WORLD-WIDE DECLINES IN SARDINE CATCHES 185 



For the three sardine fisheries mentioned, we may safely rule out (1) as 

 a cause of the decline in the catches (barring war-caused interruptions 

 in fishing); one or more of the other three must be responsible. 



In order to examine the possibility of the other causes being oper- 

 ative, various types of information are needed. In the case of natural 

 mortality rates, some estimates have been made, but they are not very 

 precise. In addition, for sardines, or practically any marine fish for that 

 matter, we have only general ideas about the various sources of natural 

 mortality and practically no ideas about how they operate. In any case, 

 they are not subject to modification by man, except as fishing mortality 

 may replace natural mortality. 



Variations in rate of recruitment are probably one of the two major 

 causes of fluctuations in catch (the other being variations in availabil- 

 ity). Here again, we have only general ideas about the natural causes 

 of variation in year-class size and how these causes operate. So far as all 

 present evidence goes, such variations are natural. However, in theory 

 at least, the size of the spawning stock could influence the size of the 

 resulting year-classes. If the theory holds, Man could control year-class 

 size by controlling stock size, but this remains to be demonstrated in 

 practice. Data on relative year-class sizes as they appear in the catches 

 are of record, at least for the North American sardine fishery. What 

 we need, however, are data on year-class sizes as they exist in the popu- 

 lation. ^ 



Variations in the availability of fish to the fishery are, as indicated, 

 a major cause of fluctuations in catch. Accurate availability data are 

 extremely difficult to obtain and, in general, availability phenomena 

 are among the least understood in the field of fishery biology. For most 

 marine fisheries, however, it is difficult to see how they could be any- 

 thing but natural and not subject to control by Man. 



So, the problem of the causes of these declines falls into two major 

 divisions: (1) What are the effects of Man's activities and how do these 

 operate, and (2) What are the effects of natural phenomena and how 

 do these operate? Solution of these two sets of problems will require 

 not only additional observation, but also the development and testing 

 of theory. 



Returning to our original question, if the declines in catches of 

 sardines were caused solely by Man, then the declines are indeed re- 

 lated. Such a relationship is not within the province of the fishery bi- 

 ologist, although the mechanics of its operation obviously must be. 



On the other hand, if the declines are natural ones, how could they 

 be related? Such relationship could only be through a pandemic, which 

 has greatly increased the rate of natural mortality and for which, inci- 

 dentally, there is not much evidence, or, through some rather world- 



