DISTRIBUTION OF EGGS AND LARVAE OF JACK MACKEREL 



275 



6 8 10 _ 



SIZE OF LARVAE IN MM. 



Figure 13. — Ratio of the average number of jack mackerel 

 larvae per night haul to the average number taken per 

 day (N/D), by size class, 1952-54. 



implication was that as the larvae grew larger and 

 stronger they were better able to dodge the net. 

 This increase in ratio with size is true not only of 

 the sardine, but also of the anchovy, the hake, the 

 Pacific mackerel, et cetera (Ahlstrom, personal 

 communication; Bridger, 1956). 



It is, indeed, surprising to find that jack mack- 

 erel do not behave in this manner. All jack 

 mackerel data for 3 years have been combined in 

 figure 13. The N/D ratio for the 7.75-mm. size 

 class has been deleted because one of the samples, 

 which was unusually large, prevented comparison 

 of this size class with the other size classes. A 

 least-squares line has been fitted to the data 

 Y=a + bX, where X=size in millimeters and 

 Y=N/D ratio. The regression statistics are 

 a=1.06; 6 = 0.006; ^^.= 0.196; X=6.0; T=1.10- 

 S6=0.057. 



The slope of the regression is not significantly 

 different from zero. The interpretation placed 

 on these data is that jack mackerel do not dodge 

 the net despite their apparent ability to do so. 

 The eyes are pigmented and presumably func- 

 tional about the time the yolk is absorbed (at 

 approximately 3.5 mm.), and larvae at yolk-sac 

 absorption are capable of movement. Ahlstrom 

 (personal communication) believes that jack 

 mackerel larvae can swim as well as the sardine 

 larvae. 



SUMMARY 



The distribution and abundance of jack mack- 

 erel eggs is described for 4 years, 1951 through 

 1954. The early survival of jack mackerel larvae 

 is described for 1952, 1953, and 1954. The data 



were obtained from monthly cruises during which 

 an average of 150 stations was occupied. 



Jack mackerel spawned in an area bounded by 

 the 26lh parallel on the south, the 45th parallel 

 on the north, the west coast of North America on 

 the east, and the 150th meridian on the west. 

 Most of the spawning occurred in a more restricted 

 area, the boundaries of which varied from year to 

 year. Eggs were mainly confined to the upper 40 

 meters of water. 



Spawning usually began in February, reached a 

 peak in May, and ceased by October. The peak 

 of spawning in 1951, which occurred in March, is 

 considered abnormally early. 



The temperature coefficient for the rate of egg 

 development was derived by a regression of log 

 hours of development on temperature in degrees 

 centigrade. Jack mackerel eggs kept under con- 

 trolled temperature conditions (in an incubator) 

 hatched at the time predicted by the derived 

 temperature coefficient. 



The estimates of egg abundance for 1951, 1952, 

 1953, and 1954 are 8.7X10'*, 5.9X10'*, 7.4X10'*, 

 and 4.6X10'*, respectively. 



The survival at the end of a 30-day period for 

 1952, 1953, and 1954 was 131, 112, and 179 larvae 

 per 100,000 eggs spawned, respectively. The 

 variation was considered insignificant. An in- 

 crease in surAaval rate during the second v>-eek of 

 larval life was noted. 



The relative growth rate of jack mackerel 

 larvae was approximated from observations on 

 laboratory populations. The relative growth dur- 

 ing the first 3 days is more rapid by a factor of 5 

 than the relative growth of the succeeding 4 days. 

 The onset of the slower growth is correlated in 

 time with yolk-sac absorption. 



An area which is bounded by line 80 on the 

 north, line 110 on the south, stations 90 on the 

 west, and the coast of California and Baja Cali- 

 fornia on the east has been shown to contain 

 almost all the month -old larvae. 



LITERATURE CITED 



Ahlstrom, Elbert H. 



1943. Studies on the Pacific pilchard or sardine 

 (Sardtnops caerulea). 4. Influence of tempera- 

 ture on the rate of development of pilchard eggs 

 in nature. U.S. Fish and Wildlife Service, Special 

 Scientific Report No. 23, 26 p. 

 1948. A record of pilchard eggs and larvae collected 

 during surveys made in 1939 to 1941. U.S. Fish 



