60° N 



55° N 



165° E 170° E 175° E 180° 175° W 170° W 165° W 160° W 



Figure 1.— Map of the Bering Sea showing statistical grids used in the Japanese groundflth data base. 



EXAMINATION FOR BIAS IN 

 THE 1964-72 DATA BASE 



Since the data base for 1964-72 included only a third of 

 the daily catch-effort records, we wanted to determine 

 whether the partial data base is biased towards high or 

 low catch levels by the subsampling procedures used. It 

 was necessary to examine this question indirectly by us- 

 ing the complete 1973-76 data base because the missing 

 records in the earlier 1964-72 data could not be easily 

 restored. First we subsampled the 1973-76 data on pair 

 trawlers in the same manner in which the 1964-72 data 

 was created so as to create an equivalent file of one-third 

 the daily fishing records. Pair trawl data were used 

 because we determined that the vessels consistently ac- 

 counted for the largest proportion (38'r) of the walleye 

 pollock caught, and should be most representative of pol- 

 lock fishing activities (Table 2). As a result of this sub- 

 sampling, two data files were created: a one-third pair 

 trawl file and an all pair trawl file. 



We compared these two files by testing if their 

 monthly mean catch and effort data were statistically 

 the same. This was achieved by (-tests of two means 



when their variances were unknown (Snedecor and 

 Cochran 1967). The tests showed that the null hypothesis 

 (that mean catch and effort of both data files were the 

 same) was not rejected at the 5' r level in all cases. Based 

 on these tests, we concluded that subsampling of the 

 1964-72 data base probably resulted in a random sample 

 of the population and was not biased towards high nor 

 low catches. 



Table 2.— Percentage of walleye pollock catches relative to total 

 catches accounted for by major Japanese vessel-gear categories in 

 the eastern Bering Sea, 1973-76. 



GRT = gross registered tons. 

 Source: Fisher>' Agency of Japan. 



