Resource Sharing Among Darters in an Ohio Stream

Abstract

This study quantified predator-prey interactions involving three darter species (Etheostoma caeruleum, E. zonale and E. blennioides) and macroinvertebrates of the upper Little Miami River, Ohio, and the extent of seasonal food and habitat partitioning among these fishes. Food overlap values (CA) based on biomass were generally lower than those computed from the numbers of prey items in the annual diets (CA = 0.41-0.75 and 0.72-0.84, respectively). These differences reflected size-selective feeding behavior and variability in the dry weights of food organisms. Feeding intensity was influenced by water temperature, while the contributions of the various invertebrate taxa to the diets shifted with changes in the abundances of these forms. Etheostoma zonale and E. blennioides selected food items in the size ranges of 1-5 and 1-6 mm, respectively; the range for E. caeruleum was 1-9 mm. Darter numbers in the riffles changed seasonally in response to the timing of reproduction. These shifts resulted in low overlap values (CA = 0.41-0.44) based on habitat use. We concluded that the darters coexist through resource sharing which reduces interspecific competition for food and space. INTRODUCTION Fish faunas of riffles in small streams of the lower Midwest are often dominated by species of the genus Etheostoma (darters). In niost cases, two or more darter species reside together, suggesting interspecific differences in life history strategies, particularly those relating to food preference, reproduction and habitat use. Though some life history characteristics of darters have been studied in detail (e.g., Fahy, 1954; Roberts and Winn, 1962; Page and Smith, 1971; Scalet,1972; Schenck and Whiteside, 1972; Page and Burr, 1976; Cordes and Page, 1980), little emphasis has been given to mechanisms that allow them to coexist in the riffle habitat. Some investigators have noted that darters are frequently concentrated in the fast, rocky portions of riffles, where they exhibit no spatial habitat partitioning except during the spawning season (Trautman, 1957; Winn, 1958). Stiles (1972) reported that habitat selection by three species of the subgenus Nothonotus in the Little River, Tennessee, is influenced by substrate type and current velocity. The present study was designed to measure ecological differences among three species of darters (Etheostoma caeruleum, E. zonale and E. blennioides) in the upper Little Miami River, Ohio. We quantified major predatorprey relationships involving the darters and macroinvertebrates in the riffle habitat, and identified and analyzed seasonal patterns of food and habitat use of the three species. MATERIALS AND METHODS Samples of fish and macroinvertebrates were taken monthly (November 1977-October 1978) at four riffles in the upper 35 km (Greene County) of the stream. The substrate at each site was composed primarily of gravel and cobblestone. Water depth ranged from 15-25 cm during most of the year; however, occasional flooding, particularly in the spring, resulted in depths 2 2 m. Sampling was delayed whenever high water occurred; no samples were taken in March 1978 because of flooding. Fish were collected with a drag-seine (2.4 x 1.2 m; square mesh = 4.8 mm). A standard area of riffle (29 m2) was sampled to establish consistency in fishing effort throughout the study. Body cavities of fish were injected with 10% formalin, and fish were placed in 10% formalin and transported to the laboratory for gut analyses. 'Present address: Department of Environmental Science, Nichols College, Dudley, Mass. 01570.