GROWTH RATE AND SHEDDING OF VIBRISSAE IN THE GRAY SEAL, HALICHOERUS GRYPUS: A CAUTIONARY NOTE FOR STABLE ISOTOPE DIET ANALYSIS

Stable isotopes have become powerful tools for gathering information on food webs in marine ecosystems. The method is based on the concept that the ratio of Nitrogen-14 to ¹⁵N (or Carbon-12 to ¹³C) in the tissues of animals is directly related to the ratio found in their diet. Vibrissae provide a time series of stable isotope data as tissue is laid down sequentially over time. Here we examine the growth rate of 283 mystacial (muzzle) vibrissae of four gray seals, Halichoeruas grypus , over a five-month period to investigate their applicability for stable isotope diet analysis. The individual vibrissae did not grow at a constant rate during the study, Fifty-nine actively growing vibrissae were modeled to quantify the growth pattern using a three-parameter von Bertalanffy curve, with the parameters estimated using non-linear mixed-effects models. This model incorporated the inherent serial correlation of these data. The growth rate was 0.024 cm/d (95% CI = 0.019–0.030), the asymptotic length differed significantly by location ( F _3,56=9.64, P < 0.001), but no significant trend was found with muzzle location ( F _3,56= 0.15, P = 0.93). The Δlength/Δtime between each measurement was calculated and most of these data fell at or near zero growth (median = 0.04 cm/d, range = 0–0.78). Individual vibrissae were shed asynchronously and without any seasonal growth trend. This has serious implications for researchers attempting to extrapolate diet data from vibrissae. Because the growth is neither continuous nor synchronous, it will be a challenge to accurately identify the dates when the isotopes were incorporated into the tissue.