Gut content biomass of stream fishes: associations with streamflow velocity, population density, and prey abundance at two spatial scales

Abstract

Although food consumption rate has been identified as an important biological variable informing growth rates and population dynamics in fisheries management, equivalent measures receive little attention for native stream fishes. This research investigates whether spatial variation in fish gut fullness can be explained by streamflow velocity, population density, or prey abundance. We tested hypotheses associated with outcomes of habitat-specific energetic demands, exploitation competition, and resource-driven habitat use. Gut content biomass (GCB) of insectivorous fish species was compared at reach and at landscape scales in the Meramec River basin (Missouri, USA). Explanatory variables were selected using ridge regression with cross-validation. GCB of widely distributed species was correlated with body size, streamflow velocity, conspecific fish density, and prey abundance. At the reach scale, GCB was positively correlated with streamflow velocity in three species, supporting a prediction of an optimal foraging model. Negative relationships between GCB and density of Luxilus zonatus may be due to exploitation competition, while positive relationships between GCB and density of Etheostoma flabellare likely result from resource-driven habitat use. We found few relationships between GCB and explanatory variables at the landscape scale, suggesting that unmeasured biological and habitat variables are more important determining gut fullness at broad spatial scales.