204 
Fishery Bulletin 112(2-3) 
model was chosen for all 3 groups on the basis of AIC 
values and visual inspection (Thomas et al., 2010). 
Line-transect theory assumes that the probability of 
detecting an animal on the transect line, g(0) equals 
1.0 (perfect detectability), although this assumption is 
rarely true for marine mammals and can be relaxed if 
a correction factor is estimated. Estimation of a cor- 
rection factor was beyond the scope of this study, and 
g(0) is assumed to equal 1.0 and to be constant across 
sea states. The 5% of sightings made at the greatest 
distances to the vessel were assumed to be outliers and 
were truncated to improve the ability to fit the prob- 
ability density function f( 0) (Buckland et al., 2001). 
Truncation distance was 2348 m for large whales and 
1098 m for delphinids and porpoises. There were 7 
whale, 11 dolphin, and 2 porpoise sightings recorded 
beyond the truncation distance and these were exclud- 
ed from density and abundance analyses. We calculated 
density, Dp for a given species within the study area i 
as 
D = J_y-i f {0 
1 2 Lj ^ =1 Sj<0) ’ 
where L[ = 
A0|zj) = 
gj(0> - 
n = 
the length of on-effort transect lines within 
the study area /; 
the probability density function at zero with 
associated covariates z for group; 
the number of individuals of that species in 
group j; 
the transect line detection probability of 
group j; and 
the number of groups of that species encoun- 
tered in the study area i. 
Group abundance for each species in each stratum was 
estimated as 
N r 
A x~^ n 
Z-/i = 1 n 
2 wL 
( 2 ) 
where A 
L 
n 
w 
Pi 
= the area of the stratum; 
= the total search effort in the stratum; 
= the number of unique groups; 
= the truncation distance by species group; and 
= the estimated probability of detecting group i 
obtained from the fitted detection model. 
Results 
Survey effort and sightings 
Line-transect surveys were conducted during 16 cruis- 
es over 5 years, with 3 years of 4-season effort and 2 
years of 2-season effort (Table 1). Including all survey 
effort from the southern CalCOFI transect lines, ob- 
servers collected visual data on marine mammals over 
267 days and searched 25,079 km of transect lines. Of 
that total distance of transect lines covered, 19,996 
km was surveyed on the 6 southern CalCOFI lines 
(hereafter referred to as “the study area”) in accept- 
able weather conditions (sea state of 0-5). Within the 
study area, on-effort transect line kilometers surveyed 
did not vary significantly by season (ANOVA, F= 0.078, 
P= 0.97). Sea states varied by season, with greatest 
sea states during spring and summer and lowest sea 
states during winter (Ivruskal-Wallis one-way ANOVA, 
P<0.001). The median sea state was 3 in all seasons, 
except for summer, when it was 4. We found no sig- 
nificant difference in perpendicular distance to tran- 
sect line for any species group by vessel (large whales, 
ANOVA, D=2.08, P=0.16; delphinids and porpoises 
ANOVA, F=1.01, P- 0.39) or by vessel-season (large 
whales, ANOVA, F= 2.76, P=0.15; delphinids and por- 
poises ANOVA, F= 0.36, P- 0.56). 
As stated in the Materials and methods section, 
acceptable survey conditions required >0.9 km of es- 
timated visibility on the transect line. Only 0.67% of 
effort was conducted with a visibility of <900 m, and 
that effort resulted in 4 sightings of common dolphins; 
because those dolphins were not identified to species 
level, their sightings were not used in the detection 
function. All encounters used in detection functions for i 
the 3 species groups were made with at least 2.77 km 
visibility, with 93% of large whales, 94% of delphinids, 
and 100% of Dali’s porpoises encountered with visibil- 
ity >7.4 km. 
In the study area during the 16 survey cruises, 29 
marine mammal species were encountered, including 
22 cetaceans, 6 pinnipeds, and a single mustelid spe- 
cies (Table 3). There were 931 on-effort sightings in 
the study area, with California sea lions ( 154 recorded 
sightings) the most commonly encountered species, fol- 
lowed by short-beaked common dolphins ( 122 sight- 
ings), northern fur seals ( Callorhinus ursinus, 59 sight- 
ings), and fin whales (53 sightings). The most common- 
ly encountered large cetaceans in the study area were 
fin, humpback (34 sightings), blue (25 sightings), and 
sperm (20 sightings) whales (Fig. 3), and the most com- 
monly encountered small cetacean species were short- 
beaked common dolphin, Pacific white-sided dolphin 
(46 sightings), and Dali’s porpoise (49 sightings) (Fig. 
4). The ratio of on-effort sightings to opportunistic and 
off-effort sightings of cetaceans (1:0.76) in this study is 
higher than would be expected for other line-transect 
surveys where very little sighting effort is conducted 
off transect or in poor sea conditions. The large number 
of off-effort and opportunistic sightings in our study 
is mostly due to the large amount of opportunistic ef- 
fort between the primary transect lines or at the water 
sampling stations along the coast. 
Multispecies sightings were observed on 15 occa- 
sions for 7 dolphin species; the northern right whale 
dolphin and Pacific white-sided dolphin mixed most 
frequently (5 times), bottlenose dolphin and common 
dolphins mixed 3 times, Pacific white-sided dolphin and 
short-beaked common dolphin mixed 2 times, striped 
dolphin (Stenella coeruleoalba ) and unidentified com- 
mon dolphins mixed 2 times, and single occurrences 
