68 
Abstract — Rockfishes ( Sebastes 
spp.) tend to aggregate near rocky, 
cobble, or generally rugged areas 
that are difficult to survey with 
bottom trawls, and evidence indi- 
cates that assemblages of rockfish 
species may differ between areas 
accessible to trawling and those ar- 
eas that are not. Consequently, it 
is important to determine grounds 
that are trawlable or untrawlable 
so that the areas where trawl sur- 
vey results should be applied are ac- 
curately identified. To this end, we 
used multibeam echosounder data 
to generate metrics that describe 
the seafloor: backscatter strength at 
normal and oblique incidence angles, 
the variation of the angle-dependent 
backscatter strength within 10° of 
normal incidence, the scintillation of 
the acoustic intensity scattered from 
the seafloor, and the seafloor rugos- 
ity. We used these metrics to develop 
a binary classification scheme to 
estimate where the seafloor is ex- 
pected to be trawlable. The multi- 
beam echosounder data were verified 
through analyses of video and still 
images collected with a stereo drop 
camera and a remotely operated ve- 
hicle in a study at Snakehead Bank, 
-100 km south of Kodiak Island in 
the Gulf of Alaska. Comparisons of 
different combinations of metrics 
derived from the multibeam data 
indicated that the oblique-incidence 
backscatter strength was the most 
accurate estimator of trawlability at 
Snakehead Bank and that the addi- 
tion of other metrics provided only 
marginal improvements. If success- 
ful on a wider scale in the Gulf of 
Alaska, this acoustic remote-sensing 
technique, or a similar one, could 
help improve the accuracy of rock- 
fish stock assessments. 
Manuscript accepted 21 November 2012. 
Fish. Bull. 111:68-77 (2013). 
doi:10.7755/FB. 11 1.1.6 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessar- 
ily reflect the position of the National 
Marine Fisheries Service, NOAA. 
Seabed classification for trawlability determined 
with a multibeam echo sounder on Snakehead 
Bank in the Gulf of Alaska 
Thomas C. Weber (contact author ) 1 
Christopher Rooper 2 
John Butler 3 
Darin Jones 2 
Chris Wilson 2 
Email address for contact author weber@ccom.unh.edu 
1 Center for Coastal and Ocean Mapping 
University of New Hampshire 
24 Colovos Road 
Durham, New Hampshire 03824 
2 Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
3 Southwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
8604 La Jolla Shores Drive 
La Jolla, California 92037 
Rockfish ( Sebastes spp.) stocks are 
difficult to assess because of their 
propensity to aggregate near the 
seafloor in areas that are difficult to 
trawl, such as rocky, cobble, or gener- 
ally rugged areas. Consequently, data 
from bottom-trawl surveys conducted 
in trawlable areas typically are ex- 
trapolated to all areas within the 
boundaries of a survey, regardless of 
whether the seafloor is trawlable or 
not (Wakabayashi et ah, 1985). Such 
extrapolation may result in biased 
biomass indices if, for example, there 
is a shift in biomass between strata 
with variable but unknown amounts 
of untrawlable seafloor (Cordue, 
2006). Evidence also indicates that 
species assemblages differ between 
trawlable and untrawlable areas 
(Matthews and Richards, 1991; Ja- 
gielo et al., 2003; Rooper et al., 2010), 
and remote-sensing techniques with 
acoustic or optical sensors may be 
able to help identify these differ- 
ences. Equally important is the need 
to have a quantitative assessment of 
those grounds that are trawlable or 
untrawlable to more accurately esti- 
mate the areas where the results of 
different stock assessment methods 
are valid. 
In many bottom-trawl surveys, 
trawlability has been assessed 
through the subjective interpreta- 
tion of normal-incidence backscatter 
(echoes) from downward-looking sin- 
gle-beam echo sounders. These back- 
scatter echoes are examined by vessel 
captains with different levels of ex- 
perience, with different echo sound- 
ers, and with different echosounder 
settings. Multibeam echo sounders 
(MBES), which have been successful 
previously for characterizion of the 
seafloor for the purposes of mapping 
habitat and surficial geology (e.g., 
Kostylev et al., 2001; Goff et ah, 
2004; Brown and Blondel, 2009), may 
offer an alternative solution for as- 
sessment of trawlability. In addition 
to the wider, high-precision coverage 
of the seafloor that results from the 
use of multiple beams, MBES offer 
the potential for more accurate dis- 
crimination between different types 
of seafloor substrate (e.g., silt, sand, 
cobble, and rock) than does the use 
of downward-looking single beams 
because of the angle-dependent na- 
