Vibrissae growth rates and foraging and migration patterns for juvenile male northern fur seals (Callorhinus ursinus) from St. Paul Island, Alaska determined with stable isotope analysis
The causes of recent declines of the population of northern fur seals (Callorhinus ursinus) on the Pribilof Islands, Alaska are unknown. Estimations of northern fur seal migratory patterns could contribute to a better understanding of the reasons for their decline and allow for better management of their population. A method used to reconstruct marine vertebrate foraging and movement patterns is stable isotope analysis of animal tissues. Vibrissae in Otariidae grow continuously throughout their lives beginning in utero, thereby incrementally accumulating stable isotope data that reflect their food sources and migratory patterns from before birth to the time of vibrissae collection (Sherer et al. 2015). I analyzed the stable carbon (δ13C) and nitrogen (δ15N) isotope values at intervals along entire vibrissae collected from 8, known age (2, 3, and 4 year-old), juvenile male northern fur seals from St. Paul Island, Alaska. This provided the δ13C and δ15N values along a temporal scale from embryogenesis to whisker collection in August of 2014. I determined vibrissae growth rates to match stable isotope patterns to specific time periods in the animals' lives. I also analyzed the δ13C and δ15N values of the segments from the longest and shorter vibrissae from the same individuals (n=3) to determine if equivalent data were reflected in both sizes. This could allow for fewer analyses per whisker, saving resources. The mean vibrissae growth rate (±SD) per month for juvenile males (3.5 ± 1.4 mm/month) had a fairly high standard deviation. The average vibrissae growth rate per month for the first year of life (4.6 ± 1.1 mm/month) indicates a faster growth rate for young of the year animals than 1-4 year-olds (3.0 ± 1.1 mm/month). I found annual oscillations in the δ13C and δ15N values, indicating that juvenile male fur seals are traveling south of the Pribilof Islands during the non-breeding season and returning every year to the Pribilof Islands. The δ13C patterns for 0-1 year of age do not match those of older years, indicating that 1 year-olds have different seasonal foraging patterns than older animals. The δ13C oscillations seem to provide a more accurate representation of variations in foraging location over time than the δ15N values. This was expected as δ13C values more accurately reflect foraging location and δ15N values reflect animal trophic position. Shorter vibrissae matched the isotopic values of the longest vibrissae indicating full analysis of the longest whisker is unnecessary.