NOAA Technical Memorandum NMFS-AFSC-136


Spatial and Temporal Analysis of


Eastern Bering Sea Echo Integration-


trawl Survey and Catch Data of


Walleye Pollock, Theragra chalcogramma,


for 2001 and 2002


by


S. J. Barbeaux and M. W. Dorn


U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Alaska Fisheries Science Center 

July 2003 
NOAA Technical Memorandum NMFS 

The National Marine Fisheries Service's Alaska Fisheries Science Center 

uses the NOAA Technical Memorandum series to issue informal scientific and 

technical publications when complete formal review and editorial processing 

are not appropriate or feasible. Documents within this series reflect sound 

professional work and may be referenced in the formal scientific and technical 

literature. 

The NMFS-AFSC Technical Memorandum series of the Alaska Fisheries 

Science Center continues the NMFS-F/NWC series established in 1970 by the 

Northwest Fisheries Center. The new NMFS-NWFSC series will be used by 

the Northwest Fisheries Science Center. 

This document should be cited as follows: 

Barbeaux, S. J., and M. W. Dorn. 2003. Spatial and temporal analysis 

of eastern Bering Sea echo integration-trawl survey and catch data of 

walleye pollock, Theragra chalcogramma, for 2001 and 2002. U.S. 

Dep. Commer., NOAA-TM-AFSC-136, 34 p. 

Reference in this document to trade names does not imply endorsement by 

the National Marine Fisheries Service, NOAA. 
NOAA Technical Memorandum NMFS-AFSC-136


Spatial and Temporal Analysis of Eastern


Bering Sea Echo Integration-trawl Survey


and Catch Data of Walleye Pollock,


Theragra chalcogramma,


for 2001 and 2002


by


S. J. Barbeaux and M. W. Dorn


Alaska Fisheries Science Center


7600 Sand Point Way N.E.


Seattle, WA 98115


www.afsc.noaa.gov


U.S. DEPARTMENT OF COMMERCE 

Donald L. Evans, Secretary 

National Oceanic and Atmospheric Administration 

Vice Admiral Conrad C. Lautenbacher, Jr., U.S. Navy (ret.), Under Secretary and Administrator 

National Marine Fisheries Service 

William T. Hogarth, Assistant Administrator for Fisheries 

July 2003 
This document is available to the public through: 

National Technical Information Service


U.S. Department of Commerce


5285 Port Royal Road


Springfield, VA 22161


www.ntis.gov 
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iii 

Abstract 

The National Marine Fisheries Service (NMFS) is charged with determining 

whether fisheries for walleye pollock, Theragra chalcogramma, affect ecosystem 

function, specifically, any adverse impacts on endangered species. Although the spatial 

and temporal scale of fisheries impacts are an important concern, analyzing fisheries 

interactions at fine spatial scales is hindered by the resolution of available data. Our 

study had two goals: to analyze echo integration-trawl (EIT) survey data in association 

with observer catch records to evaluate the intensity of the walleye pollock fisheries, and 

to define the spatiotemporal resolution reasonable given the dynamic nature of the 

resource and the limitations of the data. In this analysis we used the concept of Observed 

Catch to Survey Biomass Ratio (OCSBR) as a tool to measure fishery intensity at local 

levels. The OCSBR is the ratio of observed catch to EIT survey estimated biomass in a 

given area for a specified period of time. The OCSBR should not necessarily be 

considered a proxy for the local exploitation rate. This analysis addresses how varying 

temporal and spatial resolution changes the OCSBR value. A correlation analysis is used 

to determine the temporal and spatial scale at which OCSBR is a reasonable measure of 

fishing intensity. 
v 

Contents 


Page


Abstract ............................................................................................. iii 


Introduction .......................................................................................... 1 


Methods and Materials ............................................................................. 2 


Data Sources ................................................................................... 2 


Spatial Analysis ................................................................................ 4 


Statistical Analysis ........................................................................... 5 


Results ................................................................................................ 8 


2001............................................................................................. 9 


2002 ............................................................................................. 10 


Discussion .......................................................................................... 12 


Correlation Analysis .......................................................................... 13 


OCSBR Analysis .............................................................................. 15 


Conclusion .......................................................................................... 16 


Citations ............................................................................................... 17 

Introduction 

Walleye pollock, Theragra chalcogramma support the largest single species 

fishery in the North Pacific Ocean. The National Marine Fisheries Service (NMFS) is 

charged with determining whether fisheries for walleye pollock affect ecosystem 

function, specifically, any adverse impacts on endangered species (e.g., Steller sea lions). 

The spatial and temporal scale of fisheries impacts are an important concern of NMFS 

investigations. However, analyzing fisheries interactions at fine spatial scales is hindered 

by the resolution of available data. Here we analyze 2001 and 2002 echo integrationtrawl (EIT) survey data using contemporaneous observer data to investigate the intensity 

of the walleye pollock fisheries at local scales. An attempt was made to define an 

appropriate spatiotemporal scale to address this issue given the dynamic nature of the 

Bering Sea pollock stock and the limitations of our current data. 

In this analysis, the concept of Observed Catch to Survey Biomass Ratio 

(OCSBR) is introduced as a possible tool to measure fisheries intensity at local scales. 

The OCSBR is the ratio of observed catch to EIT survey estimated biomass for a given 

area for a set period of time. The OCSBR should not be considered a proxy for local 

exploitation ratio; observed catch does not include all catch, and the EIT survey biomass 

estimates are not a full accounting of local biomass. This analysis addresses how varying 

temporal and spatial resolution changes the OCSBR value and through correlation 

analysis, we attempt to determine at what temporal and spatial scale the OCSBR is a 

reasonable measure of fishing intensity. 
2 

Materials and Methods 

Data Sources 

The study area is located in the section of the designated Steller sea lion critical 

habitat north of Unalaska and Unimak Islands on the eastern Bering Sea Shelf (Fig. 1). 

Estimates of walleye pollock biomass were obtained from EIT surveys of the 

southeastern Bering Sea shelf in winter 2001 and 2002. The surveys were conducted by 

the Resource Assessment Conservation Engineering (RACE) Division of the Alaska 

Fisheries Science Center. The EIT surveys provided estimates of walleye pollock 

biomass from 14 m below the surface to 0.5 m above the bottom with transect spacing of 

14.8 km (8 nmi) (Honkalehto et al. 2001, Honkalehto et al. 2002). Biomass estimates 

were based on mean density along the transect at 0.9 km (0.5 nautical miles (nmi)) and 

the transect spacing, thus biomass is estimated for 0.9 × 14.8 km (0.5 × 8 nmi) areas 

(Honkalehto et. al. 2002). The 2001 EIT survey was conducted between 20 February and 

3 March. The 2002 EIT survey was conducted between 23 February and 3 March. 

Estimates of targeted walleye pollock catch for the 2001 fishing season were 

obtained from two sources. Estimates of catch for all catcher-processor vessels and 

observed catcher-only vessels were obtained from the North Pacific Groundfish Observer 

Program database. In this case, we defined targeted pollock as any pollock caught while 

the vessel fished using pelagic trawl gear, as targeting of walleye pollock with nonpelagic gear was illegal. In 2001 and 2002, walleye pollock was the only pelagic target 

in the study area. We did not include bycatch of walleye pollock from other targeted 

fisheries in our analyses because our study was limited to effects of the directed walleye 

pollock fishery. In 2001 and 2002, observed pollock bycatch in the study areas for 
3 

January through April was 3,935 metric tons (t) and 6,594 t (2.7% and 2.2% of the 

targeted removals), respectively. Catcher-processor vessels fishing walleye pollock in 

the Bering Sea are required to carry two observers and observer duties include sampling 

all hauls for species composition. Catcher-only vessels 125 ft length over all (LOA) and 

larger are required to carry an observer at all times. Vessels under 125 ft to 60 ft LOA 

are required to carry observers for 30% of their fishing days. Vessels under 60 ft LOA 

are not required to carry observers and therefore observer data are not available for these 

vessels. We calculated observed walleye pollock catch estimates using the weight of 

pollock from the species composition sample extrapolated up to the official total catch for 

each observed haul. We assigned the location at which the net was pulled from fishing 

depth (end latitude and longitude) as the catch location. Catch estimates obtained solely 

from observed catch information are referred to as observed catch estimates throughout 

this document. 

To account for unobserved catch, we obtained data on total pollock deliveries to 

shore-based processing plants and floating processors from the Alaska Department of 

Fish and Game’s (ADF&G) landings database. Here we defined targeted walleye pollock 

as all walleye pollock delivered from vessels reporting the use of pelagic trawl gear. 

Vessels fishing groundfish in the Bering Sea are required to retain and deliver all walleye 

pollock caught. Processing plants are required to report landings by ADF&G reporting 

area (30' latitude × 60' longitude cells) and by fishing start date and delivery date. We 

summarized walleye pollock delivery data by the week fishing began and ADF&G 

reporting area. We also summarized the observed catcher-only vessel data by ADF&G 

reporting area and the week fishing began. A ratio of landings to observed catch (LOCR) 
4 

was calculated for each week and ADF&G area (Table 1). We then multiplied the catch 

weight from each observed catcher-only vessel haul by the appropriate landings to 

observed catch ratio to obtain a projected overall catch estimate. We used the “raw” 

observed catch weight in our analysis when we found ADF&G reporting areas and weeks 

without reported landings. Catch data obtained using this method are referred to as 

projected catch estimates throughout this document. 

The estimates of targeted walleye pollock catch for the 2002 fishing season were 

obtained exclusively from North Pacific Groundfish Observer Program database. 

ADF&G landings data for 2002 were not available for this analysis. 

Spatial Analysis 

We employed ARCGIS software to analyze the EIT survey and targeted walleye 

pollock catch data. All data points from the two EIT surveys and targeted walleye 

pollock catch data from 2001 and 2002 were imported into ARCGIS. We created 

prediction maps from the EIT survey biomass estimates using the kriging tools in 

ARCGIS. The prediction maps were used as visual aids in identifying pollock 

concentrations but were not used in the quantitative analysis of the EIT survey biomass 

data. Specifications of the two ordinary kriged prediction maps can be found in Table 2. 

For each EIT survey, we employed the ARCGIS Thiessen function to create two 

grids with approximately 219 km2

 (8 × 8 nmi), and 1,971 km2 (24 × 24 nmi) cells. The 

grid cells in both size resolutions were somewhat irregular due to edge effects and 

although the actual areas of the cells vary in size, we refer to the grids as 219 km2 and 

1,971 km2 resolution grids throughout this document. We joined the EIT survey biomass 
5 

estimates and observed pollock catch data for the time period during the surveys, for the 

week before the EIT surveys, and the week following the EIT surveys to the grid cells 

using the “join” feature of ARCGIS. The time period during the survey differed between 

2001 and 2002. In 2001 the survey lasted 12 days while in 2002 the survey lasted 9 days. 

We chose to analyze the time frame during the survey as a unit because the spatial area 

surveyed was nearly the same between the 2 years. An OCSBR was calculated for each 

grid cell where fishing was present. The OCSBR is the ratio of observed walleye pollock 

catch divided by the EIT survey biomass estimate. To aid in the analysis each grid cell 

was color coded by OCSBR value into one of seven categories; 0 or no fishing (dark 

green), 0.01 to 0.05 (light green), 0.06 to 0.10 (bright green), 0.11 to 0.20 (yellow-green), 

0.21 to 1.00 (yellow), and greater than 1.0 (red). For 2001 the cells were labeled with 

observed catch in blue, estimated catch (observed catch multiplied by the LOCR) in cyan, 

and survey estimated biomass in black. For 2002 the cells were labeled with the 

observed catch in blue and EIT survey estimated biomass in black. 

We do not provide OCSBR maps for the periods one week prior to and one week 

following the EIT surveys at the 219 km2

 resolution. We believe that the high migration 

rate of pollock at this time of year made OCSBR analysis at the 219 km2

 resolution level 

inappropriate. This issue is considered in more detail in the Discussion section. 

Statistical Analysis 

Theoretically, the largest catches would come from areas where the biomass is 

highest. If fishing is perfectly proportional to the biomass present in an area, local 

harvest rates would be the same as global harvest rates and no “localized” depletion 
6 


would be occurring. Of course fishermen do not have perfect knowledge of targeted 

species location, the survey is not a perfect estimate of the biomass, many high 

concentrations of pollock are closed to fishing, and significant emigration from the 

surveyed areas may be occurring. Therefore, we would expect the correlation to be less 

than 1.0. However, given a small enough temporal resolution and large enough spatial 

distribution to account for migration and the imperfect knowledge of the fishermen, we 

would expect a statistically significant positive correlation. 

Following the logic above, we may be able to investigate possible signs of 

“localized” depletion. If there was significant depletion of the biomass due to catch prior 

to the EIT survey, the correlation of prior catch to biomass would be negative given a 

small enough temporal resolution and large enough spatial distribution to account for 

migration and the imperfect knowledge of the fishermen. If catch did not significantly 

deplete the available biomass then the correlation should be positive. Insignificant 

findings may indicate either that the catch did not significantly deplete the available 

biomass or that our analysis resolution is inadequate. 

For the weeks prior to, during, and after the EIT surveys the correlation 

coefficients were calculated using the Spearman rank correlation coefficient: 

n 

6∑ di 

2 

r = 1 − i =1 , s 3

n − n 

where di is the difference between ranks of EIT biomass estimates by cell and observed 

catch estimates and n is the number of cells for which we are calculating rs (Zar 1999). 
7 

For tied ranks, the assigned rank to each of the tied values is the mean of the ranks 

that would have been assigned to these values had they not been tied. Tied analyses are 

corrected using: 

( )c =

[( − − ∑ x [( − − ∑ y n n / 6 2 t n n / 6 2 t 3 3 ) ] ) ] 

(n3 − n)/ 6 − ∑ di 

2 − ∑t x − ∑ t y r . s 

Here ∑ t x = ∑ (ti 

3 − ti )

, 12 

where ti is the number of tied values of EIT survey biomass in a group of ties, and 

∑ t y = ∑ (	ti 

3 − ti )

, 12 

where ti is the number of tied values of observed catch estimates in a group of ties (Zar 

1999). 

The Spearman rank correlation coefficients were calculated for all the cells in the 

study area, all the cells fished, and then for all the cells fished with a sizable catch 

(greater than 250 t for the 219 km2

 cells, and greater than 1,000 t for the 1,971 km2 cells). 

This resulted in 18 separate analyses. 

The Spearman rank correlation coefficients were tested against critical values for 

the Spearman rank coefficient (α = 0.05 ) (Zar 1999). 

For each test with n greater than 10, the Fisher z-transformation was used to 

further provide a two-tailed P-value. The formula for the Fisher z- transformation was 

0.5ln (1 + rs ) 

Z =

3 

1.060 

n − 

(1 − rs ) . 
P = 1 −
 2 

1 c e− − , 


8 

The two-tailed P-values were calculated to 3 significant digits using 

where c = 0.806Z (1 − 0.018Z ) (Zar 1999). 

Results 

Visual inspection of the 2001 and 2002 EIT survey results and kriged prediction 

maps reveal a patchy distribution of walleye pollock with inter-annually consistent 

concentrations of biomass in the east surrounding Amak Island and following the 200 m 

isobath near the western edge of the study area (Figs. 2 and 3). Both surveys revealed 

higher concentrations of walleye pollock in the eastern half of the study area. In 2001 

the pollock biomass estimated to be within pollock fishing exclusion zones (20 nmi 

pollock trawl and 10 nmi trawl exclusion zones) in the study area (Fig. 1) was 321,355 t 

or 38% of the EIT survey estimated biomass for the study area. In 2002 the pollock 

biomass estimated to be within pollock fishing exclusion zones (the Bering Sea pollock 

restriction area and 10 nmi trawl exclusion zones) in the study area (Fig.1) was 393,200 t 

or 29% of the EIT survey estimated biomass for the study area. Analysis of the 

concentration of pollock biomass and catch by 219 km2

 cells revealed that although 

walleye pollock biomass was relatively less concentrated in 2001 than in 2002, the 

proportion of catch in 2001 was concentrated in relatively fewer cells (Fig. 4). 
9 

2001 

In 2001 the EIT survey biomass estimate for the study area was 852,254 t for 

44,600 km2 (19.1 t/km2). Targeted pollock catch in the study area prior to the survey 

(20 January - 19 February) is estimated to be 93,608 t observed. Applying the landings to 

observed catch ratios results in an estimate of 124,170 t of pollock catch prior to the 2001 

survey. During and after the EIT survey an additional 52,078 t observed (63,569 t 

projected ) were removed in the Pollock A-season and following the Community 

Development Quota (CDQ) fishery through 28 April 2001 (Table 3). 

The correlation analysis for 2001 resulted in only three statistically significant 

findings at α = 0.05. The 1,971 km2 and 219 km2

 grids during the survey and the 

219 km2 grid for the week prior to the survey resulted in significant positive correlations 

at the analysis level of all cells in the study area (ALL) (Table 4). There were no other 

statistically significant correlations, positive or negative, for any other time period, grid 

resolution, or analysis level. 

The OCSBR analysis of the 2001 observed catch and EIT survey data were 

limited to the week prior to the survey, 12 days during the survey, and the week after the 

survey (Fig. 5). During the 12 days of the survey (20 February - 3 March) a total of 

18,249 t observed of walleye pollock were removed from the study area. During the 

survey at the 1,971 km2 resolution, 17 of 32 cells were fished. Two of the cells had an 

OCSBR greater than 0.20, three cells had OCSBR values of between 0.05 and 0.10, and 

the majority of cells (n=12) had OCSBR values of less than 0.05 (Table 6 and Fig. 7). 

During the survey at the 219 km2

 grid resolution, 49 of 215 cells were fished. There were 

3 cells with OCSBR values greater than 1.00, 7 cells had OCSBR values between 0.20 
10 

and 1.00, 16 cells with OCSBR values between 0.05 and 0.15, and 23 cells had OCSBR 

values of less than 0.05 (Fig. 9). 

In 2001 there were 11,138 t observed of walleye pollock removed from the study 

area the week prior to the beginning of the EIT survey (13 February - 19 February ). At 

the 1,971 km2 resolution, 13 of 32 cells were fished. One cell had an OCSBR greater 

than 0.20, 2 cells had OCSBR values between 0.05 and 0.15, and the majority of the 

fished cells (n=10) had OCSBR values of less than 0.05 (Fig. 11). 

There were 6,970 t observed of walleye pollock removed from the study area the 

week following the end of the EIT survey (4 March - 10 March). At the 1,971 km2 

resolution, 10 of 32 cells were fished. There were no cells with OCSBR values greater 

than 0.15, two cells with OCSBR values between 0.05 and 0.15, and eight cells with 

OCSBR values of less than 0.05 (Fig. 13). 

2002 

In 2002 the EIT survey biomass estimate for the study area was 1,354,659 t for 

41,610 km2

 (32.6 t/km2). Targeted pollock catch in the study area prior to the survey 

(20 January - 22 February) is estimated to be 197,203 t observed. During and after the 

EIT survey an additional 99,360 t observed were removed in the Pollock A-season and 

following CDQ fishery through 28 April 2001 (Table 3). 

The correlation analysis for 2002 resulted in eight statistically significant findings 

at α = 0.05. At the analysis level of all cells in the study area, all of the correlation 

coefficients for both grid resolutions for before, during, and after resulted in significantly 

positive correlations. In addition, for the 1,971 km2 grid resolution during the survey 
11 

both the fished cells and cells with greater than 1,000 t of observed catch analysis 

resolutions resulted in significant positive correlations. The other 10 time periods, grid 

resolution, and analysis level combinations were not found to have statistically significant 

correlations (Table 5). 

The OCSBR analysis of the 2002 observed catch and EIT survey data were 

limited to the week prior to the survey, 9 days during the survey, and the week after the 

survey (Fig. 6). During the 9 days of the survey (23 February - 3 March) a total of 

67,391 t observed of walleye pollock were removed from the study area. During the 

survey at the 1,971 km2 resolution, 21 of 38 cells were fished. There were 2 cells with 

OCSBR values greater than 1.00, 2 cells with OCSBR values between 0.20 and 1.00 , 10 

cells with OCSBR values of between 0.05 and 0.20, and 7 cells with OCSBR values of 

less than 0.05 (Table 7 and Fig. 8). During the survey at the 219 km2 grid resolution 70 

of 205 cells were fished. There were 7 cells with OCSBR values greater than 1.00, 22 

cells with OCSBR values between 0.20 and 1.00, 20 cells with OCSBR values between 

0.05 and 0.15, and 21 cells with OCSBR values of less than 0.05 (Fig. 10). 

In 2002 there were 37,206 t observed of walleye pollock removed from the study 

area the week prior to the beginning of the EIT survey (16 February - 22 February). At 

the 1,971 km2 resolution there were 17 of 38 cells were fished. Two cells had OCSBR 

values greater than 1.00, two cells had OCSBR values greater than 0.20, 1 cell had an 

OCSBR between 0.05 and 0.10, and the majority of the fished cells (n=12) had OCSBR 

values of less than 0.05 (Fig. 12). 

There were 17,524 t observed of walleye pollock removed from the study area the 

week following the end of the EIT survey (4 March - 10 March). At the 1,971 km2 
12 

resolution, there were 12 of 38 cells fished. There were no cells with an OCSBR value 

greater than 1.00, one cell with an OCSBR between 0.20 and 1.00, three cells with 

OCSBR values between 0.05 and 0.15, and eight cells with OCSBR values of less than 

0.05 (Fig. 14). 

Discussion 

Analyzing fisheries interactions at fine-scale resolutions is hindered by the 

temporal and spatial resolution of available data. The EIT survey estimates of biomass 

are at a relatively fine spatial resolution (0.9 × 14.8 km) but are limited in that they only 

provide a snapshot of the stock’s distribution. For pelagic stocks such as walleye 

pollock, concentrations identified at small spatiotemporal resolutions will not be 

representative for an entire season and due to migration may not be representative at 

smaller time scales. Radchenko and Sobolevskiy (1993) report that walleye pollock 

aggregations in areas of the western Bering Sea may cover 15 to 17.6 km (8.1 to 9.5 nmi) 

per day while feeding and 27.8 km (15 nmi) per day while returning to spawning sites. 

Assuming similar rates of travel in the Eastern Bering Sea, the EIT survey biomass 

estimates for a 0.9 × 14.8 km resolution could be inaccurate in less than a day for a given 

sampling location. 

In addition, the EIT surveys do not include fish located below 0.5 m from the 

bottom and 14 m from the surface and therefore is a minimum estimate of true biomass. 

Surveying along transects yields an inexact estimate of true biomass. For these reasons 

our OCSBR should be considered an index of fishing intensity and should not be 

interpreted as an actual measure of the local exploitation rate. 
13 

Determining the spatial resolution of the observed catch data is hampered by long 

towing periods and the lack of information concerning the trawl path between starting 

and ending positions. Observers do not provide estimates of catch as it enters the net nor 

do they provide minute-by-minute position reports as a vessel fishes. An arbitrary choice 

must be made assigning the catch to either the start position or end position. In the 

Eastern Bering Sea walleye pollock fishery tow lengths often exceed 20 km. Thus, if 

data are assigned to either starting or ending positions the spatial analysis of catch data 

versus biomass estimates at a smaller resolution than 20 km may provide erroneously 

high OCSBR values. 

Correlation Analysis 

The correlation between catch and survey biomass was higher in 2002 than in 

2001. This finding may be explained in part by the differences in distribution and 

concentration of walleye pollock between the 2 years. In 2001 the EIT survey estimated 

that there were 503,400 fewer tons of walleye pollock in the study area than in 2002 and 

that the fish were less concentrated. The lower correlation between catch and biomass in 

2001 may be explained by the fishermen being unable to routinely locate concentrations 

of fish due to the wider dispersion of pollock in 2001 and thus may have chosen to fish 

on lower concentrations of pollock. 

For 2001 and 2002 we found positive correlations between the EIT survey 

biomass estimates and observed catch during the survey for both the 219 km2

 and 

1,971 km2

 grid resolutions at the analysis level of all cells in the study area. Based on 

this result we may conclude that fishermen caught walleye pollock in cells where the EIT 
14 

survey indicated there were higher concentrations of walleye pollock. Although this 

conclusion would seem to be intuitive, the catch and biomass estimates could have been 

greatly disparate given a possible pollock migration speed of up to 27.8 km per day 

(Radchenko and Sobolevskiy, 1993) and the patchy concentrations of pollock observed in 

the EIT survey. 

For 2001, we found positive correlations between the EIT survey biomass and 

observed catch at the 219 km2 grid resolution at the analysis level of all cells in the study 

area for the week prior to the survey. For 2002, we found positive correlations between 

the EIT survey biomass and observed catch at both the 219 km2 and 1,971 km2

 grid 

resolutions at the analysis level of all cells in the study area for the week prior to the 

survey. There were no statistically significant correlations, positive or negative, prior to 

or after the survey at the analysis level of fished cells in the study area at the 1,971 km2 or 

the 219 km2 grid resolutions. No firm conclusions can be drawn from these findings. 

Although there is no strong indication that catch measurably depleted pollock biomass in 

the study area the week prior to the EIT survey, the lack of significance for many of the 

correlations may suggest that our grid resolutions are too fine for analyzing data greater 

than one week duration from the beginning or end of the survey. 

In both 2001 and 2002 the correlation coefficients were higher at the 1,971 km2 

resolution than at the 219 km2 resolution and the pre- and post-survey correlation 

coefficients were lower than the correlations coefficients during the EIT survey. At 

larger grid resolutions and less duration between survey estimates and catch estimates, 

we obtained higher correlation coefficients; at smaller grid resolutions and longer 

duration between catch and survey estimates, we obtained lower correlation coefficients. 
15 

These results support the conclusion that the quality of our analysis is affected by grid 

and temporal resolution. We equate this to mean there is less uncertainty in our estimates 

of OCSBR at higher spatial scales and shorter durations between catch and survey 

biomass estimates. 

OCSBR Analysis 

The OCSBR analysis revealed that in 2001 and 2002, fishing was not evenly 

distributed throughout the study area and that fishing pressure may differ considerably 

from one location to the next. As one would expect, the spatial distribution of fishing 

catch tended to follow the spatial distribution of estimated biomass. In 2001 and 2002, 

we found patchy biomass distributions and patchy catch distributions in the study area. 

The 2002 data show a higher biomass and also a much more intense pollock 

fishery in the study area. In 2002, fishermen harvested 67,400 t observed of walleye 

pollock from the study area during the 9 days of the EIT survey, compared to 18,400 t 

observed during the 13 days of the 2001 EIT survey. The increase in the volume of 

pollock harvested in 2002 was reflected in an overall increase in the OCSBR values even 

though the fishery was less concentrated (Fig. 4). The overall OCSBR value for the 

study area during the 2001 survey was 0.022 and during the 2002 survey was 0.050. The 

median OCSBR value in 2001 during the survey period was 0.012 at the 1,971 km2 

resolution and 0.059 at the 219 km2 resolution, while during the 2002 survey period the 

median values were 0.077 and 0.122, respectively (Fig. 5 and Fig. 6). 

In both 2001 and 2002, high OCSBR values (greater than 0.20) tended to be 

associated with cells with low biomass estimates, adjacent to cells with high biomass 
16 

estimates. In 2002 at the 1,971 km2 resolution every cell with an OCSBR value greater 

than 0.20 were cells having below the mean estimated biomass (35,600 t) and were 

directly adjacent to cells having 2 to 5 times the mean estimated survey biomass (Fig. 8). 

This is also true for 2001 except for one cell located in the western portion of the study 

area (Fig. 7). These findings suggest that emigration from the high to low biomass cells 

may have skewed our OCSBR values, that using the ending position for observed catch 

may have overestimated OCSBR values for these cells, or that our spatial resolution even 

at 1,971 km2

 is too fine for the available data. 

Conclusion 

This analysis was undertaken in an attempt to better define what analysis 

resolution was reasonable given the dynamic nature of the walleye pollock distribution 

and the limitations of our current data. We found that smaller spatial scale analyses (i.e., 

smaller than 1,971 km2

) of EIT survey data and Observer data were too fine for our 

current data set and that even at the larger spatial scales, analyses should be restricted to 

very near the time the EIT survey data were collected. Our analysis does not help define 

what scale, temporal or spatial, is important to Steller sea lions (Eumetopias jubatus). At 

the finest spatial and temporal resolutions, that of the tracks of the trawl nets the 

exploitation rate of walleye pollock approaches 100% (minus possible escapement), 

while at the other end of the scale, the entire range of walleye pollock, the exploitation 

rate approaches the annual harvest rate of eastern Bering Sea walleye pollock. Defining 

what spatial and temporal scale is important to walleye pollock stocks, to dependent 
17 

species such as Steller sea lions, and to the Bering Sea ecosystem as a whole is a difficult, 

but necessary step to fulfill NMFS’ stewardship responsibilities. 

Citations 

Honkalehto, T., P. Walline, D. McKelvey, and N. Williamson. 2001. Appendix I. 

Results of the echo integration-trawl survey of walleye pollock (Theragra 

chalcogramma) conducted on the southeastern Bering Sea shelf and in the 

southeastern Aleutian Basin near Bogoslof Island February and March, 2001. In: 

Stock assessment and fishery evaluation report for the groundfish resources of the 

Bering Sea/Aleutian Islands regions. Section 1.A, p.8. Available North Pac. Fish. 

Mgmt. Council, 605 W. 4th Ave. Suite 306, Anchorage AK 99501-2252. 

Honkalehto, T., N. Williamson, D. Hanson, D. McKelvey, and S. de Blois. 2002. 

Results of the echo integration-trawl survey of walleye pollock (Theragra 

chalcogramma) conducted on the southeastern Bering Sea shelf and in the 

southeastern Aleutian Basin near Bogoslof Island February and March, 2002. AFSC 

Processed Rep. 2002-02, 49 p. Alaska Fish. Sci. Cent., Natl. Mar. Fish. Serv., 

NOAA, 7600 Sand Point Way NE, Seattle WA 98115 

Radchennko, V.I., and Ye. I. Sobolevskiy. 1993. Seasonal spatial distribution dynamics 

of walleye pollock, Theragra chalcogramma in the Bering Sea. J. Ichthyol. 33(2): 

63-76. 
18 

Zar, J. H. 1999. Biostatistical Analysis - 4th Edition. Prentice Hall, Inc. Upper Saddle 

River, NJ. 663 p. 
19 

Table 1. Landings to Observed Catch Ratios (LOCR). Light shaded cells indicate more 

observed catch than reported landings. Dark shaded cells indicate observed catch, but no 

landings data (NL). 

ADF&G Start Week 

Area 2/03/01 2/10/01 2/17/01 2/24/01 3/03/01 3/10/01 3/17/01 3/24/01 

635530 0.8713 0.2921 4.2972 6.8946 

645434 1.0471 1.0560 


645501 2.0032 2.2414 2.7479 2.4607 7.5073 16.6095 1.9529 1.1916 


0.1404 0.8570 

645530 2.2357 1.3674 3.9986 12.5248 2.8353 4.0801 1.0630 


655430 1.5317 1.9981 1.9654 3.4900 


655500 1.3334 1.5003 3.0589 27.3701 


655530 1.0421 4.5478 
 NL 

665401 1.2881 0.9437 2.2366 

665430 NL 

665530 

665630 1.2017 

0.2350 

675430 82.6681 

685500 1.8875 

685530 NL NL 

Table 2. Ordinary kriging specifications for Echo Integration-trawl (EIT) survey data. 

2001 2002 

Selected Method: Ordinary Kriging Ordinary Kriging 

Output: Prediction Map Prediction Map 

Number of Datasets Currently in Use: 1 1 

Number of Points: 2838 3676 

Semivariogram/Covariance Model: 3e8*Spherical(300000)+ 3e8*Spherical(400000) 

1000000*Nugget +1000000*Nugget 

Error Modeling: Microstructure: 1000000 (100%) 1000000 (100%) 

Measurement Error: 0 (0%) 0 (0%) 

Searching Neighbors to Include: 10 or at least 1 for each 10 or at least 1 for each 

Neighborhood: angular sector angular sector 

Searching Angle: 0 0 

Ellipse: Major Semiaxis: 300000 400000 

Minor Semiaxis: 300000 400000 

Angular Sectors: 4 4 

Table 3. Echo Integration-trawl (EIT) survey biomass estimates and catch estimates for 

the study area. 

2001 2002 

EIT Survey Dates 20 Feb. – 3 Mar. 23 Feb. – 3 Mar. 

EIT Survey Biomass (t) 851,254 1,354,659 

EIT Survey Area (km2

) 44,600 41,610 

Observed W/ LOCR Observed W/ LOCR 

Pre-Survey Removals (t) 93,608 124,170 197,203 NA 

During and Post-Survey Removals (t) 52,078 63,569 99,360 NA 

Overall CSBR 0.1542 0.1925 0.1911 NA 
20 


Table 4. Spearman rank correlation coefficient analysis (Zar 1999) conducted for 2001. 

Shaded rows indicate rs values significant to 0.05. All biomass estimates came from the 

2001 EIT Survey 20 Feb. - 3 Mar. Resolution ALL means all cells in the study area, 

Fished means all cells where fishing was present in the study area, and >1,000 and >250 

means cells where greater than 1,000 or greater than 250 t of pollock were caught. 

2001 Resolution rs n r0.05(2),n P 

1 Week Before 1,971 km2 ALL 0.202 32 0.350 0.285 

(13 Feb. – 19 Feb.) Fished -0.030 14 0.538 0.922 

>1,000 0.600 4 1.000 -

219 km2 ALL 0.173 215 0.138 0.014 

Fished 0.215 38 0.321 0.209 

> 250 -0.156 16 0.503 0.574 

During 1,971 km2 ALL 0.403 32 0.350 0.026 

(20 Feb. – 3 Mar.) Fished 0.120 17 0.485 0.660 

>1,000 0.543 6 0.886 -

219 km2 ALL 0.226 215 0.138 0.001 

Fished 0.180 49 0.282 0.230 

> 250 -0.056 17 0.485 0.835 

1 Week After 1,971 km2

 ALL 0.242 32 0.350 0.197 

(4 Mar. – 10 Mar.) Fished 0.182 11 0.618 0.612 

>1,000 NA - - -

219 km2

 ALL 0.082 215 0.138 0.243 

Fished 0.103 28 0.375 0.613 

>250 -0.217 12 0.587 0.511 

Table 5. Spearman rank correlation coefficient analysis (Zar 1999) conducted for 2002. 

Shaded rows indicate rs values significant to 0.05. All biomass estimates came from the 

2002 EIT Survey 23 Feb. - 3 Mar.. Resolution ALL means all cells in the study area, 

Fished means all cells where fishing was present in the study area, and >1,000 and >250 

means all cells where greater than 1,000 or greater than 250 t of pollock were caught. 

2002 Resolution rs n r0.05(2),n P 

1 Week Before 1,971 km2 ALL 0.333 38 0.321 0.047 

(16 Feb. – 22 Feb.) Fished 0.314 17 0.485 0.239 

>1,000 0.214 8 0.783 -

219 km2 ALL 0.346 205 0.138 0.000 

Fished 0.003 50 0.279 0.983 

> 250 -0.246 32 0.350 0.168 

During 1,971 km2 ALL 0.563 38 0.321 0.000 

(23 Feb. – 3 Mar.) 

219 km2 

Fished 0.679 21 0.435 0.001 

>1,000 0.832 12 0.587 0.000 

ALL 0.507 205 0.138 0.000 

Fished 0.114 70 0.235 0.363 

> 250 0.132 55 0.266 0.354 

1 Week After 1,971 km2 ALL 0.519 38 0.321 0.001 

(4 Mar. – 10 Mar.) Fished -0.119 12 0.587 0.723 

>1,000 NA - - -

219 km2 ALL 0.322 205 0.138 0.000 

Fished 0.315 31 0.356 0.094 

> 250 0.429 21 0.435 0.059 
21 


0.01-0.05 

0.01-0.05 

10 

16 

12 

23 

12 

19 

21 

15 

8 

9 

7 8 

0.05-0.10 0.05-0.10 

11 

10 

1 

9 

1 

9 

1 

5 

1 7 2 

5 

Table 7. Observed catch to survey biomass ratio (OCSBR) analysis conducted for 2002 with number of cells. Table 6. Observed catch to survey biomass ratio (OCSBR) analysis conducted for 2001 with number of cells. 

OCSBR 

0.10-0.15 0.10-0.15 

1 

3 

2 

7 

1 

6 

0 

1 

2 

5 

1 

2 

OCSBR 

0.15-0.20 0.15-0.20 

0 

2 

0 

0 

0 

1 

0 

3 

1 

5 

0 

3 

0.20-1.00 0.20-1.00 

22 

1 

7 

2 

7 

0 

5 

2 

9 

2 1 

4 

>1.00 >1.00 

0 

1 

0 

3 

0 

1 

2 

7 

2 

7 

0 

2 

Cells Fished Cells Fished 

13 

38 

17 

49 

10 

27 

17 

50 

21 

70 

12 

31 

Total Cells Total Cells 

215 215 215 205 205 205 

32 32 32 38 38 38 

Resolution Resolution 

1,971 km2 

1,971 km2 

1,971 km2 

1,971 km2 

1,971 km2 

1,971 km2 

219 km2 

219 km2 

219 km2 

219 km2 

219 km2 

219 km2 

Catch ( 67,391 t) 

Catch (11,138 t) 

Catch (18,429 t) 

Catch (37,206 t) 

Catch (17,524 t) Catch ( 6,970 t) 

1 Week Before 1 Week Before 

1 Week After 1 Week After 

2001 

During During 

2002 
22 


Figure 1. 2001 and 2002 study areas on the eastern Bering Sea shelf. 
23


Figure 2. 2001 echo integration-trawl (EIT) survey biomass ordinary kriged prediction 

map with overlay bubbles of relative biomass. Green areas are relatively low estimates 

of biomass, and orange areas are areas of relatively high estimates of biomass. The solid 

blue line is the 200 m isobath. 

Figure 3. 2002 echo integration-trawl (EIT) survey biomass ordinary kriged prediction 

map with overlay bubbles of relative biomass. Green areas are relatively low estimates 

of biomass, and orange areas are areas of relatively high estimates of biomass. The solid 

blue line is the 200 m isobath. 
24 


Figure 4. Fish biomass concentrations from observed catch during the survey periods and echo integration-trawl (EIT) surveys and for 2001 and 2002 by 219 km2 cells. 

2001 Catch 2002 Catch 2001 EIT Survey 2002 EIT Survey 

1 0.9 0.8 0.7 0.6 

Proportion of Cells 

0.5 0.4 0.3 0.2 0.1 0 

1 

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 

0 

k lloco P of Tons ofnoti oporrP
25 


Figure 5. 2001 distribution of observed catch to survey biomass ratios (OCSBR) with medians plotted. Please note that the OCSBR 

axis is on a logarithmic scale. 

� Median at 1971 km 2 

� Median at 219 km 2 resolution 

resolution 

2 1 

Week Duration from Survey 

0 -1 -2 

10.00000 

1.00000 0.10000 0.01000 0.00100 0.00010 

R BS OC
26 


Figure 6. 2002 distribution of observed catch to survey biomass ratios (OCSBR) with medians plotted. Please note that the OCSBR 

axis is on a logarithmic scale. 

� Median at 1971 km 2 � Median at 219 km2 resolution 

resolution 

2 1 

Week Duration From Survey 

0 -1 -2 

100.00000 

10.00000 

1.00000 0.10000 0.01000 0.00100 0.00010 

R BS OC
27 


Figure 7. 2001 observed catch to survey biomass ratios (OCSBR) for 1,971 km2 cells for observed catch and echo integration-trawl (EIT) survey estimates from 20 February to 3 March. 
28 


Figure 8. 2002 observed catch to survey biomass ratios (OCSBR) for 1,971 km2 cells for observed catch and echo integration-trawl (EIT) survey estimates from 23 February to 3 March. 
29 


Figure 9. 2001 observed catch to survey biomass ratios (OCSBR) for 219 km2 cells for observed catch and echo integration-trawl (EIT) survey estimates from 20 February to 3 March. 
30 


Figure 10. 2002 observed catch to survey biomass ratios (OCSBR) for 219 km2 cells for observed catch and echo integration-trawl 

(EIT) survey estimates from 23 February to 3 March. 
31 


Figure 11. 2001 observed catch to survey biomass ratios (OCSBR) for 1,971 km2 cells for observed catch from 13 February to 19 February and echo integration-trawl (EIT) survey estimates from 20 February to 3 March. 
32 


Figure 12. 2002 observed catch to survey biomass ratios (OCSBR) for 1,971 km2 cells for observed catch from 16 February to 22 February and echo integration-trawl (EIT) survey estimates from 23 February to 3 March. 
33 


Figure 13. 2001 observed catch to survey biomass ratios (OCSBR) for 1,971 km2 cells for observed catch from 4 March to 10 March and echo integration-trawl (EIT)survey estimates from 20 February to 3 March. 
34 


Figure 14. 2002 observed catch to survey biomass ratios (OCSBR) for 1,971 km2 cells for observed catch from 4 March to 10 March and echo integration-trawl (EIT) survey estimates from 20 February to 3 March. 
RECENT TECHNICAL MEMORANDUMS 

Copies of this and other NOAA Technical Memorandums are available from the 

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(web site: www.ntis.gov). Paper and microfiche copies vary in price. 

AFSC135	 DIETER, B. E., D. A. WION, and R. A. MCCONNAUGHEY. 2003. Mobile fishing gear effects on benthic 

habitats: A bibliography (second edition), 207 p. NTIS number pending. 

134 ROBSON, B. W. (editors). 2002. Fur seal investigations, 2000-2001, 80 p. NTIS No. PB2003-103825. 

133	 ANGLISS, R. A., and K. L. LODGE. 2002. Alaska marine mammal stock assessments, 2002, 224 p. 

NTIS number pending. 

132 	 DOYLE, M. J.., M. S. BUSBY, J. T. DUFFY-ANDERSON, S. J. PICQUELLE, and A. C. MATARESE. 

2002. Aspects of the early life history of capelin (Mallotus villosus) in the northwestern Gulf of Alaska: A 

historical perspective based on larval collections October 1977- March 1979, 32 p. NTIS No. PB2002-

102535. 

131 	 SEASE, J. L., and C. J. GUDMUNDSON. 2002. Aerial and land-based surveys of Steller sea lions 

(Eumetopias jubatus) from the western stock in Alaska, June and July 2001 and 2002, 45 p. NTIS No. 

PB2003-102164. 

130 	 AYDIN, K. Y., V. V. LAPKO, V. I. RADCHENKO, and P. A. LIVINGSTON. 2002. A comparison of the 

eastern and western Bering Sea shelf and slope ecosystems through the use of mass-balance food 

web models, 78 p. NTIS No. PB2003-100518. 

129	 FERRERO, R. C., and L. W. FRITZ. 2002. Steller sea lion research coordination: a brief history and 

summary of recent progress, 34 p. NTIS No. PB2002-107912. 

128	 WEINBERG, K. L., M. E. WILKINS, F. R. SHAW, and M. ZIMMERMANN. 2002. The 2001 Pacific 

west coast bottom trawl survey of groundfish resources: Estimates of distribution, abundance, and 

length and age composition, 140 p. + Appendices. NTIS No. PB2002-108221. 

127	 MARQUETTE, W. M. 2002. Annotated bibliography of the bowhead whale, Balaena mysticetus, 

1767-1983, 325 p. NTIS No. PB2002-108033. 

126	 HONKALEHTO, T., W. PATTON, S. DE BLOIS, and N. WILLIAMSON. 2002. Echo integration-trawl 

survey results for walleye pollock (Theragra chalcogramma) on the Bering Sea shelf and slope during 

summer 2000, 66 p. NTIS No. PB2002-104686. 

125	 HONKALEHTO, T., N. WILLIAMSON, and S. DE BLOIS. 2002. Echo integration-trawl survey results 

for walleye pollock (Theragra chalcogramma) on the Bering Sea shelf and slope during summer 1999, 

77 p. NTIS No. PB2002-104686. 

124	 ANGLISS, R. P., D. P. DEMASTER, and A. L. LOPEZ. 2001. Alaska marine mammal stock 

assessments, 2001, 203 p. NTIS No. PB2002-105390. 

123 ROBSON , B. W. (editor). 2001. Fur seal investigations, 1999, 52 p. NTIS No. PB2002-100418. 

122 	 SEASE, J. L., W. P. TAYLOR, T. R. LOUGHLIN, and K. W. PITCHER. 2001. Aerial and land-based 

surveys of Steller sea lions (Eumetopias jubatus) in Alaska, June and July 1999 and 2000, 52 p. NTIS 

No. PB2001-107277. 

121	 BRITT, L. L., and M. H. MARTIN. 2001. Data report: 1999 Gulf of Alaska bottom trawl survey, 249 p. 

NTIS No. PB2001-105324