BIOLOGY OF THE ATLANTIC MACKEREL 



173 



It is improbable that the daytime descent was beyond the 20 meter level at this 

 station or was ever beyond the thermocline. During 1930, 1931, and 1932, when 

 the nets were hauled obliquely below as well as above the thermocline, the lower tows 

 seldom caught larvae that could not be accounted for as contaminants resulting from 

 passage through the upper layers. 



From the length-distribution of the larvae it appears (table 4) that the larger 

 individuals (6 to 9 mm.) were more stongly inclined to migrate, reaching the surface 

 at night, while the smaller ones (4 to 5 mm.) tended to stay in the intermediate 

 5- to 10-meter levels. 



Though these observations do not provide a precise description of vertical dis- 

 tribution and migration, they do demonstrate the necessity of sampling all levels 

 down to the thermocline to get the representative statistics needed for the studies 

 on growth and mortality to follow. 



120 



t 



2E 



-i 1 r- 



MIONIGHT 



Dawn 



S 10 12 14 18 18 



T gMPER«TU3r., *c. 



Figure 5.— Vertical distribution of mackerel larvae at several points of time in the dirunal cycle in relation to temperature. Tn 6 

 solid lines connect observational points. The broken lines indicate the probable vertical position of the bulk of the population 

 of larvae. 



GROWTH 



Very little has been published on the growth of marine fishes during that early 

 period of the life history spent in the plankton community, and nothing on the 

 growth of the mackerel during this stage. Of the data collected during the present 

 investigation, only those of 1932 were collected in a manner sufficiently quantitative 

 and at short enough intervals of time to be used in deducing growth rate. 



The method of analysis consists, essentially, in following the advance in position 

 of the mode of homologous groups of larvae by comparing sizes collected in successive 

 cruises. But this cannot be done in a simple and direct manner. Mackerel eggs 

 are spawned over a period of several months. The larvae are subject to high mor- 

 tality. As a result, almost always there are vastly more small larvae than large ones, 

 and the predominance of small larvae is so great during most of the season that the 

 groups of larger ones do not form distinct modes. Instead, in ordinary arithmetic 

 frequency distributions they are apparent principally as a lengthening of tho "tail" 

 of the distribution at its right-hand side (table 5). 



