72 ILLINOIS BIOLOGICAL MONOGRAPHS [72 



period being the first two weeks of the life of the minnow. This period of 

 rapid growth gradually passes into one of slower increase, which lasts 

 from about the second week in July through August. A second rapid 

 growth period starts about the first week of September and continues for 

 about three weeks, ending with the cooling of the water late in that month 

 (see figures 11 and 12, which should be considered together). By the end of 

 September the fish have reached a length of 6.5 cm, and this length is not 

 increased during the winter period, as specimens caught in December, 

 February and April show no appreciable increase in length. As the water 

 warms up toward the end of April, however, a new growth stage is inaugu- 

 rated with the increased activities of the fish, so that the mature size of 

 7.62 cm is reached at about the spawning season, though data indicate 

 that there is often a slight growth even after the deposition of the eggs. 

 In obtaining data from which these conclusions could be drawn regarding 

 the growth rate of the species two hundred and fifty individuals were 

 measured and the average of the lot was taken as the mean size on the date 

 of capture. Figure 13 shows the number of individual fish of different 

 lengths as taken on August 1, 15, 29, September 12 and 26. Two pre- 

 cautions were taken to unify the results: all adults captured were excluded 

 from the tabulation, and the individuals used for measurements were 

 obtained from as many schools of fish as practicable without too much 

 waste of time in hunting for them, as the schools of very young Labidesthes 

 are at times provokingly difficult to locate in a large lake. During the 

 months of ice, the species is rather difficult to capture, as the number of 

 individuals is greatly reduced (the reason for which I shall discuss shortly) 

 the schools are very much broken up and the fish somewhat sluggish in 

 movement. The following table shows the number of fish measured on 

 different dates, together with the average size of the individuals involved. 

 In all cases the measurements were taken from the tip of the snout to the 

 base of the caudal fin. 



In this table correction must be made for those variations which have 

 been starred because of the small number of individuals involved. The 

 small numbers are due to the difficulties encountered in securing the fish 

 during the period when the lake is covered with ice. The figures, however, 

 tend to indicate that there is no appreciable increase in size during these 

 months. 



It is interesting and important to note that the change in growth rate 

 which occurs toward the end of July, as shown in figure 22, is intimately 

 correlated with two important changes in the habits of the fish: 1) the 

 assumption of a shallow water habitat in place of the deep water situation; 

 and 2) the change from an entomostracan diet to a mixed ration of entomos- 

 traca and insects, which is merely a transition stage leading to a culmina- 

 tion in an almost entirely insectivorous diet which is characteristic of the 



