Demer et al.: Seasonal migration of Sardinops sagax in the California Current Ecosystem 
63 
o 
10 15 20 25 30 
o 
10 15 20 25 30 
Standard length (cm) 
Figure 4 
Estimated length distributions for the “northern stock” of Pacific sardine ( Sardinops 
sagax ) in the California Current Ecosystem (filled bars) during the 2008 spring 
(upper graph) and summer (lower graph) surveys. The dashed line represents 
the estimated length composition from the corresponding assessments (Hill et 
ah, 2010). 
Discussion 
Target detection 
The depth range of the echosounder sampling for CPS 
(10-70 m) encompassed the daytime vertical extent 
of the CPS in the CCE, particularly for sardine, jack 
mackerel, and Pacific mackerel (Squire, 1972; Hill et al., 
2010) and it was shallow enough to exclude the majority 
of hake (Dorn et al., 1994) and rockfishes (Butler et ah, 
2003). Most of the CPS backscatter sampled during these 
surveys (not shown) occurred in the upper 40 m, deeper 
during the day and rising near the sea surface at night — 
a finding that is consistent with early sonar observations 
of CPS in the Southern California Bight (Holliday and 
Larsen, 1979). Therefore, it is likely that the stocks of 
sardine, jack mackerel, and Pacific mackerel were effec- 
tively sampled at depths shallower than 70 m. 
Fish may react to an approaching vessel (Ona et al., 
2007), or not (Fernandes et al., 2000), and they may 
react more to larger “quiet” vessels (Ona et al., 2007), 
although not always (De Robertis et al., 2010). In other 
words, fish behavior in response to research vessels is 
poorly understood, and it likely varies among species 
and with many other factors (Vabo et al., 2002), such as 
ontogeny, time of day, season, region, depth, sampling 
platform, and stimuli. In the CCE, a significant propor- 
tion of CPS schools reside near the sea surface (Mais, 
1974; Holliday and Larsen, 1979). However, although 
schools of epipelagic fish may dive in response to a 
survey vessel, perhaps altering their TS and inducing 
some measurement error (Holliday and Larsen, 1979; 
Vabo et al., 2002; Patel and Ona, 2009), this behavior 
may position them deeper and allow their detection 
with down-looking echosounders. For example, schools 
of Spanish sardine ( Sardinella aurita) located in the 
path of the vessel, initially in the upper 20 m, invari- 
ably descended a few meters before the passage of the 
survey vessel; the effect diminished with school depth, 
and the lateral movements were negligible (Gerlotto 
and Freon, 1992). If this behavior is consistent for CPS 
in the CCE surveyed with down-looking echosounders 
from NOAA ships, the sampling bias due to this factor 
may be negligible. 
To investigate this hypothesis, measurements were 
made of CPS schools during a 2006 survey of CPS 
by using a side-looking multibeam echosounder, pole- 
mounted on the RV David Starr Jordan (Cutter Jr. and 
Demer, 2008). Results supported earlier observations 
that near-surface fish dove beneath the vessel, well in 
