FISHERY BULLETIN: VOL. 74, NO. 3 



Baltic (Meek 1916). The stimulus for this size- 

 related behavioral difference remains unknown. In 

 Great Central Lake some survival or feeding 

 advantage may accrue to smaller individuals 

 remaining close to shore but the affinity for shore 

 shown by large individuals in July is probably 

 associated with reproduction because virtually all 

 these fish were physically mature or gravid. 



FOOD AND FEEDING 



Methods 



Feeding Relationships 



percent of body weight was used as an index of 

 feeding intensity. Gravid females were excluded 

 from the analyses because they appeared to feed 

 less intensively, judging from the occluded 

 stomachs of many individuals. Supplementary 

 information on feeding periodicity was also ob- 

 tained by subjectively classifying stomachs as 

 either full, three-fourths full, one-half full, one- 

 fourth full, trace of food, or empty, and noting 

 whether the contents were fresh, partially digest- 

 ed, or digested and therefore unidentifiable. The 

 basic data are reported by Manzer (1971, 1972). 



Three methods were used to determine the 

 importance of organisms as food: 



Seasonal and spatial differences in stickleback 

 diet were determined from catches or samples of 

 catches, if large, made during each fishing survey 

 in 1970 and 1971. By coincidence, stomachs from 

 544 stickleback, or approximately 5% of the total 

 number caught in each year, were examined for 

 content. Stickleback examined in 1970 ranged in 

 length from 15 to 78 mm; in 1971, from 14 to 86 

 mm. The numbers of fish examined from each 

 station and by survey in the 2 yr are given in Table 

 2. 



Fork length (millimeters), body weight (milli- 

 grams, minus the weight of the body cavity para- 

 site, Schistocephalus, if present), and stomach 

 content weight (to nearest 0.2 mg) were obtained. 

 Stomach content weight was determined by first 

 weighing the stomach with food and then without. 

 The stomach contents were identified to species 

 when possible, and counted using a binocular 

 microscope. The content weight expressed as a 



a. Occurrence-the percent of stickleback feeding 

 on a particular organism. 



b. Numerical-mean number of a particular or- 

 ganism per stomach. 



c. Points— relative importance of organisms con- 

 sidering size and numbers. 



The relative merits of these methods have been 

 discussed by Hynes (1950) and Windell (1968). For 

 the points method, the equivalent units assigned 

 different organisms are given in Table 3. The units 

 for common planktonic Crustacea are in the ratio 

 of their wet weight, as determined from zoo- 

 plankton studies in Great Central Lake (LeBras- 

 seur and Kennedy 1972). Equivalent units for 

 other organisms, including insects, were deter- 

 mined by inspection and assigned the same unit 

 value as other organisms or groups of organisms of 

 similar volume, assuming a common specific 

 gravity. Since individual size of a given organism 



1970: 



1 



2 



3 



4 



5 



6 



7 

 Total 



1971: 



1 



2 



3 



4 



5 



6 

 Total 



22, 30 Apr. 

 24, 25 June 

 8, 9 July 

 22, 23 July 

 5, 6 Aug. 

 19, 20 Aug. 

 2 Oct. 



12,20 May 

 10, 17 June 



9 July 



10 Aug. 

 14 Oct. 

 30 Nov. 



15 — — — — — — — — — — — — — — — — _ 15 



— — 23 8 — — — — — — — — — — — 15 10— 56 



— 13 — — — — — — — — — — — — — — — — 13 



— 30 20 10 10 — — — 10 — — — — — — — — — 80 



19 — 15 32 36 — — 10 25 15 13 12 10 10 — — — — 197 



— 13 20 13 22 10 — — 10 10 10 — — 10 — — — — 118 



— 15 10 10 10 10 — — — 10 — — — — — — — — 65 



34 71 88 73 78 20 — 10 45 35 23 12 10 20 — 15 10 — 544 



— 20 20 3 8 



10 10 — 10 10 10 



— 10 — 



10 



9 



11 



10 20 15 6 



10 10 12 11 



10 10 12 10 



8 8 10 — 



10 — 



— 11 



3 



— 10 10 

 10 10 10 



— 10 10 

 10 10 



10 10 



10 10 



6 8 



10 



10 



9 



9 



15 



40 68 68 62 45 20 31 40 43 



26 28 — 53 — 



— 74 

 10 110 

 10 130 



— 105 



— 99 



— 26 

 20 544 



654 



