Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
3.2 SEA TURTLES
3.2.1 Introduction
Sea turtles are long-lived reptiles that can be found throughout the world’s tropical, subtropical, and
temperate seas (CCC and STSL 2003). There are seven living species of sea turtles from two distinct
families, the Cheloniidae (hard-shelled sea turtles; six species) and the Dermochelyidae (leatherback sea
turtle; one species). These two families can be distinguished from one another on the basis of their
carapace (upper shell) and other morphological features. Sea turtles are an important marine resource in
that they provide nutritional, economic, and existence (non-use) value to humans (Witherington and
Frazer 2003). Over the last few centuries, sea turtle populations have declined dramatically due to
anthropogenic activities such as coastal development, oil exploration, commercial fishing, marine-based
recreation, pollution, and over-harvesting (NRC 1990; Eckert 1995). As a result, all six species of sea
turtles found in U.S. waters are currently listed as either threatened or endangered under the ESA.
Sea turtles are highly adapted for life in the marine environment. Unlike terrestrial and freshwater turtles,
sea turtles possess powerful, modified forelimbs (or flippers) that enable them to swim continuously for
extended periods of time (Wyneken 1997). They also have compact and streamlined bodies that help to
reduce drag. Additionally, sea turtles are among the longest and deepest diving of the air-breathing
vertebrates, spending as little as 3 to 6% of their time at the water’s surface (Lutcavage and Lutz 1997).
These physiological traits and behavioral patterns allow for highly efficient foraging and traveling. Sea
turtles often travel thousands of kilometers between their nesting beaches and feeding grounds, which
makes the aforementioned suite of adaptations very important (Ernst et al. 1994; Meylan 1995). Sea turtle
traits and behaviors also help protect them from predation. Sea turtles have a tough outer shell and grow
to a large size as adults; mature leatherback turtles can weigh up to 916 kg (Eckert and Luginbuhl 1988).
Sea turtles cannot withdraw their head or limbs into their shell, so growing to a large size as adults is
important. As juveniles, some species of sea turtles evade predation by residing in habitats that are either
structurally complex or moderately shallow. This prohibits marine predators such as sharks, marine
crocodiles, and large fishes from easy access (Musick and Limpus 1997).
Although they are specialized for life at sea, sea turtles begin their lives on land. Aside from this brief
terrestrial period, which lasts approximately three months as eggs and an additional few minutes to a few
hours as hatchlings scrambling to the surf, sea turtles are rarely encountered out of the water. Sea turtles
return to land primarily to nest, although some species in Hawai’i, Australia, the Galapagos Islands, and
the Mexican Pacific also bask on land (Carr 1995; Spotila et al. 1997). Sea turtles observed on land are
almost always females since males are not involved in the nesting process and likely gain fewer benefits
from basking on land. Females bask to thermoregulate, elude predators, avoid harmful mating encounters
with male sea turtles, and possibly to accelerate the development of their eggs (Spotila et al. 1997). On
occasion, sea turtles can unintentionally strand on land if they are dead, injured, or cold-stunned.
Female sea turtles nest in tropical, subtropical, and warm-temperate latitudes, often in the same region
where they were born (Miller 1997). Upon selecting a suitable nesting beach, sea turtles tend to re-nest in
close proximity during subsequent nesting attempts. Some individuals fail to nest when emerging from the
ocean. These non-nesting emergences, or false crawls, occur when sea turtles are either obstructed from
laying their eggs by debris, rocks, or roots or are distracted by conditions on the nesting beach (e.g.,
noise, lighting, or human presence). Individuals that are successful at nesting usually lay several clutches
of eggs during a nesting season with each clutch containing between 50 and 200 eggs depending upon
the species (Witzell 1983; Dodd 1988; Hirth 1997). Most female sea turtles, with the possible exception of
Kemp’s ridleys, do not nest in consecutive years; instead, they will often skip two or three years before
returning to the nesting grounds (Márquez-M. 1994; Ehrhart 1995). Nesting success is vital to the longterm
existence of sea turtles since only one out of every one thousand hatchlings survives long enough to
reproduce (Frazer 1986).
During the nesting season, daytime temperatures can be lethal on tropical, subtropical, and warmtemperate
beaches. As a result, adult sea turtles most often nest and hatchlings most often emerge from
their nest at night (Miller 1997). After emerging from the nest, sea turtle hatchlings use visual cues (e.g.,
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