Fish Production 73 



to compute the total quantity of minnows which a 10-inch bass could 

 digest in a year at each of these locations. Since the maximum yield is 

 probably 2^i"f>portioiial to the total of potential digestion, it may be 

 possible to show the relationship between carrying capacity and potential 

 yield at diflFerent latitudes. Carrying this idea further, Thompson pub- 

 hshed a table showing the theoretical eflFect of latitude on potential 

 annual sustained yield (Table 4.3). 



Table 4.3 Effect of latitude on annual yield as esti- 

 mated FROM MEAN MONTHLY TEMPERATURES OF 



different localities.^^ (From Needham, J. G., 

 "A Symposium on Hydrobiology," the University 

 of Wisconsin Press, Madison, Wisconsin, 1941.) 



Relation to Standing Crop 



Aetual data on the relationship between production (as expressed by 

 fish yields ) and the standing erops of fishes are as yet inadequate to test 

 Thompson s theory as expressed in Table 4.3. Where data on yields and 

 standing crops are available for comparison, there are other factors that 

 obscure a clear relationship, such as fishing pressure, apparent unco- 

 operativeness on the part of fish, known differences in food chains, etc. 

 For example, the yields of bass at Ridge Lake were influenced more by 

 the presence or absence of available foods than by fishing pressure ^^; but 

 even in years when the highest yields were taken ( about 65 or 70 per cent 

 of the available weight of bass in the lake), there was no indication that 

 these yields reduced potential yields for following seasons. 



A recent study of a smallmouth bass population in a gravel-pit pond ^^ 

 that produced hook-and-line yields of more than 100 pounds per acre 

 for two successive years and then in the third year produced a yield of 

 80 pounds per acre, suggests that the maximum annual yield of fish in 

 the region of central Illinois may be nearer 100 per cent of the carrying 

 capacity than 50 per cent as given by Thompson (Table 4.3). The small- 



