Hobbs and Waite: Abundance of Phocoena phocoena in three Alaska regions 
253 
2 to produce a correction factor for perception bias that 
was specific to harbor porpoise in Alaskan waters 
and to the surveys presented here and to develop 
methods to test observer performance, by using data 
collected during these surveys; and 
3 to generate abundance estimates for the three stocks 
of harbor porpoise in Alaska during 1997-99. 
Materials and methods 
Survey design 
Aerial surveys were conducted during June and July 
beginning with the SEA stock in 1997, proceeding west- 
ward through the GOA stock in 1998, and on to the 
BS stock in 1999. Each study region was divided into 
areas; 70 areas in Southeast Alaska, 39 areas in the 
Gulf of Alaska, and 4 areas in the Bering Sea (primar- 
ily in Bristol Bay) based on geographical features for 
inside waters, such as straits and inlets, and two depth 
zones for offshore waters (Fig. 1). Southeast Alaska 
was divided into more areas because of its complicated 
system of waterways, whereas Bristol Bay has rela- 
tively homogenous features and therefore was divided 
into fewer survey areas. Survey effort was stratified 
by area in Southeast Alaska based on harbor porpoise 
encounter rates calculated from sightings made in previ- 
ous surveys (Dahlheim et ah, 1993, 1994 2 ). The survey 
transect design each year varied depending on the body 
of water. In general, the transects in offshore waters 
were stratified by depth and distance from shore, after 
an alternating two short and one long sawtooth transect 
pattern, so that survey effort in the nearshore strata 
was about three times that in the offshore strata. The 
1991-93 surveys were designed with fixed distances of 
28 km offshore for the short and 74 km offshore for the 
long sawtooth tracklines. Our surveys were designed 
to include the area surveyed in 1991-1993 but also to 
cover the continental shelf if it extended beyond the 
original survey. Each set of sawtooths had two crite- 
ria and the further offshore of the criteria determined 
the length of the line. Specifically, in 1997, the short 
transects in the sawtooth transect pattern extended 
to a distance of 31 km offshore or the 183-m (100 fm) 
depth contour, and the long transects extended 74 km 
or to the 1829-m (1000 fm) depth contour, whichever 
was farthest from shore. In the 1998 GOA survey, the 
shelf fell much more gradually in places and funding 
limited the total survey time. Therefore, the nearshore 
strata transects were reduced to a distance of 28 km 
or to the 91-m (50 fm) depth contour, whichever was 
2 Dahlheim, M., A. York, J. Waite, and R. Towel], 1993. Abun- 
dance and distribution of harbor porpoise (Phocoena phocoena) 
in Southeast Alaska and Western Gulf of Alaska, 1992. 
1992 Annual report to the Marine Mammal Protection Act 
(MMPA) Assessment Program, 52 p. Office of Protected 
Resources. NMFS, NOAA, 1335 East-West Highway, Silver 
Spring, MD. 
farthest from shore, whereas the long transects followed 
the same criteria as in 1997. Because the entire Bering 
Sea survey region is shallower than 183 m (100 fm), it 
was covered equally with short transects out to 30 km 
along the shore and with parallel north— south lines 
through the center approximately 18 km apart. Smaller 
bays and inlets were treated separately and stratified 
by the width of the mouth of the bay or inlet. A subset 
was chosen to approximate the survey effort by area for 
the other survey regions, and selection was made on the 
basis of convenience (i.e., bays and inlets close to the end 
of survey effort lines were chosen). 
Line-transect surveys were flown at an altitude of 
152.5 m and a speed of 185 km/h in a DeHavilland 
Twin Otter aircraft. Survey areas were chosen each 
day to complete coverage of contiguous areas during 
weather with winds below 15 knots and at a ceiling 
above 1000 ft (305 m). Survey lines were broken off and 
other tracklines with better conditions were sought if 
the Beaufort sea state exceeded 3 or if visibility dropped 
to poor for a significant period (at the discretion of the 
team leader). A primary observer (also referred to as 
a “side observer”) was stationed at the left and right 
bubble windows of the plane; these positions allowed 
them to see water directly below the plane. To collect 
additional sightings and data to estimate perception 
bias for this study, an independent observer was sta- 
tioned at a belly window located in the floor at the 
back of the plane (this observer is also referred to as 
the “belly observer”). This window provided a circular 
field of view 100 m (30°) to either side of the trackline 
and 200 m along the trackline. Five observers rotated 
in 40-minute shifts through five positions: the right 
and left bubble windows (primary observers), the belly 
window (independent observer), a computer station, 
and a rest position. A headset system was used by the 
primary observers and computer operator to communi- 
cate openly, and the independent observer was isolated 
and used a string attached to the arm or ankle of the 
computer operator to indicate a sighting and a notepad 
to relay information. A simple short hand was developed 
so that the belly observers would not need to take their 
eyes off of the trackline. 
Survey data were recorded directly to a laptop com- 
puter in the airplane using a Turbo PASCAL (vers. 5.0, 
Borland Software Corp., Austin, TX) language-based 
software customized for the survey. The software in- 
cluded a proprietary routine (Survey, vers. 3.2, Cascadia 
Research, Olympia, WA) which read the text output of a 
global positioning system (GPS) unit connected directly 
to the serial port of the computer. The date, time, and 
position of the aircraft were automatically entered into 
the survey data every minute or whenever other data 
were entered by the recorder. At the start of each tran- 
sect, waypoint numbers, observer positions, and envi- 
ronmental conditions were recorded. Environmental con- 
ditions included percent cloud cover, Beaufort sea state, 
visibility (a subjective rating of sighting conditions by 
each observer at the following levels (excellent, good, 
fair, poor, and unacceptable), and glare (none, minor, 
