Douglas et al. Seasonal distribution and abundance of cetaceans off Southern California 
201 
termined with an angle board (zero at bow, negative 
to port, positive to starboard), sighting number, spe- 
cies, group-size estimate (best, high, low), presence of 
calves, general behavior of animals, photographs (if 
taken), and comments pertaining to the sighting. Be- 
cause of the surfacing behavior of cetaceans and lack 
of visibility while submerged, animal counts were only 
estimates, with the recorded “low” estimate being the 
minimum number of individuals observed during the 
sighting, the “high” estimate being the maximum, and 
the “best” estimate was always recognized as the value 
closest to the actual number of individuals. Group- 
size estimates and species confirmation were generally 
made by the lead observer but were agreed upon by all 
observers present. 
Transect lines were surveyed in “passing mode,” 
which does not allow for any alteration of course for 
closer examination of groups encountered. Barlow 3 
stated that surveys conducted in passing mode yield 
less biased estimates of encounter rates but result in a 
higher number of unidentified groups and more biased 
estimates of group size and species percentages com- 
pared with surveys conducted in closing mode. Clos- 
ing mode allows for all observers to go “off effort” and 
to adjust the course and speed of the vessel in order 
to approach animals sighted at a distance from the 
transect line. To assist with species identification and 
group-size estimation, 25x binoculars were available 
for all surveys on the Roger Revelle , for most surveys 
on the David Starr Jordan , and occasionally on the 
New Horizon. The 25x binoculars were used only as 
an aid once a group had been located with the naked 
eye or 7x50 binoculars to maintain a consistent search 
method between surveys. 
Observers recorded effort (on or off), weather (sea 
state, swell height, visibility [or estimated distance 
that observers can detect a small cetacean], and pre- 
cipitation), changes in course and speed, and sight- 
ing information onto data sheets. Observations were 
considered on effort if observers were actively search- 
ing with the unaided eye and 7x50 binoculars in a 
sea state 0-5, the vessel was traveling no less than 6 
km/h, and there was a minimum visibility of 0.9 km 
(0.5 nmi) in front of the vessel. All sightings of all spe- 
cies of marine mammals were recorded with the excep- 
tion of California sea lions ( Zalophus californianus), 
which were sighted most often near the coast because 
estimation of group size and documentation of sight- 
ing details of high numbers of California sea lions in 
coastal waters would have compromised the ability of 
observers to sight and record other species that occur 
in the same area. 
Because these surveys were conducted in pass- 
ing mode and with limited use of 25x binoculars, 
3 Barlow, J. 1997. Preliminary estimates of cetacean abun- 
dance off California, Oregon, and Washington based on a 
1996 ship survey and comparisons of passing and closing 
modes. Southwest Fish. Sci. Cent. Admin. Rep. LJ-97-11, 
25 p. 
some common dolphins ( Delphinus sp.) that were en- 
countered could be confirmed only to the genus level. 
Short-beaked common dolphins ( D . delphis) and long- 
beaked common dolphins ( D . capensis) have very simi- 
lar morphological features and pigmentation, and they 
are difficult to distinguish at a distance (Rosel et al., 
1994); therefore, observers were encouraged to obtain 
photographs if there was doubt about the identification 
of these or other species. Photographs were reviewed 
onboard, compared with identification guides (Reeves 
et al., 2002), and occasionally shared with experienced 
colleagues for species confirmation. Additionally, there 
are 2 forms of Pacific white-sided dolphins ( Lagenorhyn - 
chus obliquidem) along the coast of California (Walker 
et al., 1986; Lux et al., 1997; Soldevilla et al., 2011); 
because these forms are indistinguishable from a dis- 
tance, density and abundance for this species likely in- 
cludes both forms. 
Analytical methods 
Sighting and effort data from the 16 CalCOFI cruises 
were split into 2 effort categories: 1) on-effort sight- 
ings on the 6 CalCOFI transect lines, the sightings 
that form the basis of all analyses and findings; and 2) 
opportunistic effort (sightings made off effort or when 
a vessel was not on a CalCOFI transect line), which 
is presented to show species diversity and presence or 
absence of species. We calculated encounter rates by 
season (number of on-effort sightings per 1000 km of 
transect line surveyed) for each species with 10 or more 
sightings. Distance r of the animal(s) sighted from the 
vessel and sighting position were determined from the 
reticle value (or estimated distance), height of observer 
platform, sighting angle (A) to animal from the bow 
of the vessel, and position of the vessel. To character- 
ize the depth distribution of effort, sample points were 
created at 1-km intervals along transect lines within 
ArcGIS (vers. 9.2; Esri, Redlands, CA). 
Sighting data also were plotted in ArcGIS, with 
sighting and effort data linked to a coastline shape- 
file and bathymetry data set from the ETOPOl global 
relief model (Amante and Eakins, 2009) with 1850-m 
resolution (National Geophysical Data Center [NGDC], 
http://www.ngdc.noaa.gov/mgg/global/global.html); 
where highest resolution was not available from ET- 
OPOl, the NGDC coastal relief model (NGDC, http:// 
www.ngdc.noaa.gov/mgg/coastal/crm.html ) provided 
90-m resolution (73% of sighting depths and 49% of 
effort depths came from the 90-m resolution data set). 
Distance to the closest point of land, distance to the 
mainland, depth, and distance to the shelf break (200- 
m isobath) were also calculated for each sighting and 
effort location. 
Using the cold (winter-spring, defined as Janu- 
ary-April) and warm (summer-fall, defined as July- 
November) water distinctions in Forney and Barlow 
(1998), we tested for differences between the number 
of encounters of each positively identified species by 
