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Fishery Bulletin 103(1) 



percent sample volume at 14.4%. The percent volume of 

 Janthina was generally high; 15 of 37 turtle stomachs 

 had greater than 30% volume of this species. Only 4 of 

 the 13 stomachs with Velella velella had greater than 

 30% sample volume; yet Velella made up almost 11% of 

 total sample volume, and one of the stomach samples 

 was almost entirely filled (84% volume) with Velella 

 prey. In the samples that contained pyrosomas, this 

 prey item often comprised a high percent of the total 

 gut content — up to 88% stomach volume — and 7 out of 

 11 stomachs had greater than 30% stomach volume of 

 pyrosomas. Planes spp. comprised more than 30% of 

 stomach volume in only 2 of the 29 stomachs contain- 

 ing this species. Lepas spp. often occurred in very high 

 percent volumes (up to 99% of total gut content in one 

 sample), although only 6 of 21 stomachs had percent 

 volumes greater than 30% for Lepas. 



Discussion 



Prey items 



Loggerhead sea turtles in North Pacific oceanic habi- 

 tats are opportunistic feeders that ingest items floating 

 at or near the surface. Availability of prey in the oce- 

 anic realm is generally characterized as patchy. This 

 means that the majority of the ocean contains little to 

 no forage, but in some areas high densities of prey can 

 be found. This unpredictability of prey availability likely 

 contributes to the opportunistic feeding behavior of the 

 loggerhead sea turtle. The TZCF, an area of convergence 

 created within the subtropical frontal zone by cooler 

 denser water masses converging and sinking below 

 warmer lighter water masses (Roden, 1991), may serve 

 to help concentrate different prey items. Prey items such 

 as Velella can often concentrate in large numbers in such 

 areas (Evans, 1986). All size classes of this sea turtle 



collected in our study were found between 16° and 21°C 

 (Fig. 3), which typically are the temperatures that define 

 the subtropical frontal zone and TZCF (Roden, 1991). 

 Eighty-three percent of prey items that were recorded 

 were found floating on the surface or were found on 

 floating objects and would also likely be concentrated 

 at convergent fronts such as the TZCF, driven there by 

 the currents and winds (Polovina, et al., 2000; Polovina 

 et al., 2004). It is suggested that this concentration of 

 prey, along the convergent fronts, may be aggregating 

 the loggerhead sea turtles traveling along this area, 

 which are likely foraging on the increased densities 

 of prey (Polovina et al., 2003a). Turtles in our study 

 smaller than 30-cm CCL had very low volumes of prey 

 in their stomachs. It is unknown whether the paucity of 

 prey items in these turtle stomachs was related to the 

 individual's size, e.g. they were physically not able to 

 capture or ingest certain types of prey items, or perhaps 

 to a lack of experience in foraging due to youth, given 

 that turtles in this size range were determined to be 

 between 1 and 4 years of age by Zug et al. (1995), or to 

 other mitigating factors. 



Another indication that loggerhead sea turtles are 

 opportunistic feeders is the presence of oceanic, me- 

 sopelagic fish as prey items. The total number of fish 

 (lanternfish and hatchetfish) in the samples was low 

 (only 0.1 % of total stomach volume). These species of 

 fish tend to stay below the photic zone usually at depths 

 greater than 300 m during the day and migrate up near 

 the surface at night. Lanternfish make diel vertical mi- 

 grations where they reach maximum densities at 100 m 

 at night. During nightly movements some species can 

 also come directly to the surface (Hulley, 1990). Some 

 species of hatchetfish also make diel vertical migrations, 

 which would bring them to within 100 m of the surface 

 at night (Weitzman, 1986; Froese and Pauly, 2003). 

 Because of the low numbers, it is likely that loggerhead 

 sea turtles ingest only dead or debilitated fish rather 

 than actively hunt and chase such spe- 

 cies. The presence of these species also 

 indicates that the turtles may be feeding 

 at night when they would be more likely 

 to encounter the fish during their diel 

 movement. Another prey item exhibiting 

 diel vertical migration is the pyrosomas. 

 Pyrosomas, which are a part of Pacific 

 leatherback sea turtle diets (Davenport 

 and Balazs, 1991), were also present in 

 loggerhead sea turtle stomach samples. 

 Pyrosomas are colonial tunicates com- 

 prising individual zooids embedded in 

 the walls of a gelatinous tube. These 

 colonies can become quite large (some 

 greater that 4 meters in length) and 

 tend to drift with ocean currents and 

 accumulate along frontal zones which 

 make them accessible to the sea turtle 

 that forages opportunistically. At least 

 one species (P. atlanticum) has been re- 

 corded to stay below 300 m during the 



