76 
Fishery Bulletin 111(1) 
threshold of 1.2 would result in a high classification 
error rate). 
Rugosity derived from the data collected with the 
Simrad ME70 was a poor discriminator of trawlable 
versus untrawlable seafloor, generally with lower val- 
ues (e.g., smoother seafloor) in areas where the valida- 
tion data from the SDC and ROV surveys indicate that 
the seafloor is untrawlable. The areas that contained 
high values of rugosity generally were dominated by 
larger scale features: the ridgeline on the northern 
edge of the bank, the sand waves in the southeast, or 
the pockmarks in the southwest. It is likely that the 
spatial resolution of the MBES was insufficient to pro- 
vide a useful estimate of the rugosity level and that an 
MBES with higher frequencies and higher resolution 
might provide more useful results. 
The oblique-incidence S b alone provided a low er- 
ror rate as a discriminator between trawlable and un- 
trawlable seafloor. When combined with the other met- 
rics, it was possible to slightly lower the error rate, 
but an examination of the scatter plots in Figure 5 in- 
dicates that the error rates were not been lowered in 
any meaningful way. For example, the best-fit line that 
discriminates between the combination of oblique-inci- 
dence S b and normal-incidence S b shows that a com- 
bination of high oblique-incidence S b and low normal- 
incidence S h gives a better indication of untrawlable 
seafloor than high oblique-incidence S b on its own. 
This finding is contrary to what the modeled seafloor 
return (Fig. 2) would predict: high oblique-incidence S b 
and high normal-incidence S b are a better predictor of 
an untrawlable seafloor. Therefore, it is likely that the 
marginal improvement in classification error rate with 
these extra parameters combined is simply a result of 
variations in the tails of the underlying data distribu- 
tions. With only marginal improvements (5. 6-3. 8%) in 
classification error rate when up to 4 parameters are 
combined, with a hyperplane separating the 2 classes, 
it is reasonable to choose the simpler approach of using 
only the oblique-incidence S b as a predictor of traw- 
lable or untrawlable seafloor. 
Conclusions 
The results described here indicate that acoustic re- 
mote sensing of substrate type with an MBES, and 
oblique-incidence acoustic S b in particular, offer useful 
insight into whether the seafloor is untrawlable. This 
conclusion is in qualitative agreement with the work 
of Jagielo et al. (2003), who used seafloor backscatter 
collected with a sidescan sonar as part of an a priori 
assessment of trawlability (note that much of the sid- 
escan record was collected at oblique incidence angles). 
Whether these types of acoustic metrics can provide a 
similar level of confidence regarding the distribution 
of untrawlable seafloor in areas throughout the entire 
Gulf of Alaska needs to be determined. If successful on 
a wider scale, this type of acoustic remote sensing can 
help refine the interpretation of bottom-trawl surveys. 
In particular, techniques such as those described here 
could increase the accuracy in identification of areas 
with seafloor characteristics similar to areas where 
bottom-trawl surveys of rockfish were conducted (i.e., 
areas where results frojm the trawl surveys can be ap- 
plied). As a result, the precision and accuracy of bio- 
mass estimates from bottom-trawl surveys and their 
resultant stock assessments would be improved. 
Acknowledgments 
Support for this work was provided by the North Pa- 
cific Research Board (contribution no. 373). Additional 
support for T. Weber was provided by NOAA (grant 
NA05N0S4001153). We would like to acknowledge the 
crews of the NOAA Ship Oscar Dyson and FV Epic Ex- 
plorer for their help during data collection. We would 
also like to thank M. Martin, D. Somerton, and W. Pal- 
sson for their thoughtful reviews of this manuscript. 
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