Harbor Seal Abundance and Distribution in Cook Inlet
The National Marine Mammal Laboratory’s (NMML) Polar Ecosystem
Program recently received funding from the U.S. Minerals Management
Service to assess the abundance and distribution of harbor seals in Cook
Inlet, Alaska, in preparation for upcoming lease sales in the region for
oil and gas development.
At present, little is known about Cook Inlet
harbor seal abundance and distribution outside the molting period in
August. Seal abundance and distribution can change dramatically
throughout the year in response to the seals’ life history cycle and
seasonal changes in prey availability.
Biologists will conduct aerial
surveys in Cook Inlet during the seals’ pupping (June) and molting
(August) seasons as well as during early spring (March) and late fall
(October), when foraging and haul-out behavior are not constrained by
these important life history events. In addition, biologists will
assess harbor seal haul-out behavior year-round using time-lapse digital
photography collected by autonomous camera systems deployed at haul-out
sites in Cook Inlet.
The survey counts will be analyzed and adjusted to remove effects of
many covariates that affect harbor seal haul-out behavior. The adjusted
counts will form the basis of spatial and temporal analyses summarizing
the relationships between habitat characteristics in Cook Inlet and
harbor seal abundance and distribution.
By Mike Simpkins and Peter Boveng.
Improvements to Aerial Surveys of Alaskan
Harbor Seals
The Polar Ecosystems Program at NMML annually censuses Alaskan harbor
seals by mapping and photographing haul-out sites at low tide during
overflights. The seals are later counted from images and, upon
integrating a suite of variables that affect haul-out behavior, the size
of the population is estimated.
In August 2002, the northern portion of
Southeast Alaska, from Cape Suckling to Frederick Sound, was surveyed
using eight aircraft. The eight observers conducted four to six
replicate surveys of several hundred haulouts resulting in a total of
more than 7,000 photographic images.
In the past, the slides were
projected and counted on a screen, but this method had disadvantages: 1)
it was difficult to determine the overlap between consecutive images of
a single haulout; 2) poor quality images (usually resulting from low
light photography) sometimes made camouflaged seals difficult to
discern; and 3) the vast numbers of slides were difficult to organize
and store and had a limited archival life.
In an effort to resolve these problems, slides from 2002 were converted
to digital format using a high resolution slide scanner. Once images
were scanned and could be manipulated digitally on a workstation,
adjoining images could more easily be analyzed; image quality could be
enhanced (particularly contrast and brightness) to aid in distinguishing
seals; and images could be archived digitally for long-term use. The
increase in processing time due to slide scanning was clearly offset by
the ability to manipulate slides efficiently and by the potential to
enhance image quality when needed to improve counting accuracy.
Moreover, an easily accessible photo-archive of individual haulouts
will allow for comparisons across multiple years regarding the spatial
use of sites and perhaps the status of animals at those sites.
Once
the photograph counts (and the digital images) are entered into a
relational database including other survey data (e.g., aircraft
altitude, weather, visibility, time, date, tide state), the next step
will be to verify site locations using the positions recorded by
portable GPS (Global Positioning System) receivers used by observers.
By Dave Withrow, Shawn Dahle, and John Jansen.
Harbor Seal and Cruise Ship Interactions in
Disenchantment Bay, Alaska
The Polar Ecosystem Program at NMML conducted research in 2002 on the
potential disturbance of harbor seals by cruise ships entering
Disenchantment Bay (near Yakutat) during the mid-May to July pupping
season. Cruise tourism in Alaska has been growing rapidly since the
early 1970s and there is increasing interest in evaluating the possible
impacts on sensitive coastal ecosystems.
Native Alaskans from nearby
Yakutat have raised concerns that cruise ships may be causing a decline
in harbor seal abundance in Disenchantment Bay, thereby impacting an
important subsistence resource. Preliminary results regarding the
behavioral observations of seals have been released in a draft report
available on the NNML web site.
Additional reports will be released as the other phases of the analysis
are completed, culminating in a final report summarizing all of the
findings.
Analyses of the seal observations conducted from cruise ships revealed
that the likelihood of harbor seals vacating ice floes rose steeply as
ships approached seals at 500 m (about 1,600 feet) or less. Seals were
also more prone to enter the water when ships approached them directly,
rather than passing abeam. The proportion of seals that entered the
water when ships passed within 200 m was nearly 75% compared to less
than 10% entering the water at greater than 600 m where seals showed no
overt response to ships.
A more complete picture of interactions
between ships and seals will become available when analyses are
completed of broader-scale data from aerial surveys. Researchers hope
to be able to determine whether or not cruise ships may be having
large-scale effects on harbor seals by changing where and how often the
seals haul out on ice in the bay.
By John Jansen, John Bengtson, Peter Boveng, and Shawn Dahle.
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