Effects of Turbidity on Prey Consumption by Prairie Stream Fishes

—Reduced suspended-sediment loads (i.e., turbidity) in many Midwestern prairie rivers have been hypothesized as contributing to the replacement of species that historically occupied highly turbid main-channel habitats by visually feeding species that are competitively superior in less-turbid waters. We examined the relationship between prey consumption and turbidity for six fish species from the Canadian River (New Mexico, Oklahoma, and Texas) and found experimental support for this hypothesis. Among species adapted to highly turbid main-channel habitats, we found that prey consumption by the peppered chub Macrhybopsis tetranema and flathead chub Platygobio gracilis was unaffected (P . 0.12) by elevated turbidity, whereas prey consumption by the Arkansas River shiner Notropis girardi was reduced (P , 0.01). Among species characteristic of less-turbid habitats, prey consumption by the emerald shiner N. atherinoides, red shiner Cyprinella lutrensis, and sand shiner N. stramineus was reduced (P , 0.01) by elevated turbidity. Compared with prey consumption at 0 nephelometric turbidity units (NTU), prey consumption at 4,000 NTU decreased 21% among peppered chub, 26% among flathead chub, and 59% among Arkansas River shiners, which was less than that observed among emerald (73%), red (84%), and sand shiners (89%). In general, elevated turbidity had less effect on the prey consumption of species that are adapted to highly turbid habitats than on those characteristic of less-turbid habitats. The high suspended-sediment loads that historically were characteristic of many prairie streams may have excluded emerald, red, and sand shiners from main-channel habitats. Groundwater pumping, water diversion, and reservoir construction have dramatically altered aquatic habitats throughout the Missouri and Arkansas river basins (Williams and Wolman 1984; Cross and Moss 1987; Pflieger and Grace 1987; Stinnett et al. 1988; Limbird 1993; Friedman et al. 1998). Streams and rivers have been reduced in size and volume, channel morphology has changed so that many streams no longer meander or form multiple channels, frequency and intensity of flood events has decreased, and suspended-solid loads generally have decreased. These changes are associated with changes in stream fish assemblages: many species that historically inhabited the larger streams and rivers of the Arkansas and Missouri river drainages now are reduced in distribution and abundance (Cross and Moss 1987; Pflieger and Grace 1987; Limbird 1993; Bonner and Wilde 2000). In general, there has been a replacement of species that historically inhabited the main channels of these fluctuating, turbid rivers by species that were low in relative abundance or restricted * Corresponding author: [email protected] 1 Present address: Department of Biology/Aquatic Station, Southwest Texas State University, San Marcos, Texas 78666, USA. Received July 20, 2000; accepted May 25, 2002 to river margins and tributary streams (Cross and Moss 1987; Pflieger and Grace 1987). For example, Bonner and Wilde (2000) found that the Arkansas River shiner Notropis girardi, flathead chub Platygobio gracilis, peppered chub Macrhybopsis tetranema (Eisenhour 1999), and plains minnow Hybognathus placitus, which historically composed 98% of the fish assemblage, have decreased in abundance or been extirpated from the Canadian River (an Arkansas River tributary) downstream from Lake Meredith, Texas, where discharge has been reduced 76% since the river was impounded in 1966. Conversely, the red shiner Cyprinella lutrensis, sand shiner N. stramineus, and emerald shiner N. atherinoides have increased in abundance, presently composing more than 90% of the fish assemblage in the affected area. Species replacements such as that described by Bonner and Wilde (2000) were hypothesized by Cross and Moss (1987) and Pflieger and Grace (1987) to be due, in part, to decreases in suspended-sediment loads (turbidity) that allow fishes characteristic of less-turbid waters to displace (presumably via competition for food) species that are adapted to more-turbid conditions (Moore 1950; Davis and Miller 1967; Branson 1979; Huber and Rylander 1992). If this hypothesis is correct, we would predict a differential response in prey 1204 BONNER AND WILDE consumption to elevated turbidity between species that are adapted to very turbid waters (i.e., the Arkansas River shiner, flathead chub, and peppered chub) and those characteristic of less-turbid waters (i.e., the emerald, red, and sand shiners). Herein, we provide a test of this prediction. First, we test the null hypothesis that turbidity has no effect on the prey consumption of each species considered separately. Second, we test the null hypothesis that the effects of turbidity on feeding efficiency do not differ among species.