118 
Fishery Bulletin 110(1) 
Pacific mackerei 
126 
120 
126 
120 
Longitude (°W) 
Figure 5 
Pacific mackerel (Scomber japonicus) biomass densities estimated by combining information from the acoustic and trawl 
samples. The northern and southern boundaries of the strata are shown (dashed lines). 
of their stock biomasses were high, but decreasing, with 
relatively low CV values (9% to 43%). Their distribu- 
tions were in close agreement with the predictions of 
their potential habitat which, during spring, tends to 
be located off central and southern California (Zwolin- 
ski et al., 2011; Demer et al., 2012). Because the large 
majority of the sardine habitat was encompassed by the 
survey boundaries, sampling bias appears to be negli- 
gible. Acoustic estimates of sardine abundance compare 
well with those of the 2010 assessment but were higher 
than those of the DEPM (Fig. 6). Although these higher 
estimates may be attributed, in part, to the fact that 
the DEPM provides estimates only of the SSB and the 
acoustic-trawl surveys allow estimates of total sardine 
biomass, the discrepancy requires further analysis. 
Irrespective of the actual sardine biomass, all three 
time-series share a common, steadily decreasing trend 
since 2006. The rate of the decay of the sardine popu- 
lation (i.e., the net mortality rate) estimated at -0.56 
year -1 (Fig. 6) is in close agreement to the summation 
of the natural mortality rate (-0.4 year -1 ; corresponding 
to 33% of the population dying naturally each year) and 
the fishing mortality rate (-0.26 year -1 ) estimated for 
2010 (Hill et al., 2010). 
The acoustic-trawl estimates of sardine demography 
differ from those of the assessment by the lack of age-0 
and age-1 sardine. The latter were rarely caught in the 
survey trawls and consequently were not represented 
in the weighted-length distributions. This discrepancy 
may be related to the very patchy coastal distribution 
of the younger sardine that are mainly exploited in the 
California and Ensenada fisheries. Consequently, these 
fish may be over-represented in the landing statistics, 
which do not provide a biomass-density-weighted dis- 
tribution, and are under-sampled in the acoustic-trawl 
surveys. However, because there is no evidence of small 
fish growing into the larger size classes during the 2006 
-10 period, it is more likely that the assessment model 
is confounded by a variable number of small sardine 
belonging to the “southern stock” (Felix-Uraga et ah, 
2005). Like the northern stock, the southern stock also 
migrates seasonally, and enters the Ensenada and oc- 
casionally the southern California fisheries during the 
summer (Clark and Janssen Jr., 1945; Felix-Uraga et 
ah, 2005). 
In the past, when the northern stock of sardine 
declined, jack mackerel increased rapidly in the CCE 
and were targeted by the purse-seine fishery (Smith 
and Moser, 2003; Mason, 2004). Currently, jack mack- 
erel is a monitored species and its stock biomass is 
largely unknown. Thus, these acoustic-trawl survey 
results comprise the most comprehensive informa- 
tion on their distribution and abundance in the CCE 
and may be useful for managing future exploitation 
