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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.

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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