340 
Fishery Bulletin 96(2), 1 998 
Discrepancies exist between the NMFS trawl esti- 
mates of hake size and our acoustic estimates at sta- 
tions in the south of the study area. Only the larger 
size class of hake was detected acoustically whereas 
both large and small size classes of hake were caught 
in the trawls. Several explanations are suggested. 
One possibility is that large hake mask the scat- 
tering from small hake. However, if ratios of small 
to large hake are anywhere close to those seen in the 
Sebastian and Mendocino trawls (Fig. 2), our inverse 
solution of the acoustic data would easily have de- 
tected both size classes. 
Another possibility is that a physiological phenom- 
enon caused the observed absence of a north-south 
trend in swimbladder size even though the southern 
fish were smaller. Such a phenomenon would have 
to cause the swimbladder-volume-to-body-volume 
ratio to increase in smaller hake. Differences in stom- 
ach fullness could cause such a trend. Food evacua- 
tion times are different for different food types: large 
hake eat fish that are evacuated slowly, small hake 
eat crustaceans that are evacuated quickly (Durbin 
et al., 1980, in Livingston, 1983) In addition, hake 
swimbladders could regress slightly in the larger fish. 
Although these hypotheses are possible, they would 
need to result in a complete cancellation of the north- 
south trend in fish size. This result is unlikely. 
