t?rraridt illd Biolog). 1003..1(3):612 619 Status of the Ftatback Sea Turtle ;;ffiJl)J1 nooo"", on Crab Istand, Austratia, with Notes on Predation by Crocodiles RoNlr,n W. SursnnLANDr'2lun Elrzlnnrn G. Surnnnr,lNnr.3 I Institute for Environmental Studies, [Jniversit-t of Wisconsin, Madison, Wisconsin 53706 USA; zPresent Address: Nicholas School ofthe Environment and Earth Sciences, Box 90328, Duke Universirv, Durhant, North Carolina 27708 USA [E-mail: [email protected]]; 3Present Address: Department of Maternal and Child Heatth, University of North Carolina at Chapel Hitl, Rosenau Hall, CB 7445, Chapel Hill, North Carolina 27599 USA IE-mail: [email protected]] AssrnA,cr. - The conservation status of the world's largest rookery of the flatback sea turtle (Natator depressus) on Crab Island, Australia, was assessed. During six weeks in the austral winter of 1997, female flatbacks came ashoreon4234occasions, with amean of L03.3 turtle tracks pernight. Almost 300 more turtle tracks were observed from G19 July than during the same two-week period of a previous study in 1991, lending credence to the notion that this nesting aggregation remains robust. Saltwater crocodiles (Crocodylus porosus) consumed the turtles at a minimum rate of one per week, and we describe one incident of crocodile predation witnessed directly. Human harvest of turtle eggs was of low intensity and appeared sustainable. We recommend continued protection of Cape York and its associated marine ecosystems in order to ensure the persistence of Crab Island's highly significant turtle rookery. Knv Wonos. - Reptilia; Testudinesl Cheloniidae; Crocoditia; Na tator depressus; sea turtle; nesting; conservation; predation; Crocodylas porosus; Cape York; Australia The flatback sea turtle, Natator depres.rrr.r, is re- Flatback turtles nesting on Crab Island are also poten- stricted in range to the continental shelf of Australia, tially threatened by regular egg harvesting by the local occurring as far north as Irian Jaya (Limpus et al., 1993), aboriginals (Limpus et al., 1983 a,, 1993; Miller and Limpus, with nesting ranging down the northern coastline of 1991). Despite the low intensity of this harvest in previous Australia to Bundaberg on the east coast and to Barrow years, egg-collecting is both mechanized(via gasoline pow- Island on the west (Limpus, l97l; Limpus et al., 1993; ered boats) and unmonitored, factors which could lead to Prince , 1994). The largest known rookery of the flatback overexploitation. The indigenous communities also occa- occurs on Crab Island, an uninhabited sand cay in the sionally catch and eat adult female flatbacks at Crab Island, Gulf of Carpentaria belonging to the aboriginal commu- though the meat of green turtles is preferred over that of nity of Injinoo (Limpus et al., 1983a). Because of the flatbacks in the Torres Strait (Limpus et al., 1983a). The only remote location and inhospitable reputation of the island, published feeding ground recovery of a tagged Crab Island sea turtle research at this site has been limited to three flatback occurred in southern Irian Jaya,where the turtle was studies, widely separated in time. Bustard (19i2) de- consumed (Limpus et al .,1993). The number of N. depressus scribed the results of a month-long stay on Crab Island by thus caught and eaten in Indonesia is presumably far smaller one of his assistants, who tagged 180 turtles in Novem- than the many thousands of C. mydas harvested in that nation ber-December 1970. Drawing upon a series of short (Daly, 1994),, but may still represent an important source of visits in the late 1970s, Limpus et al. (1983a) reporred mortality for the Crab Island flatbacks. that the turtles nesting on Crab Island were flatbacks, not In light of these threats, and given the well-known plight green turtles (Chelonia mydas) as indicated by Bustard of sea turtle populations worldwide, it is important to per- (1912), and that these turtles nest on Crab Island year- form occasional field studies at significant, yet remote, round, with a peak of hundreds per night occurring in the rookeries such as Crab Island to monitor them for otherwise middle of the austral winter. Limpus et al. (1993) con- unobserved degradation (Godley et a1.,2001). Conducted firmed high reproductive productivity for the flatbacks six years after the last reported research in 1991, the present on Crab Island during two weeks of July 1991, and study evaluates the continued nesting success and further described various aspects of the turtles' nesting ecology. describes the nesting ecology of Crab Island's major popu- Flatbacks have been shown to perish in significant lation of sea turtles. Predation by saltwater crocodiles, numbers in the nets of the Australian northern prawn fishery Crocodylus porosLts, upon the flatbacks is also described (including the Gulf of Carpentaria), where they have been and quantified for the first time. The status of the flatback identified as the most frequent marine turtle bycatch (Poiner nesting population, along with the significance of the croco- and Harris, 1996). This fishery tripled in size from the 1970s dile-turtle interaction, are discussed, and preliminary rec- to 1980s (Somers, 1990), and the effects of the resulting ommendations are made for ensuring the future of Crab increased annual take of turtles are unknown. Island as an internationally significant natural area. SuTHEnLAND AND SuTHnnLAND Flatback Turtles on Crab Island 613 - METHODS and any evidence of predation upon eggs by crabs (Limpus et al., 1993). Any live hatchlings found in the nests were Limpus et al. (1993) provided a description of the released on the beach. geology, climate, and flora of Crab Island, and include a Each night, beginning one hour before high tide, we detailed vegetation map.The island (10o59'S., 142"06'E) is patrolled the beach between sectors 4 and 7 , an areacentered a crescent-shaped sand cay less than 100 m wide at the around the largest frontal sand dune on the island. As noted center, with a 5.5 km long beach on the west, and an by Limpus et al. (1993), two workers could not safely or expansive tidal mud flat in the eastern bay (Fig. I ). effectively patrol the entire beach for nesting turtles at night, We camped on Crab Island from 14 Jun e to 25 July due to the size of the island and the presence of foraging 1991 , a total of 42 nights. Immediately upon arrival the crocodiles in the adjacent surf. Patrolling continued until existing turtle tracks on the entire western beach were two subsequent inspections of the target sectors failed to counted, with each set of one up-track and one down-track reveal any newly emergent nesting turtles. Each turtle lo- taken as representing the emergence of a single female turtle. cated was measured with a flexible tape for curved carapace The tracks were marked through in the sand after being length (CL) and curved carapace width (CW), according to counted, preventing tracks from being counted more than the methodologies set by the Queensland Turtle Research once (Limpus et al., 1993). Each subsequent morning this Project, described in Limpus et al. ( 1983a). Tagged turtles process was repeated, with fresh turtle crawls from the night were noted as recaptures and the tag numbers were recorded. before being easily distinguished. The western beach was Unmarked turtles were not tagged. The success of each divided into I 6 sectors paced at roughl y 340 m each along turtle's nesting attempt (whether or not they deposited eggs) the spring high tide line (Fig. I ), and the number of turtle was noted, and as many freshly-laid clutches of eggs as tracks per sector each morning was counted. Turtle tracks possible were marked. After the departure of the nesting were identified to species by the relative width of the crawl turtle, these fresh nests were measured for depth and the through the sand. Narrow flipper tracks were counted as contained eggs were counted with a minimum of rotation. those of hawksbill (Eretntochelys imbricata) or olive ridley The eggs were replaced and reburied in the nests within two (Lepidochelys olivaceo) sea turtles and were easily distin- hours of deposition, a period in which such handling appears guishable from the wide tracks left by flatbacks. No other sea to cause little egg mortality (Parmenter, 1980). turtle species has been observed nesting on Crab Island Aspects of the nesting ecology of the flatbacks quanti- (Limpus et al., 1983a, 1993). fied during the present study were compared when possible Newly emerged N. clepressus clutches were located to similar data taken by Limpus et al. (1993), using two- during the morning track counts by following fan-shaped sample independent t-tests with pooled estimates of vari- arrays of hatchling tracks to their sources. Nests located near ance. The mean CL growth rate calculated from recaptures camp were excavated in the afternoon. The numbers of was also compared using a one-sample t-test to a hypotheti- empty eggshells remaining at the bottom of the nests were cal mean of zero (Zar, 1996). Two-tailed probabilities of recorded, providing a measurement of the number of turtles obtaining the calculated t-ratios were generated using Re- that had hatched. Also recorded from excavated nests were lease 12.21 of the statistical program Minitab (Minitab, the number of live and dead hatchlings left in the nest, the 1998), and all probabilities calculated were considered sig- status of any unhatched eggs (undeveloped or developed), nificant if p < .05. f\ ,------Y,-\=-lRlAN P.N.G i ] JAYA {- -\. lsland i* 4,,-' '-t t',.. ? AfIA FURA Po;it'- Moresby s fi'A Crab -r' ,1 '*u&11*'LU/ I \s llrl*t Ys Prs i )-i. GU I.t" Itt Of \T. (:ARPENTARIA -b't t"+- -t NJ -"-i*''^.--- : dTot* ,-" AUSTRALIA Figure 1 Maps of Crab Island, Australia.