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

AUGUST 2005 FINAL REPORT
Most scientists agree that the endangered leatherback and threatened olive ridley turtles are infrequent
visitors to the region. Leatherback encounters in the waters of the study area are presumed to involve
individuals that are in transit to distant foraging or nesting areas in the Pacific Ocean, while olive ridley
encounters likely involve individuals that are blown off course (also known as waifs) (Pritchard 1995;
Kolinski 2001). It is also possible that loggerhead turtles could occur in the Marianas MRA study area,
with nesting populations located in Japan and eastern Australia, a life cycle that carries individuals
throughout the North Pacific Ocean, and recent sighting records from the Philippines (Sagun et al. 2005).
The distribution of all available sea turtle occurrence records by season (dry=December through June;
rainy=July through November) is presented in Figure C-1. Occurrence records include sightings from
aerial (helicopter), marine tow, SCUBA, snorkel, and shoreline surveys as well as opportunistic sightings
during recreational SCUBA diving trips. It should be noted that the number of sea turtle records in a given
season or portion of the study area is often a function of the source or type of data, level of effort, and
sighting conditions.
Unidentified sea turtles (individuals that could not be identified to species) often account for a good
number of sighting and nesting records. Hard-shelled sea turtles (which include the green, hawksbill,
loggerhead, and olive ridley turtles) are often difficult to distinguish to species, particularly when they are
young (i.e., small size classes) and especially during aerial and boat surveys. Sea turtles may respond to
aircraft overflights and vessel approaches by making a quick dive, even before being sighted by
observers, which makes both sighting and identifying a sea turtle difficult. Species identification is less
reliable when individuals from the general public (e.g., recreational divers, beachgoers) sight sea turtles.
The reliability of species recognition may also be in question when sea turtles strand or nest, especially if
qualified individuals are not present to make an accurate identification (Lund 1985).
Occurrence data from surveys at four of the five islands in the southern Marianas arc between 1999 and
2001 reveal that the small uninhabited islands of FDM and Aguijan likely sustain on the order of tens of
sea turtles while the larger inhabited islands of Saipan and Tinian probably support on the order of
hundreds of sea turtles. It is estimated that the CNMI portion of the southern Marianas arc, which also
includes Rota, likely supports between 1,000 and 2,000 resident sea turtles. Turtle densities and
abundances in the CNMI appear to be highest at Tinian, despite its smaller size relative to Saipan and its
apparent lack of seagrass forage. Juvenile sea turtles are the most abundant life stage found in the study
area and vicinity; they accounted for between 60% and 82% of all sea turtle sightings at Saipan in 1999
and between 67% and 85% of all sightings at Tinian and Aguijan in 2001 (Kolinski et al. 1999; Pultz et al.
1999; Kolinski 2001).
There are no recent estimates of the number of sea turtles inhabiting the nearshore waters around Guam,
although survey data from the 1970s and 1980s suggested that sea turtle numbers had been reduced
over that time period (Davis n.d.; Wiles et al. 1995). During the 1990s, however, aerial survey sightings of
sea turtles gradually increased from year to year (Cummings 2002). Historically, sea turtles were known
to be highly abundant at several locations including off the northern and eastern coasts. Groups of 40 to
50 individuals were often observed between Ritidian Point and Pati Point during aerial surveys. From
1975 to 1979, approximately 70% of all sea turtles observed during aerial surveys around Guam were
seen along the island’s eastern coast (from Inarajan Bay to Ritidian Point). According to Pritchard (1995),
greater numbers of sea turtles likely occurred there due to lower levels of development and fishing
pressure. From 1989 to 1991, 58% of all aerial survey sightings of sea turtles occurred in northern Guam
between Tanguisson Beach and Pago Bay (Wiles et al. 1995). Sea turtles have also been commonly
observed inside Apra Harbor (DoN 2003a; Gutierrez 2004).
Although the resident juvenile foraging population in the study area and vicinity is large, the adult nesting
population is small (Wiles et al. 1989; Pultz et al. 1999; Kolinski et al. 2001). Rota is thought to support
few nesting turtles (Wiles et al. 1990), while the beaches on FDM and other islands of the northern
Marianas arc are likely unsuitable for sea turtle nesting (DoN 2004). Regular nesting activity in this region
is only prevalent on Guam, Tinian, and Saipan (Figure 3-3). However, nesting activity at these three
islands is infrequent when compared to other areas in the Pacific Ocean such as Mexico, Costa Rica,
Japan, Australia, and the Northwestern Hawaiian Islands (Kolinski et al. 1999; Kolinski 2001).
3-90