Parker et al : Diet of Caretta caretta in the central North Pacific 



147 



day and move up near the surface at night (Andersen 

 and Sardou, 1994); this activity again may indicate ac- 

 tive night foraging by the loggerhead sea turtle. 



Loggerhead sea turtles may feed by swallowing float- 

 ing prey whole and also by biting whole prey (or por- 

 tions off a whole prey) found on large floating objects. A 

 commonly ingested prey item, Velella velella, known as 

 "by-the-wind-sailor" (Eldredge and Devaney, 1977), typi- 

 cally was found intact. Janthina spp.. predatory gastro- 

 pods whose main prey item is Velella velella, were also 

 frequently found whole in stomachs. Small Janthina 

 spp. have been observed directly on Velella, and it has 

 been hypothesized that Janthina use Velella to settle 

 on and use the Velella as floatation until they become 

 too large for the host (Bayer, 1963). This behavior may 

 be a reason why whole Janthina and Velella were often 

 found together in stomach samples. Janthina spp. had 

 been previously noted as a prey item of loggerhead sea 

 turtles in the Azores and South Africa (Dodd, 1988) 

 but was first identified as a prey item in the Pacific 

 Ocean in a preliminary unpublished report by Cooke in 

 1992 2 — data that are included in the present study. The 

 high frequency of occurrence of Velella velella and whole 

 Janthina spp. support the hypothesis that loggerhead 

 sea turtles will feed on the surface, swallowing their 

 prey whole. Distribution of Velella velella is patchy; den- 

 sities range from <1/1000 m 3 to 1000/1000 m 3 and den- 

 sities of Janthina spp. are considerably less than those 

 of Velella. When optimum combinations of prevailing 

 winds and currents converge, densities of Velella velella 

 have been observed to be in concentrations upward of 

 10,000/1000 m'-, forming patches so large and dense 

 they have been likened to oil tanker sludge by mariners 

 (Evans, 1986; Parker, personal observ.). It is possible 

 that the one turtle that had a stomach volume of 84% 

 Velella found one of these patches on which to feed. 

 Velella velella was the one common prey item that was 

 not found in stomachs of turtles less than 30-cm CCL. 

 Because Velella were commonly swallowed whole, it is 

 possible that an average size Velella, which range from 

 5 to 10 cm (Evans, 1986), might have been too large for 

 a 13-29 cm CCL turtle to swallow whole. 



The epibiotic oceanic crabs and the gooseneck bar- 

 nacles (Lepas spp.) usually occur on floating objects; 

 Planes sometimes even rides on Velella (Chace, 1951). 

 Planes spp. also have been observed and collected from 

 the tail area of loggerhead sea turtle themselves (Dav- 

 enport, 1994; NMFS observers 3 ). Although approximate- 

 ly 80% of stomach samples with Planes spp. contained 

 whole crabs, which were identified as P. cyaneus, there 



2 Cooke, W. J. 1992. A taxonomic analysis of stomach contents 

 from loggerhead turtles (Caretta caretta ). AECOS report no. 

 697, 12 p. Prepared for NOAA, NMFS, Honolulu Laboratory, 

 2570 Dole Street, Honolulu, Hawaii 96822. (Available from 

 AECOS, Inc., 45-939 Kamehameha Hwy., Rm. 104, Kaneohe, 

 Hawaii 96744.] 



:) NMFS (National Marine Fisheries Service) observers. 1997- 

 2000. Personal commun. Pacific Islands Fisheries Science 

 Center. 2570 Dole Street, Honolulu, HI 96822-2396. 



were also numerous masticated crabs and pieces of 

 crabs. These pieces could have been P. marinus because 

 whole specimens are necessary to identify Planes spp. 

 (Spivak and Bas, 1999); therefore the lowest taxonomic 

 identification for this study was limited to Planes spp. 

 Densities of Planes spp. and Lepas spp. are not well 

 documented but are likely limited by the amount of 

 substrate on which they can settle or on the amount of 

 floating objects available. Natural drifting objects such 

 as tree logs or pumice from volcanic eruptions have 

 been documented since the nineteenth century (Kew, 

 1893, cited in Jokiel, 1990). The "floating islands," as 

 they have been called, continue to be important for 

 transporting organisms, from corals to reef fish across 

 the oceans (Jokiel, 1990). Man-made objects also sup- 

 ply substrate and habitat on which different organisms 

 can settle. Buoys and logs that wash ashore often have 

 Lepas spp. attached to them, some with Lepas spp. cov- 

 ering 100% of the area that was underwater (Parker, 

 personal observ.). Although the frequency of occurrence 

 of Planes spp. in stomach samples was high, the percent 

 sample volume of Planes was relatively low (1.2% total 

 volume) and the mean volume of Planes found was also 

 low (5.6%, Table 2), indicating that this prey was either 

 taken opportunistically or accidentally. It is not known 

 whether the Planes were ingested along with other 

 prey items or were actually grazed from larger floating 

 objects. In contrast, Lepas spp. often occurred in very 

 high percent volumes, indicating that the turtles were 

 actively grazing these prey. The constant presence of 

 Lepas spp. in samples strongly supports the hypothesis 

 that loggerhead sea turtles feed not only by swallowing 

 prey whole, but also by biting prey off larger floating 

 objects. Small chunks of Styrofoam were still attached 

 to the bases of some Lepas specimens indicating that 

 the turtle had bitten off some of the floating object itself 

 while grazing on prey found on the floating debris. 



Among other floating items that often occurred in the 

 turtles' stomachs, one common element was fish eggs. 

 Some of these fish eggs were identified as Hirundicthys 

 speculiger or flying fish eggs. Amphipods were another 

 common item but comprised a very small fraction of 

 total gut content (<1%), indicating that they were not a 

 targeted prey item. Amphipods were possibly ingested 

 incidentally as epiphytes on other items or as part of 

 the gut contents of other prey items. The proportion 

 of man-made drift debris in our sample was low in 

 contrast to prior studies (Balazs, 1985; Allen, 1992; 

 Bjorndal et al., 1994; Kamezaki, 1994; Tomas et al., 

 2002). Plastics and other man-made debris were com- 

 monly found, occurring in about 35% of stomachs, but 

 they comprised a very small fraction of the total gut 

 content (<1%). 



Loggerhead sea turtles also actively forage at deeper 

 depths if high densities of prey items are present. An 

 initial study of pelagic dive behavior of this species 

 (Polovina et al., 2003) indicates that they regularly 

 dive down to depths of 100 m and may also forage 

 at those depths, which may account for the high fre- 

 quency of occurrence and high total percent volume of 



