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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



school and were no longer susceptible to capture in plankton nets. Second, a school 

 of small mackerel was observed and sampled in Woods Hole Harbor in July 1926, 

 containing individuals between 35 and 65 mm. in length (table 21). The first evi- 

 dence shows that the schooling habit did not involve fish under 30 mm. in length; 

 the second proves that some fish, at least, begin schooling as soon as they exceed that 

 size. 



Vertical distribution. — From series of horizontal hauls at 0, 5, 10, 20, and 35 

 meters at early morning, midday, evening, and midnight, at a station (Albatross II 

 No. 20552) southeast of Fire Island Lightship (latitude 40°20' N., longitude 70°57' 

 W.) visited on July 13 and 14, 1929, there is evidence that the larvae of the mackerel 

 do not descend far below the surface, probably being limited by the thermocline, and 

 that they perform a diumal vertical migration (fig. 5). 



Table 4. — Vertical distribution of mackerel larvae] at various limes of the day as indicated by horizontal 

 lows with a closing half-meter plankton net at Station 20662 (Albatross II), latitude 40°20' N., 

 longitude 72"69' TV., July IS and 14, 1929 



' Midpoint of the 20-minnte,haul31s'given. 



In detail it will be noted (table 4) that in any one series of hauls the larvae were 

 caught mostly at only one or two levels; indicating that they were confined to such 

 thin strata that the entire population could easily, at times, be situated between the 

 levels of the hauls, and hence at those times be missed. Accordingly, it is probable 

 that in the evening the larvae were nearly all at the 10-meter level, probably traveling 

 upward, and by midnight some had reached the 5-meter level and some the surface. 

 The deeper ones probably continued upward so that nearly all reached the surface 

 shortly after midnight; and by 3 a. m., when the next series began, they had begun 

 to descend so that they were between the surface and the 5-meter level, and few were 

 taken in the hauls at either level. By noon, they probably had descended beyond 

 10 meters and were located between the 10 and 20 meter hauls, and none was caught. 



