FISHERY BULLETIN: VOL. 71, NO. 4 



tribution. After hatching, the young sticklebacks 

 congregate in the shallow sandy areas where 

 they are unavailable to lake trout and most 

 other species. By the time they leave the shal- 

 low water in late fall, they have grown to 45 mm 

 or more. It is at this length that they begin to 

 appear in trout stomachs the following spring. 



Smaller trout cannot feed well on stickle- 

 backs once the latter become available after a 

 summer's growth, because of the protective 

 spines. This inability explains the relative un- 

 importance of sticklebacks in the diet of small 

 trout. Similarly, Hoogland, Morris, and Tinber- 

 gen (1957) found young northern pike, Esox 

 lucius L., rejected sticklebacks, first exhibiting 

 a negative reaction to mechanical stimuli upon 

 contact with the spines and then a visual avoid- 

 ance. Upon reaching a certain size, the pike pref- 

 erentially take sticklebacks (Frost, 1954). A 

 discussion by Fortunatova (1959) supports this 

 theory. The dietary shift to smelt by larger 

 trout can be explained by assuming the optimum 

 size of the prey increases as the trout grow 

 larger. 



Compared to all lake trout over 28 cm long 

 taken from a number of trout index stations, 

 fish from stations 75 and 86 ate a high percent- 



age of sticklebacks (Table 21, Figure 9). 

 At station 86, considered the best trout nur- 

 sery area in the Apostle Islands, sticklebacks 

 were the predominant food item even though 

 juvenile smelt were more abundant in the index 

 trawl samples (Table 9). At station 75, the 

 second most important nursery area, stickle- 

 backs were very abundant, and they made up 

 more than 60% of the trout diet. Station 12 

 sticklebacks and smelt were about equal in the 

 index catch, but sticklebacks were much more 

 important in the trout diet. At station 44, the 

 predominant forage fish in the trawl samples 

 was sculpins, and these were dominant in trout 

 stomachs. At this station, sticklebacks were 

 comparatively rare in occurrence, but still 

 made up 21% of the volume of the trout diet. It 

 might be assumed from station 44 data that 

 sculpins are an important item in the diet and 

 that trout will substitute sculpins when stickle- 

 backs and smelt are not readily available. How- 

 ever, the stations with high sculpin abundance 

 relative to stickleback abundance do not support 

 large numbers of trout. From these data, it 

 appears that the availability of sticklebacks at 

 the various stations is probably important in 

 the selection of these areas by lake trout as 



Table 21. — Percent frequency of occurrence and (in parentheses) percent total volume for each food 

 item found in lake trout 28 cm and longer at various sampling stations in the Apostle Islands. Fish were 

 taken in June and September during the two annual Bureau of Commercial Fisheries index trawling 

 cruises. 



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