Samples for the eight Si.,'asons of the :.arly period v/ore corabint^d to dis- 

 count the effect of fluctuating recruitment. P'or the latter period, v/hen 

 recruitment was more uniform, ago readings for the last season (19U1-U2) 

 only were used._/ 



Table 1. — Numerical values detennined for age compos ition, relative 

 fishery effort, and catch. 



The numorical values given in Table 1 represent the results of a 

 canvass of the data available at present. Many shortcuts and approxima- 

 tions have entered into their derivation, but they have been used in 

 the computations to obtain a preliminary estimate of the changes which 

 have taken place in the pilchard stock, and to shovi the way in vifhich 

 more precise data may be used when available. The analyses necessary 

 to the derivation of such precise data are nov^r being carried on by the 

 United States Fish and VlCildlife Service and the California State Fish- 

 eries Laboratory. 



DETERI.IINING TOTAL MORTALITY 



The rate of disappearance, or total mortality, of the year classes 

 may be determined from the relative age composition of the stock. As 

 pointed out by Baranov (1913), in a stabilized population, the numbers 

 of fish of each age, starting with the youngest commercial -size, are the 

 same as the numbers of a given year-class surviving each year since its 

 entry into the fishery. Therefore, the effective rate of disappearance 

 may be determined from the age composition of the stock at any time* after 

 stabilization at that rate. The simplest method is to plot the logarithms 

 of the numbers of fish of each age, so that a constant percentage decrease 



w 



A recent study of the method of taking scale samples for use in the , 

 age analysis indicated that there v/as a tendency to select the larger 

 fish of each sample (scales were taken from only 10 fish out of each 

 50 measured) for the removal of scales. This may have caused an in- 

 dicated mortality rate lower than the actual. Computations to correct 

 for this selection are now being madet 



171 



