FISHERY BULLETIN: VOL. 74, NO. 3 



tivity by stickleback in 1970 can be made using 

 results of zooplankton studies by LeBrasseur and 

 Kennedy (1972) (Figure 7). A more precise method 

 of measuring the use of major planktonic forms in 

 relation to availability would have been the em- 

 ployment of Ivlev's( 1961) "electivity index," taking 

 into account the comments of O'Brien and Vin- 

 yard (1974) regarding distribution of predator and 

 prey. Bosmina, Holopedium, and Diaptomus were 

 consumed approximately in relation to their 

 abundance, although in the early part of the year 

 relative utilization was highest for Boxmina. 

 Cijclops and Bosmuia were approximately equally 

 abundant and exhibited somewhat similar sea- 

 sonal fluctuations but utilization of Bosmina was 

 sharply restricted during July and early August 

 whereas Cyclops was relatively unutilized 

 throughout the summer. Consumption of Epis- 

 chura, a less abundant form which occurred 

 mainly between May and September, was highest 

 in June during the early part of the "bloom." 



The reasons for the apparent differences in the 

 relative utilization of the major food items would 

 appear to differ. The shift from Epischura, despite 

 rather uniform abundance, to smaller organisms, 

 mainly Holopedium and Bosmina, through the 

 season may be due to the decrease in average size 

 of stickleback that occurred in midsummer. Epis- 

 chura, which equals 11 bulk units compared with 3 

 and 1 for Holopedium and Bosmina, respectively, 

 may have been too large an item to be consumed 

 by the majority of stickleback present after July. 

 Greenbank and Nelson (1959) and Rogers (1968) 

 observed that feeding habits of G. aculeatus in 

 Alaskan lakes changed through the summer and 

 differed between individuals of different size. The 

 disparity in relative utilization of Bosmina and 

 Cyclops, which were of comparable abundance and 

 individual size, cannot be thus explained. Rather, 

 it would appear that the difference in their dietary 

 importance may be explained by differences in 

 spatial distribution affecting availability: Cyclops 



1000 



DIAPTOMUS 



HOLOPEDIUM 



OUG SEPT OCT NOV DEC 



(r 

 LU 

 m 

 5 



o 

 < 



ir 



MAY JUNE JULY AUG SEPT OCT NOV DEC MAY JUNE JULY AUG SEPT OCT NOV DEC 



Figure 7.— Seasonal change in the biomass (unbroken line) of important prey species for threespine stickleback and the average number 

 present per stomach. Graphs representing biomass were taken from LeBrasseur and Kennedy (1972) and are shown in logarithmic scale. 



664 



