Using Photogrammetry to Estimate Population Demographics of Alaskan Steller Sea Lions (Eumetopias jubatus)
Abstract
Steller sea lions (Eumetopias jubatus) experienced a precipitous 80% decline in abundance
over a 25-year period starting in the mid-1970s. The decline occurred at different
rates over the range of the species, which is comprised of two Distinct Population
Segments (DPS). The endangered western DPS stabilized in 2004, but stark regional
differences have been observed within the six regions that comprise this DPS; increases
in abundance in the eastern four regions of the range have been largely offset by
significant declines in the two western regions. In contrast, the threatened eastern
DPS, which occurs in southeast Alaska, is recovering and currently being considered
for delisting under the Endangered Species Act. Aerial surveys of haul out and rookery
sites (sites with >50 pups born annually) offer snapshots of abundance and population
health, but ecological mechanisms driving population dynamics cannot be inferred from
these surveys. Current methodologies employed to study vital rates (i.e., survival,
recruitment, and natality) are limited to a relatively small number of rookeries,
leaving gaps in age-structure information across the Alaskan range —most notably in
the central and western Aleutian Island regions, which continue to experience significant
declines.
In the present study, photogrammetric methodologies were developed which allowed the
measurement of lengths of individual Steller sea lions from images captured during
a 2008 aerial survey. These length data were then used to provide estimates of the
proportion of different age and sex classes in various regions. Standard lengths of
sea lions were measured from vertical digital images. All sea lions observed in close
proximity to a pup or juvenile were considered adult females, thus providing a category
of “known” adult females. All standard lengths assigned a Straightness Classification
of 3 - lying straight along their long axis with only minor curvature at either the
posterior or anterior portion of the body - were used in analysis. An altitude calibration
flight was performed in Sitka, Alaska in which images of the runway number were obtained
(for which true dimensions were known) and linear regressions were performed to correct
for bias in altitude measurements collected from the radar altimeter. The resulting
regression equations were combined to produce an equation to convert measured lengths
(Lm; pixels) to “true” lengths of sea lions (Lp; m) using altitude measurements from
the radar altimeter (Hm; ft): Lp= Lm ( 3.742E-3Hm + 8.003E-3 ).
Converted length data were applied to a finite mixture distribution model to estimate
the proportion of the population comprised of three age-sex classes—juvenile (males
and females), adult female and adult male. To minimize bias due to the effects of
sample size and site selection, broad regional comparisons of rookery, haul out, and
combined sites were conducted for the eastern and western DPS, as well as a comparison
of two areas within the range of the western DPS experiencing contrasting abundance
trends. Females in the eastern DPS were significantly smaller than those in the western;
and amongst the western DPS regions, females in the center portion of the range were
larger than in two fringing regions. Range-wide estimates of vital rates and age structure
of Steller sea lions may help to identify environmental or anthropogenic drivers that
are impeding the recovery of the western DPS of Steller sea lions in Alaska.
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https://hdl.handle.net/10161/5242Citation
Sweeney, Katie (2012). Using Photogrammetry to Estimate Population Demographics of Alaskan Steller Sea Lions
(Eumetopias jubatus). Master's project, Duke University. Retrieved from https://hdl.handle.net/10161/5242.Collections
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