GRISWOLD and SMITH: LIFE HISTORY OF NINESPINE STICKLEBACK 



Table 18. — Food of Apostle Islands sticklebacks expressed as percentage frequency of occurrence and (in parentheses) 



percentage of total volume for each food item. 



destructive to fish eggs and fry in Lake Superior 

 as Bigelow and Schroeder (1953) thought was 

 generally the case. Neither eggs nor fry of any 

 other kind were found in stickleback stomachs, 

 although gravid females and young-of-the-year 

 of smelt, trout-perch, slimy sculpins, and pygmy 

 whitefish were taken in the same areas at about 

 the same time as the stickleback stomach sam- 

 ples. 



The percentage volume of total stomach con- 

 tents for each day and for each food item taken 

 in the mysis sled is given in Table 17. Ponto- 

 poreia was the most important food item except 

 in the November sample. In that month, mysids 

 were the most important food and were more 

 abundant in plankton samples than at any other 

 time. Copepods were most abundant in the June 

 and July samples, and their relative contribu- 

 tion to stickleback diet was also highest in these 

 months. In general, when mysids or copepods 

 became relatively more abundant with respect 

 to themselves or to other zooplankters in the 



plankton samples, they also become more im- 

 portant in the stickleback diet. This suggests 

 that the stickleback shows no strong food pref- 

 erences but is adaptable and eats what is most 

 available. 



Relation to Food Utilization of 

 Other Forage Species 



Juvenile smelt (less than 12.5 cm), trout- 

 perch, and slimy sculpins were found to have 

 diets similar to sticklebacks in Lake Superior 

 (Anderson and Smith, 1971). Since all these 

 species were common in the Apostle Islands, 

 competitive relationships between them and the 

 stickleback could exist. A rank correlation coef- 

 ficient was used to measure the similarity of food 

 composition between two samples and thus 

 permit examination of interspecific and intra- 

 specific feeding relationships. 



Table 19 is an example of the computation 

 as discussed by Snedecor (1956). In the case of 



1055 



