BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 



283 



otolith examination. Smith found that for 

 the inshore area, several year-classes are 

 represented in the catch. The distribution 

 of these classes in the entire population is 

 as follows: 



Smith says of this table: "... Two-yeai 

 old fish constitute the bulk of the trawl 

 catch . . . and . . . few older plaice are 

 taken. One-year old fish are probably the 

 most abundant on the grounds, but the 

 majority of them are too small to be taken 



tory populations of the vole, Microtus 

 agrestis. In one table they contrast age- 

 structure in populations that are "stable or 

 Malthusian" with those that are "life-table 

 or stationary." The former type was defined 

 by Dublin and Lotka (1925), for human 

 populations, as " . . . the relative rate at 

 which a population would alternately in- 

 crease or decrease, if the observed mortality 

 and fertility were to remain constant in a 

 stable and unlimited environment." The lat- 

 ter or life-table population is the form that 

 would be " . . . ultimately established in 

 a population where the birth-rate and 

 death-rate were equal ..." and in a lim- 

 ited environment. Table 20 shows the com- 



Tahle 20. Age-Structure in Vole Populations 



by the net." The sex ratio of the inshore 

 samples is forty-nine males to fifty- one fe- 

 males. 



During the February and March spawn- 

 ing season the ripe fish congregate in an 

 offshore area. This population consists of 

 males two to six years old and females four 

 to twelve years old. These males are three 

 times as numerous as the females. Smith 

 finds that the three-year females are essen- 

 tially absent from the offshore area catches. 

 For some unknown reason this age-class of 

 females passes its third year presumably in 

 another, and unknown, offshore area. A 

 somewhat similar study is that of Jones 

 (1939) on age-classes of salmon. 



Leslie and Ranson (1940) present some 

 observations on age distribution in labora- 



puted age-structure in vole populations on 

 the basis of these two assumptions (Leslie 

 and Ranson, 1940). 



For the Malthusian distribution, the 

 birth rate is 0.1127, and the death rate 

 0.0250, "That is to say, taking one week as 

 a unit of time, and given a population of 

 1000 females of all ages distributed in the 

 stable [Malthusian] form, we should expect 

 25 of these to die and some 113 daughters 

 to be born." Thus we have additional evi- 

 dence for the principle that when births ex- 

 ceed deaths a population has a youthful age 

 composition. In the hfe-table population in 

 which the births are equal to the deaths 

 the range of age is much greater. 



Emlen (1940) presents an illustration of 

 age shifts in populations based on actual ob- 



