Assessing seasonal trends in harbor seal (Phoca vitulina) diet using stable isotope analysis along the whisker

• Main Author: Smith, Justin Arthur
• Other Authors: O'Brien, Diane, Horstmann, Larissa, Breed, Greg, Karpovich, Shawna
• Format: Thesis
• Language: English
• Published: 2017
• Subjects: Harbor seal; Food; Alaska; Gulf of; Fairbanks; Gulf of Alaska; glaciers; harbor seal; Phoca vitulina
• Online Access: http://hdl.handle.net/11122/8140

Thesis (M.S.) University of Alaska Fairbanks, 2017 Harbor seals (Phoca vitulina) that use tidewater glaciers in the summers to molt, pup, and breed have declined in recent years, which could be attributed to reduced prey availability caused by regime shifts in the coastal marine environment. Recording seasonal dietary trends of harbor seals in different years could improve our ability to test if regime shifts caused these declines. However, such analysis has not been possible, because wild harbor seals are difficult to recapture. Stable isotope analysis of serial sections of growing whiskers (mystacial vibrissae) can be used as a tool to assess diet over different seasons, but uncertainty about whisker growth status and shed dates have prevented accurate estimates of stable isotope deposition date in the past. In Chapter 1, I characterized harbor seal whisker morphology to improve estimates of stable isotope deposition date. First, I measured 567 whiskers collected from wild harbor seals in the Gulf of Alaska from 2003 to 2012. Measurements included the length of a smooth root section (SRS), the length of the bumpy section, and the distance between each bump (inter-bump length; IBL). I found that the SRS was longer for spring-collected whiskers than fall-collected whiskers and matched the length of fully-grown, shed whiskers. These results suggest that the SRS can be used to differentiate whisker shed and growth status, and can be used to determine the sequence of whisker shedding by cohort in summer-captured seals. I also found that the mean IBL was correlated with whisker length and provides a proxy for whisker growth rate. I compared stable carbon isotope ratios along the three longest whiskers from 10 harbor seals and found that intra-individual patterns of whisker stable carbon isotope ratios became more synchronous when expressed by deposition date rather than by position along the whisker. In Chapter 1, I proposed a method to improve deposition date estimates by applying individually adjusted growth ...