temperature at each station by a single-;' value. For that .purpose the av- 

 erage of the temperatures at 10 and 20 meters was used. 2' Ti.j.s choice 

 v;as based on the results of a series of horizontal hauls made with closing 

 nets (cf. SillLTian, 19U3), which showed that although a few pilchard eggs 

 are occasionally found deeper than 50 meters, nearly all occur above 30 

 meters, and by far the greatest concentration- is usually between 10 and 

 20 meters or even nearer to the surface. Surface temperatures, however, 

 were not included in the average, because of their rather marked fluctua- 

 tions . 



To bo sure, the tcnperature readings may not represent exactly the 

 average temperature to which the eggs had been subjected from spavming 

 to the time of collection. HoTrever, we can assume, relatively safely, 

 that the temperature of the sea water usually had not changed very much 

 during this period of time, which, on the average, would have boon only 

 about a day and a half. 



Pilchard eggs vrcre found developing at teir^eratures ranging from 

 12.5° C. to 17.6 C. as measured by the mean of the 10 - 20 meter levels; 

 this is an over-all range of slightly more than 5.0° G. Hovrever, the 

 majority of samples containing pilchard eggs wore taken at temperatures 

 betvraen 13.5° C. and l6.Q° C, or v;ithin a range of ^,5° C. 



INFLUENCE OF TE].P:mA.tURS ON RATE OF DEVELOPLLiNT jF PILCHARD EGGS 



The priBiary information upon which the analysis of the relation be- 

 tween temperature and rate of development of pilchard eggs in nature can 

 be made arc the following: (1) The estimated age, in hours, of most pil- 

 chard eggs from the time of spawning to the time of collection and (2) 

 the temperature of the water at the depths where the majority of eggs, 

 were developing, taken at the time of collection of the eggs. 



An analysis could not be made of the influence of ten^Jerature on 

 the length of time required by the pilchard egg to develop completely 

 to hatching, because of the difficulty of determining, from the preserved 

 material, when hatching would have occurred. However, a correlation was 

 possible between temperature and length of time of development of the 

 pilchard egg to the stage immediately preceding hatching (stage XI, cf. 

 appendix). 



Not all records of stage XI could be used in the correlation, since, 

 in s'ome samples, only a small percentage of the eggs referred to a common 

 day of spawning had reached this stage, the majority still being in the 

 preceding gtage; also, in a few samples the age of the eggs could' not be 

 determined vdth certainty. Therefore, the folloi/ing criteria were set 



- Tomporaturos at 10 and 20 meters wore usually closely similar, and both 

 depth usually occurred in the stratum above the thermocline. For more 

 than 50 percent of the items used in the subsequent correlations, the 

 difference in temperature between these tvro depths v;as less than 0.1° C, 

 and for only' one -sixth of the items vras the difference greater than 0.5° C. 



159 



