National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
é established in 1881 «<< 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Abstract—Many rockfishes (Sebastes 
spp.) inhabit rugged areas of seafloor 
that are inaccessible to survey trawl 
gear. Their utilization of such habitat 
makes estimation of their abundance 
difficult. Furthermore, it is often diffi- 
cult to assess whether habitat is traw- 
lable or untrawlable and to estimate the 
spatial extent of both habitat types. To 
help determine trawlability for the con- 
tinental shelf in the Gulf of Alaska, we 
used multibeam sonar data collected in 
the area during 2011, 2013, and 2015. 
These data were used to derive 3 char- 
acteristics of the seafloor: oblique inci- 
dence backscatter strength (S,, oblique), 
seafloor ruggedness, and bathymetric 
position index. Habitat type was cat- 
egorized as trawlable or untrawlable 
through analysis of video from deployed 
drift cameras. We tested the effective- 
ness of the use of these seafloor char- 
acteristics in prediction of habitat 
trawlability with 4 types of models: 
generalized linear model, generalized 
additive model, boosted regression 
tree, and random forest. All 4 models 
perform moderately well at predicting 
trawlability across the shelf, and results 
from all of them indicate that S, oblique 
is the most important characteristic 
in discriminating between trawlable 
and untrawlable habitat. These results 
indicate that multibeam sonar data 
can help determine habitat type, infor- 
mation that in turn can help improve 
habitat-specific estimates of biomass of 
marine fish species. 
Manuscript submitted 11 September 2020. 
Manuscript accepted 18 August 2021. 
Fish. Bull. 119:184-196 (2021). 
Online publication date: 13 September 2021. 
doi: 10.7755/FB.119.2-3.7 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Comparison of model types for prediction 
of seafloor trawlability in the Gulf of Alaska 
by using multibeam sonar data 
Sarah C. Stienessen (contact author)! 
Christopher N. Rooper” 
Thomas C. Weber® 
Darin T. Jones' 
Jodi L. Pirtle* 
Christopher D. Wilson' 
Email address for contact author: sarah.stienessen@noaa.gov 
" Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
2 Stock Assessment and Research Division 
Pacific Biological Station 
Fisheries and Oceans Canada 
3190 Hammond Bay Road 
Nanaimo, V9T 6N7 Canada 
The bottom-trawl surveys conducted 
by the Alaska Fisheries Science Center 
(AFSC) provide estimates of biomass 
of numerous rockfishes (Sebastes spp.) 
in the Gulf of Alaska (GOA) (Aydin 
et al., 2019). An area-swept method is 
used for the bottom-trawl surveys con- 
ducted over trawlable areas within the 
GOA (von Szalay and Raring, 2018). 
Results (e.g., biomass and numbers of 
various fish species) obtained from sur- 
veys in trawlable areas are expanded 
to the entire GOA, including untraw- 
lable areas (i.e., those locations where 
the structure of the seafloor prevents 
survey gear from being properly or 
successfully deployed according to sur- 
vey protocols). The biomass estimates 
from bottom-trawl surveys are essen- 
tial input for stock assessment efforts 
and data-based fisheries manage- 
ment. However, many rockfish species 
prefer rocky and rugged habitat that 
is often inaccessible to bottom-trawl 
gear; in other words, many rockfish 
3 Center for Coastal and Ocean Mapping 
University of New Hampshire 
24 Colovos Road 
Durham, New Hampshire 03824 
* Alaska Regional Office 
National Marine Fisheries Service, NOAA 
P.O. Box 21668 
Juneau, Alaska 99802 
species reside at higher densities in 
untrawlable habitats (Jagielo et al., 
2003). Acoustic backscatter attributed 
to individual rockfish in untrawlable 
areas is estimated to be 3 times that of 
backscatter attributed to individuals in 
trawlable areas (Jones et al., 2021). 
By not sampling the preferred 
(untrawlable) habitat of some rockfish 
species, the biomass of these species 
in the GOA can be underestimated. 
Similarly, by sampling only the pre- 
ferred (trawlable) habitat for other 
rockfish species, the biomass of these 
species can be overestimated. Survey 
efforts targeting only one of the hab- 
itat types occupied by a species can 
add non-random error to biomass esti- 
mates (Cordue, 2007). Consequently, 
reliable assessments of rockfish spe- 
cies are needed for both trawlable 
and untrawlable habitat types in the 
surveyed area. Acoustic methods, with 
complementary direct sampling tools, 
are being used to assess semi-pelagic 
