Cohort and tag-site specific tag-loss rates in mark-recapture studies : a southern elephant seal cautionary case

• Published in: Marine Mammal Science
• Main Authors: Oosthuizen, Wessel Christiaan, De Bruyn, P.J. Nico, Bester, Marthan Nieuwoudt, Girondot, Marc
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley -Blackwell 2010
• Subjects: Mirounga leonina; Mark–resight; Cohort heterogeneity; Double tagging; Marker-loss; Marion Island; Phocids; Tag shedding; Tag placement; Southern elephant seal > Marking; Animal marking; Elephant Seal; Elephant Seals; Southern Elephant Seal; Southern Elephant Seals
• Online Access: http://hdl.handle.net/2263/15930; https://doi.org/10.1111/j.1748-7692.2009.00328.x

Marker-loss is a common feature of mark–recapture studies and important as it may bias parameter estimation. A slight alteration in tag-site of double tagged southern elephant seals (Mirounga leonina), marked at Marion Island from 1983 to 2005 in an ongoing mark–recapture program, had important consequences for tagloss. We calculated age-specific tag-retention rates and cumulative tag-retention probabilities using a maximum likelihood model selection approach in the software application TAG LOSS 3.2.0. Under the tag-loss independence assumption, double tag-loss of inner interdigital webbing tags (IIT; 17 cohorts) remained below 1% in the first 5 yr and increased monotonically as seals aged, with higher tag-loss in males. Lifetime cumulative IIT tag-loss was 11.9% for females and 18.4% for males, and equivalent for all cohorts. Changing the tag-site to the outer interdigital webbing (OIT; 6 cohorts) resulted in increased and cohort-dependent tag-loss, although the variation (mean ± 95% CI) in cumulative tag-loss probabilities never exceeded 5.3% between cohorts at similar age. Although different studies may homogenize techniques, we advocate the importance of data set-specific assessment of tag-loss rates to ensure greatest confidence in population parameters obtained from mark–recapture experiments. Permanent marking should be implemented where feasible. The Department of Science and Technology, through the National Research Foundation (NRF), provided financial support. WCO received financial support from a NRF Grantholder-linked bursary within the project “Conservation of Seabirds, Shorebirds and Seals” led by L. Underhill of the Animal Demography Unit, Department of Zoology, University of Cape Town