Rooper et al Estimating species and size composition of rockfishes to verify targets in acoustic surveys 
329 
optical gear types and therefore they were either 
unavailable to or not retained by the bottom trawl; 
the smallest rockfish (<150 mm) could probably 
escape more readily through the bottom trawl. 
Escapement of this kind has been observed in the 
Gulf of Alaska and Aleutian Islands bottom trawl 
surveys where the smallest (<100 mm) rockfish 
are often not captured (von Szalay et a!., 2010, 
2011). Because juvenile fish and smaller species, 
such as harlequin rockfish were observed primari- 
ly in untrawlable areas, it is also possible that the 
smallest fish seek out shelter among the rocks and 
are not available to the bottom trawl. The accu- 
racy of the fish length measurements also differed 
by gear type. The fish captured in the bottom 
trawl are generally assumed to be measured with 
only a minimal amount of error because each mea- 
sured fish is individually handled, measured, and 
recorded. However, fish caught by bottom trawl 
are only measured to the nearest cm. The error 
rates for the SDC in measuring the size of known 
targets have previously been estimated to be less 
than 8.2%, or less than 2.5 cm for a 30-cm fish 
(Williams et al., 2010). Other stereo video systems 
have generally produced smaller error rates <1% 
of length (Harvey et al., 2002, 2003; Shortis et 
al., 2009). The higher error rates for the SDC are 
probably due to the need to remove cameras from 
the housing unit after each deployment, which 
possibly causes a slight misalignment of the cam- 
eras in relation to the position at calibration re- 
ducing the precision of measurements (Williams et 
al., 2010). The accuracy of length measurements 
from the ROV parallel laser measuring system 
was not determined; however, previous research 
with parallel laser systems have indicated length 
measurements are accurate to 1-5% of the to- 
tal length of a rigid object (Rochet et al., 2006). 
Because fish lengths are translated directly into 
target strength estimates for acoustic biomass 
estimation, errors and biases in fish length from 
the target verification tools are important to de- 
termine so that the effect on total fish biomass 
can be known. 
Thus, the results of this study indicate that the 
method chosen for target verification in acoustic 
assessments depends on the fish species to be 
assessed, their size, and the substrate type to be 
examined. Advantages of the bottom trawl over 
the optical methods are that it allows identifica- 
tion and measurements of all the rockfish species 
collected. Specimens collected with the bottom 
trawl also provide auxiliary information important 
to stock assessment, such as diet, age, and stage 
of maturity. The advantage of the optical methods 
is that they provide data for discriminating spe- 
cies assemblages in untrawlable areas or areas 
with potentially vulnerable habitats such as deepwater 
corals and sponges that could be damaged by further 
trawling (Heifetz et al., 2009). Habitat-specific densities 
Composition of rockfish (based on percentages from stereo 
drop camera estimates of height distribution from each spe- 
cies) at stations by height off the seafloor in the categories 
>1 m off the seafloor and <1 m off the seafloor. Stations 
were surveyed by remotely operated vehicle, bottom trawl, 
and stereo drop camera. Some sites have been slightly offset 
to show species composition charts. Solid line indicates the 
extent of the acoustic transects, the shaded area shows the 
area that was considered predominantly untrawlable in the 
analysis of acoustics (Weber et al. 2 ). Dusky rockfish ( Sebastes 
variabilis ), northern rockfish (S. polyspinis ), harlequin rockfish 
(S. variegatus), and Pacific ocean perch (S. alutus) are shown 
individually. The other rockfish species group comprises yei- 
loweye rockfish (Sebastes ruberrimus), redstripe rockfish (S. 
proriger), redbanded rockfish (S. babcocki), dark rockfish (S. 
ciliatus), tiger rockfish (S. nigrocinctus ), pygmy rockfish (S. 
wilsoni), silvergrey rockfish ( S . brevispinis ), and rosethorn 
rockfish (S. helvomaculatus). 
and associations can also be collected by video meth- 
ods — factors that are masked by the bottom trawl that 
integrates the catch over a large and unobserved area 
