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OCR for page 61
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5
Nalural Mortality and
Critical Life Stages
his chapter summarizes current information on the causes and mag-
nitude of natural mortality of sea turtles, and discusses how sea tur-
tles at different life stages contribute to the population or to the
reproductive value. Recent analyses of loggerhead populations and
reproduction (Crouse et al., 1987) are especially useful for making
decisions about conservation of sea turtles, because they help to identify
life stages in which reduced mortality can have the greatest influence on
the maintenance or recovery of endangered or threatened sea turtle pop
ulations.
From models developed by Frazer (1983a), female loggerheads proba-
bly first nest when about 22 years old, and survivors continue nesting
every few years until they are about 54. Most mature female loggerheads
nest every second or third year and deposit several clutches of eggs dur-
ing a nesting season. Thus, an individual is estimated to lay on the aver-
age 80 eggs each year for 30 years. The eggs and hatchlings have high
mortality rates, but as the survivors grow, natural mortality declines
markedly. About 80% of the nesting females studied for many years at
Little Cumberland Island survive from one year to the next. (Chapter 2
presented variations on the pattern of life history of the several species of
sea turtles.) These general patterns of mortality and reproduction form a
61
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62
Decline of the Sea Turtles
basis for the insight needed to devise a rational program for sea turtle
conservation (Crouse et al., 19871.
Sea turtles are killed by various animals and environmental phenome-
na. Nests and eggs are destroyed by predators, erosion, and inundation
by rain or tides. After hatching, turtles of all ages, both at sea and on
land, are consumed by predators. They are also subject to debilitating
parasites and diseases and are killed by various abiotic factors, including
hurricanes and thermal stress. However, quantitative accounts of sea tur-
tle mortality in the wild are few.
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) or wild, nondomesticated animals introduced by humans
(mongoose) or enhanced by human activities (raccoons). Beach erosion
is a natural source of mortality that has also been altered by human activi-
ties.
Predation
BIOTIC SOURCES OF MORTALllY
Many species, from ants to jaguars, prey on sea turtles. Excellent
reviews (which include lists of predators) by Hirth (1971), Stancyk (1982),
Witzell (1983), and Dodd (1988) categorize predators by the life stage of
sea turtles on which they prey, and the following presentation below fol-
lows that pattern.
Eggs and Hatchlings on the Becch
Predators of the Kemp's ridley at Rancho Nuevo, Mexico, include coy-
otes, raccoons, coatis, skunks, ghost crabs, and ants (Marquez M. et al.,
19899. Some predators, such as the black vulture, feed on eggs from
nests already opened by other predators or erosion. Hatchling Kemp's
ridleys are caught and eaten on the beach by ghost crabs, vultures, grack-
les, caracaras, hawks, coyotes, raccoons, skunks, coatis, and badgers
(Marquez M., in prep.~.
The major loggerhead egg predator in the southeastern United States is
the raccoon (Dodd, 19881. Before protective efforts were initiated, rac-
coons destroyed nearly all the nests at Canaveral National Seashore, Flori-
da (Ehrhart, 1979), and at Cape Sable, Florida, raccoons destroyed 85% of
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63
Natural Mortality and Critical Life Stages
the nests in 1972 and 75% in 1973 (Davis and Whiting, 19779. The high
rate of predation might have resulted from the unusually large raccoon
populations, which were augmented by such human activities as habitat
alteration, food supplements (garbage), and removal of natural predators
of the raccoon (Carr, 1973; pers. comm., L. Ehrhart, University of Central
Florida, 19891. Not all nesting beaches in Florida suffer such high losses
from raccoons; for example, only seven of 97 nests on Melbourne Beach,
Florida, were destroyed by raccoons in 1985 (Withering/on, 19861. Other
nest predators are ghost crabs, hogs, foxes, fish crows, and ants (Dodd,
19881. From 1980 to 1982, nonhuman predators destroyed up to 80% of
the loggerhead clutches laid on two barrier islands in South Carolina
(Hopkins and Murphy, 19831.
Management practices have eliminated nearly all the beach predation
of Kemp's ridleys at Rancho Nuevo, and reduced predation significantly
on most of the important loggerhead nesting beaches.
Hatchlings as They Leave the Beach
Once in the ocean, Kemp's ridley hatchlings are eaten by a large vari-
ety of predatory birds and fish (Marquez M., in prep.~. Loggerhead hatch-
lings at this time in their lives also fall prey to a similar array of predators,
including gulls, terns, sharks, and other predatory fish (Dodd, 19881.
Many Atlantic sharpnose sharks captured in a commercial fishery off Flori-
da during the turtle hatching season in 1988 had loggerhead hatchlings in
their stomachs (pers. comm., A. Bolten, University of Florida, 19891.
Larger Juveniles and Adults in the Water
Sharks-and other large predatory fish are important predators of
Kemp's ridleys in all oceanic life stages (Marquez M. et al., 19891. Tiger
sharks might be selective predators of large cheloniid sea turtles; analyses
of stomach contents of 404 tiger sharks showed that 21% of the sharks
with food in their stomachs had eaten large turtles (Witzell, 19871. Balazs
(1980) has summarized data on predation of juvenile and adult green tur-
tles in Hawaii by tiger sharks; turtles were found in 7-75% of tiger sharks
sampled in Hawaiian waters inhabited by sea turtles.
Nesting Females on the Beach
There is no evidence of nonhuman predation of adult loggerhead
females on U.S. nesting beaches, but it might have occurred in the past.
Reported predators of leatherbacks, green turtles, and hawksbills are
similar to those of loggerheads and Kemp's ridleys at each life history
stage (Hirth, 1971; Pritchard, 1971; Fowler, 1979; Balazs, 1980; Stancyk,
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64
Decline of the Sea Turtles
1982; Bjorndal et al., 1985; Witzell, 19871. The actual predator species
change with geographic region, but are from the same feeding guilds.
Diseases and Parasites
Most reported diseases in sea turtles have been described in captive
animals (Kinne, 19851. Diseases induced by stress or improper diet in
captivity and not known to occur in wild sea turtles (Glazebrook, 1980;
Kinne 1985; Lauckner 1985) will not be discussed here. An excellent
7 7
. 1 ·, ~. . 1 1 ~1 · ~_ 1
review or the diseases ancl parasites of sea turtles can [~e rouna In LaucK-
ner (1985), and specific parasites of sea turtles are identified in the
reviews by Hirth (1971), Witzell (1983), 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) and the Hawaiian Islands (Balazs,
19861. In the Indian River, 40-52% of the green turtles captured in 1983-
1988 had fibropapillomas. In Hawaii, 10% of the nesting females at
French Frigate Shoals had fibropapillomas, as did 35% of 51 stranded
green turtles in 1985 (Balazs, 19861. Recaptured turtles have demonstrat-
ed further proliferation of the fibropapillomas, although in other cases
regression occurs (Witherington and Ehrhart, 1989a). Tumors can cause
mortality indirectly. Turtles whose vision is blocked by tumors are unable
to feed normally, and turtles with fibropapillomas are more prone to
entanglement in monofilament line and other debris (Balazs, 1986; With-
erington and Ehrhart, 1989a). Research on the cause of the disease is in
progress (Jacobson et al., 19891.
Spirorchidiasis has been reported in loggerheads (Wolke et al., 19821.
Severe infestations of spirorchids (blood flukes) result in emaciation, ane-
mia, and enteritis, or conversely, emaciation and anemia could make a
turtle more susceptible to spirorchid infestation. Three genera of blood
flukes were identified in 14 of 43 loggerheads stranded or floating dead
from Florida to Massachusetts (Wolke et al., 19821. Spirorchidiasis can
result in death or make turtles more susceptible to succumb to other
stresses (Wolke et al., 19821.
A macrochelid mite (Macrocheles sp.) has been found on Kemp's ridley
hatchlings emerging from relocated nests (Mast and Carr, 19851. Mites of
the same genus, considered to be nonparasitic, were found on loggerhead
hatchlings in South Carolina (Baldwin and Lofton, 19591.
Bacterial and fungal infections of eggs can be a major source of mortal
93 ~- -- , --- D - ' ' _ A .
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65
Natural Mortality and Critical Life Stages
ity. Bacteria and fungi are implicated as a major cause of death of olive
ridley eggs at Nancite, Costa Rica, where hatching success averages only
5% (Cornelius, 1986; Mo, 19881. Microbial pathogens are believed to
cause mortality of loggerhead embryos (Wyneken et al., 19881.
Other Nesting Turdes
Eggs and emerging hatchlings are sometimes killed when their nest is
dug into by a nesting female of either the same or a different species.
Bustard and Tognetti (1969) described this activity as a density-dependent
mortality factor. 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 densi-
ties that are relatively low. However, during the mass nestings (arrib-
adas) of olive ridleys, large numbers of nests can be destroyed. Cor-
nelius (1986) estimated that 7% of the nests of the olive ridley colony at
Nancite, Costa Rica, were destroyed by other females' digging in the
same arribada, and another 10% were destroyed by females' digging in
subsequent arribadas. In contrast, at Tortuguero, Costa Rica, of 587 green
turtle nests monitored, none was destroyed by nesting activities of other
turtles (Fowler, 19791. At Mon Repos, Australia, an average of 0.43% of
the total seasonal egg production in five consecutive seasons was
destroyed by nesting loggerheads (Limpus, 19851.
Vegetation
Although usually a minor cause of death, plant roots can invade turtle
nests and cause mortalities. Invasion by roots of beach morning glory
(Ipomoca pes-caprae) and sea oats (Uniola paniculata) killed 275
embryos in three of 97 loggerhead nests on Melbourne Beach, Florida
(Withering/on, 1986), and at Cape Romaine, South Carolina, 5% of the
eggs laid among sea oats were destroyed by the roots (Baldwin and
Lofton, 19593. Destruction of marine turtle nests by sea oat roots also has
been reported by Raymond (19841.
Plants can also entrap sea turtles. Hatchlings get entangled on their
way to the sea (Limpus, 1985) and adult females sometimes become fatal-
ly trapped in vegetation or by logs washed onto the nesting beach
(Pritchard, 1971; Cornelius, 19861.
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66
Decline of the Sea Turtles
ABIOTIC SOURCES OF MORTAU1Y
Erosion, Accretion, and Tidal Inundafion
In almost every nesting colony, some nests are lost to erosion, accre-
tion, and tidal inundation. The extent of mortality varies widely among
beaches, years, and species. Nests deposited on shifting beaches are
more susceptible to destruction from erosion or accretion. In each
species, some turtles deposit nests below the high-tide line. Leatherbacks
often nest in areas vulnerable to erosion or inundation: 40-60% of the
nests in Surinam were in such areas, compared with 12% of green turtles
on the same beach (Whitmore and Dutton, 1985), the Guianas, and St.
Croix (Eckert, 1987), but less than 3% in Malaysia (Mrosovsky, 19831.
Erosion and inundation destroyed 3-25% of the loggerhead nests deposit-
ed each year on two barrier islands in South Carolina in 1980-1982 (Hop-
kins and Murphy, 1983), and on Melbourne Beach, Florida, 17 of 97 log-
gerhead nests in 1985 were lost to erosion, accretion, and surf action
(Withering/on, 19869.
Heavy Rains
Heavy rain can destroy large numbers of nests. Ragotzkie (1959)
reported that all embryos in 15 of the 17 loggerhead nests deposited on
Sapelo Island, Georgia, in 1955 and 1957 were drowned by heavy rain.
Kraemer and Bell (1978) also reported heavy loggerhead egg and hatch-
ling 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.
Thermal Stress
Hypothermia in sea turtles causes a comatose condition and can result
in death. Perhaps the best-documented events are those that occurred in
recent years in Long Island Sound, New York (Meylan and Sadove, 1986),
and in the Indian River lagoon system, in Florida (Wilcox, 1986; Wither-
ington and Ehrhart, 1989b). Both areas can act as natural "traps," because
of their geographic configurations (Witherington and Ehrhart, 1989b). Of
52 turtles (41 Kemp's ridleys, nine loggerheads, and two green turtles)
stranded in Long Island Sound in the winter of 1985-1986, 18 were alive
when discovered and 11 (nine ridleys, one loggerhead, and one green
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67
Natural Mortality and Critical Life Stages
turtle) survived after gradual warming at rehabilitation centers (Meylan
and Sadove, 19861.
Morning surface water temperatures below 8°C in 1977, 1978, 1981,
1985, and 1986 caused hypothermic stunning of sea turtles in the Indian
River lagoon system, in Florida (Witherington and Ehrhart, 1989b). Those
events involved 342 green turtles (25-75 cm SCL), 123 loggerheads (44-91
cm), and two Kemp's ridleys (55-63 cm).
Among the stranded turtles, a
greater proportion of green turtles than of loggerheads died, and smaller
turtles were more susceptible to hypothermia. Most of the turtles were
released alive, and many were recaptured months or years later. We have
no way of estimating the mortality that would have occurred without
human intervention (Witherington and Ehrhart, 1989b).
QUANTITATIVE STUDIES OF NAVAL MORTAUh
The only life stage for which natural mortality of sea turtles has been
quantified is the egg and hatchling stage, including the brief period when
hatchlings emerge from the nest and make their way down the beach to
the water. Percentage of emergence of hatchlings is measured and report-
ed in the literature in two ways. In the first, egg clutches are marked as
they are laid and followed through the season; that results in an emer-
gence percentage for eggs in all clutches laid. In the second, the emer-
gence success of hatchlings from clutches that successfully produce hatch-
lings is determined. The former value is the best measure of survivorship.
Results in Table 5-1 indicate the range of survivorship values for the egg
stage. Of necessity, some studies include sources of mortality related to
human activities for example, predation by humans, formerly domestic
animals, and wild animals introduced by humans.
The rate of mortality resulting from predation is assumed to be much
higher for eggs and very small turtles than for larger turtles, because the
lists of predators on eggs and hatchlings are much longer than those of
predators on larger juveniles and adults. However, there are no quantita-
tive studies of predation away from the nesting beach, so the assumption,
although a reasonable one, has not been tested.
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 "repro-
ductive value." Reproductive value is the relative contribution of an indi-
vidual of a given age to the growth rate of the population (see Mertz,
1970, for a description of reproductive value). The more offspring an
individual is expected to produce, the higher its reproductive value. The
life stages that we consider below are eggs and hatchlings, small juveniles,
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68
Decline of the Sea Turtles
TABLE 5-1 Emergence success of sea turtle egg clutches presented as mean (range). Emer-
gence success is the percentage of eggs that produce hatchlings that reach the surface of the
sand above the nest chamber. Data are presented only for natural nests (nests not moved or
protected) from studies that included those clutches that produced no hatchlings.
Clutches Emergence
Species and Location (Number) Success (%) Reference
loggerhead
Tongaland 72 78 (0-99) Hughes, 1974
Brevard Co., Florida 97 56 (0-99) Witherington, 1986
Cape Canaveral (1982) 310 1 (0-90) McMurtray, 1982
Cape Canaveral (1983) 76 3 (0-?) McMurtray, 1986
Green turtles
Bigisanti, Sur~nam 57 84 Schulz, 1975
Hawaii 40 71 (0-93) Balazs, 1980
Tortuguero, Costa Rica 318 35 (0-?) Horikoshi, 1989
Florida 25 57 (0-94) Witherington, 1986
Hawlesbill
U.S. Virgin Islands 61 60 (0-100) Small, 1982
U.S. Virgin Islands 88 81 Hillis and Mackay, 1989a
Tortuguero, Costa Rica 5 36 (0-94) Bjorndal et al., 1985
Antigua, West Indies (1987) 99 79 (0-100) Corliss et al., 1989
Antigua, West Indies (1988) 156 85 (0-100) Corliss et al., 1989
Leatherback
Bigisanti, Surinam 52 50 Schulz, 1975
Culebra, Puerto Rico 429 71 (0-100) Tucker, 1989b
St. Crow (1983) 98 25 (0-95) Eckert and Eckert, 1983
St. Croix (1984) 123 26 (0-97) Eckert et al., 1984
large juveniles, subadults, and nesting adults (breeders). The life stage
with the highest reproductive value is the one for which greater protec-
tion can contribute the most to the maintenance or recovery of a popula-
tion.
Reproductive value can be estimated with population models. Those
models have a long history in population ecology, perhaps beginning
with Lotka (19221. The models traditionally combine information on age-
specific fecundity and age-specific survivorship to yield population pro-
jections where survivorship is the percentage of individuals that survived
the year and fecundity is the average number of eggs produced per
female. Other important factors in the calculations are the number of
years required for an animal to reach its reproductive age and the ratio of
females to males in the population.
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69
.
Natural Mortality and Critical Life Stages
The concept of a mathematical value for reproductive value arose from
Cole's use of demographic models (Fisher, 19581. A reproductive value of
1 is assigned to a newly laid egg, and all other ages receive valuations rel-
ative to that. The idea of reproductive value is fundamental to conserva-
tion biology, because it helps to identify the age classes of most signifi-
cance for determining future population size.
Population modeling is also useful in assessing whether a particular
population is growing or declining and at what rate. Its greatest useful"
ness, however, might be in sensitivity analysis (Cole, 1954), the estimation
of the magnitude of change in the growth rate of the population for each
of several changes in such factors as fecundity and survivorship. A sensi-
tivity analysis can evaluate, for example, whether a 10% increase in sur-
vivorship could have the same effect on population growth as a 50%
increase in fecundity. If it did, then the growth of the population would
be 5 times more sensitive to survivorship changes than to fecundity
changes. Sensitivity analysis is also useful for predicting which of several
life stages would be most responsive to a particular management tool.
The loggerhead is the sea turtle whose demographics are best known,
because loggerheads nest in sufficient numbers along the southeastern
U.S. coast to be accessible to scientists, and because one nesting popula-
tion on Little Cumberland Island, Georgia, has been subject to intensive
tagging since 1964 (Richardson and Hillestad, 1978; Richardson and
Richardson, 19821. Frazer has conducted an exhaustive analysis of the
Cumberland loggerhead population (Frazer, 1983a,b; 1984; 1986; 1987;
Frazer and Ehrhart, 1985; Frazer and Richardson, 1985a,b; 1986) and has
provided the algebraic notation for the standard age-based population
model of Lotka (19221.
Survivorship and fecundity in loggerheads are best estimated by life
history stages (eggs, hatchlings, small pelagic juveniles, large coastal juve-
niles, subadults, and adults), rather than years of age, so Crouse et al.
(1987) used Frazer's demographic data from Cumberland Island logger-
heads to apply a stage-based demographic technique for analyzing popu-
lation dynamics. The approach, as developed by Werner and Caswell
(1977), is analogous to the traditional age-based life-table analysis, but
does not require age-specific information.
Population factors (Table 5-2, columns 1-4) used by Crouse et al.
(1987) in the analyses were calculated by Frazer (1983a). Predictions for
reproductive value (column 5) and sensitivity (column 6) were derived
from the model of Crouse et al. (19871. Five life stages are represented.
Annual survivorship is lowest in eggs and hatchlings-67% per year and
in large juveniles-68%. Large juveniles are the dominant size group (55-
75 cm) of the turtles stranded on the beaches of North Carolina (Crouse et
OCR for page 70
70
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OCR for page 71
71
Natural Mortality and Critical Life Stages
al., 1987) and other beaches from Florida to North Carolina (Schroeder,
1987; Schroeder and Warner, 1988; Schroeder and Maley, 19891. Survivor-
ship is estimated to be highest for nesting females 81% per year.
Because they do not breed until they are 12-30 years old, and 22-33% die
each year, few loggerheads reach reproductive age. The reproductive
value of individual surviving turtles is greatest for breeders, which, once
they reach maturity, can continue to breed for many years. Each individ-
ual breeder's reproductive value is estimated to be about 584 times
greater than that of an egg or hatchling. Few turtles, however, survive to
adulthood and reproduce. As Crouse et al. (1987) noted, "By increasing
the survival of large juveniles (who have already survived some of the
worst years) a much larger number of turtles are likely to reach maturity,
thereby greatly magnifying the input of the increased reproductive value
of the adult stages."
i,
The analyses of Crouse et al. (1987) suggested that the greatest
ncrease in growth rate of the Little Cumberland Island population could
be achieved by increasing the survivorship of the large juveniles and
subadults. Increasing fecundity or survivorship of eggs had less influence
on population growth than increasing survivorship of older turtles. This
conclusion was not especially sensitive to uncertainties in the parameter
estimates. 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. No conservation effort can be successful
without adequately protecting all stages in the life cycle, but the analyses
of Crouse et al. (1987) strongly suggest that efforts to reduce mortality of
larger juvenile and adult loggerheads will be more effective at promoting
loggerhead population growth than efforts to increase the numbers of
hatchlings leaving the beaches. The analyses also predict that efforts to
protect eggs on nesting beaches and efforts at "headstarting" loggerheads
would by themselves be insufficient to reverse the observed decline in
the population of loggerheads nesting on Little Cumberland Island (Figure
3-14).
Although the results of such population models clearly depend upon
necessary assumptions regarding poorly known demographic characteris-
tics, the general conclusions of the Crouse et al. (1987) model of the log-
gerhead are robust. Of the poorly known demographic characteristics,
age at sexual maturity is the one to which the model is most sensitive.
But large changes in maturation rate and in other imprecisely known
demographic characteristics did not alter the general conclusion that
increasing the survivorship of juveniles and young adults would promote
population growth far more than increasing survivorship of eggs and
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72
Decline of the Sea Turtles
hatchlings. However, the imprecision of our knowledge of necessary
demographic characteristics for loggerheads prevents us from specifying
how many hatchlings would have to be spared to equal the effect of spar-
ing the life of a single large juvenile, although we know that the number
is large.
Crouse et al. (1987) modeled only the loggerhead, but there are rea-
sons to believe that aggregate reproductive value in Kemp's ridley and
other sea turtles is also greater for larger juveniles and young adults than
for earlier and later stages. The key demographic characteristics that lead
to this pattern in how reproductive value varies with life stage are the rel-
atively long time to sexual maturity and the extremely high mortality rate
from birth to age of sexual maturity. Those characteristics ensure that
reproductive value of individual hatchlings will be relatively low. To the
degree that all sea turtles share those two traits with the loggerhead, the
conclusion that reproductive value of hatchlings is relatively low will
apply generally. The implication for conservation efforts, too, is general:
Increasing survivorship of older juvenile and young adult sea turtles is the
most effective means of increasing population sizes. Because mature sea
turtles age without ceasing to reproduce, reproductive value will remain
high until late in adult life, thus suggesting that continued protection of
adult sea turtles will be an important conservation measure. However, if
there are few or no hatchlings, there will inevitably be few or no adults
ultimately. Therefore, relative reproductive values will be useful in man-
agement decisions only if there is a certainty that large numbers of hatch-
lings are being produced.
SUMMARY
Sea turtles lay great quantities of eggs throughout their life, particularly
if mortality is low for adults. Predators consume many turtle eggs on
most unprotected beaches. Demographic analyses suggest that the repro-
ductive 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 nor-
mal conditions appear to be adapted to withstanding substantial egg loss.
However, demographic analyses suggest that the reproductive value of a
large juvenile, subadult, or adult sea turtle is higher than that of an egg.
Because population growth is most sensitive to changes in survivorship of
large juveniles and subadults, we conclude that reduction of human-
induced mortality in these life stages will have a significantly greater
effect on population growth than reduction of human-induced mortality
of eggs and hatchlings.
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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 repro-
ductive values are relatively low.
Representative terms from entire chapter:
reproductive value
