DISTRIBUTION OF EGGS AND LARVAE OF JACK MACKEREL 



261 



there are more occurrences of eggs than of larvae, 

 while in region 5, the converse is true. This may 

 indicate southern transport of the larvae by the 

 California Current. 



The high proportion of stations occupied in 

 region 2 at which eggs or larvae were taken is a 

 further indication (see also tables 3, 4, and 5) that 

 this is the region of maximum spawning activity. 



A comparison of total occurrences of eggs with 

 total occurrences of larvae is interpreted as 

 indicative of a distribution more regular than that 

 encountered for other pelagic eggs and larvae 

 (e.g., sardines). 



SAMPLING OF THE HORIZONTAL RANGE OF 

 SPAWNING 



To determine the proportion of the total annual 

 spawning which might be missed by failure to 

 extend the sampling far enough seaward, the 

 proportion of eggs taken beyond stations 90 (the 

 usual seaward extent of sampling) was computed 

 (see column 100 — seaward in tables 6, 7, 8, and 9). 

 This areal proportion was multiplied by the pro- 

 portion of annual spawning that occurred during 

 the appropriate 2-month interval (tables 2, 3, 4, 

 and 5) to give a bimonthly estimate of the pro- 

 portion of annual spawning which might be 

 missed by failure to extend the sampling suffi- 

 ciently seaward (table 12). It would appear 

 that at least 21 percent of the annual spawning 

 has occurred seaward of stations 90, and that a 

 portion of the eggs has been missed in those years 

 when monthly sampling was not extended beyond 

 that point. 



Table 12. — Proportion of jack mackerel spawning occurring 

 seaward oj station 90, by 2-month intervals, 1951-54 



1 Region not occupied. 



Data from the previously mentioned Norpac 

 indicate that the proportion of eggs spawned in 

 northern waters (north of line 40) may be between 

 1 and 2 percent of the annual total. This pro- 

 portion is minimal, since spawning occurs in 

 periods other than that covered by Norpac. This 

 estimate was determined by estimating the total 



spawning in the region for the period August-early 



September (8,655 billion oggs) and comparing that 

 figure with the estimated number of eggs spawned 

 in 1954 (464,452 billion) and 1951 (874,322 billion). 

 It is therefore inferred that an appreciable amount 

 of annual spawTiing may occur west of stations 90 

 but a lesser amount takes place north of line 40. 



EFFECTS OF TEMPERATURE ON 

 SPAWNING 



Temperature may have at least two effects on 

 the jack mackerel. It influences the rate at which 

 the eggs develop (see p. 250), and it may well 

 govern when and where the adults spawn. 



To determine the effect of temperature on the 

 spawning jack mackerel, temperatures at 10-meter 

 depths were tabulated by 0.5° C. intervals for all 

 stations where jack mackerel eggs were taken in 

 1951 through 1954 (table 13). The temperature 

 at tjiis depth is generally representative of the 

 strata in which jack mackerel eggs are most 

 abundant. These data were examined also for 

 seasonal effect by dividing the year into an early 

 (January-May) and a late (June-December) 

 period. The effect of temperature on the size of 

 haul was also examined by dividing the samples 

 into two categories: hauls containing 1-100 eggs 

 and hauls containing 101 eggs and more. 



A seasonal trend toward higher spawning 

 temperatures in the late summer with a greater 

 temperature range was indicated (table 14 and 

 figure 8). The data were then tabulated by 



14 25 15:25 



TEMPERiTURE *C 



Figure 8.— Percentage of early (January-May) stations, 

 of late (,Iune- December) stations, and of the total 

 stations having jack mackerel eggs, shown by 0.5° C. 

 temperature intervals measured at 10- meter depth. 



