BIOLOGY OF THE ATLANTIC MACKEREL 



205 



elimination of mortality from any of these stages, but if a year class is to be successful 

 there is obviously greatest opportunity for improved survival in the noncritical stages, 

 for they contributed most heavily to the failure of the year class. For tbis reason, one 

 must look with at least as much suspicion on the mortality during non-critical stages 

 as on the mortality during critical stages when in search for casual agencies that may 

 have been operative during 1932. 



In looking for such agencies, there are two features of the 1932 season that ap- 

 peared to be unusual and of the sort likely to have affected survival. One of these 

 was the relative paucity of zooplankton in the area of survey during the spring and 

 early summer (i.e., May and June). The zooplankton catches averaged only 280 cc. 

 per haul, as compared with 556 cc. in 1931 and 547 cc. in 1930 (Bigelow and Sears, 

 1939, p. 200). Both of the last named seasons produced good year classes, and there 

 is, therefore, an indication of correlation between zooplankton abundance and the 

 survival of a mackerel year class. If failure to survive in good numbers in 1932 was 

 in fact due to dearth of food, and the dearth was continuous throughout the season 



Fiocbe 18.— Resultants of wind movement, as recorded at Wlnterquarter Lightship daring May of each year 1930-1933. 



of larval development, as the data indicate, it could easily affect the mortality through 

 virtually all stages, for the smaller fish larvae probably feed on the young stages, and 

 larger larvae on the adult stages of zooplankton forms. 



The other distinctive feature was the prevalence of northeasterly winds during 

 the period of larval development in 1932. Figure 18, in which are plotted the result- 

 ants of wind movement of force 3 Beaufort scale or higher, during May of each year, 

 1930-33, demonstrates how 1932 differed from the other years in having an excess 

 of northeasterly over southwesterly winds. That this may well be related to the 

 production of successful year classes is indicated by the fact that 3 years, 1930, 1931, 

 and 1933, all with an excess of winds from the southwest, gave rise to successful year 

 classes, while 1932, the only one with an excess from the northeast, failed to produce 

 a successful year class 24 (Sette, 1938, p. 19). 



Since the discovery of this relation between successful mackerel year classes and 

 wind movement, similar phenomena have been reported for other fishes. Carruthers 



" The wind directions in 1928 and 1929 were not consistent with this rule of correspondence of southwesterlies and successful 

 year classes, but there were other unusual features of the year classes from these seasons and therefore consideration of them will be 

 left to a subsequent paper of this series. 



