Error patterns in age estimation of harp seals (Pagophilus groenlandicus): results from a transatlantic, image-based, blind-reading experiment using known-age teeth

Abstract

Frie, A. K., Fagerheim, K-A., Hammill, M. O., Kapel, F. O., Lockyer, C., Stenson, G. B., Rosing-Asvid, A., and Svetochev, V. 2011. Error patterns in age estimation of harp seals (Pagophilus groenlandicus): results from a transatlantic, image-based, blind-reading experiment using known-age teeth. – ICES Journal of Marine Science, 68: 1942–1953. Blind readings of known-age samples are the ultimate quality control method for age estimates based on hard tissues. Unfortunately, this is often not feasible for many species because of the scarcity of known-age samples. Based on a unique collection of known-age teeth of harp seals (age range: 1–18 years), ageing errors were evaluated in relation to true age, reader experience, sex, and tooth format (images vs. originals). Bias was estimated by linear models fitted to deviations from true age, and precision was estimated as their residual standard error. Image-based blind readings of 98 tooth sections by 14 readers, representing different levels of experience, generally showed high accuracy and precision up to a seal age of ∼8 years, followed by an increasingly negative bias and increased variance. Separate analyses were therefore conducted for young seals (1–7 years) and older seals. For young seals, moderate associations were found between reader experience and levels of bias, precision, and proportions of correct readings. For older seals, only precision levels showed a significant association with reader experience. Minor effects of sex and tooth format are unlikely to affect these main patterns. Observed errors, even for highly experienced readers, may affect important age-related parameters, emphasizing the importance of known-age calibration of the output from all readers.

Aspartic acid racemization rate in narwhal (Monodon monoceros) eye lens nuclei estimated by counting of growth layers in tusks

“…1a, b). A single, experienced tooth-reader trained in reading beluga (Delphinapterus leucas) and narwhal teeth (not tusks) was chosen to read the narwhal tusks as less-experienced tooth-readers of harp seal (Pagophilus groenlandicus) teeth have been shown to be less accurate and more negatively biased (Garde et al 2010;Frie et al 2011). GLGs were counted on both sides of each of the longitudinal sections, and each GLG was marked with a pencil for bookkeeping and to check for consistency in their classification.…”

Two techniques of age estimation in cetaceans: GLGs in teeth and earplugs, and measuring the AAR rate in eye lens nucleus

“…Studies on GLG readings in harp seals (Pagophilus groenlandicus) indicate a tendency towards greater disagreement in the older animals (>8 yr.) probably due to a more complex layering structure with age (Frei et al 2011). In order to correctly interpret the layers a high degree of experience is needed.…”

A review of age estimation methods in marine mammals with special reference to monodontids

“…Inter-and intra-reader variation has been discussed extensively in various publications (e.g., Mansfield 1991, Bjørge et al 1995, Evans et al 2002, Christensen-Dalsgaard et al 2010, Frie et al 2011, Brill et al 2015. The cyclic GLG pattern is not always distinct and anomalies such as accessory and marker lines might be counted as annual layers in younger individuals, while in older individuals, the GLGs become compact and difficult to differentiate ( Fig.…”

“…The cyclic GLG pattern is not always distinct and anomalies such as accessory and marker lines might be counted as annual layers in younger individuals, while in older individuals, the GLGs become compact and difficult to differentiate ( Fig. 1) (Grue and Jensen 1979, Hohn 1990, Mansfield 1991, Stewart et al 1996, Evans et al 2002, ChristensenDalsgaard et al 2010, Frie et al 2011. Generally, however, GLG counts tend to be negatively biased because it is more likely for growth layers NAMMCO Scientific Publications,Volume 10 4 to be lost, boundary layers of GLGs to not be detected, or compressed GLGs in old animals not be sufficiently visible than for significant supernumerary lines to be counted (Stewart and Stewart 2005).…”

“…Increased precision or within-reader consistency is generally positively correlated with experience (Campana 2001). However, both experienced and inexperienced readers showed significant levels of bias and imprecision in harp seal (Pagophilus groenlandica, formerly Phoca groenlandicus) age readings when age estimates were compared to the known age (Lawson et al 1992, Frie et al 2011. Similar studies with known-age grey seal teeth however, revealed no particular correlation of ageing error with reader experience and for all readers 89.3% of errors were within +1-2 years of known age (Frie et al 2013).…”