MANZER: DISTRIBUTION AND FOOD OF STICKLEBACK 



were hypolimnetic whereas Bos^mina were mainly 

 epilemnetic (LeBrasseur and Kennedy 1972, 

 Figure 2). If temperature influences their dis- 

 tribution as is suggested by their distribution in 

 relation to the thermocline, one might reasonably 

 infer that Cyclops was less available than Bosm ina 

 to stickleback inhabiting the littoral and near- 

 shore areas where water temperatures generally 

 are highest. Consumption of Holopedium in- 

 creased rapidly as the summer progressed. Con- 

 sumption of Diaptomus increased during August 

 and September. The appearance of these approx- 

 imately similar sized species in the diet of stick- 

 leback paralleled their occurrence in population 

 succession and maximum abundance in the rela- 

 tively warm surface waters. 



The diel feeding rhythm observed during July 

 and October has not been described for G. actdea- 

 tus in fresh water but the pattern is exemplary of 

 feeding periodicities described for a variety of 

 freshwater and marine fishes. The association of 

 peak feeding with postdawn and predusk periods 

 in summer and late fall when the number of 

 daylight hours differs suggests that feeding is 

 light-dependent. 



The literature on meal size and daily ration for 

 G. aculeatus is rather scant considering the 

 number of studies on the feeding biology of the 

 species. Krokhin (1957) using the O2 consumption 

 method estimated that stickleback averaging 4.5 g 

 in summer (August) consumed 5.1% of their body 

 weight daily. Beukema (1968) feeding stickleback 

 (2.5 g mean weight) Tubifex worms concluded that 

 the contents of a well-filled stomach equalled 5.5% 

 of the body weight, and that daily intake amount- 

 ed to 12% of the body weight. Beukema recognized 

 that the daily ration was rather high for adult fish 

 and suggested that rapid digestibility of the food 

 offered may have been responsible for the rather 

 high food intake value obtained. The mean daily 

 ration estimated in the present study from diel 

 feeding rhythm curves for stickleback in July 

 (mean length = 55 mm, mean weight = 2.4 g) and 

 in October (mean length = 39 mm, mean weight = 

 0.7 g) was 6.5% and 7.8%, respectively, of their body 

 weight. These estimates are only slightly less than 

 those derived by doubling the maximum meal size 

 of individuals of corresponding length (see Figure 

 6A), namely 7.8% and 10%. Considering that food 

 intake is influenced by several factors such as size, 

 physiology and behavior of individual, food de- 

 privation, previous meal size, temperature, and 

 prey digestibility (Darnell and Meierotto 1962; 



Davis and Warren 1968; Keast and Welsh 1968; 

 Swenson and Smith 1973), one may conclude that 

 the mean daily rations determined in this study 

 are in close agreement with those obtained from 

 experimental studies. 



FEEDING RELATIONSHIP 

 BETWEEN STICKLEBACK AND 

 JUVENILE SOCKEYE SALMON 



Information on competition between stick- 

 leback and juvenile sockeye salmon for food must 

 be based on samples of each species from the same 

 catch. Further, it must be assumed that in- 

 dividuals of each species taken together fed in the 

 same area. In 1970, 7 of 105, or 6.6%. of the sets 

 yielded both species. Sockeye salmon equalled 5% 

 of the two species combined. In 1971, the two 

 species were caught together in 18 of 89, or 20.2% 

 of the sets, and sockeye salmon equalled 2.2% of 

 the combined catch. 



Sockeye salmon and stickleback caught in the 

 littoral zone in October 1970 and May-July 1971 

 were used in this comparative study (Tables 11 and 

 12). Only catches containing 5 or more individuals 

 of each species were considered and a maximum 

 number of 10 individuals of each species was 

 examined from any one catch. For convenience, 

 the catches were grouped according to the follow- 

 ing time periods: October 1970, May-June 1971, 

 and July 1971. 



Stickleback through this period increased in 

 average size as a result of seasonal growth. By 

 contrast, sockeye salmon, although larger, 

 decreased in average size. This decrease in size 

 reflects the emigration from the lake of the larger 

 individuals as smolts in the following spring. The 

 relatively high percentage (20%) of stickleback 

 with empty stomachs in July can be explained by 

 the presence of the gravid females. 



In general, stickleback and young sockeye sal- 

 mon taken together exhibited considerable die- 

 tary overlap (Tables 11 and 12). Stomach contents 

 of sockeye salmon were treated and analysed in 

 accordance with methods used for stickleback. The 

 degree of similarity in diet during each period was 

 determined from occurrence data using Spear- 

 man's rank correlation coeflficient, r, (Siegel 1956). 

 The r, value indicates agreement in rank of food 

 items and can range from -i- 1.0 for complete 

 agreement to -1.0 for total disagreement. The 

 tests were restricted to items which were not 

 rendered unidentifiable through digestion and 



665 



