317 
Estimating species and size composition 
of rockfishes to verify targets 
in acoustic surveys of untrawlable areas 
Email address for contact author: Chns.Rooper@noaa.gov 
' Alaska Fisheries Science Center 
National Marine Fisheries Service 
National Oceanic and Atmospheric Administration 
7600 Sand Point Way NE 
Seattle, Washington 981 1 5 
2 Southwest Fisheries Science Center 
National Marine Fisheries Service 
National Oceanic and Atmospheric Administration 
8604 La Jolla Shores Dr 
La Jolla, California 92037 
Abstract — Rockfish (Sebastes spp.) 
biomass is difficult to assess with 
standard bottom trawl or acoustic 
surveys because of their propensity 
to aggregate near the seafloor in high- 
relief areas that are inaccessible to 
sampling by trawling. We compared 
the ability of a remotely operated vehi- 
cle (ROV), a modified bottom trawl, 
and a stereo drop camera system 
(SDC) to identify rockfish species 
and estimate their size composition. 
The ability to discriminate species 
was highest for the bottom trawl and 
lowest for the SDC. Mean lengths and 
size distributions varied among the 
gear types, although a larger number 
of length measurements could be 
collected with the bottom trawl and 
SDC than with the ROV. Dusky (S. 
uanabilis), harlequin (S. variegatus), 
and northern rockfish (S. polyspinis ), 
and Pacific ocean perch (S. alutus) 
were the species observed in greatest 
abundance. Only dusky and north- 
ern rockfish regularly occurred in 
trawlable areas, whereas these two 
species and many more occurred in 
untrawlable areas. The SDC was able 
to resolve the height of fish off the 
seafloor, and some of the rockfish 
species were observed only near the 
seafloor in the acoustic dead zone. 
This finding is important, in that 
fish found exclusively in the acoustic 
dead zone cannot be assessed acous- 
tically. For these species, methods 
such as bottom trawls, long-lines, or 
optical surveys using line transect 
or area swept methods will be the 
only adequate means to estimate the 
abundance of these fishes. Our results 
suggest that the selection of appro- 
priate methods for verifying targets 
will depend on the habitat types and 
species complexes to be examined. 
Manuscript submitted 15 June 2011. 
Manuscript accepted 17 February 2012. 
Fish. Bull. 110:317-331 (2012). 
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. 
Christopher N. Rooper (contact author)' 
Michael H. Martin' 
John L. Butler 2 
Darin T. Jones' 
Mark Zimmermann' 
Rockfishes (Sebastes spp.) are a 
group of species with a predilection 
for high-relief, rocky habitats, where 
biomass estimation from traditional 
bottom-trawl survey data is difficult 
or impossible. However, many of these 
rockfishes also occur semipelagieally, 
so that acoustic biomass assessments 
are possible (Wilkins, 1986; Demer et 
al., 2009; Ressler et ah, 2009; Rooper 
et ah, 2010). Acoustically estimat- 
ing fish abundance requires accurate 
target verification of species composi- 
tion and size distribution — verification 
that is typically achieved with midwa- 
ter or bottom trawls. Because bottom 
trawling is hampered in high-relief 
areas, so too are acoustic abundance 
estimates from these habitats, owing 
to inadequate information describing 
species-specific abundance and size 
composition for fishes on or near the 
seafloor. Therefore, habitat-specific 
rockfish distribution patterns have the 
potential to affect the accuracy and 
precision of survey biomass estimates 
when traditional bottom trawl or com- 
bination acoustic-bottom trawl survey 
methods are used (Cordue, 2006). 
Evidence suggests that untrawlable 
areas can support different species 
assemblages than those found in 
trawlable areas (Matthews and Rich- 
ards, 1991; Jagielo et ah, 2003; Zim- 
mermann, 2003). Untrawlable areas 
can also have different size classes 
or abundances of the same species 
(Matthews, 1989; Stein et ah, 1992; 
O’Connell and Carlile, 1993; Rooper 
et ah, 2007). The primary species 
thought to inhabit untrawlable ar- 
eas in high abundance in Alaska are 
northern rockfish ( Sebastes polyspi- 
nis ), dusky rockfish (S. variabilis), 
juvenile Pacific ocean perch (S. alu- 
tus), and black rockfish (S. melanops ; 
Clausen and Heifetz, 2002; Rooper et 
ah, 2007). Additionally, some rock- 
fish species that occur in Alaska are 
rarely encountered in bottom trawl 
surveys (e.g., tiger rockfish [S. ni- 
grocinctus]), possibly because of their 
preference for rough, rocky, and 
therefore untrawlable habitat. For 
these reasons, there is a clear need 
for alternative assessment methods 
to accurately and precisely estimate 
rockfish distribution and abundance 
over untrawlable areas, so that, in 
conjunction with similar estimates 
from trawlable areas, rockfish stock 
assessments can be improved. 
Critical for an accurate acoustic as- 
sessment of rockfishes is determin- 
ing the vertical distribution of species 
and sizes and their relation to the 
