The Relationship Between Seasonal Distribution of Steller Sea Lions
(Eumetopias jubatus) and Weather During the Non-breeding Season in
the Western Aleutian Islands, AK
Ashley Imhof
Introduction
Methods
Steller sea lion populations in the U.S. are separated into two distinct population Segments at 144°W, the Eastern and Western Stock. The Western Stock is listed as endangered and the Eastern Stock as threatened under the Endangered Species Act..
The Western Stock is broken down into six sub-areas. While most sub-areas tend to show stable or erratic population trends, those in the western Aleutians continue to decline at ~10%
despite
the shutdown
off
10% per year, d
it th
h td
commercial fisheries and other conservation measures enacted through the species’
Recovery Plan. A variety of factors likely contribute to population declines, but data about
seasonal distribution during the non-breeding season and the effects of weather patterns
on distributions are limited.
Severe weather may act as a disturbance at terrestrial sites, limiting use during the winter
months when weather is most extreme. This study analyzed how seasonal distribution
was correlated with weather at three haul-outs or rookeries in the western Aleutians and
one in the central Aleutians during the non-breeding season using wind gusts and average
wind speed as proxies for severe or mild weather. Understanding how weather patterns
affect sea lion distribution and habitat use will likely be important in coming years, since
some predictions claim that climate change will cause an increase in the frequency and
intensity of storms and cyclonic events in the North Pacific.
Data Collection:
Using wind gust and average wind speed as a proxies for mild or severe weather, what is
the relationship between seasonal distribution of Steller sea lions and weather patterns in
the Western Aleutian Islands?
Trends for each site
One or more remote cameras were set up at each site: Gillon Point on Agattu,
Cape Sabak on Agattu, Alaid, Cape Wrangell on Attu, and Hagox Point on Ulak.
Photographs were taken at 30-minute intervals between sunrise and sunset
between 17 October 2012 and 2 July 2013. After the field data collection period, I generated weekly counts for each site
from a single photograph beginning after the first sea lion attendance at the site.
Brand re-sights were also recorded, as well as various observations including first
animal attendance, first breeding female attendance, first pup birth, dead/
entangled animals, as well as any other significant observations or fluctuations in
sea lion attendance. Sites were mapped using Google Earth. Weather data were
obtained through OGIMET website for Adak (latitude 51-52-40N, longitude
176-28-45SW) and Shemya (latitude 52-43-00N, longitude 174-07-00E) at
13:00UTC. Adak is the closest weather station to Ulak and Shemya is the
closest weather station to the Near Island haulouts: Agattu, Alaid, and Attu. Analysis:
Cape Sabak was left out of analysis due to inaccessibility during the non-breeding
season. All data after 15 June 2013, the date of first breeding female attendance,
were removed to account for fluctuations in attendance due to breeding activity.
Any counts missing data for average wind speed were removed from wind speed
analysis and counts missing wind gust were removed from wind gust analysis. Overall trends in sea lion weekly counts, average wind speed, and average wind
gust were analyzed. These averages were used to set parameters for mild and
severe weather. Wind gusts > 50km/hour were designated as high and average
wind speed > 31km/hour were considered severe. Effects of weather (mild versus severe based on average wind speed or wind gust
velocity) on sea lion counts were analyzed using generalized linear models with
site as an additional factor and a Poisson error distribution (SPSS). Results
Trends for all sites
Study Sites
Table 1: Average sea lion counts, wind speed and wind gusts for all islands. Fig 2. Examples of haul-out sites/rookeries at Ulak (right) and Agattu (left)
Acknowledgements
Lowell Fritz, NOAA Fishery Biologist, NMML, AFSC/NMFS
Kathryn Sweeney, NOAA Affilliate, NMML Alaska Ecosystems
Program
Aaron Wirsing, Assitant Professor, University of Washington
Mild:
Average wind speed < 30km/hr
Severe:
Average wind speed > 30km/hr
Low:
Max wind gust < 50km/hr
High: Max wind gust > 50km/hr
When data from all sites
were combined, Steller sea
lions were more abundant
when wind was less intense,
using both average wind
speed (Fig. 3) and maximum
wind gusts (Fig. 4) as proxies.
20
Sea Lions Present
Ulak tended to have both
milder weather and higher
average sea lion counts than
the Near Islands (Table 1).
These averages were used to
set the following parameters
for wind speeds:
16
12
8
4
0
15.37
mild
7.84
severe
Fig. 4 Average Sea Lions Counts in
Low and High Wind Gusts at All
Sites Combined
40
30
20
10
0
Alaid mild
6.25
Alaid severe
6.65
Agattu mild
25.44
6.21
Agattu severe
Analysis of sea lion presence
relative to maximum wind
gust also showed a great deal
of inter-island variation (Fig.
5). With the exception of
Attu, all sites showed lower
average counts at high wind
gusts, though no trends were
significant. Modeling showed
no significance for
relationships between sea
lion counts and site
(p=0.235), wind gusts
(p=0.078) or for site and
wind gust interaction
(p=0.579). 11.52
Attu mild
7.93
18.75
Attu severe
28.67
Ulak mild
Ulak severe
Fig. 6 Average Sea Lions Counts in Low
and High Wind Gusts at Each Site
40
32
24
16
8
0
14.33
Alaid low
4.41
Alaid high
20.25
Attu low
6.26
Attu high
11.36
Agattu low
6.91
Agattu high
18.69
Ulak low
20
Ulak high
Conclusions
Average speed and wind gust velocity did not have a consistent effect on Steller sea lion counts. However, the effect of the interaction between average wind speed and site was significant. In other words, counts were higher when wind speeds were greater on some islands, but the reverse was true on others. This interaction could be due to microclimatic differences between sites, where one site may have more cover or protection against weather
These findings are especially interesting considering studies
predict
udies
i that
h pr
edict
di an iincrease
ncrease in
i
storm and winter cyclone activity in the North Pacific and other areas. One such study
notes that cyclone activity in the North Pacific has been markedly increasing since 1948
(Graham and Diaz 2001). If sea lions are being affected by severe weather, an increase in
storms could alter their distribution and abundance across their Alaskan range
Further research should examine differences in geography of sites and other microclimatic
features that could influence the use of haul-out sites in severe weather. Also, I believe that
with more data points more significance could have been shown at more sites, as some had
very few counts when broken down into mild/severe weather and high/low wind gusts. 25
Sea Lions Present
Fig 1. Maps show weather stations in red and study sites in yellow both zoomed out (left)
and zoomed in (right). Shemya weather station was used for weather for Near Islands:
Agattu, Alaid and Attu. Adak weather station was used for weather for Ulak. Fig. 3 Average Sea Lions Counts in
Mild and Severe Average Wind
Speeds at All Sites Combined
50
When analyzing sea lion
presence relative to average
wind speed at each site
separately, there was
considerable inter-island
variation (Fig. 5). Ulak
actually had higher counts at
severe wind speeds. Linear
regression showed that,
while average wind speed
(p=0.41) and site (p=0.13)
did not significantly affect sea
lion counts, the interaction
between site and average
wind speed did (p=0.031). Fig. 5 Average Sea Lions Counts in Mild
and Severe Average Wind Speeds at
Each Site
Se Lions Present
Research question
Results (cont.)
Sea Lions Present
20
References
15
10
5
0
15.95
low wind gust
6.42
high wind gust
Graham N.E. and Diaz H.F. 2001. Evidence for Intensification of North Pacific Winter Cyclones
since 1948. Bulletin of the American Meteorological Society 82(9): 1869-1893
Sease J.L., and York, A.E. 2003. Seasonal distribution of Steller Sea Lions at rookeries and
haul-out sites in Alaska. Marine Mammal Science 19(4): 745-763
Trites A.W. and Porter, B.T. 2002. Attendance patterns of Steller Sea Lions (Eumetopias
jubatus) and their young during winter months. Journal of Zoology, London 256:
547-556