114 
Fishery Bulletin 110(1) 
uted in 2006. Subsequently, their spatial distributions 
(Fig. 1) and average s A values decreased by approxi- 
mately 70% (Fig. 2). Concurrently, the total area occu- 
pied by CPS decreased, but the densities of fewer CPS 
aggregations remained relatively constant. 
The acoustically detected CPS during daytime were 
well matched spatially to the presence of CPS in the 
night-trawl catches (Fig. 1). CPS catches were located 
in regions with significantly higher acoustically de- 
tected CPS than the overall average, except for 2006, 
when acoustically detected CPS were abundant 
throughout the study area and the number of 
trawls was the lowest (Fig. 2). Therefore, both 
trawl sampling at night and acoustic sampling 
during day are effective detectors of CPS when 
they occur in the same areas. Echoes that 
were acoustically ascribed to CPS but were not 
matched with CPS catches comprised a very 
small fraction of the total s A (Fig. 1) and were 
excluded from further analysis. 
Sardine were caught offshore of central and 
southern California, partially overlapping with 
catches of other species, especially jack and Pa- 
cific mackerel (Fig. 1). Sixty-eight percent of the 
catches containing CPS contained sardine (Table 
1) . This was the highest “species proportion.” 
The weight of the total sardine catch was also 
the highest of the CPS catches, except in 2008 
when two catches of anchovy totaled more than 
500 kg. Sardine ranged in SL between 11 and 
27 cm and averaged approximately 21 cm (Table 
2) . The potential sardine habitat encompassed 
the area containing echoes attributed to sardine 
(Fig. 3). In 2006, almost the entire potential 
sardine habitat was surveyed, but some sardine 
may have resided to the south of the sampling 
area (Fig. 3). At that time, sardine were evenly 
distributed in high densities throughout the po- 
tential habitat, allowing the use of a single stra- 
tum for estimation. In contrast, in 2008, sardine 
occupied roughly one fourth of the north-south 
extent of the survey area, and were concen- 
trated in the southern region of their potential 
habitat. The potential habitat extended beyond 
the survey area, mainly offshore, but sardine 
densities diminished gradually and completely 
towards the survey-area boundary, indicating 
that the stock was surveyed entirely. For the 
2008 survey, two strata were defined and used 
for the biomass estimations. In 2010, the esti- 
mated sardine densities were again low' or null 
close to the survey boundaries, indicating that 
most or all of the sardine stock was sampled. In 
contrast to spring 2008, in 2010, the potential 
sardine habitat was located farther north and 
closer to the shore, mainly off San Francisco and 
Monterey Bay. A single stratum was used for the 
biomass estimation. 
Jack mackerel were common in the trawl 
catches during each of the surveys (Fig. 1), ac- 
counting for the second largest species propor- 
tion, and the third largest catches (Table 1). Jack 
mackerel were the largest of the CPS, some more 
than 50 cm FL (Table 2). The estimated distri- 
butions of jack mackerel were normally more re- 
250 - 
2006 
200 - 
'°' "Q o- -o -o -a 
150 - 
p. ' o- o- u -o 
* 0 
0 
100 - 
n ^ 
50 - 
0 - 
250 - 
2008 
oT 200 - 
I 
CM 150 - 
e '°* ©- -o. 
E 
03 100 - 
■O -o -0 ®- o- -O -°- -O -o -8- ©- o_ ^ 
50 - 
®— — ® a-"'"® -® ® 0 ® 
0 - 
250 - 
2010 
200 - 
150 - 
100 - 
50 - 
ft* ^ 9 
O' ° 
0 -e a s a 0 — — « 0 © 
0 - 
1 l 1 1 1 1 — 
40 80 120 160 200 240 
Window size (nmi) 
Figure 2 
Nautical area scattering coefficient (s A ; m 2 nmi -2 ) values, sam- 
pled during daytime, attributed to coastal pelagic fish species 
(CPS) in the vicinity of night-time CPS catches, for sampling 
windows with increasing sizes (dashed line). The solid line 
represents the mean s A attributed to CPS for any set of points 
randomly selected from the survey track with size equal to that 
of the number of catches with CPS. The shaded area represents 
the 95% confidence interval of the mean s A attributed to CPS 
calculated through resampling. The s A of CPS in the vicinity 
of CPS catches is significantly higher than the average, except 
for 2006, in which CPS were ubiquitous in the survey area. 
The average s A for the entire survey area decreased by 70% 
in the study period. 
