SAMPLING LOCATIONS 



AUGUST 1976 



130" 178°W I76"W 174"W 172°W 170°W 



I68"W 166"W 



162°W 160"W 158°W 



Figure 9. — Location of successful trawling stations in the Bering Sea during August 1976. 



Table 9.- 



-Summary of subsampling for processing successful trawl 

 catches during the 1976 Bering Sea surveys. 



Percentage of catch 



Number of 



Cumulative proportion 



processed 



trawl samples 



of total number 



100 



547 



0.824 



90-99 







0.824 



80-89 







0.824 



70-79 







0.824 



60-69 



5 



0.831 



50-59 



8 



0.843 



40-49 



19 



0.872 



30-39 



25 



0.910 



20-29 



28 



0.952 



10-19 



24 



0.988 



<10 



8 



1.000 



Total 



664 



1.000 



General Distributions of Faunal Abundance 



Sampling biases. — Two approaches have been used to describe 

 the distributions and abundances of species during the 1976 spring 

 trawl survey. For most summaries of the data, an assumption has 

 been made that the target populations maintained stationary geo- 

 graphical distributions throughout the 3-mo duration of the 

 survey. However, because some species populations were ap- 

 parently undergoing relatively rapid, long-distance (100-300 km) 

 migrations between different regions of the study area, other sum- 

 maries of the data are presented by individual months. 



Measures of abundance during the survey were apparently af- 

 fected by two major sources of error: The progression of the 



survey following the dispersal of some populations from deep to 

 shallow waters (particularly yellowfin sole and Alaska plaice), 

 with repeated sampling of high fish densities; and possible 

 seasonal variations in the vulnerability of some populations (most 

 notably walleye pollock) to the trawl. 



The effects of migration apparently influenced substantial 

 overestimates of true population (primarily flounder) abundance 

 within the study area. These biases were caused by exceptionally 

 high flounder densities following the recession of the pack ice 

 edge into Bristol Bay during April-May. Although estimates of 

 yellowfin sole and Alaska plaice abundance were apparently most 

 severely biased, the overall estimates for other migrating fish 

 species may also have been affected. 



Seasonally varying vulnerability to the trawl may have caused 

 substantial underestimates of true walleye pollock abundance 

 within the survey area. In particular, changes in the vertical 

 distribution of walleye pollock within the water column may have 

 caused a varying (but perhaps sometimes large) proportion to be 

 missed by the bottom trawl. Spring spawning behavior of walleye 

 pollock is reported to include schooling of large individuals high 

 in the water column (Serobaba 1974). 



Overall catches. — A total of 78 fish species distributed among 

 22 families was recorded from the 435 trawl samples used for pop- 

 ulation analyses (Table 10). In general, overall trawl catch rates 

 were highest north and northeast of Unimak Island, along the 

 Alaska Peninsula, and east of the Pribilof Islands (Fig. 13). The 

 overall observed abundance of major taxonomic groups are sum- 

 marized in Table 1 1 and expanded to apparent biomasses in Table 

 12. Fish accounted for approximately 73% of the mean total 

 catch, and invertebrates 27%. 



15 



