BIOLOGY OF THE ATLANTIC MACKEREL 191 



average rate of movement is sometimes about 3% nautical miles per day and may at 

 times, on the part of the largest individuals, attain eleven nautical miles per day. 

 In 1932, the combined drift and swimming movements brought the larvae to the 

 shores of Long Island and southern New England. 



MORTALITY 



Outstanding in the early life history of marine fishes is the high mortality in 

 early stages. At sea, this is evident from the low numbers of larvae compared to 

 the high number of eggs taken in plankton tows. In marine fish hatcheries, it has 

 been evident from the high loss of larvae in all attempts to keep them beyond absorp- 

 tion of the yolk sac. It is probable that the fish cultural experience led to the gen- 

 erally accepted theory that the time of yolk sac absorption is the most critical period, 

 and that it is so because the fish at that time must find proper food or die as soon as 

 all the yolk is gone. Moreover, Hjort (see p. 207) believed that annual variation in 

 the times and places of plankton increase during spawning might be such that an 

 abundance of the right kind of food might coincide with this critical stage in one 

 year and not in another. The coincidence of the two would produce a successful 

 year class; the non-coincidence, a failure. 



However elaborate the theory, it has yet to be proved at sea that the yolk sac 

 stage is critical or that the annual variation of mortality in this stage is responsible 

 for the variation in year-class strength. Thus, a determination of mortality of the 

 young stages of mackerel in 1932 is not only of interest in itself, but has an important 

 bearing on the general theory of fluctuations in fish populations. Inasmuch as the 

 year class of 1932 has subsequently faded to appear in the commercial stock in impor- 

 tant numbers (Sette, 1938), the present examination of mackerel mortality in the 

 season of 1932 deals with the record of a failing year class and should bring to light 

 the stages that were critical in its failure. 



Determination of mortality. — There is at hand a simple way of determining the 

 mortality rate of that year if it may be assumed that all the various egg and larval 

 stages were sampled in proportion to their abundance in all parts of the spawning 

 grounds, and during the entire period of planktonic existence. Then a frequency 

 distribution of the summed numbers at each stage through the season would express 

 their average relative numbers and constitute a survival curve. Although the 

 sampling in 1932 approached a stage of perfection warranting treatment based on this 

 general plan, there were nevertheless imperfections requiring secondary modifications, 

 as will be explained. 



The actual drawing of hauls appears to have been qualitatively and quantitatively 

 adequate. At each station, all levels at which eggs or larvae might be expected to 

 occur were sampled uniformly, and the subsequent adjustment for volume of water 

 strained per meter of depth provided totals at each station which may be taken as 

 the summation of individuals below 17.07 square meters of sea surface, irrespective 

 of their level in the water. Comparison of 1-meter and 2-meter net hauls indicated 

 that there was relatively little selective escapement from the nets (p. 215). Also, the 

 towing stations formed a pattern reasonably well covering all parts of the important 

 spawning grounds off the United States coast. 



On the other hand, in some respects the samples did not adequately cover the 

 entire season. At the time of the first cruise, spawning had already begun and 

 larvae were taken for which there were no corresponding eggs. Simdarly, force of 

 circumstances prevented cruises from being taken as frequently in July as earlier in 



