FISHERY BULLETIN: VOL. 71, NO. 4 



habits of sticklebacks and several associated 

 species with which they are most likely to com- 

 pete or which may be involved in predator-prey 

 relationships with sticklebacks. 



In the stomach analyses, contents were re- 

 moved from the upper esophagus to pyloric 

 sphincter and measured by water displacement 

 in IS-cm^ centrifuge tubes. The contents were 

 then emptied into a gridded petri dish for indenti- 

 fication under a binocular microscope at 10 x 

 magnification. Organisms present in small 

 numbers were counted, but larger numbers were 

 estimated from counts in randomly selected 

 squares of the dish. Percentage of each item in 

 the total volume of each stomach was estimated. 

 The data are reported as percentage frequency 

 of occurrence and percentage of total volume for 

 each food item in stomachs containing food. 



Invertebrate Abundance 



Invertebrates important in the stickleback 

 diet were sampled with the mysis sled concur- 

 rently with sticklebacks used in food analysis; 

 therefore, it is assumed that the gear sampled 

 food organisms that were available to stickle- 

 backs. One-mile tows were made with the sled 

 and the invertebrate catch was preserved in 

 10% Formalin. Invertebrate samples were identi- 

 fied and enumerated on a gridded petri dish in 

 the same manner as stomach contents. 



The average number of organisms per 100 m 

 of tow is shown in Table 17 for 18 tows on five 

 dates. The data indicate the extreme abundance 



of Pontoporeia affi)ns adjacent to the bottom. 

 Although copepods were also abundant, they 

 were usually not as important in mass as either 

 Pontoporeia or Mysis. Coregonid eggs and fry 

 were found occasionally in fall plankton samples. 



Food of the Stickleback 



Food analyses of 421 stickleback stomachs in 

 nine different months were made (Table 18). 



Fish under 50 mm long fed primarily on 

 Pontoporeia and copepods, which made up 79 

 and 13% of the total food volume, respectively. 

 Cladocerans were of minor importance, and 

 mysids were also present infrequently. Other 

 organisms present in these smaller fish were 

 chironomid and diptera larvae and a filamen- 

 tous green alga. 



Food of sticklebacks 50 mm and over were 

 characterized by a variety of larger food items, 

 suggesting that one factor limiting the diet of 

 young fish is size of food items. Pontoporeia 

 taken by the under 50-mm sticklebacks were 

 small in size (4.0 mm average) compared with 

 those found in adult stomachs (6.5 mm). 



In these larger fish, Pontoporeia made up 

 61% of the total stomach volume, Mysis 21%, 

 copepods 8% , and miscellaneous other items 

 10% . Pontoporeia was the major food item in all 

 months but November. There is a suggestion 

 from late fall and early spring samples that core- 

 gonid eggs may be an important food item in 

 the winter. Except for these eggs, there is little 

 to suggest that P. pioigitius is significantly 



Table 17. — Average number of invertebrates caught per 100 m in tows along bottom with mysis sled in the Apostle Islands. 

 [The percentage volume of each item found in stickleback stomachs is in parentheses.! 



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