810 
Fishery Bulletin 96(4), 1998 
strata, hereafter referred to as sections, were sur- 
veyed on the same day. Sections were chosen in a 
random starting order and every attempt was made 
to complete all sections before surveying the same 
sections again. 
Harbor porpoise were surveyed from a 7.3-m alu- 
minum marine patrol vessel during Beaufort sea 
state 0 (wind speed=0-1.8 km/h), 1 (wind speed=1.8- 
5.6 km/h), or rarely Beaufort 2 (wind speed=7.4-ll 
km/h). Each transect was completed in approximately 
52 min, at an average boat speed of 9 km/h. Date, 
time, and tidal phase (flood or ebb) were recorded 
before each transect was surveyed. At the beginning 
and end of each transect, Secchi disk readings were 
recorded to the nearest 0.1 m and SST was recorded 
(by a calibrated thermometer located on the trans- 
ducer of the survey vessel) to the nearest 0. 1°C. Dur- 
ing surveys, two observers divided the field of view 
across the forward 180° of the transect path. Obser- 
vations were made from the roof of the vessel (height 
above the waterline=2.68 m, measured to the observ- 
ers’ eyes in surveying position) with unaided eyes 
and with Fujinon 7 x 50 reticle and compass bin- 
oculars. When an individual or group of harbor por- 
poise was located, an observer recorded time; group 
size and composition; compass bearing to the por- 
poise; ocular reticle marks from the horizon to the 
porpoise; Beaufort sea state; number of boats, birds, 
and marine mammals within 1 km of vessel; and di- 
rection of harbor porpoise travel. A group of harbor 
porpoise was defined as two or more porpoise visible 
at the water’s surface within three body lengths 
(5 m) of each other, having nearly synchronous div- 
ing patterns ( <15 seconds between sightings of each 
individual). Observers were trained and tested in the 
use of reticle binocompasses and in calibrating their 
readings on buoys and points of land and comparing 
distance accuracy to National Oceanic and Atmo- 
spheric Administration (NOAA) navigational charts 
and vessel’s radar. Compasses on binoculars were 
also tested and were found not to be significantly 
affected by metal on the survey vessel. Although not 
common, the horizon was sometimes obscured by 
land in the observers’ viewing area. To compensate 
for this, we estimated the number of reticles from 
the land-water interface (directly beyond the por- 
poise) to the porpoise sighting, then carefully rotated 
the binoculars from the land to the horizon. We then 
determined the number of reticles the horizon was 
beyond the land and added this amount to the reticle 
reading of the harbor porpoise sighting. Loran coor- 
dinates, Beaufort sea state, visibility, SST, and num- 
ber of boats within 1 km of the vessel were recorded 
every ten minutes during surveys and for each har- 
bor porpoise sighting. Depth and seafloor slope 
(at each harbor porpoise sighting) were determined 
from NOAA navigational charts and bathymetric 
charts. 
Locations of harbor porpoise were determined with 
the aid of Fujinon 7 x 50 reticle (one reticle=17 min 
or 0.283°) and compass binoculars. Vertical angle was 
calculated as the angle between the horizon and the 
harbor porpoise. Distance to harbor porpoise was 
calculated as 
' tan(G! ) ’ 
where D r = the radial distance from the vessel to the 
porpoise; 
H = the eyeheight of observers; and 
a = the vertical angle between the horizon 
and porpoise. 
Locations were plotted on NOAA navigational charts 
by using Loran (latitude and longitude) coordinates 
of the vessel at the time harbor porpoise were sighted, 
and distance and bearing to the sighting. 
Perpendicular distance from the trackline to har- 
bor porpoise was determined by using 
D p = D r x sin(a), 
where D p - perpendicular distance; 
D r = the radial distance to the harbor por- 
poise; and 
a = the angle off the trackline (the difference 
between the trackline heading and the 
bearing to porpoise). 
Seafloor depth and slope were determined by us- 
ing a NOAA navigational chart and bathymetric map. 
Percentage slope was calculated as 
dz 
% slope = — x 100%, 
ds 
where dz = the difference between the two closest 
depths (m) printed on the chart on ei- 
ther side of a harbor porpoise location 
(with contour lines drawn among depths ); 
and 
ds - the distance (m) between those two 
depths. 
Bathymetric charts with contour intervals of 10 m 
were used to verify angle of slope between depths. 
To determine if harbor porpoise occurred over 
depths and slopes in proportion to available depths 
