379 
Application of an acoustic-trawl survey design 
to improve estimates of rockfish biomass 
Email address for contact author: dana.hanselman@noaa.gov 
1 Marine Ecology and Stock Assessment Program 
Auke Bay Laboratories 
Ted Stevens Marine Research Institute 
Alaska Fisheries Science Center, NMFS, NOAA 
17109 Pt. Lena Loop Road 
Juneau, Alaska 99801-8626 
2 Resource Ecology and Fisheries Management Division 
Alaska Fisheries Science Center, NMFS, NOAA 
7600 Sand Point Way, NE 
Seattle, Washington 98115-6349 
3 Resource Assessment and Conservation Engineering Division 
Alaska Fisheries Science Center, NMFS, NOAA 
7600 Sand Point Way, NE 
Seattle, Washington 98115-6349 
Abstract — Biomass estimates of sev- 
eral species of Alaskan rockfishes 
exhibit large interannual variations. 
Because rockfishes are long lived 
and relatively slow growing, large, 
short-term shifts in population abun- 
dance are not likely. We attribute the 
variations in biomass estimates to 
the high variability in the spatial 
distribution of rockfishes that is 
not well accounted for by the survey 
design currently used. We evaluated 
the performance of an experimental 
survey design, the Trawl and Acoustic 
Presence/Absence Survey (TAPAS), to 
reduce the variability in estimated 
biomass for Pacific ocean perch 
( Sebastes alutus ). Analysis of archived 
acoustic backscatter data produced 
an acoustic threshold for delineating 
potential areas of high (“patch”) and 
low (“background”) catch per unit of 
effort (CPUE) in real time. In 2009, 
we conducted a 12-day TAPAS near 
Yakutat, Alaska. We completed 59 
trawls at 19 patch stations and 40 
background stations. The design per- 
formed well logistically, and Pacific 
ocean perch (POP) accounted for 
55% of the 31 metric tons (t) of the 
catch from this survey. The resulting 
estimates of rockfish biomass were 
slightly less precise than estimates 
from simple random sampling. This 
difference in precision was due to the 
weak relationship of CPUE to mean 
volume backscattering and the rela- 
tively low variability of POP CPUE 
encountered. When the data were 
re-analyzed with a higher acoustic 
threshold than the one used in the 
field study, performance was slightly 
better with this revised design than 
with the original field design. The 
TAPAS design could be made more 
effective by establishing a stronger 
link between acoustic backscatter and 
CPUE and by deriving an acoustic 
threshold that allows better identi- 
fication of backscatter as that from 
the target species. 
Manuscript submitted 15 September 2011. 
Manuscript accepted 11 July 2012. 
Fish. Bull. 110:379-396 (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. 
Dana H. Hanselman (contact author ) 1 
Paul D. Spencer 2 
Denise R. McKelvey 3 
Michael H. Martin 3 
Typically, surveys of resource biomass 
are designed around simple random 
sampling (SRS), stratified simple 
random sampling (SSRS), or system- 
atic sampling (Thompson, 2002). One 
of these standard designs will perform 
adequately when the resource is rela- 
tively uniformly distributed or when 
the areas where variability in biomass 
is highest are static and well known. 
In practice, many resources, such as 
fish populations, exhibit highly vari- 
able and complex spatial structure, 
and standard survey methods lead 
to extremely imprecise estimates of 
biomass (Hanselman and Quinn, 
2004). Novel sampling designs have 
been developed to improve abundance 
estimation under these circumstances. 
One example is adaptive cluster 
sampling (ACS; Thompson, 1990; 
Thompson and Seber, 1996), which 
has been explored both in the field 
(e.g., Lo et al., 1997; Woodby, 1998; 
Conners and Schwager, 2002; Han- 
selman et ah, 2003) and in simula- 
tion studies (Christman, 1997; Brown, 
1999; Christman and Pontius, 2000; 
Christman and Lan, 2001; Brown, 
2003; Su and Quinn, 2003). Other 
methods have been used: double sam- 
pling, ratio, and regression estima- 
tor approaches to improve precision 
(Eberhardt and Simmons, 1987; Han- 
selman and Quinn, 2004; Fujioka et 
al., 2007). These approaches improve 
precision by relating a variable that 
is expensive or difficult to collect (e.g., 
trawl catches) to a correlated auxil- 
iary variable of which many samples 
can be collected quickly or inexpen- 
sively (e.g., acoustic data). 
A resource for which standard sur- 
vey methods have proven inadequate, 
Alaskan rockfishes ( Sebastes spp.) 
are abundant and supported a valu- 
able commercial trawl fishery with 
an average exvessel value of US$ 15 
million between 2008 and 2010. Sur- 
vey estimates of biomass for many 
Alaskan rockfish species exhibit large 
interannual variations that are not 
consistent with the longevity (>80 
years) and relatively low productiv- 
ity of these species (Hanselman et 
al., 2003; Fig. 1). One of the causes 
of imprecision in survey estimates 
of biomass is the high variability in 
the spatial distributions of rockfish 
populations. For example, the biomass 
estimate of Pacific ocean perch ( Se- 
bastes alutus) from the survey con- 
