74 Carrying Capacity, Productivity, and Growth 



mouth bass yield from this pond for any one season was almost wholly 

 dependent upon fish spawned during the two preceding years. When the 

 population was censused after 4 years of high yields, the standing crop 

 at the beginning of the fifth year (fish caught by fishermen in April, May, 

 and early June plus the fish taken in a June census of all remaining fish ) 

 was approximately 100 pounds per acre which is considered relatively 

 high for this kind of fish in this type of water (gravel pit) in central 

 Illinois. 



Largemouth bass are less predictable than are smallmouth bass. 

 Certainly, the fishing pressure was heavy enough at Onized Lake (II- 

 hnois ) ^ in 1939 and 1940 to insure a crop of fish by hook-and-line methods, 

 large enough to tax the production capacity of that body of water. The 

 yield of largemouth bass here was 53.0 pounds per acre in 1939 and 19.8 

 pounds per acre in 1940. The weight of bass caught in April, May, and 

 June of 1941, when added to the weight of bass taken in the total fish 

 census of June 24, 1941, suggested that the standing crop of bass at the 

 beginning of 1941 may have been between 50 and 60 pounds per acre 

 or about 2.5 times the weight of bass taken in 1940 when the total fishing 

 pressure exerted on the lake was 1647 man-hours per acre. On the basis 

 of the 1939 yield of 53.0 pounds per acre, the carrying capacity of Onized 

 Lake for bass could be estimated to lie between 50 and 100 pounds per 

 acre, which is anything but specific. However, the fact that yields of 

 largemouth bass are influenced largely by factors other than angling 

 pressure makes it unsafe to estimate its turnover, production, or carrying 

 capacity in a mixed population of fishes. 



Bluegill yields from Onized Lake for 1939 and 1940 were 174 and 66 

 pounds per acre, respectively. The estimated standing crop at the begin- 

 ning of 1941 was around 153 pounds per acre, which must have been con- 

 siderably under the carrying capacity of Onized Lake for bluegills. More 

 than 6500 bluegills were taken in the final census, June 24, 1941, which 

 was more than 3200 per acre, a sufficient number of active digestive 

 tracts to convert food to flesh quickly if an excess of food (over mainte- 

 nance needs) was available. 



These feeble attempts to unravel the basic relationships between stand- 

 ing crops, observed yields, and productive capacity in populations com- 

 posed of several species of fishes emphasize the need for more data on 

 fish populations composed entirely of one species. Current information 

 demonstrates the validity of these concepts and their importance in 

 understanding fish-population dynamics. 



Estimating ProdtjCtion 



A fairly close estimate of total production of fish flesh may be obtained 

 for one or two growii.g seasons provided the fish are tagged or marked 



