252 
Fishery Bulletin 108(3) 
Uixon ~ 
Entrance 
60'N- 
55°N' 
140 W 
1 30 W 
Alaska 
Prince 
William 
Canada 
Bering Sea Bristol 
Stock Bay ^ 
Southeast 
Alaska stock 
Gulf of Alaska 
stock 
Gulf of Alaska 
260 
520 
1,040 
tm Kilometers 
Figure 1 
The three harbor porpoise ( Phocoena phocoena ) stock regions in Alaska (Southeast Alaska, Gulf of 
Alaska, and Bering Sea). The gray shaded areas represent the areas surveyed in 1997- 99, subdivided 
into areas based on geographical features and depth zones. Dark gray offshore areas were surveyed 
at one third the effort level per square km than lighter gray nearshore areas. Black lines represent 
boundaries between stocks. 
estimates are required to be based on data not more 
than 8 years old (Wade and Angliss, 1997). To meet this 
requirement, abundance surveys of the harbor porpoise 
stocks in Alaskan waters were conducted from 1997 
through 1999. 
An important consideration when conducting multi- 
year surveys is that animals may move from one sur- 
vey area to another among years and therefore may 
be counted more than once. Little is known about the 
year-to-year changes in the distribution of harbor por- 
poises in Alaska. For two studies of harbor porpoise 
(Phocoena phocoena ) on either side of the Atlantic, in 
the Danish Belt seas (Teilmann et al. 1 ) and in the Gulf 
of Maine (Read and Westgate, 1997), it was concluded 
1 Teilmann, J., R. Dietz, F. Larsen, G. Desportes, and B. 
Geertsen. 2003. Seasonal migrations and population 
structure of harbour porpoises (Phocoena phocoena) in 
the North Sea and inner Danish waters based on satellite 
telemetry. Abstract in proceedings of annual meeting of 
the European Cetacean Society, Tenerife, Spain. 
that porpoises follow similar movement patterns from 
year to year and typical ranges of up to 200 km. This 
finding indicates that a net movement in response to 
interannual variation in habitat could occur over a 
range of 100 km. Although this could result in a bias in 
the estimation of abundance for each stock, depending 
on the year of the survey, it is not likely that this is a 
significant occurrence. Each stock region comprises 800 
to 1200 km of shoreline so at most approximately 10% 
to 15% of the region is potentially subject to a net shift 
in distribution from or into the adjacent stock. Also, the 
stock boundaries have been chosen to correspond with 
areas of low harbor porpoise density and therefore there 
are few animals available to make a shift. These two 
arguments suggest that if a net shift does occur, it af- 
fects at most a small percentage of the population. 
This study had three objectives: 
1 to present the results of an aerial survey of three 
harbor porpoise stocks in Alaskan waters during the 
summers of 1997, 1998, and 1999; 
