FISHERY BULLETIN: VOL. 74, NO. 3 



Table 11. -Stomach contents of threespine stickleback in Great Central Lake, October 1970-July 1971. 



'Mainly D. pulex. 



'Based on stomachs in whicfi condition of contents permitted counts of various dietaries. 



3T = Trace = < 1% of bulk. 



which were present in at least 10% of the stomachs 

 of one or the other foraging species. Infrequent 

 ties in rank were broken in favor of the larger food 

 item. 



The r, values for May-June and July samples 

 were significant at P = 0.05 but that for October 

 was not (Table 13). In October Bosmina, Cyclops, 

 and copepodids were common items in the diet of 

 stickleback compared to the larger Epischura and 

 Holopedium in the sockeye salmon diet. A possible 

 explanation for the diff'erence between stickleback 

 and sockeye diets in October may be that larger 

 predators feed on larger prey: in October, sockeye 

 salmon on the average measured 74.6 mm, stick- 

 leback 39.5 mm. 



The observed dietary overlap indicates the 

 existence of potential competition between stick- 

 leback and sockeye salmon for food in May-June 

 and July. Accurate assessment of actual competi- 

 tion is contingent not only on information on food 

 and feeding habits of the two foraging species but 

 on other factors, such as their temporal and spatial 

 associations during different life history stages 

 and their abundance and growth in relation to 

 food supply. For this study, data essential for 



quantitative assessment of competition during 

 different seasons are inadequate or unavailable, 

 although competition in winter is precluded by the 

 apparent absence of stickleback. It is known 

 however that when the two species occur together 

 it is near shore or in the littoral zone, and that 

 relative to stickleback sockeye salmon are few in 

 number: sockeye salmon are almost the exclusive 

 inhabitants of the limnetic zone (D. Robinson, 

 pers. commun.). From the distribution patterns of 

 the two species, it can be inferred that stickleback 

 in Great Central Lake are not serious competitors 

 of sockeye salmon for food despite their similarity 

 in diet. Additionally, during this study the zoo- 

 plankton abundance had increased substantially 

 as a result of nutrient additions (LeBrasseur and 

 Kennedy 1972) and the growth rate in sockeye 

 salmon was faster than that observed under 

 untreated lake conditions (Barraclough and 

 Robinson 1972). However, in lakes where both 

 species are abundant and overlap extensively in 

 spatial distribution, utilization of a common food 

 resource may affect production of one or both of 

 the foraging species, especially during periods of 

 reduced or limited food supply. 



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