Sea turtle populations are overestimated worldwide from remigration intervals: correction for bias

P Casale,SA Ceriani

doi:10.3354/esr01019

Abstract

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 41:141-151 (2020) - DOI: https://doi.org/10.3354/esr01019 Sea turtle populations are overestimated worldwide from remigration intervals: correction for bias Paolo Casale1,*, Simona A. Ceriani2,3 1Department of Biology, University of Pisa, Via A. Volta 6, 56126 Pisa, Italy 2Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Saint Petersburg, FL, USA 3Department of Biology, University of Central Florida, Orlando, FL, USA *Corresponding author: paolo.casale1@gmail.com ABSTRACT: Estimating population abundance is key for species of conservation concern. This is particularly challenging for marine animals, like sea turtles, with ocean-scale distribution and migratory nature. However, sea turtles lay clutches on land where they can be easily counted; thus, clutch number has always been the most common index of population abundance. A female typically lays >1 clutch per year and does not reproduce every year. Therefore, 2 conversion factors are needed to convert the number of egg clutches to the number of adult females: the number of clutches laid by a female in a nesting season and the fraction of adult females reproducing in a season, which is linked to the breeding periodicity. The effects of breeding periodicity, probability of detection and annual survival probability on the derived adult female abundance were investigated by simulating a virtual population of adult females over a 15 yr beach monitoring period. The results indicate that current methods may greatly overestimate the abundance of sea turtle populations, especially in situations with a low detection probability, including temporary emigration. The factors involved and ways to minimize biases and errors are discussed, including a method which is easy to implement using existing datasets. A careful reassessment of current estimates of sea turtle abundance derived from nest counts and capture-mark-recapture data would be appropriate, and the potential error associated with such estimates should be considered when they are used in conservation status assessments. KEY WORDS: Sea turtle · Remigration interval · Abundance · Breeding proportion · Mortality · Detection · Simulation Full text in pdf format Supplementary material PreviousNextCite this article as: Casale P, Ceriani SA (2020) Sea turtle populations are overestimated worldwide from remigration intervals: correction for bias. Endang Species Res 41:141-151. https://doi.org/10.3354/esr01019 Export citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 41. Online publication date: January 30, 2020 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2020 Inter-Research.

Highlights

Abundance is one of the most fundamental attributes that describe an animal population
When the 4 combinations of realistic low and high values of S (0.70 and 0.95; see a review by Casale et al 2015) and p (0.1 and 1) are used in simulations with the 13 different values of M considered, the results indicate a stronger effect of p than S on the estimation of the adult female assemblage size by the 2 methods currently used and that a low S can only partially compensate for a low p (Fig. 2)
This study provides information and methods that can contribute to future assessments of abundance and conservation status of sea turtle populations as well as to the re-assessment of previous estimates

Summary

Introduction

Abundance is one of the most fundamental attributes that describe an animal population. Endang Species Res 41: 141–151, 2020 turtle meat and eggs has been greatly reduced, several other anthropogenic threats are cause for increasing concern, such as incidental capture in fisheries (e.g. Wallace et al 2013a), habitat destruction Annual clutch counts represent the most common index of sea turtle population trends (e.g. National Marine Fisheries Service & U.S Fish Wildlife Service 2008, Witherington et al 2009, Wallace et al 2013b, Casale & Tucker 2015, Mazaris et al 2017). 3 conversion factors (clutch frequency, breeding proportion and sex ratio) are needed to estimate the number of adults, this is rarely attempted, for instance for specific criteria of the IUCN Red List Assessments (Wallace et al 2013b, Casale & Tucker 2015). It is more common to use 1 or 2 conversion factors to estimate the number of adult females nesting annually and the total number of adult females, respectively

Objectives

Methods

Results

Conclusion