Zwolinski et at: Distributions and abundances of Sardinops sagax and other pelagic fishes in the California Current Ecosystem 
111 
(Crone et al., 2009). Young Pacific and jack mackerel 
tend to reside in coastal waters and nearshore banks, 
whereas the older mackerel reside mostly offshore ( Mac- 
Call and Stauffer, 1983). Pacific and jack mackerel may 
also migrate seasonally north-south, although not as 
far north as sardine (Demer et al., 2012). 
When the anchovy stocks are low, they tend to remain 
in certain areas, e.g., in the Southern California Bight 
(SCB), Monterey Bay, and coastal regions off Oregon 
and Washington near river plumes. However, when the 
subpopulations of anchovy increase, their distributions 
expand to adjacent areas (Messersmith et al., 1969). 
Off the U.S. west coast, the fisheries of the aforemen- 
tioned species are regulated under the Pacific Fishery 
Management Council (PFMC) CPS management plan. 
Sardine and Pacific mackerel are “actively managed 
species,” which means that they are regulated by the 
setting of annual quotas founded on periodic assess- 
ments of their populations. In contrast, jack mack- 
erel and anchovy are “monitored species.” Although, 
this status does not require formal assessment and 
the setting of quotas (which can be set by each state), 
knowledge of their dynamic stock biomasses is desirable 
because their exploitation rate and thus their manage- 
ment status may change. 
Assessments of actively managed species rely on sin- 
gle-species models that combine catch-at-age statistics 
from the commercial and sport fisheries, and abundance 
and demographic information from fishery-independent 
surveys, when available (Crone et al., 2009; Hill et al., 
2010). Although catches may indicate changes in the 
structure of the CPS community, they are not unbiased 
indicators of the state of the ecosystem (Pennington 
and Stromme, 1998; Cotter et al., 2009). This is due 
to landing data that are affected by both natural vari- 
ability and market demand (Mason, 2004). For sardine, 
estimates of spawning-stock biomass (SSB) from sur- 
veys with the daily egg production method (DEPM; Lo 
et ah, 2010) comprise the longest fishery-independent 
time-series. However, owing to uncertainties in the 
DEPM estimates of SSB, managers called for additional 
fisheries-independent abundance estimates to include 
in the stock assessment model (Hill et ah, 2006). In 
response, an acoustic— trawl method (i.e., a method com- 
bining echosounder and trawl sampling) was developed 
and used to survey the abundances and distributions 
of the dominant CPS in the CCE (Demer et ah, 2012). 
Acoustic-trawl surveys, conducted periodically and 
synoptically over the scales of the stocks, can simulta- 
neously provide biomass estimates of multiple actively 
managed and monitored species, accurately track their 
distributions and demographics, and provide estimates 
of recruitment and mortality. Data from these mul- 
tispecies surveys may be used to monitor epipelagic 
communities, and provide information for precaution- 
ary and ecosystem approaches to the management of 
exploited and emerging fisheries (FAO, 2003; Rice et 
ah, 2005). 
Our goal was to demonstrate the successful use of an 
acoustic-trawl method to monitor the distributions and 
abundances of multiple epipelagic fish species in the 
CCE. Estimates are derived for the most abundant spe- 
cies, i.e., sardine, jack mackerel, and Pacific mackerel. 
However, the emphasis is on sardine, the dominant, 
actively managed CPS off the west coast of the United 
States during these surveys. The resulting estimates 
of their abundances and demographics are compared to 
those of the most recent assessment (Hill et ah, 2010). 
Methods 
Three acoustic-trawl surveys of CPS were conducted 
off the west coast of the United States during April to 
May of 2006, 2008, and 2010. The surveys were con- 
ducted from the U.S. National Oceanic and Atmospheric 
Administration (NOAA) research vessels Oscar Dyson 
(2006), David Starr Jordan (2008), and Miller Freeman 
(2010), and a contracted fishing vessel Frosti (2010). 
The surveys extended south to the Mexican border, and 
north to the westernmost part of Vancouver Island, 
Canada, in 2006, and to the Strait of Juan de Fuca in 
2008 and 2010 (Fig. 1). The survey transects extended 
to 250 nmi offshore, south of Point Conception, and to 
140 nmi offshore, farther north. Transect spacing varied 
between 40 and 80 nmi, occasionally with denser sam- 
pling off southern California, which is the area of higher 
expected sardine biomass during the spring (Zwolinski 
et al., 2011). 
Acoustic sampling 
Measurements of volume backscattering strength ( S v ; dB 
re 1 m -1 ) and target strength (TS; dB re 1 m 2 ) were made 
by using calibrated echosounders (Simrad EK60, Kongs- 
berg, Norway) configured with split-beam transducers, 
operating at 38, 70, 120, and 200 kHz (RV David Starr 
Jordan and FV Frosti), 18, 38, 70, 120, and 200 kHz (RV 
Oscar Dyson), and 18, 38, 120, and 200 kHz (RV Miller 
Freeman). The echosounder systems were calibrated 
immediately before the surveys with a 38.1-mm-diameter 
sphere made from tungsten carbide with 6% cobalt 
binder material (Foote, 1983). Throughout the surveys, 
pulses of 1024 ps were transmitted at 0.5-s intervals, 
except in 2006 when the pulse interval was 2 seconds. 
Transmit powers were 2000, 2000, 1000, 500, and 120 W 
at 18, 38, 70, 120, and 200 kHz, respectively (Demer et 
al., 2012). Received powers were sampled every 256 ps, 
indexed by time and geographic position, and recorded 
to at least 250-m range (500 m in 2006). 
Trawl sampling 
During the night, CPS tend to migrate closer to the 
sea surface and form loose aggregations (Cutter Jr. and 
Demer, 2008) facilitating capture and providing better 
estimates of the proportions of CPS in the area than esti- 
mates from directed daytime trawling. Each night during 
the survey, beginning 30 to 60 min after sunset, as many 
as four surface trawls were set to sample CPS for the pur- 
