is approximately 1 to 1 . Alternatively the eggs 

 0.20 to 0.38 mm. may be only an extension of 

 the large mode of nonyolked eggs that is 

 resorbed following spawning of the more 

 advanced yolked eggs. The presence of a 

 bimodal size distribution of yolked eggs in the 

 developing ovaries of fish species that spawn 

 pelagically seems to be a typical condition, 

 but whether this bimodality means the fish 



will necessarily spawn more than once is an 

 unsettled question. 



Plankton tows taken at monthly intervals at 

 the CalCOFl stations off the coasts of Califor- 

 nia and Baja California reveal the following 

 seasonal occurrence for the 105,776 jack 

 mackerel larvae taken in the 7-year period 

 1951-57: 



The average age of the above larvae is 6.6 

 days from spawning, and therefore the larval 

 occurrences should adequately indicate the 

 spawning season. 



Farris (1961) deternnined the daily distribu- 

 tion of spawning by plotting the relative 

 abundance of precleavage eggs against time 

 of collection. He found that about one- third of 

 these newly spawned eggs were taken in the 

 hour between 2330 and 0030, and almost two- 

 thirds in the 4 hours between 2030 and 0030; 

 spawning activity seemingly reaches a peak 

 shortly before midnight. 



On monthly survey cruises, 1950 through 

 1952, Ahlstrom and Ball (1954) took jack 

 mackerel larvae at water temperatures (20- 

 meter depth) of 10° to 19.5° C, but over 70 

 percent of the larger concentrations of larvae 

 (50 or more larvae per standardized haul) 

 occurred within a 2° range, 14° to 16° C. 

 They also found that 80.1 percent of the larvae 

 occurred offshore between Point Conception, 

 California, (about lat. 35° N.) and San Quentin 

 Bay, Baja California, (about lat. 30° N.); 10.7 

 percent of the larvae were taken north of this 

 area and 9.2 percent south. The spawning area 

 did not appear to extend south of the area 

 surveyed, but there was probably some spawn- 

 ing to the north outside the survey area. 



Spawning as indicated by percentage occur- 

 rence of larvae (adjusted to number of sta- 

 tions) relative to distance offshore was as 

 follows: 



80 miles 9.9 



160 do 32.7 



240 do 39.9 



320 do 15.0 



400 do 2.5 



Spawning probably also extended farther sea- 

 ward than the survey area. 



3.17 Spawn 



On the basis of 538 eggs taken during April 

 and May 1950 from various parts of the spawn- 

 ing range, Ahlstrom and Ball (1954) described 

 the jack mackerel egg as pelagic, nonadhesive, 

 spherical, 0.98 mm. (range 0.90-1.08) in di- 

 ameter, with a yolk 0.80 mm. (0.68- 0.88) in 

 diameter, a single oil globule 0.26 mm. (0.18- 

 0.35) in diameter, and a perivitelline space 

 0.09 mm, wide (fig. 4). The eggshell is clear, 

 tough and unsculptured. The irregularly seg- 

 mented yolk mass appears yellow to amber in 

 preserved material. 



Ahlstrom (1959 table 7) showed that 97 per- 

 cent of the jack mackerel eggs and 88 percent 

 of the larvae are found in the upper 50 meters 

 of water. Few eggs or larvae are found below 

 100 meters and none below 140 meters. 



3,2 Pre-adult phase 



3.21 Embryonic phase 



Embryonic development of the jack mackerel 

 is typical of most fish with pelagic eggs. One 

 identification feature present throughout em- 

 bryonic development is the segmentation of yolk 

 naaterial which is usual in the eggs of iso- 

 spondylid fishes, but unusual in those of 

 percomorph fishes. Ahlstrom and Ball (1954) 

 gave a detailed description of the embryonic 

 development of the jack mackerel (Fig. 4), 



According to data presented by Farris 

 (1961), incubation tinne from spawning to the 

 last stage before hatching follows the formula 

 log Y_ = 3.257-0.088X in which Y = hours and 

 X - temperature in degrees Centigrade. Thus 

 incubation time would be 2 days at 17.9° C, 

 3 days at 15.9° C, and 4 days at 14.5° C. 



