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Habitat Assessment & Marine Chemistry Program

Identifying Regional Variation in Harbor Seal Fatty Acid Signatures Using Analysis of Similarity (ANOSIM)

Figure 1, young harbor seal
Figure 1.  A young harbor seal ashore on a cobble beach.  We will be examining dietary variation between age classes.  Photo by John Moran
 

The abundance of harbor seal (Phoca vitulina) (Fig. 1) has declined in several regions of Alaska over the past 2-3 decades, while in other regions of the state the numbers are increasing. A decrease in prey quality and availability is a common hypothesis for explaining the decline in harbor seal populations (as well as other marine mammal populations in Alaska). In response to this trend, the Habitat and Marine Chemistry Program’s Nutritional Ecology Laboratory at Auke Bay Laboratories is collaborating with the Alaska Department of Fish and Game (ADF&G) to investigate the relationship between regional trends and diets by analyzing the fatty acid (FA) composition of harbor seal blubber.

Inferring marine mammal diet from scat or stomach contents is logistically difficult and may yield biased results. Fatty acid patterns of prey species are reflected in the storage lipids of their predators, thus inferences can be made about an animal’s diet and foraging ecology. Blubber samples taken from free-ranging animals can supply information about diet that is not limited to the last meal or dependent on the recovery and identification of undigested material or by durable prey pieces.

 
  BB GB PES SE
GB 0.568
(0.000)
     
PWS 0.644
(0.000)
0.355
(0.000)
   
SE 0.666
(0.000)
0.769
(0.000)
0.779
(0.000)
 
TUG 0.526
(0.000)
0.820
(0.000)
0.837
(0.000)
0.590
(0.003)
Table 1.  ANOSIM R values for fatty acid signatures from five regions in Alaska:  Glacier Bay (GB), Prince William Sound (PWS), Southeast Alaska (SE), Bristol Bay (BB), and Tugidak (Tug); p values in parenthesis.  (R values range from -1 to 1, positive values indicate a difference between groups, if there is no difference R = 1, negative value indicate a greater difference within groups than between groups).

We used analysis of similarity (ANOSIM) to compare blubber fatty acid signatures from 352 harbor seals in five regions of Alaska with differing population trends: Glacier Bay, where harbor seal numbers are decreasing rapidly (-14.7% 1992-2005); Prince William Sound, where numbers are stabilizing with an increasing short-term trend (-2.4% in 1990-2005; +9.0% in 2000-05); Southeast Alaska, which is stable (+0.6%, Sitka, in 1984-2005) or increasing (+5.0%, Ketchikan, in 1983-2003); Bristol Bay, which is increasing (+7.0% in 1995-2005); and Tugidak Island, which is increasing (+7.5%, Kodiak, 1993-2004).

We found significant differences in fatty acid signatures between all five regions (Table 1). Prince William Sound and Glacier Bay were most similar (ANOSIM R = 0.355, P < 0.001) and both had decreasing population trends during years when blubber was collected (1997-2001). We found the greatest difference in fatty acid signatures when comparing Tugidak to Glacier Bay (ANOSIM R = 0.826, P < 0.001) and Tugidak to Prince William Sound (ANOSIM R = 0.837, P < 0.001). These regions had opposite population trends during the sampling period. Our analysis indicates that harbor seals from these five sites differ in diet composition, which supports the hypothesis that prey availability or quality may be influencing regional population trends.

We are continuing to investigate dietary variation within harbor seal populations by examining factors such as age, sex, and haul-out substrate (glacial ice versus land). As additional samples become available, we believe that this interagency effort will provide not only information on population trajectories but valuable insight in the feeding ecology of Alaska harbor seals.

By John Moran
 

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