1959 



Catch efficiency curve 



38 44 50 



FORK LENGTH (CM.) 



56 



Figure 2. — Catch efficiency curve (sum of ordinate heights 

 at each length) for three mesh sizes combined, pink 

 salmon, 1957 and 1959. 



it shows a relative catch efficiency of gill nets over 

 fish length range. Curves for 1957 and 1959 are 

 shown. 



In both years the catch efficiency curves show 

 a dip at 41 cm. Catch efficiency at 41 cm. is lower 

 because of the 1%-inch gap between the 3%- and 

 4^-inch mesh sizes. The gap between the 4K- and 

 5^-inch meshes is % inch. 



The final step in applying the method was to 

 reconstruct the length frequency curve of the 

 available fish population, adjusting for effect of 

 gill net selectivity. The uncorrected catch was 

 divided by the sum of ordinates at each length 

 class (table 7). The corrected catch for all length 

 classes was the length frequency curve adjusted 

 for effect of gill net selectivity. 



When the uncorrected and the corrected length 

 frequency distributions of pink salmon taken by 

 combined mesh sizes of gill net in 1957 and 1959 



Figure 3. — Length frequency distribution of pink salmon 

 adjusted for effect of gill net selectivity, 1957 and 1959. 



are plotted, a single mode of maturing 2-year-old 

 fish is evident (fig. 3). 



Adjustments for gill net selectivity in 1957 were 

 minor. In the uncorrected catches the 41-crn. and 

 44-cm. length classes were slightly under-repre- 

 sented and the 47-cm., 50-cm., and 53-cm. classes 

 were slightly over-represented. The corrected 

 catch curve adjusts for these conditions. The 

 mode, after I adjusted for selectivity, remains 

 unchanged at 44 cm. 



In 1959 the amount of correction- was somewhat 

 greater than in 1957. As in 1957, the 41-cm. 

 and 44-cm. lengths were under-represented, the 

 47-cm., 50-cm., and 53-cm. lengths over-repre- 

 sented. Adjustments in 1959 changed the posi- 

 tion of the mode from 47 cm. to 44 cm. 



SOCKEYE SALMON 



Least squares lines were fitted to the catch 

 ratios of sockeye salmon taken in 1959 by the 

 four gill net mesh sizes (fig. 4). Catch data are 

 in table s. Catch ratios were computed for 



384 



U.S. FISH AND WILDLIFE SERVICE 



