160 



< 

 IT 



e 



3 

 O 



o 



bJ 



m 



PM 9tolO lOtoll ntolZ l2tol lIoZ 2to3 3ta4 A.M. 



TIME NETS EMPTIED S RESET 



Figure 14.--Migration of sockeye salmon juveniles for 

 two nights in 1957, Brooks Lake outlet. 



BNO.S 



Figure 15. — Hourly catch of yearling sockeye salmon 

 caught in each net expressed as a percentage of each 

 net's total hourly catch. Brooks Lake outlet, night of 

 May 31 to June 1, 1957. 



On the basis of these night-long samples, 

 the 1-hour period from 10 to 11 p.m. was 

 selected for subsequent sampling. Later in 

 the summer when darkness fell earlier, sam- 

 pling was moved first from 9 to 10 p.m. and 

 then 8 to 9 p.m. This early schedule avoided 

 conflict with brown bears that fished for 

 spawning adult salmon at the net sites below 

 the weir later in the night during September. 



On the night of May 30-31, the wing leads 

 of net 1 were removed at 11 p.m. to see what 

 effect this reduction in efficiency would have 

 on the catch. The effect was immediate and 

 drastic — although the catch of net 1 was usually 

 the highest, it dropped to the lowest for the 

 remainder of the night. This great reduction 

 in catch was brought about by reducing the 

 effective width of the net from 6.5 to 3 feet. 



To detect relative differences in the propor- 

 tions of yearlings and 2-year-olds caught by 

 each of the three nets, the hourly catch of 

 yearlings on the night of May 31 to June 1 was 

 plotted as a percentage of the combined catch 

 of yearlings and 2-year-olds for each net 

 (fig. 15). For example, in net 2 from 10 to 

 11 p.m., 28 percent of that hour's catch con- 

 sisted of yearlings and 72 percent of 2-year- 

 olds. No major differences were apparent. 

 The percentage of yearlings showed a slight 

 increase during the middle of the night, but 

 the range of the means was only 9 percent 

 through the entire night. The catch of year- 

 lings was progressively less from net 1 to 



net 3. Fluctuations in the hourly percentages 

 by net were so slight that no adjustment in 

 the sampling technique was made. Sampling 

 during any single hour yielded representative 

 samples. 



Net 1 was the only net fished for the entire 

 season; its total catch for each nightly 1-hour 

 sampling period provided an index to seasonal 

 abundance (fig. 16). Probably some migration 

 had already occurred when fishing began on 

 May 20, since migrants were caught the first 

 night of fishing. Peak migration occurred on 

 May 29 when 343 sockeye salmon were cap- 

 tured during a 1-hour period. 



In previous years, migrant sampling had 

 been discontinued in mid- June, but in 1957 test 



390 



I,, I rr:^^! 



20 29 

 MAY 



1— 1-^— ^"T— p-r-^ 

 9 19 29 9 19 29 9 19 29 9 19 29 

 JUNE JULY AUGUST SEPT 



Figure 16. --Catch of downstream migrant sockeye 

 salmon. Brooks Lake outlet. May 20 to September 25, 

 1957 (net 1 only, l-hour catch each night). 



23 



