264 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



14,25 1625 



TEMFtRATURE »C 



Figure 10. — Percentage of stations having jack mackerel 

 eggs, grouped by 0.5° C. temperature intervals measured 

 at 10-meter depth for stations with hauls of 1-100 eggs, 

 with hauls of 101 eggs and more, and for all stations 

 where eggs were taken. 



All jack mackerel data were then combined for 

 each year and the mean, median, and modal tem- 

 peratures at 10 meters were computed (table IS). 

 The mean and median temperatures for the 4 



years are remarkably constant (15.5° C.) with 

 about 60 percent of the annual spawning occurring 

 within 1° of the median and mean. Less than 40 

 percent of all the stations occupied during the 

 spawning season (February through July) had 

 temperatures within 1° of 15.5° C. (table 19). 

 The constancy of annual temperature medians and 

 means would indicate a sharp temperature opti- 

 mum for spawning were it not for the within-year 

 temperature shift. The within-year temperature 

 shift suggests that a physiological temperature 

 optimum for jack mackerel is a function of many 

 environmental factors, such as condition of the 

 fish and availability of food, to mention two 

 possibilities. Furthermore, if temperature is the 

 controlling factor, spawning should occur more or 

 less imiformly throughout the area having the 

 optimum temperature. A temperature of 15.5° C. 

 is usually present in the waters off California or 

 Baja California, but spawning occurs only during 

 spring and summer. This may indicate that the 

 length of day has some regulatory effect on 

 spawning. 



Table 17. — Summary: Distribution of temperatiires at 10 ineters, by season, at stations occupied, 1951-S4 



[In 0.5° C. intervals] 



