216 
Fishery Bulletin 112(2-3) 
124°W 123°W 122°W 121°W 120°W 119°W 118°W 117°W 116°W 
i i i i i i i i i 
Figure 7 
Map of on-effort encounters with the Pacific white-sided dolphin ( Lagenorhynchus obliquidens ) by 
season during the 16 shipboard line-transect surveys conducted quarterly during 2004-08 as part 
of the California Cooperative Oceanic Fisheries Investigation. The color of the triangle indicates 
the season: blue=winter, green=spring, red=summer, and yellow=fall. 
In addition to the high CV value associated with our 
abundance estimate, a likely cause of this discrepancy 
between the 2 studies is the difference in estimated 
group size, where we observed an average of 40.5 in- 
dividuals in a group and Barlow and Forney (2007) re- 
ported 13.4 individuals in a group. 
The Northern right whale dolphin and Dali’s porpoise 
are known to favor cold waters, and we found both spe- 
cies to have the greatest abundance estimates during the 
winter-spring period over all depths. Although encoun- 
ters with northern right whale dolphins in the summer- 
fall period were few, an increase in density during the 
winter-spring surveys in shallow water was observed — a 
finding that is consistent with earlier records that found 
this species beyond the continental slope for warm-water 
seasons and in shelf waters of the Southern California 
Bight for the cold-water season (Barlow, 1995; Forney 
et al., 1995; Forney and Barlow, 1998). 
Although seasonally abundant, Dali’s porpoises are 
often initially sighted when they react to survey ves- 
sels, thereby biasing abundance estimates upward. To 
compensate for vessel attraction, Barlow and Forney 
(2007) included only Dali’s porpoise sightings made in 
sea states of 0-2 — an approach that they noted limited 
sample size. On the basis of the detection model for 
Dali’s porpoises (Fig. 2), which showed an even taper- 
ing of sightings with distance from the vessel, we in- 
cluded sightings in sea states of 0-5, assuming that 
it would be better to have a greater number of sight- 
ings than an insufficient number to estimate abun- 
dance. Spatially, our analysis of encounters with Dali’s 
porpoises in the CalCOFI study area agrees with the 
finding of Morejohn (1979) that Dali’s porpoises were 
commonly seen in small groups along the shelf and 
slope and in offshore waters. Dali’s porpoises were con- 
sistently found in recently upwelled waters near shore 
(Peterson et al., 2006). In the CalCOFI study area, the 
highest encounter rates of Dali’s porpoises occurred in 
spring, when upwelling waters were active. 
As with the Dali’s porpoise, many of the delphinids 
are known to react to a vessel before visual observers 
can detect them; this behavior is especially a concern 
when the naked eye and low-power binoculars are used 
in the search method, as they were in the CalCOFI 
surveys used in this study. Although reaction to ves- 
sel cannot be ruled out as a factor in our results, our 
decision to keep all on-effort sightings in the analyses 
was based on the detection model for delphinids (Fig. 
