134 
Fishery Bulletin 115(2) 
function (Table 2). For the species pool that includes 
killer whales {Orcinus orca) and sperm ¥/liales {Physe- 
ter microcephalus), the low number of “other” sightings 
(n=l) prevented testing the species covariate. Upon fur¬ 
ther examination, this sighting was found to contain 
a species co-occurrence not observed in the Hawaiian 
Islands EEZ and not represented in any of the other 
pooled sightings. Therefore, this sighting was removed 
from the pool used to estimate the detection function so 
that a species effect could be evaluated. Although the 
sample size was sufficient to model the detection func¬ 
tion of pantropical spotted dolphins (Stenella attenua- 
ta) separately, the species covariate and the “other” fac¬ 
tor level were used to explore the influence of a large 
number of sightings in which the pantropical spotted 
dolphin was not the most abundant species. 
Given the estimated covariate detection function 
and the sightings within the established truncation 
distance from the systematic effort during the HICEAS 
in 2010, the density {D) of each species was estimated 
by using a Horvitz-Thompson-like estimator (Marques 
and Buckland, 2004): 
where L 
giO) 
/(0,Cj) 
N 
D = - 
Ef=i/(0.Cj)-Sj, 
2-L-gm 
( 1 ) 
the length of systematic-effort transect lines 
in the study area; 
the probability of detection on the trackline; 
the probability density of the detection func¬ 
tion evaluated at zero distance for sighting 
j with associated covariates c; 
the number of individuals of the species in 
sighting j; and 
the number of sightings of the species dur¬ 
ing systematic-effort within the analytical 
truncation distance. 
The value of f(0,Cj) that was applied was a weighted 
average of all covariate models within 2 AICc units of 
the best-fit model. The inverse of /(OjCj) is the effec¬ 
tive strip width (ESW), which is the distance from the 
trackline beyond which as many sightings were made 
as were missed within. 
Barlow (2006) used estimates of g(0) adapted from 
previous studies of delphinids and large whales (Bar- 
low, 1995), sperm whales (Barlow and Sexton^), and 
beaked whales and Kogia spp. (Barlow, 1999). Howev¬ 
er, results from recent work in which g(0) was derived 
from apparent densities in different Beaufort sea state 
conditions (assuming that true density is not affected 
by sea state) indicate that g(0) had been previously 
overestimated, particularly for high sea states (Bar- 
low, 2015). Barlow (2015) estimated g{0) in Beaufort 
sea states 0-6 for 20 cetacean taxa by using a model 
2 Barlow, J., and S. Sexton. 1996. The effect of diving and 
searching behavior on the probability of detecting track-line 
groups, go, of long-diving whales during line-transect sur¬ 
veys. Southwest Fish. Sci. Cent. Admin. Rep. LJ-96-14, 21 
p. [Available from Southwest Fisheries Science Center, Na¬ 
tional Marine Fisheries Service, 8901 La Jolla Shores Dr., La 
Jolla, CA 92037.] 
that accounted for spatial and temporal differences in 
density. This model was fitted to cetacean sighting data 
from the eastern Pacific, which included the on-effort 
sightings from the HICEAS in 2010. Therefore, the re¬ 
sulting estimates of glO) can be applied to the estima¬ 
tion of cetacean abundance for the HICEAS in 2010. 
The estimates ofglO) by Barlow (2015) were relative 
to a value of 1 at a Beaufort sea state of 0 for most spe¬ 
cies or species groups considered, with the exception 
of the Cuvier’s beaked whale {Ziphius cavirostris) and 
Mesoplodon spp., for which scaled absolute estimates 
of g(0) were determined for Beaufort sea states 0-6. 
In the absence of absolute estimates of g{0) for most 
of the remaining taxa, the relative values of g(0) from 
Barlow (2015) were assumed to be absolute values in 
the present study. Estimates of ^(0) for the HICEAS 
in 2010 (Table 3) were obtained by taking a weighted 
average of both the Beaufort-specific values of g{0} and 
the associated coefficients of variation (CVs) presented 
in Barlow (2015), where the weights were the propor¬ 
tion of systematic effort in each sea state category 
(0-6) during the HICEAS in 2010. 
For species not covered in Barlow (2015) because 
of small sample sizes, g{0) was assumed to be simi¬ 
lar to the g(0) estimates of associated species in the 
species pools formed to model the detection functions, 
given the similar detection characteristics (e.g., size, 
surface behavior, group sizes) of the species in each 
pool (Table 2). Therefore, g(0) for these species was ob¬ 
tained either by using the estimate of another species 
in the species pool or, if more than one estimate was 
available, by averaging the available estimates. Spe¬ 
cifically, for the HICEAS in 2010, the estimate of g(0) 
for striped dolphins {Stenella coeruleoalba) was used 
for Fraser’s dolphins (Lagenodelphis hosei) and melon¬ 
headed whales (Peponocephala electra), the estimate 
for short-finned pilot whales was used for Longman’s 
beaked whales (Indopacetus pacificus), and the esti¬ 
mates for rough-toothed dolphins (Steno bredanensis), 
bottlenose dolphins (Tursiops truncatus), and Risso’s 
dolphins (Grampus griseus) were averaged for pygmy 
killer whales (Table 3). 
The abundance of each species was determined by 
multiplying the density estimate by 2,447,635 km^— 
the area of the Hawaiian Islands EEZ minus the area 
of the land masses of the main and Northwestern 
Hawaiian Islands. However, the ranges of the pelagic 
stocks of pantropical spotted and bottlenose dolphins, 
which are the stocks involved in the estimation (Table 
1), do not span the entirety of the Hawaiian Islands 
EEZ (Carretta et aL, 2011, 2014). Therefore, the area 
of the ranges of island-associated stocks of pantropical 
spotted and bottlenose dolphins was subtracted from 
the larger area, resulting in areas of 2,392,576 km^ 
and 2,425,900 km^ for pantropical spotted and bottle¬ 
nose dolphins, respectively. The mixed parametric and 
nonparametric bootstrap routine described in Barlow 
(2006) and refined by Barlow and Rankin^ was used 
3 Barlow, J., and S. Rankin. 2007. False killer whale abun- 
