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The Pacific pond
turtle was once abundant in California, Oregon, and in the Puget
Sound area of
Washington but has been declining through most of its range since
the mid-1800's.
Prior to the arrival of people of European descent, it is
estimated that 4 million turtles lived in and around Tulare Lake
in the San Joaquin Valley. Local commercial collecting activities for the restaurant trade in the Tulare Lake
and Buena Vista basins caused a major decline in the population.
Commercial harvest from all western state lakes decimated the population long before
dams
squeezed of the water supply from millions of acres of habitat. A
report from San Francisco in the 1890's showed an average of 18,000
sold for food annually. Pond turtles reproduce very slowly and
populations cannot recover from mortality to adults from
harvesting. Continued exploitation and habitat
destruction continue to dramatically reduce pond turtle populations.
TAXONOMY
In the order Testudines and the
family Emydidae, the Pacific pond turtle (Actinemys marmorata) (although
some turtle biologists are now using Emys m. and some
continue to use the
former genus name Clemmys m.) has been known variously as the
western
pond turtle, western mud turtle, Pacific mud turtle, Pacific
terrapin, and Pacific freshwater turtle. Baird and Girard
described the Pacific pond turtle in 1852 after collecting the
type specimens during an 1841 U.S. Exploring Expedition in the
vicinity of Puget Sound, Washington.
Two subspecies: the
northern Pacific pond turtle (A. m. marmorata) and the
Southern Pacific Pond Turtle (A. m. pallida) were
described in 1945. The northwestern subspecies is distributed from
Washington's Puget Sound south to the Sacramento Valley. The
Southern Pacific Pond Turtle is found from near Monterey, California
south to Baja California, Mexico. The two subspecies are thought
to interbreed in the northern San Joaquin Valley. DNA studies may
change the assumed relationships between the two subspecies, as
the most northerly and southerly regions may show enough
distinction to be renamed as completely different species.
DESCRIPTION
The Pacific pond
turtle is a medium-sized turtle. Maximum size varies geographically, with the
largest animals (210 mm or 8.2 in) occurring in the northern part
of the range. Turtles become sexually mature at a carapace
length of about 120 mm.
Color: Carapace (upper
shell) dark brown or olive above without dark streaking - Plastron
(lower shell) cream to yellowish,
sometimes with dark blotches in the centers of the scutes
Size:
length: adults 120mm-210mm, juveniles 32-120mm, hatchlings: 25-31mm
Weight: Males: average
= 554 g (1.2 lb), Females: non-gravid average = 504 g (1.1 lb), Hatchlings:
3-7 g (0.11-0.25 oz)
NATURAL HISTORY
Behavioral Characteristics
Turtles bask in the
sun when not disturbed. Basking has been
noted in all months of the year
in some areas, but generally increases in frequency through the
spring and peaks in early to mid-June. Basking declines in summer until September, when
another peak is observed. Turtles bask in warm water concealed in
or under masses of floating vegetation or algae, or in shallow
water relatively close to shore. Aquatic basking
peaks in early to mid-July in most
areas and declines by early September.
Pacific pond turtles are wary, with a well-developed sense of
sight and a moderate sense of hearing. Escape behavior has been
observed when the perceived threat is within 100 m (330 ft) or
even further away.
In the spring, early summer, and autumn most turtle activity is
diurnal. They are mostly nocturnal in summer. During the
summer the species may be most active in early morning and
evening, and inactive during the heat of the day.
Overwintering
“Overwintering” refers to periods of reduced or no
activity during the winter
which may include periods of a hibernation-like state of reduced
physiological activity. Western
pond turtles overwinter from mid-October or November to March or
April. Pond turtles may
overwinter on land up to 500 m from the nearest watercourse. During a study in
California, 10 of 12 pond turtles overwintered at upland sites. Preliminary observations from
turtles in a pond environment suggest that juveniles overwinter in
the water.
Foraging
Pacific pond turtles locate food by sight or by smell,
and spend considerable amounts of time foraging. Under normal
conditions feeding behavior is solitary. Pacific pond turtles can
only swallow food under the water. They
normally forage along the bottom, searching
carefully in submerged leaf litter and other detritus. They feed
during the day all months except during the summer when they may
feed at night as well.
Diet
The Pacific pond turtle is a dietary generalist. They prey heavily
on aquatic invertebrates, such
as the larvae of beetles, stoneflies, caddisflies, dragonflies and
other insects. They will prey upon fish and frogs in stream environments. Scavenging has been noted on the
carcasses of various mammals, birds, reptiles, amphibians and bony
fishes. Where bullfrogs
(Rana catesbeiana) occur with Pacific pond turtles, there is no
evidence that turtles feed on either larval or post-metamorphic
bullfrogs, although they may feed on their carcasses. Unpalatable elements in the skin of
bullfrogs may deter predation by
pond turtles.
Use of plants appears to be limited except in the case of
post-partum females, who may ingest large quantities of cattail or bullrush
roots at certain seasons. Water lily pods and alder catkins are also
eaten. In certain circumstances, turtles may eat large quantities
of filamentous green algae.
Movement
Most Pacific pond turtles are somewhat sedentary, although they
are capable of moving significant distances and occasionally
travel several hundred meters in just a few days.
HABITAT REQUIREMENTS
The Pacific pond turtle is found associated with a variety of
aquatic habitats, both permanent and intermittent. They are found
from sea level to approximately 1,375 m (4,500 ft) in elevation. The name
western "pond" turtle is
something of a misnomer, as it is more often associated with
streams where ponds are scarce.
Historically, Pacific pond turtles occurred in large numbers in
the warm, shallow lakes and
sloughs on the floor of the San Joaquin and Sacramento valleys. It
is in the few remaining areas that approximate these habitat
conditions that this species reaches
its highest densities. Western
pond turtles are usually rare or
absent in reservoirs, canals, or other bodies of
water heavily altered by humans. Pacific pond turtles may inhabit
some large rivers, but are usually restricted to areas near
the banks or in adjacent
backwater habitats where the current is relatively slow and
abundant emergent basking sites and
sanctuary sites exist. They generally avoid heavily shaded areas.
Substrate and Vegetation
Habitats used by Pacific pond turtles may have a variety of
substrates including solid rock,
boulders, cobbles, gravel, sand, mud, decaying vegetation, and
combinations of these. In many
areas turtles are found in rocky streams with little or no
emergent vegetation. In other areas they
occur in slow-moving streams or backwaters with abundant emergent
vegetation such as cattails
or bulrush. Dense growths of woody
vegetation along the edges of a watercourse, which may shade
potential emergent basking sites,
and make otherwise suitable habitats unsuitable for pond turtles.
Basking Sites
Pacific pond turtles spend a considerable amount of time engaged
in emergent basking, and
they are more abundant in habitat that have basking sites. Turtles
use a variety of areas for emergent basking, such as rocks, sand,
mud, downed logs, submerged
branches, and emergent or submerged aquatic vegetation.
Water Conditions
Turtles have been observed
with water temperatures as
low as 1-2ºC (37ºF) and as
high as 38ºC (100ºF). In general,
turtles avoid prolonged exposure to water above 35ºC (95ºF).
Reproduction
Courtship and mating
behavior have been observed from February to November. Age and size at development of secondary
sexual characteristics varies geographically, but these are generally
evident in both sexes by the time an animal reaches 110 mm (4.3
in) carapace length. The time required for males to achieve
sexual maturity is not known, but is thought to be at least 10-12
years. Females as small as 111 mm (4.3 in), with an approximate age of
6-7 years, have been observed
carrying eggs in southern California.
When preparing to lay eggs, females typically leave the water in
late afternoon or early evening
and travel a considerable distance. Females moisten the soil
around the nest by urinating prior to
digging the nest chamber. Excavation of the flask-shaped nest may
require several hours to
complete and the female commonly remains on or near the nest site
overnight.
Clutch size varies
from 2 to 13 eggs with the larger females laying more eggs. Unusually
cold wet weather can cause total nesting failure. In southern California,
some hatchlings leave the nest in early fall while in the northern
part of the species range hatchlings overwinter in the nest.
Mortality
Under undisturbed conditions possibly
only 10-15% of the animals
that hatch in a given year survive until the end of the first
year. Survival from the first to second
and second to third year is similarly low, but increases slightly
by the fourth and fifth years. The average life expectancy of
adults is not known, but is at least 42 years. The estimated maximum life-span based upon an extrapolation
from known adult growth rates is 50-70 years.
Predation
There are many
predators of pond turtles. The introduced bullfrog preys on juvenile Pacific pond turtles and other small turtles. Bullfrogs are native to the eastern United
States, but have become
abundant and widely distributed in the west since their
introduction to Idaho in the 1890's, and to Oregon in the 1920's. Bullfrogs may be an
important predator on hatchlings because both frequent shallow
water habitat.
Predation by bullfrogs and other predators may be responsible for
the lack of juveniles in many
pond turtle populations.
Other known predators
include: Largemouth bass (Micropterus
salmoides),
Black bear (Ursus americanus), coyote (Canis latrans),
raccoon (Procyon
lotor), striped skunk (Mephitis mephitis), Bald Eagle (Haliaeetus leucocephalus),
Osprey (Pandion
haliaetus), gray fox
(Urocyon cinereoargenteus), and domestic dog (Canis
familiaris).
Predation by humans may take the form of wanton shooting, capture
by hook and line fishing or
entanglement in nets, collection for the pet trade or collection
for food.
Suspected predators include bobcat (Lynx rufus),
Great Blue Heron
(Ardea herodias), Black-crowned Night-heron (Nycticorax nycticorax),
Golden Eagle
(Aquila chrysaetos), Red-shouldered Hawk (Buteo lineatus),
Sierra garter
snake (Thamnophis couchii), rainbow
trout (Oncorhynchus mykiss),
and channel catfish (Ictalurus punctatus).
Drought
A prolonged drought in California (1985-1990) apparently
resulted in declines of up to 85 percent in some populations and
the outright elimination of others.
Drought may function as a direct mortality factor by eliminating
the habitat or prey base required
by turtles for survival. Without adequate body fat reserves
normally produced by late-season
feeding, turtles may be unable to survive the stress of
overwintering.
Disease and parasites
Pacific pond turtles
may develop a syndrome similar to upper respiratory disease, the causes
remains a mystery but may be a virus or mycoplasma. Pacific pond turtles essentially have
evolved in isolation from
most other turtle species for most of their history. Therefore
exotic species most likely introduce
pathogenic agents to which Pacific pond turtles have never been
exposed, and thus have had no
chance to evolve any level of resistance. If this is the case, the
introduction of exotic species,
particularly from unhealthy captive situations, (e.g. red-eared
slider) may have catastrophic
consequences for Pacific pond turtle populations. A herpes-like
virus has been reported to kill
captive Pacific pond turtles in California. |
