature turtles tend to move among foraging grounds to a greater extent than in the Pacific, is the confounding of emi gration and mortality in estimates of apparent survival (usually referred to as phi)
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From page 81... ...
Loggerhead nesting populations may show population structure (mitochondrial DNA differentiation) on a scale of less than 100 km (Bowen et al., 2005)
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From page 82... ...
Once on neritic foraging grounds, immature turtles tend to move among foraging habitats. Knowledge of movements of immature sea turtles has improved through increased flipper tagging of immatures, satellite telemetry, genet ics, and stable isotopes and has revealed a more complex series of dis persals of some turtles (Eckert and Martins, 1989; Eckert, 2002; Bolten, 2003a; Harrison and Bjorndal, 2006; McClellan and Read, 2007; Reich et al., 2007)
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From page 83... ...
SOMATIC GROWTH AND AGE AT SEXuAL MATuRITY Somatic growth has been measured in a number of seaturtle popula tions. Adult females essentially stop growing after attaining sexual matu rity, at which point resources are allocated away from somatic growth to reproduction.
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From page 84... ...
on foraging grounds may result from variation in primary sex ratios, sexspecific mortality, or sexspecific dis persal. Data on secondary sex ratios of immature and adult sea turtles are needed to develop sexspecific population models and to evaluate "opti mal" sex ratios (i.e., ratios at which reproductive output is maximized in a population)
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From page 85... ...
. Of necessity, most work on identifying density dependence in sea turtle populations has focused on processes that occur on nesting beaches.
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From page 86... ...
(2010) evaluated evidence of depensation in green turtles and loggerhead turtles.
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From page 87... ...
Concerns about these caveats have led to disagree ment about the value of strandings for population assessment (Epperly et al., 1996; Turtle Expert Working Group, 2000)
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From page 88... ...
Recently, a data review by a loggerhead turtle working group included plots of turtle sizes observed through time that showed a good correlation between the size distributions of strandings on the east coast of the United States and of turtles observed at a powerplant intake in Florida and juvenile mark–recapture surveys (Turtle Expert Working Group, 2009; Vaughan, 2009)
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From page 89... ...
Flipper collection could become standard protocol for STSSN volunteers in the southeastern and Gulf states, but a considerable investment in time and resources will be needed to process and evaluate those samples. RECOMMENDATIONS • Researchers should give high priority to generating estimates for the following parameters: survival of immature turtles and nesting females, age at sexual maturity, breeding rates, and clutch frequency.
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From page 90... ...
For example, the seaturtle stranding and salvage networks should be evaluated, per haps with the assistance of the U.S. Geological Survey's National Wildlife Health Center.
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