MANZER: DISTRIBUTION AND FOOD OF STICKLEBACK 



6.55% of their body weight. Some digestion would 

 have occurred during consumption so this is a 

 minimal value. 



For October, failure to obtain feeding indices 

 between 1900 and 0700 h over the diel cycle pre- 

 cluded similar estimation of the daily ration. 

 However, if the residual content is assumed to be 

 0.65% of the body weight during periods lacking 

 observations, the daily food consumption can be 

 estimated to be 2.60 + 2.90 + 2.30 = 7.80% of the 

 mean body weight. 



Estimates of maximum meal size were obtained 

 by plotting feeding indices for only those fish 

 which were judged to have "full" stomachs during 

 the postdawn feeding period (i.e., the most inten- 

 sive feeding time of day) against length (Figure 

 6A). Data for stickleback in July were used 



o 

 o 



X 



o 

 o 

 m 



I 

 o 

 u 



S 



a 

 o 

 O 



I 

 u 



o 

 o 

 o 



»' = 8.l8-0.077Jir 

 A =-0.788 



. Hatching size 



30 40 50 



FISH LENGTH (mm) 



Figure 6.-The relation between maximum size of single meal 

 (A) and weight of stomach contents (B) with length of threespine 

 stickleback. 



because of their wide range in length. Despite 

 considerable individual variation between fish of 

 the same length obviously feeding intensity was 

 inversely related to length (r = -0.788, df = 26, P 

 <0.01). From the regression line fitted by the 

 method of least squares, it can be predicted by 

 extrapolation that, on the average, larval stick- 

 leback, which measure approximately 8 mm upon 

 hatching, consume 7.5% of their body weight in a 

 single meal, and that consumption in relation to 

 body weight decreases 0.8% per 10 mm increase in 

 length. As would be expected, large fish in a single 

 meal eat more than do small fish and the relation- 

 ship is of the positive exponential form (Figure 

 6B). 



For stickleback in October and July (assuming 

 mean lengths of 40 mm and 60 mm, respectively), 

 the average meal size was approximately 5 and 

 3.5% of their body weight, respectively. Assuming 

 two feeding periods per day, the daily ration 

 becomes 10 and 7% of body weight. These values 

 are in reasonable agreement with daily ration 

 estimates based on diel fluctuations in stomach 

 contents. 



Discussion 



During 1970 and 1971, the first 2 yr of a fer- 

 tilization program attempting to increase sockeye 

 salmon production in Great Central Lake, stick- 

 leback were observed to feed on a variety of 

 organisms with planktonic crustaceans (cladocer- 

 ans and copepods) and insects (chironomid pupae 

 and larvae), to a lesser degree, being the main food 

 organisms. These findings are consistent with 

 observations on food of stickleback in a variety of 

 freshwater habitats made by other investigators 

 (Hartley 1948; Hynes 1950; Greenbank and Nelson 

 1959; Rogers 1968). From a trophic standpoint, the 

 species is a secondary consumer. 



The literature on feeding of fishes in both 

 laboratory and in nature is replete with evidence 

 that consumption is influenced by a multitude of 

 factors. In the present study eff"ort was focussed on 

 examining seasonal and diel changes in feeding 

 habits, possible influencing factors being limited 

 to size and sexual maturity. 



The most pronounced feature observed in the 

 feeding of stickleback was the seasonal change in 

 the importance of different kinds of organisms 

 consumed. Although the food resource was not 

 sampled in conjunction with the food studies, some 

 general comments on food availability and selec- 



663 



