382 
Fishery Bulletin 1 10(4) 
bottom trawl surveys, but this difference accounts for 
POP swimming above a trawl net that may dive down 
into the net path in response to the pressure wave of 
the trawl. This potential for “herding” may increase 
the effective height of the net. In addition, Aglen (1996) 
found that the correlation between catch and acoustic 
backscatter off the seafloor was greatest for Atlantic 
species of Sebastes and suggested that a taller acoustic 
layer should be more robust for identification of areas 
of intense backscatter. The actual size of the acoustic 
layer, however, does not contribute directly to biomass 
estimates, which are based on CPUE data from trawls. 
Patch definition was determined with the use of 2 
metrics: 1) the value of mean volume backscattering, 
S v (log decibels re 1 m -1 . MacLennan et al., 2002) that 
defines high acoustic intensity (S v threshold) and 2) 
the proportion of cells where the S v threshold was ex- 
ceeded. A proportion criterion was used to smooth the 
S v values across cells to avoid defining small areas with 
high acoustic backscatter as discrete patches. Analy- 
sis of archived data indicated that a proportion was 
preferable to a moving average that was sensitive to 
intermittent large increases in S u . The distance for 
evaluating the proportion of cells was a sampling win- 
dow that spanned 31 cells for a total of 3.1 km, which 
is comparable to the distance needed to prepare for and 
conduct a bottom trawl. For our study, an area became 
designated as a patch when the proportion of cells in 
the sampling window that exceeded an S v of -65.6 dB 
was 0.39 or higher. The criteria for patch definition 
were determined by using the 80 th percentile of values 
from acoustic backscatter data measured aboard the FV 
Sea Storm during a NMFS GOA trawl survey in 2005 in 
the same Yakutat area. The acoustic backscatter data 
were echo-integrated in Echoview, and the S v values 
were exported and analyzed with R software scripts, 
vers. 2.9.0 (R Development Core Team, 2009), which 
generated graphs showing values of S v that defined the 
start and end of patches meeting the threshold criteria 
(Fig. 3). In each identified patch, a location for a patch 
station that was at least 1 km (a single trawl length) 
from the edge of that patch was randomly selected, and 
a 10-min trawl was conducted from that random loca- 
tion as the starting point. 
The CPUE data collected from these trawls were as- 
signed to patch stations (random trawls conducted with- 
in identified patches) or background stations (trawls 
conducted within planned stations at which the acoustic 
threshold was not exceeded). It is important to note that 
if a planned station was found to be located within an 
acoustically identified patch, a trawl was conducted at 
a patch station that was randomly selected within that 
patch rather than at the preselected location. 
Data analysis 
The acoustic backscatter data were processed and then 
categorized according to vessel activity. Echoview soft- 
ware was used to correct the backscatter data for noise 
and erroneous seafloor tracking. Partitioning back- 
scatter by vessel activity was necessary to accurately 
estimate the size of patches and the total length of 
the path traveled by the FV Sea Storm inside patches. 
Hence, to eliminate double counting, we avoided track- 
line segments where the boat circled around to set 
up trawls or searched for ground suitable for trawls. 
Seven vessel-activity categories were 
assigned to each 100-m cell: 1) tran- 
siting between stations, 2) return- 
ing to set up a trawl, 3) searching for 
ground suitable for trawls, 4) trawl 
deployment, 5) trawling (with offset 
for trawl distance behind the vessel), 
6) trawl recovery, and 7) other transit 
that was not part of our study. Cat- 
egories 1 and 4-6 were included in 
this study. Overlap, defined as any- 
where the vessel path was within 50 
m of the haul or earlier vessel path, 
was measured with ArcGIS software 
(ESRI, Redlands, CA, vers. 9.2). 
CPUE was an estimate of fish den- 
sity (kg/km 2 ) at each station and was 
calculated as the catch of a species in 
kilograms divided by the area sam- 
pled (i.e., the product of the net width 
in kilometers and the trawl trackline 
in kilometers). Patch length was com- 
puted with the haversine formula to 
calculate great-circle distances (as 
implemented in the R package ar- 
gosfilter; R Development Core Team. 
2009) between GPS coordinates for 
xs 
o 
3 
08:00 
13:00 
Time 
18:00 
Figure 3 
Example of script outputs for real-time monitoring of patches during a 
2009 acoustic-trawl survey. The time series (solid wavy line) and solid 
horizontal line represent mean volume backscattering (S v ) per 100 m and 
S v threshold, respectively. The dashed time series and horizontal lines 
represent the proportion of 100-m cells exceeding the S v threshold over a 
3.1-km window and the threshold for the proportion, respectively. Time is 
given in Alaska Daylight Time. 
