Accuracy and precision of an umbilical-based method for estimating birthdates of pre-weaned harbour seal pups

Fine-scale age estimation of animals can provide insight into important biological processes but can be logistically difficult to measure in wild populations. We evaluated the accuracy and precision of an umbilical-based method for fine-scale age and birthdate estimation in a wild population of harbour seal pups (Phoca vitulina) in the St. Lawrence Estuary, Quebec, Canada. Our method consists of attributing umbilicus degeneration scores to estimate pup age in days. To assess the methods validity, we first constructed a score attribution test with field pictures of pup umbilical cords at various stages of degeneration. The score attribution test was completed by 8 observers, and we measured the accuracy and precision of the umbilicus degeneration score attribution by calculating Cohen’s kappa as well as intraclass correlation coefficient (ICC). We used data from 758 pups (captured between 1998 and 2023) for which an umbilical degeneration score was assigned in the field to evaluate the efficiency of the score test to estimate birthdate. Our findings showed that observers can accurately and precisely attribute umbilicus scores, with a Cohen’s kappa mean value of 0.93 which represent almost perfect agreement and an ICC value of 0.92 which is considered excellent precision of the method. Our results indicated that the umbilical-based method can accurately estimate pup birthdates. We suggest the scoring method we developed provides a accurate approach to estimate age, when possible, to a previously used back-calculation method based on body mass. Incorporating the measurement of umbilical score more broadly in research protocol has the potential to open the door to a wide range of ecological studies, including birth phenology, which are important to better understand population and ecosystem dynamics.