Physiological and Genetic Responses to Environmental Stress
167
Unlike certain freshwater turtles, which overwinter in frozen ponds and thus
withstand months submerged in near-freezing water (Jackson, 2000), sea turtles
(with the exception of leatherbacks) trapped in cold waters (below 8–10 ∞ C) may
become lethargic and buoyant, floating at the surface. This condition is defined as
cold stunning (Schwartz, 1978). Salt gland function may be impaired in cold-stunned
animals, as evidenced by increased blood concentrations of sodium, potassium,
chlorine, calcium, magnesium, and phosphorus (George, 1997; Carminati et al.,
1994). Affected animals may not eat for days or even weeks prior to cold stunning,
increasing overall physiological stress (Morreale et al., 1992). However, it is likely
that it is the rate of cooling below 15 ∞ C that evokes cold stunning rather than the
temperature per se. Satellite tracking studies of ocean migrating Kemp’s ridley and
loggerhead turtles indicate that they remain active in water temperatures as low as
6 ∞ C (Keinath, 1993). Sea turtles that overwinter in inshore waters are most susceptible to cold-stunning because temperature changes are most rapid in shallow water,
especially in semienclosed areas such as lagoons (Witherington and Ehrhart, 1989).
As temperatures drop below 5–6 ∞ C, death rates become significant, because the
animals can no longer swim or dive, become vulnerable to predators, and may wash
up onshore, where they are exposed to even colder temperatures.
As with other physiological stressors, cold stunning can affect specific populations of sea turtles more than others. For example, although cold-stunning events
occur in Florida as well as in northern waters, the extended exposure to frigid waters
experienced by turtles off New England or New York results in much higher mortality
rates. Morreale et al. (1992) reported overall mortality rates as high as 94% over
three winters in New York, whereas Witherington and Ehrhart (1989) reported only
10% mortality for cold-stunned turtles in a Florida estuary.
Habitat utilization is also a significant factor in differential mortality during
cold-stun events. The waters off New York and New England appear to be an
important habitat for juvenile Kemp’s ridley turtles, with the result that a large
percentage of identified cold-stunned animals are of this species (Figure 6.1). Of
the 277 total sea turtles found on Cape Cod, MA, during the 1999–2000 winter
season, 79% were Kemp’s ridley turtles, 19% loggerheads, and 2% greens (Still
et al., in press). During the 1985–1986 winter, 79% of the turtles retrieved on Long
Island (NY) were Kemp’s ridleys (Meylan and Sadove, 1986). Indeed, Kemp’s
ridleys have consistently made up more than 50% of the cold-stunned turtles found
along Cape Cod for the past 20 winters, and 67–80% of cold-stunned turtles found
off Long Island over a 3-year period were Kemp’s ridleys (Morreale et al., 1992).
By contrast, in five significant stunning events over a 9-year period in the Indian
River Lagoon (FL), 73% of 467 recovered turtles were greens (Figure 6.1), 26%
were loggerheads, but less than 1% (2 animals) were Kemp’s ridleys (Witherington
and Ehrhart, 1989).
Size is also an important factor in susceptibility to cold-stun events, because
juveniles are the primary life history stage affected. The majority of Kemp’s ridleys
retrieved off Cape Cod in the 1999–2000 season were in the 25.0–29.9 cm curved
carapace length (CCL) size class, as were many greens. Similarly, Morreale et al.
(1992) reported a mean straight carapace length (SCL) of 29.4 cm for Lepidochelys
kempii and 32.7 cm for Chelonia mydas for cold-stunned turtles collected off Long
1123 book.book Page 167 Monday, November 11, 2002 11:11 AM
167
Unlike certain freshwater turtles, which overwinter in frozen ponds and thus
withstand months submerged in near-freezing water (Jackson, 2000), sea turtles
(with the exception of leatherbacks) trapped in cold waters (below 8–10 ∞ C) may
become lethargic and buoyant, floating at the surface. This condition is defined as
cold stunning (Schwartz, 1978). Salt gland function may be impaired in cold-stunned
animals, as evidenced by increased blood concentrations of sodium, potassium,
chlorine, calcium, magnesium, and phosphorus (George, 1997; Carminati et al.,
1994). Affected animals may not eat for days or even weeks prior to cold stunning,
increasing overall physiological stress (Morreale et al., 1992). However, it is likely
that it is the rate of cooling below 15 ∞ C that evokes cold stunning rather than the
temperature per se. Satellite tracking studies of ocean migrating Kemp’s ridley and
loggerhead turtles indicate that they remain active in water temperatures as low as
6 ∞ C (Keinath, 1993). Sea turtles that overwinter in inshore waters are most susceptible to cold-stunning because temperature changes are most rapid in shallow water,
especially in semienclosed areas such as lagoons (Witherington and Ehrhart, 1989).
As temperatures drop below 5–6 ∞ C, death rates become significant, because the
animals can no longer swim or dive, become vulnerable to predators, and may wash
up onshore, where they are exposed to even colder temperatures.
As with other physiological stressors, cold stunning can affect specific populations of sea turtles more than others. For example, although cold-stunning events
occur in Florida as well as in northern waters, the extended exposure to frigid waters
experienced by turtles off New England or New York results in much higher mortality
rates. Morreale et al. (1992) reported overall mortality rates as high as 94% over
three winters in New York, whereas Witherington and Ehrhart (1989) reported only
10% mortality for cold-stunned turtles in a Florida estuary.
Habitat utilization is also a significant factor in differential mortality during
cold-stun events. The waters off New York and New England appear to be an
important habitat for juvenile Kemp’s ridley turtles, with the result that a large
percentage of identified cold-stunned animals are of this species (Figure 6.1). Of
the 277 total sea turtles found on Cape Cod, MA, during the 1999–2000 winter
season, 79% were Kemp’s ridley turtles, 19% loggerheads, and 2% greens (Still
et al., in press). During the 1985–1986 winter, 79% of the turtles retrieved on Long
Island (NY) were Kemp’s ridleys (Meylan and Sadove, 1986). Indeed, Kemp’s
ridleys have consistently made up more than 50% of the cold-stunned turtles found
along Cape Cod for the past 20 winters, and 67–80% of cold-stunned turtles found
off Long Island over a 3-year period were Kemp’s ridleys (Morreale et al., 1992).
By contrast, in five significant stunning events over a 9-year period in the Indian
River Lagoon (FL), 73% of 467 recovered turtles were greens (Figure 6.1), 26%
were loggerheads, but less than 1% (2 animals) were Kemp’s ridleys (Witherington
and Ehrhart, 1989).
Size is also an important factor in susceptibility to cold-stun events, because
juveniles are the primary life history stage affected. The majority of Kemp’s ridleys
retrieved off Cape Cod in the 1999–2000 season were in the 25.0–29.9 cm curved
carapace length (CCL) size class, as were many greens. Similarly, Morreale et al.
(1992) reported a mean straight carapace length (SCL) of 29.4 cm for Lepidochelys
kempii and 32.7 cm for Chelonia mydas for cold-stunned turtles collected off Long
1123 book.book Page 167 Monday, November 11, 2002 11:11 AM