178 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



50 



IE 

 40 bl 



30 



20; 



10 



o 



UJ 



-I 



30 



10 20 30 10 20 30 10 20 30 10 20 



MAY JUNE JULY AUGUST 



Fioitie 8.— Growth of mackerel larvae and post-larvae derived from the progression of modes of figure 7. The vertical lines at the 

 upper right represent the range of sizes and position of modes (diamonds) of young mackerel collected by dip net from pound 

 nets in the vicinity of Woods Hole, Mass., in. the years designated. The straight lines in the upper part, of the graph are on the 

 logarithmic scale. The curved line in the lower part represents the actual growth of the S series being plotted on an arithmetic 

 scale. 



Inasmuch as the S series had its origin in the area and near the time of maximum 

 spawning and formed the most distinct mode in the deviation curves, it may be taken 

 as most nearly typical of the growth of larvae in the season of 1932. In the lower part 

 of figure 8, the growth of this series has been plotted on an arithmetic scale from which 

 it is readily seen that mackerel hatching in early May attain a length of 4 mm. by 

 about May 20, 7 mm. by June 1, 12 mm. by June 15, and 22 mm. by July 1. This 

 rate projected to the 22nd of July reaches 48 mm. (nearly 2 inches), which closely 

 agrees with the largest larva of the final cruise and also with the length of individuals 

 in the dip net sample of July 22, 1926, which ranged from 35 to 65 mm. (1.4 to 2.5 

 inches). 



From the above relationship of sizes and ages, and from Worley's (loc. cit.) 

 data on rates of incubation, it is possible to compute the duration and average age of 

 each of the egg stages and of each size-class of larvae. Apart from its value per se, 

 this is of use in further computations of mortality rate. 



This was calculated as follows: the weighted mean temperature in which the 

 stage A eggs were found during the cruises of 1932 was 10.9° C. At this temperature 

 the incubation period occupies 7.23 days (Worley 1933, fig. 5). Stage A, representing 

 the development from fertilization to complete epiboly constitutes 35 percent of the 



