MANZER: DISTRIBUTION AND FOOD OF STICKLEBACK 



present in 13 stomachs, compared with 1 for 

 females. The ingestion of detritus by males is 

 probably related to its role in nest building and not 

 to feeding behavior per se. 



Diel Feeding Rhythm and Variation in Diet 



Despite some size differences in stickleback at 

 sites A and B (station 1), feeding intensity indices 

 (food weight/body weight x 100) for stickleback 

 caught at a specific sampling time were similar 

 during October and July. Active feeding took place 

 mainly during postdawn and predusk hours, lead- 

 ing to two daily alternating feeding and "non- 

 feeding" periods (Figure 5). Differences between 

 the mean indices for different times of day in the 

 October and July series were subjected to the 

 Kruskal-Wallis test (Siegel 1956) and found to be 

 significant (October, H = 25.71, 4 df , P = <0.0001; 

 July, H = 28.97, 7 df , P = <0.001). This periodicity 

 in feeding was corroborated by the mean number 

 of organisms present in stomachs at different 

 times of day (Table 10). 



The kinds of organisms consumed and their 

 importance at different times of the diel cycle are 

 presented in Table 10 for both the October and 

 July series. Information for October is based on 

 stickleback ranging in mean length from 37 to 44 

 mm. Stickleback examined in the July series were 

 less uniform in size and ranged in mean length 

 from 49 to 63 mm. 



Considering the important food items, the com- 

 position of the diet changed through the daily 

 cycle in October and July (Table 10). In October, 

 Bosmina and Holopedium occurred in a very high 

 percentage of the stomachs examined, regardless 

 of sampling time. In terms of numbers consumed 

 and bulk units, Holopedium was the dominant 

 item, especially between 0700 and 1000 h. Between 

 1300 and 1900 h the relative importance of 

 Holopedium was reduced somewhat by the in- 

 creased consumption of Bosmina, Alona, Epis- 

 chura, and eggs of zooplankton. 



In July, Holopedium was the dominant food 

 organism throughout the daily cycle. Although not 

 as important as Holopedium in terms of numbers 

 or bulk, eggs of zooplankton were present in a 

 large proportion of the stomachs examined, rang- 

 ing from 40% (0100 h) to 100% (1000 h), with 

 consumption being greatest in the morning. 

 Epischura was present in stomachs at most times 

 of the day, but their contribution to the diet was 

 highest during peak feeding times. 



1000 1300 1600 1900 2200 



HOUR OF DAILY CYCLE (PS T ) 



0100 0400 



Figure 5.- Diel fluctuations in feeding intensity of threespine 

 stickleback in October 1970 (closed circles) and July 1971 (open 

 circles). The number associated with each datum point repre- 

 sents sample size. The horizontal bars indicate periods of 

 daylight and darkness. 



Rotifera were present in a large proportion of 

 the stomachs throughout the diel cycle in October 

 and July and were numerous compared to most 

 other items. Their individual small size would tend 

 to depress their importance as a food item. 



Daily Ration and Maximal Meal Size 



The described diel fluctuations in feeding in- 

 tensity indicate that in July at least, consumption 

 and evacuation occurred alternately over periods 

 of approximately 6-h duration. On the average, a 

 particle of food required about 6 h to pass through 

 the stomach. Stomachs were least full at 0400 h 

 when the contents amounted to 0.65% of the mean 

 body weight but they were, on the average, never 

 devoid of food, suggesting that feeding was con- 

 tinuous in the population. Freshly ingested or- 

 ganisms were present in some stomachs even 

 during dark hours. 



Recognizing two periods of consumption and 

 stomach evacuation each of approximately 6-h 

 duration, and the presence of "residual" content, 

 the daily ratio (DR) in July can be calculated by the 

 formula: 



DR = R + Pi + P2 



where R = residual content x food particle evac- 

 uation time. 

 Pi = Major feeding index - residual content, 

 P2 = Minor feeding index - residual content. 



Substituting actual values indicated in Figure 5, 

 the food consumed by stickleback in July amount- 

 ed to (0.65 X 24/6) + (2.80 - 0.65) + (2.45 - 0.65) = 



661 



