BIOLOGY OF THE ATLANTIC MACKEREL 185 



The southern center shifted southward from off Delaware Bay (Cape May II) 

 half way to the Chesapeake Capes (Winterquarter I) where it remained during the 

 following cruise and possibly the next one also, though these stations were not visited 

 on the fourth cruise. During the fifth cruise it was found farther north and seaward 

 in the offing of the southern New Jersey coast (Atlantic City III and Cape May IV). 

 Next it appeared to join the northern center and was apparent as a tongue extending 

 from this center to the offing of the middle of the New Jersey coast (Atlantic City II) . 

 Thereafter its location apparently coincided with the northern center. 



During the time that the two centers were separate they moved in essentially 

 identical directions (fig. 15). Both moved southward from May 3 to May 22 and then 

 northward until June 7, apparently under a common impulse. If the resultants of 

 wind direction and force during the cruises be plotted, 16 as in figure 15, it is seen that 

 the strong winds blew in essentially the same direction as the larvae moved, southerly 

 until May 22 and then northerly until June 7. Obviously the wind, by drifting the 

 surface water, was responsible for the transport of the larvae. After June 7, however, 

 the movements of larvae did not correspond so closely with the movements of the wind 

 (fig. 16) and must have been to some extent independent of them. Thus the move- 

 ments of the population of mackerel larvae may be divided into two phases, an early 

 passive phase and a later active phase. The break between the two came, as might 

 be expected, when the larvae, at a length of 8-10 mm., developed fins (p. 171) and 

 graduated from the larval state to the post-larval stage. The movements in the two 

 stages will be considered in detail separately. 



During the passive phase, although the movements of the two centers of larvae 

 are essentially similar and both correspond to that of the wind, there are minor differ- 

 ences worthy of note. The southern center was found at the same place on cruises 

 II and III in spite of considerable sustained wind from the northeast and corresponding 

 movement of the northern center in the interim between the cruises. Later there was 

 the great shift of the southern center between cruises III and V without correspond- 

 ingly great wind movement and without correspondingly great drift of the northern 

 center. To some extent these discrepancies may be due to failure precisely to locate 

 centers of distribution with the stations as far apart as of necessity they were. 16 



But it is more likely that the peculiarity in the relation of the drifts of the northern 

 and southern centers has a physical rather than statistical basis. The outstanding 

 peculiarity was that the northern center traced a course in a southerly direction almost 

 equal in distance to its return in a northerly direction (up to cruise VI) whereas the 

 southern center moved southerly a much shorter distance and then returned northerly 

 a much greater distance. Considering now the topographical features, it is noticeable 

 that at the northern and middle portions of the area the continental shelf is broad and 

 the water relatively deep, while at its southern end the shelf narrows sharply and the 

 water is much shoaler. A water mass impelled by the wind could move in a southerly 

 direction freely until it reached the narrow, shoal southerly end where it must either: 

 (1) stream very rapidly through the "bottle neck" at the southern extremity; (2) turn 

 out to sea; or (3) pile up temporarily. 



" Records of the Winterquarter Lightship, 8 a. m. and 4 p. m., including only those winds of force 3 (Beaufort Scale) or higher, 

 were plotted in vector diagrams to determine the resultants. 



» The true position of the northern center at the time of cruise III (fig. 13) was particularly uncertain. On the chart of movement 

 (flg. 15) it seemed logical to plot it at the center of gravity between the three northern stations with largest catches, that is, Atlantic 

 City II, Cape May I, and Cape May III. but its true position most likely was between stations, there or elsewhere, and hence missed. 

 This accounts also for the almost complete obliteration of mode S on this cruise, to which attention was earlier called in discussing 

 progress of modes as indicating growth. 



