62 PISH AND GAME COMMISSION. 



An examination of the data from San Pedro by Mr. Thompson led 

 to a report npon the extent to which the changes could be followed. 

 The net result Avas to show that there is a cei*tain independence between 

 the fisheries for large and for small fish. This leaves it doubtful 

 whether a class which was dominant during passage through the smaller 

 sizes would remain so during passage through the larger sizes. A 

 certain year group might appear to be very abundant while it is 

 among the smaller sizes, simply because the few age groups with which 

 it would be compared are on a low level of abundance. But upon 

 becoming one of the large sizes which run during January and Febru- 

 ary the age groups with which it is contrasted are many more in 

 number and may include among them equally dominant ages. There 

 is, therefore, a necessity for careful observation as to just how far these 

 conditions limit the prophesying of the catch. 



There is also room for thought in regard to just how far these 

 phenomena limit our ability to detect depletion from overfishing. In 

 discovering this it is necessary to rely upon two criteria which can not 

 well be considered separately. The first of these is naturally the 

 decrease in actual numbers of fish in the ocean. The second is a decrease 

 in the average length of life of the individuals, or, in different words, 

 the decrease in relative numbers of old fish, a decrease in the proportion 

 of survivals. 



The presence of this great variation in numbers contained in a year 

 class, which we have termed the phenomenon of dominance or of sup- 

 pression, at once raises the question as to whether a lack of fish is due 

 to the "natural" lack of success in spawning the younger fish, rather 

 than to overfishing. This "natural" lack, however, would be accom- 

 panied by a greater abundance of the older fish, overfishing by a lesser 

 abundance. The phenomena of dominant age groups, which we have 

 demonstrated, therefore throws the burden of proof of overfishing upon 

 the second criterion, namely the decrease in average length of life, 

 which would accompany the greater death rate caused by intensive 

 fishing. 



The determination of the average length of life involves a com- 

 parison of the young and old age classes. In many species it may well 

 be a fair presumption that the younger age classes are always of 

 approximately the same abundance, the success of spawning not vary- 

 ing, so that at any given time the percentage of survival at a greater 

 age may be determined at least approximately by the direct comparison 

 of that older age group with the younger ages present. However, as 

 we have shown in the sardine, the nature of the fishery and the errors 

 in the necessary method of sampling, effectively preclude a reliable 

 comparison of the smaller and larger fish, Avhich represent the young 

 and old. 



There is, then, nu choice for us in our observation of the sardine 

 fishery, save to rely upon following each year class through its life 

 liistory, and to compare its abundance at each stage with an average 

 obtained by many years' observation. The difficulties in so doing are 

 manifold. We have therefore come to the conclusion that it is better 

 in the case of the sardine to frankly face the possibility that we may 

 not be able to detect overfishing until that overfishing has progressed 

 fartlier than it might be wise to allow. 



