GRISWOLD and SMITH: LIFE HISTORY OF NINESPINE STICKLEBACK 



trout-perch in spring. The significant rank cor- 

 relation does not necessarily indicate the 

 amount of comi)etition for food, but merely the 

 potential for such competition. For competition 

 to occur, there must be a demand by more than 

 one organism for the same food in excess of 

 immediate supply. It can only be said that since 

 the abundance of sticklebacks and slimy sculpins 

 are positively correlated in the index catches 

 and their food relationships are similar, the 

 possibility of competition between the two spe- 

 cies exists during some seasons. The actual ex- 

 istence of competition is highly questionable 

 because of the apparent abundance of P(nito- 

 poreia, the number one food item, and the ap- 

 parent adaptability of the stickleback in food 

 utilization. The drop in correlation in the fall 

 was due to greater use of chironomids by 

 sculpins. 



While the correlation with trout-perch also 

 indicates potential competition for food, their 

 abundance in the index trawl samples was nega- 

 tively correlated and Dryer (1966) found 

 trout-perch and sticklebacks to have different 

 bathymetric distributions. While the trout-perch 

 and sticklebacks utilized the same foods in 

 spring, their secondary foods were different in 

 summer and fall. Anderson and Smith (1971) 

 found trout-perch fed on wider variety of food 

 items than sticklebacks in the Apostle Islands. 

 This combined evidence seems to indicate that 

 competition between these two species is also 

 unlikely. 



Sticklebacks as Forage for Other Fish 



Several fish species which occur in Lake 

 Superior have been rei)orted to feed on stickle- 

 backs at various locations. These include lake 

 trout; brown trout, Salnio trutta L.; rainbow 

 trout, Salmo gairchieri Richardson; burbot. 

 Lota lota (L.); and various sculpins. Of these, 

 the lake trout was the only species which fed 

 heavily on sticklebacks in the Apostle Islands. 



Dryer, Erkkila, and Tetzloff (1965) related a 

 change in diet in juvenile Lake Superior lake 

 trout to their size. The diet, primarily a crusta- 

 cean-insectivorous type in small trout, became 

 piscivorous in larger fish. 



In the present study, sticklebacks were found 

 in 7.5% (33) of 436 lake trout less than 28 cm ( 1 1 



in.) long (TL). Only 3.5% of the volume of stom- 

 achs from 142 fish under 25.4 cm (10 in.) was 

 sticklebacks. In 294 trout from 25.4 to 28.0 cm, 

 sticklebacks made up 32% of the volume of the 

 diet. 



Sticklebacks were a major part of the diet of 

 lake trout from 28.0 to 38.0 cm (11.0 to 14.9 in.) 

 (Figure 9). P. pi<iigitii<s made up 39% of volume 

 in the diet and were found in 42% of the 176 

 trout which contained food. Smelt was the second 

 most abundant fish species in the diet of 28- to 

 38-cm trout. The food habits of trout remained 

 about the same for all ice-free months (Anderson 

 and Smith, 1971). 



The major food of trout longer than 38 cm 

 was smelt. Of 30 trout in this size range, 69% of 

 the stomach contents were smelt remains. 

 Sticklebacks were found in five of these fish and 

 made up 17% of the volume. Nineteen adult 

 sticklebacks and two sculpins were found in one 

 50-cm (19.7 in.) trout. Sticklebacks eaten by 

 trout were all age-group I or older. No stickle- 

 back eggs or age-group fish were found in 

 over 4,500 trout stomachs sampled on the Lake 

 Superior project. 



The explanation of the absence of small stick- 

 lebacks in the trout diet is probably one of dis- 



100 



»MPHIPOO; 



OTHER 



STA. 86 



STA. 75 



STA. 44 



STA. 2 



STA. 12 



Figure 9. — Percent volume of various food items found in 

 lake trout 28 cm and longer at various sampling stations 

 in the Apostle Islands. Fish were taken in June and Sep- 

 tember during the two annual BCF index trawling cruises. 



1057 



