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A persistent problem in assessing turtle populations is that tags often come off or are shed by the turtles. Mostly people just do the best they can without ever addressing this important problem directly. In fact there are.at least two practical questions here:

1) how to prevent tag loss, in particular what kind of tag is best?

2) how frequent is tag loss? This information is necessary for making population estimates based on tag returns.

On the first question, what type of tag is best, opinion seems to be divided. For instance, on a recent visit to Trengganu, Malaysia, Mr. Siow Kuan Tow (State Director for Fisheries, Kuala Trengganu, Malaysia) informed me that in the leatherback conservation programme plastic tags (Jumbo Rotatag, Dalton, Henley, England) had been substituted for monel tags because tag loss with the latter was too great. On the other hand, in S. Africa, plastic tags were given up sometime ago and monel metal substituted instead. More recently Dr. G.R. Hughes (P.O. Box 662, Pietermaritzburg, Natal, S. Africa) writes: "Regrettably a substantial number of loggerheads had lost their tags and the callusses appear to be those remaining after the loss of-plastic tags although some callusses were clearly those resulting from the loss of monel tags. A not inconsiderable number of monel tags were removel and replaced with new ones because the originals were corroded, some very badly."

It is conceiviable, of course, though not very likely, that one kind of tag would be better for one population and another kind for another. But without quantitative assessment of tag loss in these cases, it is not possible to tell whether this is the case, or whether either the monel metal or the plastic tag is superior.

There seem to be very few studies on tag loss. However, in Surinam.Schulz (1975, Zoologische Verhandelingen, 143, p 61-62) marked 80 newly tagged green turtles with paint Within one month 12 of these had been seen on the beach again with the paint mark still visible, but without the tag (metal tag). "The actual number of animals that lost their tag has been estimated at 15-20%, a figure based on a calculation which included the estimated number of turtles that had lost both paint mark and tag." In whatever way one does the calculations, at a minimum certainly 15% of the tags were lost ... within a month! Schulz points out that there are several reasons why tags are lost, including poor tagging and loss through corrosion.

Corrosion of tags has been documented recently by Mr. G.H. Balazs (Institute of Marine Biology, P.O. Box 1346. Kaneohe, Hawaii, U.S.A.). He has a collection of tags recovered from sea turtles; their disintegrating and battered state is a dismal sight for any turtle researcher. Correspondence between Mr. Balazs and the manufacturers of the monel metal tag indicated that working of the metal to and fro might cause cracks which would result in deterioration of the metal. On the positive side, this company (National Band and Tag Company, 721 York Street, Newport, Kentucky, USA) is looking into the possibility of producing a tag made from a more corrosion resistant material. This is known as Iconel, an alloy containing nickel. The cost of tags made from Iconel is higher than that of monel metal tags. The exact cost however depends on how many of these tags are ordered. Combined orders will reduce the prices. People interested should communicate direct With George Balazs. He also has information and views on what size of tag is best. However, it must be added that Iconel has not yet been given long-term trials on marine turtles, and some method of assessing the reliability of this kind of tag would surely be desirable.

How might reliability of tags be assessed then? Mr. Stanley de Silva (Office of Chief Game Warden, Peti Surat 311, Sabah, Malaysia) is launching on an experiment that should help resolve some of these issues. He has undertaken to double tag green turtles nesting near Sandakan, Sabah; on one flipper there will be a monel metal tag and on the other flipper a plastic tag. If this experiment can be carried out with large enough numbers of turtles and for long enough, it should not only establish which kind of tag is superior, at least for the Sabah turtle populations, but also provide estimates of the chances of loss occurring with each of these two kinds of tag. Such estimates will be valuable in helping to assess trends in turtle populations based on tag returns. Perhaps someone should try something along similar lines with the new Iconel tag.

Meanwhile, anyone with information or views on the tag loss problem is urged to
share them with other biologists by writing to this newsletter.

N. Mrosovsky.