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Individual identification of the young loggerhead turtles, selected from among the 1991 group of waifs and subsequently reared in captivity, was required and flipper tagging was chosen for this purpose. Because of the initial small size of the animals being retained and their uncertain viability, tagging was delayed until turtles in the study group were ca. 1 year old.

Because many of these yearlings were to be released to sea immediately, and those being retained were expected to be in captivity for an extended period, non-corrodible tags were required. The smallest titanium tags readily available for our use were 50 mm long (20 mm inside loop) x 6 mm wide and weighed ca. 0.9 g. These tags are manufactured by Stockbrands Co. Pty. Ltd., Perth, Western Australia. Similar tags are known to have been supplied for use at the Cayman Turtle Farm (Caribbean Sea), Cuba, and the Nagoya (Japan) Public Aquarium.

These small titanium tags were applied to all our juvenile turtles early in April 1992. They were fixed over the trailing edge of the fore-flipper(s) in the same axillary position used with adult turtles (see Prince 1993, after Limpus 1992). The tags, when first fixed to the still relatively small flippers of the yearlings, overlapped the flipper edge by 9-10 mm. Flipper tissue within the tag loop was not compressed at the point of attachment or elsewhere. All tags were, in fact, quite loose. The tagged yearlings varied between 400 and 1100 g, with straight carapace lengths (SCL) of 13.5-17.0 cm.

None of the tagged 1991 cohort juveniles released to the sea have been seen since.

Turtles retained captivity are now, as of August 1995, approximately 4.5 years of age. All the original tags have been lost. The first small tag to be shed was lost early in 1994. The turtle losing this tag was one of the smaller individuals at the time of tagging: it had grown to ca. 3 kg weight and 23.5 cm SCL. The other surviving turtles in the group, with tags still attached, varied between 2.4-4.0 kg, with SCLs of 22.5-26.0 cm, on 18 February 1994. Further small tags were lost through 1994. Only three of the original 14 tags applied remained attached in February 1995, and these last three tags had all gone by mid-1995.

Loss of the original 50 x 6 mm tags from the fore-flippers of our growing captive juvenile loggerheads resulted from a combination of radial growth of the flipper edge relative to the point of tag attachment and thickening of the growing flipper itself, leading to pressure necrosis of the tissue within the tag loop. Tag shedding occurred when the accreting dead tissue mass first caused the flipper edge to split, after which further normal activity of the turtle is sufficient to eventually dislodge the wad of dead tissue along with the tag.

The predictable loss of the original small tags from the still growing turtles prompted retagging mid-1994. Titanium tags used for adult turtles (see Prince 1993) were considered too large for this purpose. Tags from a small batch of #681 inconel tags provided by George Balazs (U. S. National Marine Fisheries Service, Hawaii) were used instead. All remaining turtles were double-tagged with these tags on 12 June 1994. Body weights at this time varied from 3.98-6.48 kg; SCLs varied from 26.7-30.9 cm.

Two of the 24 new inconel tags were shed during the summer of 1994/95. Another five tags had been shed by 22 May 1995, by which time the turtles had grown to body weights between 8.5 and 12.0 kg, with SCLs of 35 to 38 cm. The mode of tag shedding in this case was similar to that described above for the smaller size titanium tags. No further inconel tags had been lost as of 1 September 1995. The low water temperatures in the turtles' sea-cage over the intervening austral winter period have suppressed the turtles' appetites and metabolism sufficient to arrest further growth to date. Additional losses of these inconel tags can be expected as growth resumes with the onset of warmer weather.

I have taken this opportunity to report these observations because there are few reports on the mode of shedding of non-corrodible flipper tags of any size from sea turtles, and because large numbers of similar tags appear to have been used by other workers for tagging juvenile turtles later released to the sea. We did, of course, anticipate tag loss as our captive turtles grew, but we were surprised at how soon this occurred.

Fern Wood (in litt.) has advised me that subsequent sightings of some Cayman Turtle Farm green turtles released to the sea bearing the small (50 x 6 mm) titanium tags have been reported for turtles up to 30 kg. Tags on these Cayman turtles were affixed in the soft tissue between the two large scales nearest the axilla. Our tags were mostly fixed over the first large scale position on the small loggerheads.

The thickened scales on the trailing edge of the fore-flippers certainly do appear to thicken and expand more than the softer tissue between the scales, and the scales are less forgiving of compression than the softer tissue. The difference in initial tag placement may be responsible for at least some of the Cayman releases retaining tags to much greater size than our young loggerheads.

We have not been able to check this 'initial placement' hypothesis with other small turtles. However, one other older juvenile loggerhead in captivity that we have been able to observe was tagged with the larger (adult size) titanium tags when ca. 10 kg and 43.5 cm curved carapace length (CCL) (similar to the present size of the ca. 4.5 year turtles in our experimental group above). These large tags, when first attached to the turtle, had the same relative tissue clearance as the small tags when applied to the yearlings.

This older tagged individual has now grown to ca. 48 kg and 73 cm CCL. Both large tags have overlapped the flipper edge over soft inter-scale tissue, which has now filled the tag loop and is starting to be compressed. There is not yet any necrosis of this enclosed soft tissue. The flipper scales on either side of the tags have however grown much further radially, and are of thicker cross section. I have little doubt that tags enclosing the scale section as noted would already have induced tissue necrosis within the loop.

As we already have more than 600 growing juvenile turtles at sea tagged with these large titanium tags as part of the Western Australian Marine Turtle Project (WAMTP), we have some cause for concern. We believe there is a need to review initial tag placement. Although Limpus (1992) apparently did not detect any difference in tag retention rates for turtles tagged over the scale vs. over soft tissue, such a finding might not be entirely valid for juveniles.

Acknowledgments: I thank Jim and Fern Wood for their help with the Cayman tags, and George Balazs for the inconel tags used. Our experimental turtles were raised with the assistance of Underwater World, Hillarys for the first year of captive life. Nick Kirby's contribution was vital. Mooring for the outdoor sea-cage in which the turtles have since been accommodated was provided initially by the Department of Transport, Hillarys Boat Harbour, and later by Underwater World. Special thanks to Jim Scott, Hillarys Boat Harbour Manager, and Dean Lee and Steve Menzies from Underwater World for their help. The WAMTP is a project supported by CALM and the Australian Nature Conservation Agency.

Limpus, C. J. 1992. Estimation of tag loss in marine turtle research. Wildl. Res. 19:457-69.

Limpus, C. J., T. A. Walker and J. West. 1993. Post-hatchling sea turtle specimens and records from the Australian region, p.95-100. In: R. James (Compiler), Proc. Australian Marine Turtle Conservation Workshop, November 1990. ANCA: Canberra.

Prince, R. I. T. 1993. Western Australian Marine Turtle Conservation Project: an outline of scope and an invitation to participate. Marine Turtle Newsletter 60:8-14.

Prince, R. I. T. 1994a. Shark Bay World Heritage Area: an important loggerhead nesting site. Marine Turtle Newsletter 67:5-6.

Prince, R. I. T. 1994b. Status of the Western Australian marine turtle populations: The Western Australian Marine Turtle Project 1986-1990, p.1-14. In: R. James (Compiler), Proc. Australian Marine Turtle Conservation Workshop, November 1990. ANCA: Canberra.

ROBERT I. T. PRINCE, Wildlife Research Centre, Dept. Conservation and Land Management, P. O. Box 51, Wanneroo, Western AUSTRALIA 6065.