Weber and McClatchie: Effect of environmental conditions on the distribution of Scomber japonicus 
91 
temperature offset by reduced 
geostrophic flow and zooplank- 
ton abundance. 
The two temporal block- 
ing variables affected model 
predictions but were either 
unrelated to interannual dif- 
ferences in physical habitat 
(previous stock size), or only 
partly related (day of year). 
After the model controlled for 
stock size and time of sam- 
pling (Figs. 6 and 5H), model 
predictions indicated that the 
core area contained the best 
habitat available nearly as fre- 
quently as the southern por- 
tion of Mexican waters; the 
core area had the greatest 
mean predicted probability of 
capture in 9 of 21 years. The 
area of Mexican waters north 
of CalCOFI line 95 tended to 
be intermediate in terms of 
probability of capture, between 
the core area and the south- 
ern area near Punta Eugenia. 
The controlled probabilities of 
capture exhibited greater vari- 
ability between the core area 
and Mexican waters among 
years, indicating the two areas 
may exhibit greater differences 
in habitat-suitability trends 
than were apparent from un- 
controlled model predictions. 
Corrected predictions also in- 
dicated that habitat quality for 
larval Pacific mackerel in the 
core area generally was better 
from the 1980s to 2008 than in 
the early years of the CalCO- 
FI survey, probably because of 
more appropriate temperatures 
and geostrophic flows (Fig. 5, 
A and C). The difference was 
somewhat masked in the un- 
corrected predictions because 
stock size was a dominant pre- 
dictor, and sampling did not 
necessarily occur during the 
seasonal period when Pacific 
mackerel larvae were most 
likely to be captured (Fig. 5D). 
Discussion 
The model indicated that dis- 
tributions of Pacific mackerel 
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