The direct exploitation of prey on an inundated floodplain by cherryfin shiners (Lythrurus roseipinnis) in a low-order, blackwater stream

Martin Thomas O'Connell

Abstract

Although the importance of floodplains to aquatic organisms in large streams is well-established, little is known about the benefits provided by brief seasonal floods in low order, blackwater streams. Previous research has shown that in years of increased flooding, some species of stream fishes experience population increases, suggesting that stream fishes profit from floods. The mechanism that causes floods to be beneficial to some fishes in low order, blackwater streams is unknown, but other research suggests that benefits do not stem from fishes using the inundated floodplain for spawning. I examined the possibility that stream fishes in low order, blackwater streams benefit from floods by exploiting the increased food resources originating from the floodplain. Using cherryfin shiners (family Cyprinidae; Lythrurus roseipinnis ), a numerically dominant species known to occur on the inundated floodplain, I compared food availability, foraging behavior, current velocities, and micro-habitat choice between the inundated floodplain and the low water stream. Drift samples from both habitats indicated that more food was available on the inundated floodplain than in the low water stream. Gut samples from L. roseipinnis held in enclosures in both habitats demonstrated that fish ate more on the inundated floodplain than in the low water stream. An individual-based basic prey model incorporating both availability and gut sample data confirmed that fish on the inundated floodplain foraged more efficiently than fish in the low water stream. A comparison of natural current velocities with critical swimming velocities for L. roseipinnis showed that most natural velocities in all three habitats (inundated floodplain, flooded channel, low water stream) were non-prohibitive, suggesting that stream fishes in low order, blackwater streams do not move onto the inundated floodplain to seek refuge from high current velocities within the flooded channel. There were no differences detected in micro-habitat choice (high current velocity versus low current velocity) between fish enclosed on the inundated floodplain and the low water stream. Although, the micro-habitat choice results are inconclusive in regard to comparing the two habitats, the food availability and foraging behavior results suggest that stream fishes benefit from floods by exploiting the increased food resources originating from the inundated floodplain.