Decline of the Sea Turtles Causes and Prevention (1990) / Chapter Skim
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5. Natural Mortality and Critical Life Stages
5. Natural Mortality and Critical Life Stages
Pages 61-73
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From page 61... ...
- ~ ; ~ - - ~ A [~ ~ [:~ 5 Nalural Mortality and Critical Life Stages his chapter summarizes current information on the causes and magnitude of natural mortality of sea turtles, and discusses how sea turtles at different life stages contribute to the population or to the reproductive value.
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From page 62... ...
Some of the apparently natural factors that are lethal to sea turtles are associated with human activities. For example, sea turtles are subject to predation by wild, formerly domestic animals introduced by humans (hogs and dogs)
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From page 63... ...
Balazs (1980) has summarized data on predation of juvenile and adult green turtles in Hawaii by tiger sharks; turtles were found in 7-75% of tiger sharks sampled in Hawaiian waters inhabited by sea turtles.
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From page 64... ...
, and Dodd (19881. Cutaneous fibropapillomatosis, a disease of green turtles, has been recorded infrequently in Florida waters for many years (Smith and Coates, However, large numbers of green turtles have recently contracted the disease in the Indian River lagoon system in east-central Florida (Witherington and Ehrhart, 1989b)
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From page 65... ...
Although a thorough study of the relationship between nesting density and this mortality factor has not been carried out, clearly the greater the number of nesting females in a given area, the greater the likelihood of a female disturbing an earlier nest. In most areas, this is a minor source of mortality because most nesting populations have densities that are relatively low.
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From page 66... ...
also reported heavy loggerhead egg and hatchling mortality in Georgia resulting from heavy rains. At Tortuguero, Costa Rica, heavy rains and high groundwater drowned all embryos in many green turtle nests in 1986 and 1988 (Horikoshi, 19891.
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From page 67... ...
The value of an individual of a particular age or life stage can be stated according to its expected production of offspring, hence the term "reproductive value." Reproductive value is the relative contribution of an individual of a given age to the growth rate of the population (see Mertz, 1970, for a description of reproductive value)
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From page 68... ...
. The life stage with the highest reproductive value is the one for which greater protection can contribute the most to the maintenance or recovery of a population.
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From page 69... ...
A reproductive value of 1 is assigned to a newly laid egg, and all other ages receive valuations relative to that. The idea of reproductive value is fundamental to conservation biology, because it helps to identify the age classes of most significance for determining future population size.
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70 and 0 11 .= O .
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From page 71... ...
Because beach strandings of dead sea turtles are dominated by large juveniles (Crouse et al., 1987) , reducing strandings would affect the very life stage whose increased survivorship could increase loggerhead population growth the most.
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From page 72... ...
Predators consume many turtle eggs on most unprotected beaches. Demographic analyses suggest that the reproductive value of a turtle egg is low and that the sensitivity of population growth to the loss of an egg also is low; sea turtle populations under normal conditions appear to be adapted to withstanding substantial egg loss.
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From page 73... ...
73 Natural Mortality and Critical Life Stages However, every age and life stage has value. Given that sea turtle species are threatened with extinction, every individual in every life stage becomes important to the survival of the species and protective efforts should be focused on all life stages, even those where individual reproductive values are relatively low.
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