Nearshore Distribution and an Abundance Estimate for Green Sea Turtles, Chelonia mydas, at Rota Island, Commonwealth of the Northern Mariana Islands1 Steven P. Kolinski,2 Ronald K. Hoeke,3 Stephani R. Holzwarth,3 Larry I. Ilo,4 Evelyn F. Cox,2 Robert C. O’Conner,2 and Peter S. Vroom3 Abstract: Seventy-three green turtles, Chelonia mydas (Linnaeus, 1758), were observed in 84 sightings along 28 transects covering 67% of Rota’s shoreline and outer reef perimeter in the Commonwealth of the Northern Mariana Is- lands. No other sea turtle species were encountered. Juvenile turtles of various sizes dominated in all surveyed environments, and observations of turtles with estimated straight carapace lengths a 40 cm suggested recent and continuing recruitment at Rota. Distribution of turtles appeared temporally stable when compared with previously reported observations and data, with turtle concentra- tions highest along northeast, east, and southeast coasts of the island. Approxi- mately 118 turtles were projected to inhabit nearshore habitats at Rota. Although this population may appear minor and indistinct compared with those at nearby Tinian and Saipan, continued monitoring would be useful for compar- ison of Mariana Islands trends. Thirty-five species of cyanophytes, algae, and a sea grass noted as green turtle forage in other world regions were identified at Rota in this and previous surveys. The green turtle, Chelonia mydas (Lin- annual nests ranging to the low tens through- naeus, 1758), has been reported as the princi- out the commonwealth (Pritchard 1982, pal sea turtle species in the Commonwealth Wiles et al. 1989, 1990, McCoy 1997, Pultz of the Northern Mariana Islands (CNMI) et al. 1999, Ilo and Manglona 2001, Kolinski (Wiles et al. 1989, 1990, McCoy 1997, Pultz et al. 2001). The preponderance of turtles in et al. 1999, Kolinski et al. 2001, 2004, 2005). the region are found residing in nearshore Anecdotal observations and intermittent sur- reef areas of the southern arc islands (Figure veys suggest that nesting greens make up a 1), where identification of distributions and relatively small proportion of the local sea abundance has become critical to ascertaining turtle population, with known numbers of turtle population status and dynamics, partic- ularly in relation to human activities that in- clude efforts at resource management and 1 Manuscript accepted 20 December 2005. recovery (National Marine Fisheries Service 2 Joint Institute for Marine and Atmospheric Re- and U.S. Fish and Wildlife Service 1998, Ko- search, National Marine Fisheries Service, Pacific Islands Regional Office, 1601 Kapiolani Boulevard, Suite 1110, linski et al. 2001, 2004, 2005; S.P.K., unpubl. Honolulu, Hawai‘i 96814-0047 (e-mail: steve.kolinski@ data). noaa.gov). In 2003, surveys of reefs throughout the 3 Joint Institute for Marine and Atmospheric Re- Mariana Archipelago off the NOAA ship R/ search, National Marine Fisheries Service, Coral Reef V Oscar Elton Sette included a rapid yet com- Ecosystem Division, 1125B Ala Moana Boulevard, Ho- nolulu, Hawai‘i 96814. prehensive assessment for turtles in nearshore 4 Division of Fish and Wildlife, Department of Lands marine habitats of the southernmost CNMI and Natural Resources, P.O. Box 10007, Lower Base island, Rota. Few assessments for sea turtles Area, Saipan, MP 96950. in nearshore waters at Rota have been made (Wiles et al. 1990, Ilo and Manglona 2001), Pacific Science (2006), vol. 60, no. 4:509–522 and published observations on turtle distribu- : 2006 by University of Hawai‘i Press tions and abundance for the island are limited All rights reserved (Wiles et al. 1990, Kolinski et al. 2004). Con- 509 Figure 1. The Mariana Islands and the 2003 distribution of transects and turtles at Rota Island. Sea Turtles at Rota Island . Kolinski et al. 511 sistent with recommendations of the recovery ranging from 3 to approximately 30 m. On plan for U.S. Pacific green turtle populations one of the boats, when a turtle was sighted (National Marine Fisheries Service and U.S. its approximate position was signaled to boat Fish and Wildlife Service 1998), in this paper personnel via a magnetic-switch telegraph and we present detailed information on turtle recorded on a geographic positioning system distributions, estimated size classes, projected (GPS) (Garmin 76S). On the other boat tur- abundance, and likely food resources for tle positions were estimated by matching ob- green turtles in marine habitats surrounding servation times with points collected every 5 Rota Island. sec on a GPS set to record tow tracks. Obser- vation time, species, size (visually estimated straight carapace length [SCL] made in com- Study Area parison with 1-m-wide tow-boards delineated Rota (14 90 N, 145 12 0 E) is an 85-km2 is- with 10-cm markings), sex (when discern- land located northeast of Guam (60 km) and able), activity, and habitat characteristics were southwest of Aguijan (71 km) in the southern recorded by underwater observers. Individual arc of the Mariana Archipelago (Figure 1). tows typically lasted 1 hr and occurred at spa- The island consists of terraced limestone pla- tial intervals that allowed for characterization teaus bordering two small volcanic outcrops of all major (i.e., north, east, south, and west) and reaches 491 m in height (Eldredge 1983, coastlines (Figure 1). The second method Wells and Jenkins 1988, Wiles et al. 1990). consisted of dive surveys made between Steep limestone cliffs, rocky shores, and depths of 12 and 22 m by eight scientific intermittent benches characterize much of divers who collected data on turtles while as- the eastern and southern coasts as well as the sessing fish, invertebrate, and algal commu- southern portion of the western peninsula. nities along three 25-m transects at each of Coarse calcareous sand beaches are inter- six locations around the island. The dive sur- spersed among rock along northern, south- vey data are presented to reflect consistency ern, and western shores (Eldredge and in observations of turtle distributions. How- Randall 1980), where sea turtle nesting has ever, they were not included in abundance es- been reported as infrequent (Wiles et al. timates because most likely were resightings 1990, Grout 1997, McCoy 1997, Ilo and of turtles counted in tow surveys that over- Manglona 2001). Marine habitats are di- lapped these short transects. In all surveys, verse and include fringing reefs, shallow reef benthic habitats and the water column were slopes, spur and groove zones, extensive pave- assessed for turtles within ranges of visibility ment, boulder fields, ledges, and steep cliffs. (typically 15 to >40 m). Surface waters also Human development is concentrated mainly were searched by support boat personnel, in the southwest of the island at Songsong with pertinent information on turtle sightings (Figure 1). The 2000 census identified 3,283 (as previously described) being recorded. residents (Evans et al. 2002), approximately Turtle maturity status was not directly 5% of the CNMI population. Fisheries re- measured for any individual. Visual SCL esti- sources at Rota are exploited by local and mates were used to grossly categorize turtles neighboring island residents. as juveniles (a70 cm), juvenile/adults (>70 and a90 cm), and presumed adults (>90 materials and methods cm). The range for ‘‘presumed adults’’ may appear liberal but was based on the average Assessments for turtles at Rota were con- of mean SCL nesting sizes presented for Pa- ducted on 19 and 20 September 2003, off the cific turtles by Hirth (1997), rounded to the NOAA ship R/V Oscar Elton Sette. Two sur- nearest decimeter. Estimated turtle numbers vey methods were employed. Towed-diver were adjusted for each transect by removing surveys (Kenyon et al. in press) consisted of resightings of the same individual based on two scuba divers per boat being towed paral- size, distinguishing features, time, and specific lel to shore by each of two boats at depths locations and/or routes. Such methods are TABLE 1 Observations of Green Turtles at Rota Island, CNMI, in 2003 Estimated No. of Turtles Observed Transect Max. Time Length Total No. of Juvenile/ Presumed Turtles Site Date (hr:min) Method (km) Observations Juvenile Adult Adult Total per km North Rota Puntan Sailigai 9/20 0:15 Single tow 0.67 0 0 0 0 0 0.0 to Tatachok Tatachok to 9/20 1:00 Single tow 2.58 0 0 0 0 0 0.0 Teteto Sayan Gigani 9/20 1:35 Dive survey *0.08 0 0 0 0 0 *0.0 Sayan Gigani to 9/20 1:00 Single tow 2.62 0 0 0 0 0 0.0 Agusan I Batko to 9/20 0:33 Single tow 0.82 2 2 0 0 2 2.4 Lalayak Lalayak to 9/20 0:27 Double tow 1.05 6 3 2 0 5 4.8 Mochong Mochong to 9/20 0:32 Single tow 1.12 2 1 1 0 2 1.8 Maya Mochong 9/20 1:19 Dive survey *0.08 5 *1 *2 0 *3 *37.5 (Mean G SE) Subtotal 6:41 8.86 15 6 3 0 9 1.5 G 0.8 East Rota Puntan Fina 9/19 0:28 Single tow 0.99 4 1 2 1 4 4.0 Atkos Puntan Fina 9/19 0:29 Double tow 0.99 3 2 0 1 3 3.0 Atkos to As Dudo As Dudo 9/19 1:36 Dive survey *0.08 0 0 0 0 0 *0.0 As Dudo to 9/19 0:36 Single tow 1.34 9 6 1 0 7 5.2 Puntan As Fani I Chiugai to 9/19 0:54 Double tow 1.86 27 14 8 4 26 14.0 Puntan Saguagahga (Mean G SE) Subtotal 4:03 5.18 43 23 11 6 40 6.6 G 2.5 South Rota Puntan 9/19 0:28 Single tow 1.21 4 1 3 0 4 3.3 Saguagahga to Taksunok Taksunok 9/19 1:25 Dive survey *0.08 0 0 0 0 0 *0.0 Alaguan to 9/19 0:56 Single tow 2.31 6 1 3 2 6 2.6 Payapai Agatasi to 9/19 1:00 Single tow 2.96 2 2 0 0 2 0.7 Gaonan Guaa to Gagani 9/19 1:00 Single tow 2.30 1 1 0 0 1 0.4 South Puntan 9/20 0:30 Single tow *0.87 1 *1 0 0 *1 *1.1 Pona West