Table 3. — Catch by mesh size and catch ratio of adjacent 

 mesh sizes, pink salmon, 1959 



1957 



1959 



i Original catches of the 4'i-inch mesh were 6 times as large as shown: they 

 were divided by 6 to equalize fishing effort between mesh sizes. 



The natural logarithm of the catch ratio is 

 directly related to the length of pink salmon for 

 1957 and 1959 (fig. 1). The straight lines are 

 fitted by the least squares method. The relation 

 in both years is approximately linear. Holt 

 showed in equation (2) that this relation must 

 be linear if the mesh selection curve is normal. 

 With replicating evidence for 2 years, I feel justi- 

 fied in assuming that the mesh selection curve for 

 pink salmon is approximately normal. 



To obtain the normal frequency curve for each 

 mesh size, the mean selection length and the 

 standard deviation of the curve were needed. 

 The estimation of these parameters for 1959 pink 

 salmon is shown in table 4 and the determination 

 of the normal curve in table 5. 



A further step in applying the method was to 

 construct a composite selection curve from the 

 mesh selection curves of the three mesh sizes 

 (fig. 2). This composite curve was obtained by 

 summing at each length class the ordinate heights 

 of the individual curves (table 6), as was done by 

 McCombie and Fry (1960). The composite curve 

 can be called a "catch efficiency curve" because 



Table 4. — Estimation of mean selection length and standard 

 deviation of mesh selection curves, pink salmon, 1959 



+ 6r 

 + 5 

 + 4 

 + 3 



+ 2 



O 



» - I 



41 44 47 50 53 

 FORK LENGTH (CM] 



41 44 47 50 53 

 FORK LENGTH (CM.) 



Figure 1. — Catch ratio of adjacent mesh sizes by fork 

 length, pink salmon, 1957 and 1959. 



Table 5. — Determination of ordinate heights of normal curve 

 for each mesh size, pink salmon, 1959 



1 L„ = mean selection length (cm.), S = standard deviation (cm.). 



2 Mean of 4.9 and 4.4. 



Table 6. — Summation of ordinate heights of three mesh 

 selection curves, pink salmon, 1959 



AT=93. 50/22.23 = 4. 21. 



GILL NET MESH NET SELECTION CURVES FOR SALMON 



383 



