11 



The prespauning sample of the population had ap- 

 jjioximately a 1:1 sex ratio (Table 3). and there was 

 no indication of a difference in lontjevity between males 

 and females. 



FOOD HABITS 



Forbes (1878:75) noted "larvae of gnats and of 

 May-ffies, with a few copepoda" in the stomach of a 

 specimen of Bohichthys clegans (= E. gracUc) and later 

 (1880:24) reported that thiee specimens contained 37 

 ])ercent dipterous lar\ae and 63 percent ephemerid 

 lar\ae. 



Stomachs of 61 adult slough darters from the study 

 area, taken in February, March, April, May, and Jime, 

 and of seven juveniles, taken in June, July, September, 

 October, and December, were examined. The types 

 of food eaten, seasonal changes in feeding habits, and 

 differences in food preference between adults and young 

 were noted. Stomachs of a few adults from localities 

 outside the study area, taken chiefly in late summer, 

 were also examined. 



During late winter adults fed primarily on midge 

 larvae (Chironomidae) and microcrustaceans, especially 

 copepods and cladocerans (Table 4) . The only other 

 item in the stomachs of adults collected in February 

 was the snail (Lymnaea sp,). Midge larvae constituted 

 64 percent by volume of the food eaten in February. 

 Copepods, although more abundant numerically, made 

 up only 21 percent by volume of the stomach contents. 



Table 4. — Stomach contents of adult slough darters in 

 Dismal Creek, estimated percentages* of each item by volume. 

 Figures in parentheses represent numbers of stomachs examined. 



» 



* Rounded to nearest tenth of 1 percent. 



In the spring new food organisms were added as 

 they became available. They included other crustaceans 

 (ostracods, amphipods, and phyllopods) and insect larvae 

 (Ephemeroptera, the dipteran Sciaridae, and the coleo])- 

 teran Dytiscidae). By May mayfly lar\ae were the pre- 

 dominant food, accounting for 56 percent of the volume 

 of stomach contents, and midge larvae made up 41 

 percent. 



After si)awning season the amount ol food taken by 

 adults decreased sharply. Stomachs of a few adults from 

 localities outside the studv area were examined in Julv. 



.\ugust, and September, Each was either empty or con- 

 tained only one food item, and stomachs of all adults 

 were greatly shrunken. Whether reduced feeding is the 

 result of a small food supply or reduced activity brought 

 on by heat and low oxygen availability is not known. 



Although very young darters probably feed on dia- 

 toms and other plankton, they quickly grow large enough 

 to take adidt-sized food. Within 2 weeks the young 

 were eating midge lai\ae and microcrustaceans, items 

 which furnish most, if nut all, of the food of the juvenile 

 fish. The juveniles can eat items of surprisingly large 

 size. For example, a darter 18 mm in standard length 

 collected on Jidy 3 contained a 16-min midge larva and 

 two copepods. 



The greatest \olunie of food is apparently taken by 

 adidts during the sjiring months, for at that time inost 

 of the darters had stomachs greatly enlarged and some- 

 times containing 60—70 organisms of various sizes. The 

 second greatest volume was foimd in darters collected 

 in winter, and the sinallest in adult darters taken in 

 summer and fall. In contrast. Fahy (1954:152) found 

 that adults of the riflle-inhabiting E. bUnnioidcs eat 

 least in the winter months and most in July and August, 



The main criteria for E. gracilc appear to be sizes 

 of prey items and their availability. Almost any insect 

 larva small enough to be ingested will be eaten, and 

 even adult darters take such minute food items as cope- 

 ])ods and ostracods, 



INTERACTIONS WITH OTHER ORGANISMS 



Competition 



Interspecific competition for food does not seem to 

 be an important factor in the control of population 

 density. Stomachs were removed from two or three 

 specimens of 18 of the 23 associated species in the study 

 area, and their contents were examined for similarities 

 in food types and sizes to those of the slough darter. Like 

 the slough darter, its associates shtiwid no predilection 

 for specific foods. 



In Dismal Creek only ilir hluulnose darter, E. chloro- 

 sominn. seemed to be in direct com|)ctilion for food with 

 the siougii darter throughout the year. .Adults of most 

 other fishes fed on larger organisms in the simimer and 

 ate little or nothing in the winter. The young of several 

 species of minnows, darters, and sunfishes, however, 

 did feed on similar items. If there is competition, it 

 does not last long, for the rapidly growing young of 

 larger species quickly switch to larger foods. There is, 

 moreover, a depth separation : darters feed on the bot- 

 tom, but most minnows, tojiminnows, and sinifishes feed 

 above the bottom or near the sinface. 



Etlwostoma sjxctabilc and E. nigrum eat the same 

 general types of food, but there is a habitat segregation. 

 E. slirctabHc inhabits a substrate of gravel or rocks, and 

 nigrum usually occms in sand-bottomed or hard-bot- 

 tomed pools. Dining the winter all foin- species of dart- 

 ers retreat to the deeper pools, where they, along with 



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