Comparison of Monel 49 and Inconel 681 flipper tag loss in green turtles, Chelonia mydas, nesting at Tortuguero, Costa Rica

Sebastian Troëng1, Jeff Mangel2, and Catalina Reyes1
1 Caribbean Conservation Corporation, Apdo. Postal 246-2050, San Pedro, San José , Costa Rica
2 Nicholas School of the Environment, Duke University, Durham, North Carolina 27705, USA

Low tag loss is desirable for long-term studies of sea turtles because it will allow researchers to follow individual turtles for longer time periods (Balazs 1999). Green turtles have been tagged with flipper tags at Tortuguero, Costa Rica, since 1955 (Carr et al. 1978). The monitoring protocol for Tortuguero (CCC 1998) establishes that nesting sea turtles are tagged in order to: a) identify individual sea turtles for research purposes; b) monitor hunting pressures in feeding, migratory, and internesting habitats via tag returns; c) identify developmental habitats and migratory corridors for populations of sea turtles that nest at Tortuguero via tag returns; d) determine the longevity of sea turtles; e) determine how long female turtles are reproductively active; f) determine the retention time of tags.

The objective of the paper is to compare tag loss for Monel #49 and Inconel #681 flipper tags, both manufactured by National Band and Tag Company, KY, USA and used to tag green turtles at Tortuguero, Costa Rica.

Since 1998, at least 1,000 new green turtles are tagged each year at Tortuguero (CCC 1998). This represents a sample of the green turtles that come ashore to nest. Tags are applied to each front flipper, axillary, inside the first scute on the trailing edge of the flipper.

Probability of tag loss was calculated for double tagged green turtles that were subsequently encountered with one or two tags (Wetherall 1982). Within-season tag loss was calculated from the first to last sighting and between season tag loss was calculated from the date of tagging to the first sighting two, three, four or five years later. The probability of tag loss is defined as: 1-Ki = 1-((2rdi)/(rsi+2rdi)), where Ki is the probability of retaining a tag during the interval i rdi is the number of individuals encountered carrying two tags at interval i rsi is the number of individuals encountered carrying one tag at interval i Confidence limits (95%) were calculated according to the methodology presented by Bjorndal et al. (1996).

The probability of within-season tag loss varied between 0.019 and 0.169 (Table 1). Between season tag loss was consistently lower for Inconel #681 tags used during the 1998-1999 nesting seasons than for Monel #49 tags used in 1996-1997.

Explanations for the lower tag loss for Inconel #681 tags include less corrosion than for Monel tags, easier to check locking mechanism for Inconel #681 tags and thorough selection and training of Research Assistants during the 1998-2001 nesting seasons. Limpus (1992) concluded that tag loss was greater for the more distal tagging positions on the front flippers. The placement of tags (axillary, next to the first scale) may have contributed to the relatively low tag loss seen at Tortuguero. Passive integrated transponder (PIT) tags are being increasingly used in sea turtle tagging projects because of their perceived low tag loss. Godley et al. (1999) reported that 93% of PIT tags were detected within-season in renesting green turtles, which would suggest a 7% within-season tag loss. This is higher than the within-season tag loss observed for Inconel #681 at Tortuguero. Metal flipper tags also have the added advantage of being externally visible so that non-experts can identify tags and provide tag return information. Parmenter (1993) reported 8% tag loss over two years for PIT tags used on the flatback turtle (Natator depressus). The tag loss is lower than the loss observed in Inconel #681 tags in this study. However, the sample size (n=37) was small and there may also be species differences that make comparisons between Parmenter’s and this study inappropriate. Bjorndal et al. (1996) did not find a difference in tag loss for Monel #49 and Inconel #681 tags used in Tortuguero in 1989. It may be that the explanations mentioned above, especially the selection and training of Research Assistants may have confounded their or our study. Based on the difference in corrosion rates observed in removed Monel and Inconel tags (pers. obs.), we think that the advantage of the more resistant Inconel #681 tags will become more apparent as the study period increases to four or more years.

The most important consideration when choosing tag type should be to ensure that the tags employed will fulfill the research objectives of the study. Currently, we consider Inconel #681 tags to be adequate for the Tortuguero Green Turtle Program (CCC 1998).

All Research Assistants and participants taking part in the 1996-2001 Green Turtle Programs are gratefully acknowledged for their hard work and diligence in tagging turtles and checking them for old tags. Their dedicated efforts made this study possible. The CCC Scientific Advisory Committee members are thanked for their significant contributions to the new Tortuguero monitoring protocol.

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Abstract of paper presented at 22nd International Symposium, 2002