THE PR03LELI 



Up to the present, no method has been developed for determining 

 the age of individual specimens of the Pacific pilchard, or sardine. 

 Lacking a satisfactory substitute, it has thus not been possible to 

 isolate year classes of fish taken in samples of the commercial catch, 

 so as to trace, by serial sampling, their growth in length, or to fol- 

 low their passage through the fishery. 



Such available knov;ledge as bears on the growth and age of pilchards 

 results from the occurrence of dominant year classes in the population 

 from time to time, which can be readily recognized by their prominence 

 in systematically collected samples of the commercial catch; By this 

 method it has been shown (Clark, 1936, pp. 18-2I|.) that year classes 

 have entered the fishery when their mode i/as between 190 and 200 ram. , 

 that their approximate growth could be followed through as many as five 

 years, when their mode was in the neighborhood of 23P mm., and c"buld 

 be traced virith somewhat less certainty, if they persisted in the fishery, 

 for as many as five additional years, when their mode was near 260 mm. 

 Thereafter, according to Clark's work, the hitherto dominant groups have 

 completely lost their identity. 



This statistical method is useful only for studying those year 

 classes abundant enough to produce persistently prominent modes in the length 

 frequency curves. However, the method does not permit determining whether 

 a dominant group is composed of one or of several year classes, nor can 

 it be used for studying individually or collectively the more numerous, 

 adoLiinant age classes. Furthermore, as Thompson (1926, p. 18?) says, 

 "....our vie\T of the course of a given year-class is probably much dis- 

 torted by the conditions of selection and by the errors of sampling." 



Neglecting, for the purpose of this study, the problem of sampling, 

 an ideal method of analyzing frequency curves would expose and recognize 

 individual year classes from the youngest to the oldest, and of all 

 degrees of abimdance. To be accomplished this necessitates age deter- 

 mination of individual fish. 



Thg desirability of such a method was emphasized by Thompson (op. 

 cit. p. S2) who says, "There are two ways in which the abundance of 

 fish may be affected; first by the variations in mortality rate at 

 various stages of life preceding capture, and second by the direct 

 effect pf the enrivonment upon the movements or habits of the fish. 

 In the first of these the analysis for size or age must be of supreme 

 utility, being the sole means of determining the relative proportion 

 each age forms of the catch..." 



Since the ages of individual fish are determined usually from scales 

 or otoliths, Thompson made a preliminary examination of pilchard scales 

 in the course of his investigation on that species (Thompson, op. cit). 

 He used for this purpose a collection of 19h specimens, ranging from lUU 

 to 228 mm. in length, taken from October 28 to December 2. These were 



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