204 Fishing and Natural Mortality 



populations that are not fished tend to become numerically overabundant 

 sooner than those subjected to a substantial annual cropping. 



The fish population o£ Sugarloaf Lake ( Michigan ) , which was studied 

 intensively from 1948 through 1952/^' ^^ was found to remain quite con- 

 stant for 5 consecutive years; this meant that recruitment and total 

 mortality were nearly equal during those years. 



Table 7.2 Calculations of rates of total annual mortality, 

 exploitation, and natural mortality for bluegills 

 in muskellunge, shoe, and wawasee lakes ( indiana), 



RICKER.^S 



* Based on an estimated rate of exploitation of 20 per cent. 



Under these conditions the assumption was made ^^ that survival could 

 be expressed by the ratio of the number of fish in a particular age group 

 divided by the number in the next younger age group, thus: 



S (Survival) = 



II + III + IV + 

 1+ II -fill + 



From the rate of survival, it was possible to figure total mortality and, 

 from fishing mortality, natural mortality. These investigators calculated 

 that the total mortality for bluegills was 66 per cent. As the annual fishing 

 mortality was 21 per cent, the natural mortality rate was 45 per cent. 

 This was a little less than Ricker calculated for Indiana lakes, but still 

 in the same general range. 



Statistics on the catches of marked largemouth bass at Ridge Lake 

 (Illinois) and the survival of marked fish from one draining census to 

 another ( usually 2 years ) gave information on fishing mortality and total 

 mortality of the 1941 year class from 1943 to 1951.^ Similar (but previ- 

 ously unpublished) data on the 1947 and 1949 year classes of bass are 

 included with data for the 1941 year class first in Table 7.3. In this table 

 the number of fish shown opposite each period listing was the number put 

 back following the census at the beginning of the period. The fishing 

 mortality rate (exploitation rate) for the first year was the fraction put 



