The importance of passive integrated transponder (PIT) tags for measuring life-history traits of sea turtles

Lucy C.M Omeyer,Paolo Casale,Wayne J Fuller,Brendan J Godley,Kelle E Holmes,Robin T.E Snape,Annette C Broderick

doi:10.1016/j.biocon.2019.108248

Lucy C.M Omeyer, Paolo Casale + Show 5 more

Open Access

https://doi.org/10.1016/j.biocon.2019.108248

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Abstract

Capture-mark-recapture studies rely on the identification of individuals through time, using markers or tags, which are assumed to be retained. This assumption, however, may be violated, having implications for population models. In sea turtles, individual identification is typically based on external flipper tags, which can be combined with internal passive integrated transponder (PIT) tags. Despite the extensive use of flipper tags, few studies have modelled tag loss using continuous functions. Using a 26-year dataset for sympatrically nesting green (Chelonia mydas) and loggerhead (Caretta caretta) turtles, this study aims to assess how PIT tag use increases the accuracy of estimates of life-history traits. The addition of PIT tags improved female identification: between 2000 and 2017, 53% of green turtles and 29% of loggerhead turtles were identified from PIT tags alone. We found flipper and PIT tag losses were best described by decreasing logistic curves with lower asymptotes. Excluding PIT tags from our dataset led to underestimation of flipper tag loss, reproductive periodicity, reproductive longevity and annual survival, and overestimation of female abundance and recruitment for both species. This shows the importance of PIT tags in improving the accuracy of estimates of life-history traits. Thus, estimates where tag loss has not been corrected for should be interpreted with caution and could bias IUCN Red List assessments. As such, long-term population monitoring programmes should aim to estimate tag loss and assess the impact of loss on life-history estimates, to provide robust estimates without which population models and stock assessments cannot be derived accurately.

Highlights

Population-based conservation is dependent on accurate estimates of life-history traits (Hernández-Camacho et al, 2015; Yokoi et al, 2017)
Female identification Of the females previously tagged with both flipper and passive integrated transponder (PIT) tags at Alagadi Beach, 53% of green turtles (n = 305) and 29% of loggerhead turtles (n = 132) were identified in subsequent nesting seasons by PIT tag(s) alone between 2000 and 2017, resulting in the overestimation of neophytes, for green turtles (Fig. 1, Table S1)
Nesting female abundance would have been overestimated by 37% for green turtles (20002017, PIT tags: n = 389, no PIT tags: n = 533) and 9% for loggerhead turtles (PIT tags: n = 371, no PIT tags: n = 405)

Summary

Introduction

Population-based conservation is dependent on accurate estimates of life-history traits (Hernández-Camacho et al, 2015; Yokoi et al, 2017). Most population studies use capture-mark-recapture (CMR), which involves the release of tagged individuals and their subsequent recapture events over time. These studies rely on individual recognition to provide information about demography, behaviour and survival, assuming tags are correctly identified and retained over time (Pradel, 1996). CMR has been used extensively in sea turtle research to study behaviour (e.g. Lazar et al, 2004) and to estimate life-history traits (e.g. Stokes et al, 2014) Such studies generally rely on tagging individuals with two external flipper tags, reducing the probability of individual loss from the identifiable population. Double tagging allows tag loss estimation, the rate of which may be influenced by tag type and size, species and tagger experience (Casale et al, 2017; Limpus, 1992)

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