186 Fishing and Natural Mortality 



However, a later investigation, involving the cropping of smallmouth 

 bass from another central Illinois pond, did not bear out Thompsons 

 estimate of maximum production. ^"^ In this pond of 1.42 acres, the annual 

 yields of smallmouth bass for four successive years were 78.0, 119.0, 123.0, 

 and 81.3 pounds per acre. In June of the fifth year the pond was treated 

 witli rotenone and a census was made of the remaining smallmouth bass. 

 The total bass population, exclusive of the age class (1 to 1.5 inches 

 total length) amounted to 52.3 pounds per acre. Previous to this census 

 (from April 11 to June 6) fishermen had taken 48 pounds of bass per 

 acre. Together, the census and the pre-census catch amounted to 100.3 

 pounds of smallmouths per acre, a figure that must include the flesh added 

 to individuals of the population that were alive during April and May. 

 On the basis of this experiment, the replacement potential for fish flesh 

 at the latitude of central Illinois may approach 100 per cent during a 

 single growing season. While this high level of cropping must be con- 

 sidered a sustained yield, the future yield status of this population of 

 smallmouths was precarious in that a reproduction failure for a given 

 season probably would have seriously curtailed the yield for the fishing 

 season two years hence. Since individuals in this population were con- 

 verting food into flesh with some efficiency, any severe reduction in the 

 number of digestive tracts would be followed by a reduction in food 

 conversion. 



Certain species of warm-water fishes cannot be depended upon for 

 a sustained yield because they do not produce new year classes every 

 year. This production of intermittent year classes may prevent the en- 

 trance of new recruits into the fishable population as older fish are taken, 

 until the latter become scarce and the yield is forced downward. Failure 

 to produce annual year classes may be associated with environmental 

 conditions or with the collection of metabolic products (see Chapter 5). 

 Yields including several species of fishes usually are more constant, be- 

 cause failure of a year class in one species may be compensated for by 

 high production in other species. 



In Chapter 5, 1 expressed certain relationships between carrying capac- 

 ity, productivity, and growth. In populations of fishes composed of one, 

 two, or several kinds, a substantial sustained removal of one species should 

 result in ( 1 ) increased growth rate and improved reproduction success of 

 uncaptured individuals of that species and/or (2) the expansion of the 

 population of some other species to fill the space created by the removal 

 of the first species, with the final result that the population of the first 

 species might level off at a much lower point than formerly. This may 

 be what happened to many of the more desirable fishes inhabiting the 

 Great Lakes prior to 1947 before the sea lamprey became numerous. 



