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Marine Turtle Newsletter 13:4-9, © 1979

Double Tagging of Green Turtles in the Galapagos Islands

Derek Green
Estacion Biologica Charles Darwin, Galápagos, Isla Santa Cruz, Ecaudor

Tag loss has always been a problem for turtle researchers, so much so that it formed an important topic in four of the first five issues of the Marine Turtle Newsletter (No.1, 1976; Nos. 2,3, & 5, 1977). Partly with this in mind, in September of 1975, I started to double tag east Pacific greens, Chelonia mydas agasizi in the Galgápagos islands. In addition to the normal monel tag fixed to the second or third proximal scute (or the skin in between) of the trailing edge of the right foreflipper, I also inserted a coloured plastic tag (Jumbo Rototag, Dalton Supplies, Henley, England) into the webbing between the fourth and fifth digits of the right hindflipper. The plastic tag, which consists of a male and female portion, both embossed with a number, is inserted after a hole has first been made in the flipper with a leather punch in order to prevent buckling. By using a different colour at each tagging site, it was possible to determine the origin of a turtle without it having to be caught, this being particularly advantageous whilst in the water, the principal reason for the use of coloured plastic tags in the first place. The results can be divided into 3 sections: A) - from resident population; B) - from nesting beaches; C) - from long-distance migration and re-nesters after several years absence.

A - Resident population. Although the majority of the adults migrate away from the archipelago after the nesting season (Dec-June), large numbers can be found all year round. This population consists mainly of subadults, virgin females and juveniles, but there are quite a few larger females and a sprinkling of males. Tag returns have shown that some of these letter females are late departures from the previous nesting season and early arrivals for the next, but an as yet undetermined percentage are truly resident. Table 1 shows the tag losses 116 recaptures between Sept 1975 and the present, a period of almost 4 years. It is obvious that plastic tags have the greater staying power. Only 8 turtles out of the 116 recaptured had lost their plastic tags (of 49 who had lost the metal one). If only the metal tag had been used then only 67 (57.8%) of the recaptures could have been identified. Using only the plastic tag would have meant identification of 108 (93.1%). As both were used, 112 (96.6%) were identified. In turtles recaptured 101-200 and 201-300 days after tagging, metal tags showed a loss of 22.2%. This agrees very closely with Schulz (see Marine Turtle Newsletter No. 1, 1976, p.4) who estimates a l5-20% tag loss. This percentage loss increases dramatically with time.

B - Nesting beaches. Table 2 shows results for 1979. The results are typical of other beaches in Galápagos and of other years. Again the plastic tags showed the greater staving power, but as all cases of tag loss occurred within 62 days, I would attribute this to faulty tagging or cases where the flippers were very thick and fleshy, rather than corrosion. This also applies to the plastic tag, where only the male half was still present. The metal tag losses of 1.3% and 3.1% are probably very low as a large proportion of turtles tagged were not seen again.

C - Long distance migrations (Table 3) and re-nesters after several years' absence. A turtle nesting after a 2 year absence had the plastic tag intact but no metal one. One re-nesting after 3 years had the reverse, the metal one intact but no plastic tag. Out of interest, 3 turtles tagged only with metal tags still had them when they re-nested after 685, 2208, and 2192 days, respectively.

Table 1. Tag loss: plastic v. metal in 116 recaptures of greens double tagged in Elizabeth Bay, Isabela and Turtle Cove, Santa Cruz, 1975-1979.

Days Recaptured After Tagging

Table 2: Tag Loss: plastic v. metal of 464 nesting female greens double tagged on Quinta Playa and Baltra, 1979.

* Male portion only

Table 3: Tag Loss: Plastic v. metal of 7 female greens recaptured outside the Galápagos, 1975-1979.

? = unknown if present

Condition of tags: 1. Metal. After periods of 170, 300, 425, 432, 465 and 676 days, respectively, 6 of the tags included in Table 1 were loose and had to be replaced. In 5 of these the plastic tags were still in good condition, but with the turtle recaptured after 432 days, the plastic tag was missing. In all 6 cases the metal bar had corroded and was absent/almost absent. Corrosion was also very obvious where the metal had come into contact with internal tissue, as previously noted by Balazs (Marine Turtle Newsletter No. 2, 1977). Referring to the resident population, in most cases the metal tags had algae growing on them which sometimes obliterated the number. In some of the more severe cases, barnacles, sponges, ascidians, hydroids, sipunculids and even polychaetes were found, but this was never as serious as with the plastic tags.

Condition of tags: 2. Plastic. In only 3 cases were the plastic tags damaged enough, to warrant being replaced. In all 3 cases, damage was to the upper or male portion. One had a longitudinal crack after 495 days. In the other two, after 6-16 days and 949 days, respectively, the male part was broken in half horizontally, thus losing the third digit of the number. In the latter case (i.e. after 949 days) it was only through the embossing that the tag was able to be read, as the black paint of the numbering had disappeared. In all these 3 cases the metal tag was missing. Whereas metal tags suffer from corrosion, plastic ones do not. The most common reason for their loss is that the normally 2-3 cm from the trailing edge of the flipper, gradually becomes enlarged by the continual rubbing of the male prong so that eventually the connecting strip of flipper tears, and the tag, both halves still joined, falls out. The biggest disadvantage of the plastic tag is its tendency to become obliterated by algae, especially encrusting species such as Lithothamnion, and barnacles - much more so than the metal ones. Even after only 2 weeks a growth of green filamentous algae is often obvious and after 4-6 weeks this needs to be scraped off in order to read the number, especially as these algae tend to grow on the number itself. On the nesting beaches, tags tend to remain in this state with an occasional barnacle creeping in. In the feeding areas, however, where the turtles lead a much more sedentary existence, the green algae are gradually replaced by reds, especially Lithothamnion. During this time, barnacles, bryozoans, sponges, ascidians and hydroids often appear, completely obliterating the number. The tag, in fact, becomes quite a marine park, for polychaetes, crabs, amphipods and even sipunculids are often found among the algae and barnacles. Even so, the colours remain distinct. Plastic tag returns of more mobile turtles such as long distance migrators and re-nesters after several years' absence show much less, if any, animal or plant growth.

Conclusion. There is no doubt of the plastic tag's superior staying power. It is so superior, in fact, that in Sept 1978, I had return addresses stamped onto the back of the female half. It is also easier to check if they have been applied properly or not. In fact, experience has shown that they are much easier to apply than the metal ones. In addition, there is the advantage of colour coding. Unfortunately, they do not help solve the long-term problem. After less than three years, one of the tags was virtually impossible to read and I cannot envisage them being readable after 5 years. Even if an algae-free plastic was used, the embossing of the numbers would have to be improved. Ironically, the dense algal coat forms a protective covering for the tag and the number is plainly visible after scraping. However, although this is suitable for the resident population here, it is not suitable for migrating turtles or with long-term tag returns in mind. Perhaps the plastic tag's major asset is its method of fixation - the prong of the male portion passing through the flipper and entering the female portion on the other side. Perhaps inconel or even titanium (Cornelius, Marine Turtle Newsletter, No. 5, 1977) tags based on this type of fixation method could be the answer. Expensive I know, but it could pay dividends in the long run. After all, what price is knowledge? In the meantime, each turtle caught here will continue to receive two tags.