Demer et al.: Seasonal migration of Sardinops sagax in the California Current Ecosystem 
61 
Table 3 
Estimates of biomass in million metric tons (Mt) of Pacific sardine ( Sardinops sagax), Jack mackerel ( Trachurus symmetricus), 
and Pacific mackerel (Scomber japonicus) in the California Current Ecosystem during the 2008 spring and summer surveys, 
their coefficients of variation (CV) values (%), and 95% confidence interval ( CI 95 ) values (Mt). Catches of other coastal pelagic 
fish species were too few to enable estimations of their biomasses. The total biomass values are apportioned to strata with 
areas (nmi 2 ) defined in Figure 3. For the summer survey, the coastal stratum accounts for extrapolated sardine biomass in the 
unsampled nearshore region. Where the trawl CV is larger than the total CV, there were too few trawls to classify species and 
estimate target strength independently between transects. In those cases, the total CV values probably under-estimate the total 
random sampling error. 
Season 
Stratum 
Area (nmi 2 ) 
Trawls 
Biomass (Mt) 
Trawl CV (%) 
Total CV(%) 
0 
1 
Pacific sardine (Sardinops sagax) 
Spring 
1 
11,469 
7 
0.047 
59.2 
45.8 
0.017-0.104 
2 
22,771 
7 
0.704 
33.2 
9.3 
0.579-0.823 
sum 
34,240 
14 
0.751 
31.4 
9.2 
0.611-0.870 
Summer 
coast 
2848 
0 
0.169 
29.7 
43.6 
0.042-0.325 
1 
25,971 
29 
0.573 
22.7 
31.3 
0.266-0.952 
2 
45,477 
13 
0.059 
82.0 
81.9 
0.003-0.165 
sum 
74,296 
42 
0.801 
18.9 
30.9 
0.371-1.299 
Jack mackerel (Trachurus symmetricus) 
Spring 
i 
19,524 
8 
0.078 
30.5 
32.1 
0.032-0.129 
2 
22,771 
7 
0.069 
139.6 
47.5 
0.019-0.140 
sum 
42,295 
15 
0.147 
67.6 
28.4 
0.075-0.232 
Summer 
i 
9823 
6 
0.027 
83.4 
31.9 
0.014-0.037 
2 
78,532 
26 
0.421 
13.9 
36.0 
0.136-0.724 
sum 
88,355 
32 
0.448 
14.0 
33.9 
0.159-0.749 
Pacific mackerel (Scomber japonicus) 
Spring 
i 
19,524 
8 
0.014 
88.7 
51.8 
0.005-0.037 
Summer 
i 
49,453 
22 
0.055 
129.1 
38.3 
0.019-0.103 
Because the estimated sardine biomasses from the 
two surveys were not significantly different (Table 3), 
most or all of the stock appears to have migrated from 
the south and offshore in the spring to the north and 
inshore in the summer (Fig. 3). Of course, there is the 
possibility that nearly identical proportions of the stock 
could have been missed during the two surveys, e.g., 
not sampled off Mexico during the spring survey, and 
off Canada during the summer survey, or they resided 
too deep or offshore or both during spring and too shal- 
low or inshore or both during summer. 
The distributions of jack and Pacific mackerel also 
suggest seasonal migrations toward the north during 
the summer. However, relative to sardine, jack mack- 
erel were located closer to shore in the spring and 
farther offshore in the summer (Fig. 3). Also, landings 
data (Crone et al., 2009) provide evidence that Pacific 
mackerel were distributed well beyond the sampling 
area, and most of their biomass was located to the 
south. Jack mackerel is not the target of a fishery; 
therefore no assessment or other form of abundance 
estimation is available for comparison. 
The biomass densities of each evaluated species (Ta- 
ble 4) were not statistically correlated for any relevant 
lag (intertransect distance) in any of the defined strata, 
thereby enabling the bootstrap procedure to estimate 
the random sampling error. Thus, the CV values reflect 
the combined random sampling error of the echosounder 
and trawl sampling. For sardine and jack mackerel, 
these values may be sufficiently small to allow the re- 
sults to be used in assessments, and their populations 
were apparently sampled in entirety. The much larger 
CV values for Pacific mackerel were the result of a 
lower biomass, patchy distribution, and too few trawl 
catches (Table 3). 
The length distributions of sardine sampled during 
the 2008 spring (mean SL=21.2; standard deviation 
[SD]=1.5 cm) and summer (mean SL=20.5; SD=2.2 cm) 
had very similar modes which matched the major TL 
mode (Table 2, Fig. 4) in the assessment model (Hill et 
al., 2010). The major mode in the SL values was slightly 
larger in the summer versus spring survey data, as 
might be expected from growth. The secondary mode 
in the summer survey data is the result of one catch 
of small sardine off Monterey Bay. Because additional 
trawls were not conducted off central and southern 
California due to problems with the RV David Starr 
Jordan, the small acoustically-mapped CPS biomass 
in the central and southern California regions (Fig. 2) 
were ascribed to these sardine (Fig. 3). 
