Natural Mortality 205 



back after the March census that were caught during the fishing season 

 (June, July, and August) of that year. 

 Thus, 



(J 



( 1 ) Exploitation rate, 1st year = — 



Ci = no. of marked fish caught during summer of census year, 

 Ni = no. of marked fish returned following census. 



Some fish might be expected to die of natural causes during this period. 

 The fishing mortality rate (exploitation rate) for the second year was 

 figured in the same way except that the calculation of available fish was 

 based on the number of fish returned following the census of March of 

 the first year, minus the fish caught during the first year, minus one half 

 of the fish that disappeared between censuses ( natural mortality ) : 



(2) Exploitation rate, 2nd yr. = 



C2 



Where, Co = no. of marked fish caught during the 2nd summer 

 No = no. of marked fish in census following 2nd summer 



The total mortality for any 2-year period was the diflFerence between 

 the number of marked fish restocked after a draining census and the 

 number bearing the same mark that were captured in the next succeed- 

 ing census. These ranged from 60 to a little more than 90 per cent 

 (Table 7.3) for two years, averaging around 35 per cent for a single year. 

 When the fishing mortality of 25 to 30 per cent per year was subtracted 

 from total mortality, the remainder, natural mortality, averaged from 5 

 to 11 per cent. This 11 per cent calculation was the average of four 2-year 

 periods, all part of the life span of the 1941 year class. Table 7.3 shows 

 that the average weight of the bass at the beginning of this period was 

 0.17 pound (7.6 inches) and their natural mortality rate for the two years 

 was 33 per cent. These fish were still vulnerable to predation. The natural 

 mortality rate for this group as old fish in the 1949-1951 period was 34.4 

 per cent. Here we were dealing with fish nine to eleven years old and 

 they were beginning to die of senile degeneration. If these two groups, 

 the small (young) and the old, are omitted, the annual natural mortality 

 rate for 1941 brood fish was 6 per cent or about the same as that for the 

 1947 and 1949 broods (5.5 and 6.3 per cent, respectively). 



Table 7.4 shows annual mortality rates of bluegills and largemouth bass 

 described above, with those of largemouths, smallmouths, walleyes, and 



