Pygmy Killifish and Mosquitofish 91 



approximately 4 hours after they were put in the tanks, for comparison 

 of gut contents. In the "field" method, the decline in gut contents 

 between field samples taken at intervals during non-feeding times of day 

 yielded a second measure of evacuation rates. (See the section on 

 Feeding Dynamics for an example of evacuation analyses.) 



In the laboratory, lengths of foods in foreguts were converted to 

 weights. Foreguts were analyzed because their contents declined con- 

 sistently with time in tanks, whereas hindguts continued to receive food 

 from foreguts in some cases. I examined foregut contents under a 

 dissecting microscope and measured lengths of food items. Length- 

 weight regressions in Dumont et al. (1975) were used to calculate dry 

 weights of Ostracoda, Harpacticoida, nauplii, and most Cladocera. Hall 

 et al. (1970) gave macrothricid weights. Insect head-capsule widths were 

 converted to weights (Smock 1980). Ruttner-Kolisko (1977) and J. 

 Gerritsen and H. S. Greening (personal communication) gave rotifer 

 length-weight conversions. A regression by Gerritsen (personal com- 

 munication) for the Okefenokee Swamp was used to derive cyclopoid 

 weights. Maximum carapace widths of araneids were converted to 

 weights (Edgar 1971, Barber 1983). Weights of Acari were estimated 

 from the regression of Oribatei by Persson and Lohm (1977). 



I calculated a length-weight regression to yield weights of Gambusia 

 affinis within Gambusia foreguts. Foregut content (S) was expressed in 

 relative units, i.e. mg dry food/g dry fish, assuming 20% dry to wet 

 weight conversion for fish (e.g. Lagler et al. 1977). 



Evacuation rates, daily food consumption, and area-based con- 

 sumption were calculated for both fishes. Area-based consumption 

 equals the dry weight equivalent of fish density (Oliver and Schoenberg 

 1989, method of density measurement adapted from Freeman et al. 

 1984) times calculated daily food consumption (Staples 1975, adjusted 

 in Elliott and Persson 1978; Persson 1982; Garcia and Adelman 1985). 

 Results are based on collections of fish at about 4-hour intervals for 24 

 hours, in summer (L. ommata and G. affinis, 19-20 August 1984) and 

 winter (L. ommata on 7-8 March 1984, and G. affinis on 16-17 

 February 1985, when it was common). 



ANALYSES AND RESULTS 

 Diets 



Chironomids and Cladocera dominated the diet of Leptolucania 

 ommata. Major prey types were Chironomidae, unidentified Insecta, 

 and Cladocera (Table 1). Oribatid mites, not usually found in fish guts 

 (B. J. Freeman, personal communication), were eaten by both L. 

 ommata and G. affinis. 



