BIOLOGY OF THE ATLANTIC MACKEREL 155 



I he general rate, of doubtful significance, were during egg stages, when about ^ 

 percent per day was indicated, and during the yolk sac stage (3-mm. larvae), when 

 about 23 percent per day was suffered. 



The indicated total mortality, from the spawning of the eggs to the end of plank- 

 Ionic existence (50 mm. or 2 inches long), was 99.9996 percent. That is, the survival 

 was in the order of magnitude of only 1 to 10 fish per 1,000,000 of newly spawned eggs. 



This mortality was not due to sharply higher death rate at the yolk-sac stage — 

 a theory of year-class failure holding favor among fishery biologists. Mortality was 

 substantial in all stages. It was greatest during fin development in the transition 

 phase from larval to post-larval stages. The higher mortality at this time appears 

 to have been connected with the particular pattern of drift caused by the dominant 

 wind movement, which in 1932 left the larvae farther than usual from their nursery 

 grounds along the southern New England coast. This, together with a general 

 scarcity of plankton, is considered the cause of failure of the 1932 year class. 



SIGNIFICANCE OF RESULTS 



Most conservationists lay particular stress on the maintenance of adequate spawn- 

 ing reserves. It is important to do so. If an annual commercial crop is to be con- 

 stantly obtained, the spawning stock must be kept large enough to produce as many 

 young as are needed to replace the fish caught by man and other predators. This can 

 be done, in most cases, only by controlling the annual 3'ield. From this springs an 

 obvious, but not universally appreciated, fact that accumulating a surplus of spawners 

 is a wasteful practice, for it means holding the annual yield below the amount that 

 the resource is capable of producing. It would be simple, for instance, to insure an 

 adequate spawning reserve by allowing no fish to be caught. But this would be more 

 futile than to allow all to be caught. The latter would utilize one crop, the former 

 none. Obviously, efficient exploitation calls for an intermediate course of action, 

 one that would permit taking the maximum annual yield commensurate with the 

 maintenance of an adequate spawning reserve; no more and no less. 



But what is an adequate spawning reserve? It can be defined as one large 

 enough to reproduce the young needed to recruit the commercial stock. Its deter- 

 mination is a matter of observing the numbers of recruits produced by spawning 

 stocks of different sizes. Thus, the answer rests on knowledge of recruitment. 



Two things affect recruitment: First, the numbers of spawners; second, the 

 mortality in young stages — "infant mortality." The latter is tremendous and 

 variable. Its variability is so great that it could readily obscure such correlation 

 between number of spawners and number of recruits as might be present intrinsically. 

 For example, under a given quality of survival conditions a large spawning population 

 may produce a large number of recruits and a small population a small number of 

 recruits, but with variable survival conditions a large number of spawners might 

 produce only a small number of recruits if infant mortality be relatively high; and 

 conversely, a small number of spawners might produce a large number of recruits if 

 infant mortality be relatively low. As long as one can observe only the changes in 

 numbers of spawners and numbers of recruits, the relation between the two cannot 

 be seen, for it is obscured by the intervening infant mortality. Therefore, as long as 

 the effect of infant mortality is unknown, so long will the size of an adequate spawning 

 reserve be unknown. 



Thus the measurement of infant mortality is the key to the problem. In the 

 course of this study, a technique for making this measurement has been devised, and 



