Relative Plumpness of Fish 75 



when released at the beginning of the period, and are collected by 

 draining for a census at the end. In the final census, marked fish are 

 counted to determine natural mortalit) , and of course fishing mortality 

 within the period must be measured through a creel census. Marked fish 

 in the "draining" census are weighed individually to calculate the gain 

 in flesh made during the period. Unmarked fish are weighed, as they also 

 represent production. 



Total production equals: 



( 1 ) flesh gained by recaptured marked fish, 

 plus ( 2 ) flesh produced by new recruits entering the population 

 plus ( 3 ) flesh gained by fish lost through natural mortality 

 plus (4) flesh gained by fish captured by anglers. 



An estimation of flesh gained by marked fish eventually lost through 

 natural causes may be made if one assumes that these fish remained alive 

 for half the period in question. Under this assumption their gain per in- 

 dividual would equal one-half that of marked fish of a comparable size 

 range. Loss of production through natural deaths among new recruits 

 is an unknown quantity, although it might not represent a very large 

 poundage of fish flesh. Estimated production in long-lived species is 

 probably more accurate than that in short-lived species because the 

 annual turn-over (recruitment and death rate) is smaller. Using the 

 method outlined above for bass in Ridge Lake (Illinois) with a period 

 limit of two years, the gains are as follows: in the the 1941-1943 period, 

 58.5 pounds per acre; in the 1943-1945 period, 34.2 pounds per acre; in 

 the 1945-1947 period, 37.2 pounds per acre; in the 1947-1949 period, 59.8 

 pounds per acre; and in the 1949-1951 period, 53.5 pounds per acre.^^ 

 However, these estimates of production for two-year periods cannot be 

 used to estimate the maximum production for a single year. 



RELATIVE PLUMPNESS OF FISH 



Everyone who has an opportunity to handle large numbers of fish soon 

 becomes aware of the variation in the plumpness of individual fish in any 

 species. Usually, this variation in the relative condition of each of several 

 species of fish inhabiting a body of water is greater than that among 

 individuals of one species; for example, bass in a lake may be in good 

 flesh while sunfish may be thin. While there is some variation in the 

 plumpness of individuals of a single species at any one time, larger 

 changes in relative plumpness for that species may follow annual cycles 

 or even longer periods, in the latter instance perhaps reflecting variations 

 in feeding conditions or population densities. All of these variations may 



