Vibrissae of juvenile southern elephant seals Mirounga leonina as matrix for obtaining fine-scale, time-based dietary information

Abstract

Combining satellite-linked telemetry and stable isotope analysis provides a powerful, indirect approach to assess the trophic ecology of individuals on a spatial and temporally integrated basis. However, this approach requires accurate species-specific quantification of the period of biomolecule deposition in the sampled tissue. Sequentially sampled vibrissae (whiskers) provide a chronology of biogeochemical data; but, the vibrissal growth rate and history are required for temporally integrated data interpretations. The purpose of this study was to a) quantify the vibrissal growth parameters of southern elephant seals (SES) Mirounga leonina, and b) use serially sampled vibrissal regrowths with a known growth history to obtain a fine-scale, temporally related dietary reconstruction of the sampled individuals. Contrary to the previously described asynchronous vibrissal shedding pattern of southern elephant seals (SES) Mirounga leonina during the annua l pelage moult, 71.1 % (n = 140 individuals) displayed vibrissal shedding at Marion Island, southern Indian Ocean. Irregular vibrissal shedding, however, impedes the ability to draw general assumptions regarding the growth history of any unknown vibrissa collected, irrespective of having a detailed vibrissal growth rate model. To account for the unknown onset of biomolecule deposition in the vibrissae, vibrissal regrowths with known (mark-recapture data) growth histories were sampled. The von Bertalanffy growth function indicated that the vibrissal growth rate of known-aged SES decreases as the asymptotic length is approached. The resolution of the dietary data obtainable from a single 2 mm vibrissal segment ranged from a maximum of 3.5 days, but decreased to > 40 days as the growth rate decreased near the base of the regrowth. Nevertheless, the temporal resolution obtained spanned an entire year, demonstrating that fine-scale, long-term, temporally integrated dietary information can be obtained. Secondly, the vibrissa regrowths of underyearling SES (n = 14) (aged < 1.3 yo) were sequentially sampled along the length of each vibrissa and produced fine-scale intra- and inter-individual dietary information during their first year at sea.

The depleted ?15N (8.5 ± 0.6 ‰, mean ± SD) measured during the post-weaning foraging confirmed that underyearling SES of both sexes predated lower trophic level (TL) prey (TL = 2.7) within the Antarctic Polar Front (APF) and Polar Frontal Zone (PFZ). The Bayesian isotopic mixing model approach indicated that the underyearling SES diet consisted predominantly of Subantarctic krill species (52%), such as Euphausia vallentini. Lower TL crustacean feeding cephalopods contributed 26%, whilst myctophid fish and larger cephalopods contributed 12%. The initial krill-based diet of underyearling SES shifted to myctophid fish in older age-classes and underyearling SES utilized a unique trophic niche within the guild of marine top predators at Marion Island, suggesting low levels of intra- and inter-specific competition. This study represents the first utilization of vibrissa regrowths to increase the resolution of dietary information obtainable through SI analysis.