1952-56 (Ahlstrom, 1959, p. 204). Furthermore, a 

 coincidence of distributions of parent stocks and 

 theii- eggs was detected in the rehited species, 

 Sardinops melanosticta, inhabiting the waters adja- 

 cent to Japan (Nakai, 1960, p. 821). 



SUMMARY 



The rate of natural mortality of the Pacific 

 sardine seems to be higher in the older fish than 

 in the younger ones and higher in the southern 

 subpopulation than in the northern one. For this 

 reason it is indicated that natural mortaUty should 

 be estimated for the subpoj)idations by age from 

 the catch data of two adjacent age groups. 



Availability was found to differ by age as well 

 as by season. High correlations were obtained 

 between the rates of two adjacent age groups 

 exploited in the same fishing season ; therefore, this 

 rate also should be estimated by examination of 

 two adjacent age groups. 



The data suggest that availability was positively 

 correlated with water temperatures in July to 

 September, immediately preceding the fishing 

 season. 



In addition to the above factors, others such as 

 efficiency of gear and area of fishing grounds may 

 affect the catch curves. The most promising pro- 

 cedures may be the comparisons of the total 

 mortality coefficients, nvmiber of boats, and area 

 of fishing grounds during the shortest time periods. 

 Consideration of these facts indicates that running 

 averages for six seasons should be computed for 

 these factors. 



ACKNOWLEDGMENTS 



Elbert H. Ahlstrom and the late Clyde C. 

 Taylor of the Bureau of Commercial Fisheries 

 Biological Laboratory, La Jolla, Calif., gave me 

 instruction and guidance, and David Kramer of 

 the same Laboratory edited the manuscript. 

 Reviewers were O. Elton Sette, Bureau of (yora- 

 mercial Fisheries Biological T aboratory, Stanford, 

 Calif.; John C. Marr, Bureau of Commercial 

 Fisheries Biological Laboratory, Honolulu, Hawaii; 

 and Douglas G. Chapman of the University of 

 Washington, Seattle, Wash. 



596 



U.S. FISH AND \VITJ)LJFE SERVICE 



