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Marine Turtle Newsletter 67:13-14, © 1994

Use of Pit Tags to Identify Adult Leatherbacks

Peter Dutton and Donna McDonald
Leatherback Project, P. O. Box 1697, Frederiksted, St. Croix, U. S. Virgin Islands 00841-1697

Tags applied to the flippers of sea turtles usually do not persist longer than a few years and this tag loss problem has confounded attempts to accurately assess the size of nesting populations. One of the most intensely monitored nesting populations of leatherback turtles, Dermochelys coriacea, occurs at Sandy Point National Wildlife Refuge (SPNWR) on St. Croix, U. S. Virgin Islands. From 1977 to 1993, 293 individuals (gravid females) have been tagged with monel, titanium, and/or inconel flipper tags. However, this number may overestimate population size, since untagged turtles may actually be remigrants that have lost their tags from previous years. Only 5-48% of the nesting females in any given year are classified as remigrants based on flipper tags, but "new" untagged animals often have possible tag scars, suggesting that remigration is higher than previously thought (tag scars are extremely difficult to confirm).

In 1992 we began using PIT (Passive Integrated Transponder) tags, in addition to traditional flipper tags, to identify individual females nesting at SPNWR. The PIT "tags" consist of a small (14 mm x 2 mm) cylindrical glass-encapsulated microchip encoded with an identification number. We use AVID tags (3179 Hamner Avenue, Norco, California 91760; Tel: 1-800-336-AVID). Tags are injected into the shoulder muscle during the motionless "trance" of egg-laying, using a 2-inch 12-gauge needle so as to penetrate beyond the thick layer of blubber into the underlying muscle. Tags are sterilized in alcohol and packed in antibiotic ointment. The skin is cleaned with Betadine to prevent infection. Tags are detected using a small portable scanner passed over the shoulder area; tag number is displayed on a digital screen on the scanner. Shoulder placement (Figure 1) was chosen for several reasons:

1. the area is exposed, making it relatively easy to apply and read the tag without disturbing the turtle;
2. the area is relatively well protected, since more distal portions are generally lost during predator attacks;
3. stranded animals and carcasses are often missing flippers, making it impossible to verify the presence of traditional flipper tags.

We injected two tags into each of 15 turtles, one high on the shoulder under a thicker (ca. 5 cm) blubber layer and one lower on the shoulder under a comparably shallower blubber layer (ca. 2.5 cm deep), to test tag retention and readability at different depths of insertion. Tags were all easily readable at both locations throughout the three-month nesting season. Of 105 PIT tags injected in 1992 and 1993, only one was lost (i.e., undetectable) by the end of the season, and this was probably because the tag was not injected correctly. Most injection sites had healed by the next consecutive nesting following injection (generally 9-11 days), although two of the tag sites appeared to have been infected and an additional five tag sites were slightly swollen. Continued tagging and monitoring over the next few years will give us an indication of the reliability of this new tagging method and will also provide an independent evaluation of conventional flipper tag retention.

PIT tagging has great potential as a field identification system; however, it will be important to standardize techniques, particularly tag placement, if use is to become widespread in sea turtle research. An important consideration is manufacturer compatibility of different PIT tagging systems. We have found that AVID scanners are able to read tags manufactured by different companies, but that other scanners require compatible tags.