MANZER: DISTRIBUTION AND FOOD OF STICKLEBACK 



acarids, Araneida, planaria, Odonata, and fish 

 (cottids). 



The different food organisms differed seasonally 

 in their dietary importance. Considering items of 

 major importance, in 1970 in late April, virtually 

 all stickleback stomachs examined contained Bos- 

 mina, Cyclops, and zooplankton eggs, but Bos- 

 mina was most important, averaging 2,419 in- 

 dividuals per stomach and making up 95% of the 

 bulk. By late June, Bosmina was still the dominant 

 food item but had declined somewhat in impor- 

 tance as indicated by an increasing proportion of 

 stickleback feeding on Epischura (55%), 

 Hohpedium (41%), and chironomids, especially 

 pupae. Of these Epischura was most important, 

 forming almost 50% of the bulk. Through July and 

 August, Bosmina was consumed by a high 

 proportion of stickleback (no less than 60%) but 

 Holopedium progressively became the dominant 

 food organism (approximately 60% by bulk). Dur- 

 ing these two months, the number of stickleback 

 feeding on Alona, copepod copepodids and nauplii, 

 and Diaptonms increased but none of these items 

 was important quantitatively. In October, 

 Holopedium continued to be the dominant food 

 item in terms of bulk, but more stickleback fed on 

 Bosmina (98%). Diaptomus and Cyclops were 

 present in about 50% of the stomachs examined 

 and were of minor importance. Rotifers and eggs 

 were present virtually throughout the study pe- 

 riod, the former item occurred rather frequently 

 (26-76%), but were unimportant in terms of bulk. 

 Judging from size, the eggs were from both 

 zooplankton and stickleback. Since stickleback 

 spawn between late June and early August, eggs 

 encountered at other times of the year presumably 

 were zooplankton eggs. 



In May 1971, about one-half of the stickleback 

 had Bosmina, Cyclops, and copepodids in their 

 stomachs. Cyclops was most important in terms of 

 numbers per stomach (75) but chironomid pupae, 

 because of relative size of individuals, was impor- 

 tant in terms of bulk (38%). By mid-June, more 

 stickleback were feeding on Epischura (56%) and 

 Holopedium (34%), but Epischura was the domi- 

 nant food organism (95% of total stomach con- 

 tents). About the same number (49%) of stick- 

 leback fed on Cyclops as in May, and although the 

 item ranked second in incidence, it accounted for 

 only 2% of the total stomach content. In July, 

 Epischura declined in importance but still main- 

 tained dominant position among the other food 

 organisms. Holopedium continued to increase in 



importance. This inverse trend in the importance 

 of these two food items was observed into October. 

 In October, Holopedium was the dominant food 

 item and Bosmina ranked second in bulk and were 

 consumed by as many stickleback as were 

 Holopedium. In terms of occurrence, Diaptomus 

 (71%), Cyclops (65%), copepod copepodids (48%), 

 and zooplankton eggs (54%) were of secondary 

 importance. At the end of November, Holopedium, 

 Bosmina, and copepod copepodids formed the 

 major part of the diet of stickleback and in- 

 dividually were of about equal importance. 



The stickleback diet in 2 yr showed some marked 

 seasonal similarities and differences. Bosm ina was 

 not as important in the early part of 1971 as in 

 1970. Another difference is the greater importance 

 of Epischura later into 1971 than 1970, and the 

 greater importance of Holopedium in July and 

 August in 1970. A feature common to both years is 

 the late season resurgence of Bosmina as an 

 important food organism. It is not known for 

 certain whether these differences and similarities 

 represent annual differences in abundance levels 

 of the various kinds of organisms or in sampling 

 dates. 



Diet in Relation to Stickleback Size 



A total of 205 stickleback taken from the eastern 

 end of the lake on 22 July and 5 August 1970, and 

 ranging in length from 15 to 78 mm were ex- 

 amined for diet differences in relation to size. The 

 stickleback were arbitrarily divided into four size 

 groups: <30 mm, 30-49 mm, 50-69 mm, 70-1- mm. 

 Data on diet for the same size group for the 2 days 

 were pooled since samples were obtained in the 

 same general area within a short time interval 

 (Table 7). 



A high proportion of the stickleback (75, 65, and 

 68% respectively) in the <30 mm group consumed 

 Bosmina, Rotifera, and Holopedium. Alona, Epis- 

 chura, and chironomid larvae occurred in about 

 one-half of the stomachs. Of the remaining items 

 consumed only copepod nauplii, chironomid pupae, 

 and zooplankton eggs were of any importance, 

 occurring in 18, 16, and 13% of the stomachs, 

 respectively. Larger stickleback, excluding the 

 70-1- mm group of which only 11 were examined, 

 tended to feed more on Holopedium, Epischura, 

 chironomid pupae, and zooplankton eggs, and less 

 on Rotifera (except those in the 30-49 mm group), 

 Bosmina and Alona. Copepod nauplii apparently 

 were not consumed by larger stickleback, but fish 



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