251 
Abundance of harbor porpoise 
( Phocoena phocoena) in three Alaskan regions, 
corrected for observer errors due 
to perception bias and species misidentification, 
and corrected for animals submerged from view 
Email address for contact author: Rod.Hobbs@noaa.gov 
National Marine Mammal Laboratory 
Alaska Fisheries Science Center 
National Marine Fisheries Service 
7600 Sand Point Way N.E 
Seattle, Washington 98115 
Abstract — Estimating the abundance 
of cetaceans from aerial survey data 
requires careful attention to survey 
design and analysis. Once an aerial 
observer perceives a marine mammal 
or group of marine mammals, he or 
she has only a few seconds to iden- 
tify and enumerate the individuals 
sighted, as well as to determine the 
distance to the sighting and record 
this information. In line-transect 
survey analyses, it is assumed that 
the observer has correctly identified 
and enumerated the group or indi- 
vidual. We describe methods used to 
test this assumption and how survey 
data should be adjusted to account 
for observer errors. Harbor porpoises 
(Phocoena phocoena ) were censused 
during aerial surveys in the summer 
of 1997 in Southeast Alaska (9844 
km survey effort), in the summer oft 
1998 in the Gulf of Alaska (10,127 
km), and in the summer of 1999 in 
the Bering Sea (7849 km). Sight- 
ings of harbor porpoise during a 
beluga whale ( Phocoena phocoena) 
survey in 1998 (1355 km) provided 
data on harbor porpoise abundance 
in Cook Inlet for the Gulf of Alaska 
stock. Sightings by primary observ- 
ers at side windows were compared 
to an independent observer at a belly 
window to estimate the probability of 
misidentification, underestimation of 
group size, and the probability that 
porpoise on the surface at the track- 
line were missed (perception bias, 
g(0)). There were 129, 96, and 201 
sightings of harbor porpoises in the 
three stock areas, respectively. Both 
g(0) and effective strip width (the 
realized width of the survey track) 
depended on survey year, and g(0) also 
depended on the visibility reported by 
observers. Harbor porpoise abundance 
in 1997-99 was estimated at 11,146 
animals for the Southeast Alaska 
stock, 31,046 animals for the Gulf 
of Alaska stock, and 48,515 animals 
for the Bering Sea stock. 
Manuscript submitted 2 March 2009. 
Manuscript accepted 4 March 2010. 
Fish. Bull. 108:251-267 (2010). 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Roderick C. Hobbs (contact author) 
Janice M. Waite 
Accurate estimation of abundance of 
cetaceans from survey data requires 
careful attention to both survey design 
and analysis (Buckland et al., 2001). 
Aerial surveys of cetaceans depend 
on rapid discovery, recognition, and 
recording of sightings of individuals 
and groups of animals by observers, in 
addition to accounting for animals that 
were missed because the observers did 
not notice them (perception bias) or 
because the animals were below the 
surface (availability bias) (Buckland 
et al., 2001). Although an experienced 
trained observer is efficient at recogni- 
tion of a species and recording data, it 
is necessary to include methods that 
can measure error rates of observers 
and account for them in the estima- 
tion of abundance. We present here 
the results of a series of aerial surveys 
designed to estimate the abundance of 
harbor porpoise (Phocoena phocoena ) 
in Alaskan waters. 
When, during an aerial line-tran- 
sect survey, an object or group of 
objects is encountered, an aerial 
observer has only a few seconds to 
complete several tasks: 1) perceive 
the objects, 2) identify the objects, 3) 
enumerate the objects, and 4) deter- 
mine the distance of the objects from 
the trackline. Items 1 and 4 are the 
major concern for the estimation of 
perception bias and for line-transect 
survey analysis, and it is generally 
assumed that the observer completes 
items 2 and 3 correctly or indicates 
uncertainty correctly (e.g., species 
code “unidentified porpoise” indicates 
uncertainty between Dali’s porpoise 
[ Phocoenoides dalli] and harbor por- 
poise). We develop methods to test the 
assumptions of correct species iden- 
tification and enumeration and apply 
them to the analysis of line-transect 
survey data and the estimation of 
abundance. 
From 1991 to 1993, the National 
Oceanic and Atmospheric Adminis- 
tration (NOAA) conducted aerial sur- 
veys in three regions of the Alaskan 
coast: 1) Cook Inlet and Bristol Bay 
in 1991; 2) in the waters around Ko- 
diak Island and south of the Alaska 
Peninsula in 1992; and 3) in the 
offshore waters of Southeast Alaska 
from Dixon Entrance to Prince Wil- 
liam Sound in 1993. The inside wa- 
ters of Southeast Alaska were sur- 
veyed in each of these years by NOAA 
crews aboard the NOAA RV John 
N. Cobb. The abundance estimates 
for these regions were combined to 
produce an abundance estimate for 
the Alaska stock of harbor porpoise 
(Dahlheim et al., 2000). Since then, 
the Alaska stock has been split in- 
to three stocks: Southeast Alaska 
(SEA), Gulf of Alaska (GOA), and 
the Bering Sea (BS) stocks (Fig. 1). 
The 1991-93 abundance estimate 
was subdivided to correspond with 
the new stock boundaries (Hill and 
DeMaster, 1998). To maintain up-to- 
date stock assessments, abundance 
