Average Density Index for Walleye Pollock, 

 Theragra chalcogramma, in the Bering Sea 



LOH-LEE LOW' and IKUO IKEDA' 



ABSTRACT 



The data base and an average density index (ADD procedure for assessing walleye pollock. 

 Theragra chalcogramma. abundance in the eastern Bering Sea were evaluated. The data base con- 

 sisted of daily catch-effort records of individual fishing vessels in the Japanese groundfish fishery from 

 1%4 to 1976. Variances about the annual mean catch, effort, and catch per unit effort (CPUE) data 

 were low. Coefficient of variation of annual CPUE data was in the \-2% range for the data base after 

 1%9 but higher in earlier years when the number of fishing records was lower. An ADI procedure is de- 

 scribed which takes into consideration different types of vessels used in the fishery, species mix in the 

 catch, distribution uf pulhuk. and fishing pattern of the fieel. Data from five vessel class-gear types 

 that fished mainly for pollock were selected to compute ADIs in four area-time cells. An overall ADI 

 within these cells was determined, summarizing the results by vessel class-gear types and area-time 

 cells. From l%4 to the early IHTDs, overall ADI and CPUE trends increased as a result of increased pol- 

 lock abundance and fishing power of vessels. For 2-.'J years during the early 1970s, abundances of pol- 

 lock were at peak levels. Beginning in 1972, abundance declined but stabilized during 1975-78 at an 

 intermediate level when 1.1 million metric tons of walleye pollock were harvested annually. 



INTRODUCTION 



Catch-eflort data from commercial fisheries can be 

 very useful for monitoring abundance of fish stocks. The 

 concept for its use is simple: if conditions of fishing and 

 vulnerability of the stock to fishing remain fairly con- 

 stant, the amount of catch for a standard unit of fishing 

 effort, catch per unit effort (CPUE), should reflect abun- 

 dance of the stock under exploitation (FAO f976). 

 However, conditions of the fishery and dynamics of the 

 stock (and therefore its vulnerability to fishing) fre- 

 quently change, complicating the use of CPUE. These 

 chan'ges and the quality of data are variables that may 

 invalidate the use ot CPUE for measuring stock abun- 

 dance. It is, therefore, of great importance to collect and 

 analyze catch-effort data carefully so that these and 

 other factors do not bias indices of abundance. 



In the eastern Bering Sea, catch-effort data from the 

 Japanese trawl fishery have been the most consistent and 

 important source of information for assessing 

 walleye pollock, Theragra chalcogramma, abundance. 

 Since 1964, these data have been collected in a detailed 

 and systematic manner. From this data base, CPUE 

 computations to assess abundance of the pollock 

 resource have been made over several years and results 

 reported in documents submitted to the International 

 North Pacific Fisheries Commission (INPFC). 



At the 1974 annual meeting of the INPFC, Japanese 

 scientists presented an extension of the CPUE index 



used by U.S. scientists as a measure of pollock abun- 

 dance termed an average density index (ADI). This ADI 

 procedure involved some selection of pair trawl catch- 

 effort data from the data base for its computations. 

 Because the Japanese trawl fishery used various vessel- 

 gear combinations and took a variety of species, there 

 was some difficulty in data selection for deriving un- 

 biased measures of pollock abundance. In addition, there 

 was a need to evaluate whether the fundamental data is 

 representative of all data collected since not all daily 

 catch-effort records were incorporated into the data base 

 used to derive ADI values. To resolve these questions, the 

 Committee on Biology and Research of INPFC es- 

 tablished a working group to evaluate the data and the 

 ADI procedure for assessing pollock abundance. This 

 paper reports on results of the working group's study on 

 the bias and variability of catch-effort data and the stan- 

 dardization of ADIs as a measure of pollock abundance. 



THE WALLEYE.POLLOCK FISHERY 



Walleye pollock is of the family Gadidae and is dis- 

 tributed throughout the North Pacific. Its life span is 

 about 15 yr, but 2-6 yr olds are the major ages exploited 

 by the fishery. This age range corresponds to fish weigh- 

 ing 91 g (24 cm fork length) and 817 g (49 cm fork length). 

 Pollock is by far the most abundant demersal fish in the 

 eastern Bering Sea (Pereyra et al. 1976)^ and has been 



'Northwest and Alaska Fisheries Center, National Marine Fisheries 

 Service. NOAA. 2725 Montlake Boulevard East, Seattle, WA 98112. 



"Far Seas Fisheries Flesearch Laboratory, Fisheries Agency of .Japan, 

 1(100 Orido, Shimizu 424, Japan. 



'Pereyra, W. T., J. E. Reeves, and R. G. Bakkala. 1976. Demersal 

 fish and shellfish resources of the eastern Bering Sea in the baseline year 

 1975. Processed Rep., 619 p. Northwest and Alaska Fish. Cent., Natl. 

 Mar. Fish. Serv., NOAA. 2725 Montlake Boulevard E., Seattle, WA 

 98112. 



