Pygmy Killifish and Mosquitofish 93 



In Gambusia affinis, insects were dominant in the diet, but other 

 arthropods and some mosquitofish were also eaten. In summer, 

 Tanypodinae and odonate nymphs were quantitatively significant foods 

 (Table 1). Cladocera, Cyclopoida, and Araneae composed more of the 

 diet in summer than in winter. In both winter and summer, non- 

 tanypode Chironomidae and unidentified Insecta were dominant food 

 items. Gambusia affinis showed some cannibalism (in summer, about 

 3%). 



Feeding Dynamics 



Gambusia affinis. The balance between consumption and evacua- 

 tion may be inferred from diel changes in foregut content (solid lines on 

 Fig. 1). When the slope of the line is positive, consumption rate is 

 greater than evacuation rate during the specified time period; when 

 negative, consumption is less than evacuation. 



Analysis of evacuation rates is based on comparisons of gut- 

 content trends obtained by field and tank methods, i.e. the solid versus 

 the dashed lines of Fig. 1 and 2. During certain periods (e.g. 1055 to 

 1455 hours), fish in the field actively fed, which resulted in an increase 

 in the foregut content (Fig. 1). For such periods, it is necessary to use 

 the fish held in food-free tanks (dashed lines) to calculate evacuation 

 rates. At other times (e.g. 1850 to 2250 hours), the foregut content 

 declined in both field fish and tank-held fish. For these periods, I 

 considered field fish, which were unconfined and egesting in their 

 natural environment, to provide the better estimate of natural evacuation 

 rate. The evacuation rate is computed by 



r= ln(S +l)-ln(S t +l) 



in which S Q is relative foregut content before and S t is relative foregut 

 content after time t (adapted from Elliott and Persson 1978; 1 is added 

 to allow logarithmic transformation of zeros). The same equation 

 applies for determining evacuation in tank-held fish. When evacuation 

 data were missing for a period, evacuation rate was presumed to be an 

 average of rates before and after that period. Finally, the various rates 

 during the day were weighted by time to produce an average evacuation 

 rate (Table 2). 



Evacuation rates for each of the time periods were used in calculat- 

 ing consumption over each of these periods. Each such evacuation rate 

 was combined with average foregut content before and after the period 



