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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.
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.
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.
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.
Auke Bay Lab