To compare the effect of teraporature on pilchard development vd.th 

 that of development of other fishes, the temperature characteristic (ivfu), 

 also knovm as the thermal increment, has been calculated for each of 

 the four stages of eggs and given in table 5» Since the temperature char- 

 acteristic depends primarily on the slope of the lines, and since the 

 four fitted lines, although nearly parallel, are not exactly so, each 

 stage has a somev;hat different Mu value. These vary from 25,800 (the 

 value furthest out of line) to 22,100. Since the individual slopes, are 

 not significantly different from the common slope (four lines fitted 

 simultaneously) the latter may be taken for computing the average tempera- 

 ture characteristic for the v^rhole period of development, v^hich is 22,^00. 

 This is of similar magnitude to the fev; Ivlii values that have been deter- 

 mined for the development af eggs of other teleosts, which range between 

 16,700 and 214., 900. Hence the pilchard is not atypical, and the observa- 

 tional evidence here used gives substantially the same conclusions as 

 the more commonly used experimental evidence. 



SIMIARY 



1. The ratu of development of pilchard eggs as influenced by the 

 temperature of the water v/as studied from samples and data collected 

 in nature during the 19U0 and 19i4.1 surveys of pilchard spavming off 

 southern California. 



2. Evidence based on the time of occurrence of newly-spavmod pre- 

 cleavage eggs in the samples indicates that pilchard spavming is largely 

 limited- to the four-hour period between 8:00 p.ia. and midnight. 



3. Experiments conducted on the vessel at sea shovrcd that the pil- 

 chard egg ordinarily required about three days to develop to hitching. 



k- It is usually possible to separate the eggs according to the 

 several days of spawning represented in the samples , and to determine 

 the probable length of time each stage had been developing. 



5. The temperature range over v/hich pilchard, eggs v/ere found de- 

 veloping in nature was from 12.5°C. to 17.6°C. Hovrcvcr, the majority 



of pilchard eggs vrcre taken at temperatures bet'v.'een 13.5° C. and l6,0 C. 



6. The Correlation of temperature with the rate of development 



of pilchard eggs was analyzed for four stages of eggs, Sir.iilar results, 

 were obtained for all, 



7. There is no evidence that the relation between temperature and 

 rate of development changes during incubation. 



8. The time required to reach a stage of development increases 

 about 7.15 percent for each 0.5° C. decrease in temperature. 



9. The temperature characteristic (Mu) for the pilchard egg during 

 development is about 22,500. This is of similar magnitude to the few 



lAi values determined for the development of other fishes, 



146 



