Backwater immigration by fishes through a water control structure: Implications for connectivity and restoration

Recent efforts to reduce sediment loading in large-river backwaters have relied on inexpensive low-maintenance structures that promote river-backwater connectivity. How these structures influence ichthyofauna is not understood. We quantified the distribution of fishes through a stop-log structure installed during the Swan Lake habitat restoration project on the lower Illinois River. Modified hoop nets were systematically fished simultaneously in a two-dimensional 3 vertical X 3 horizontal grid of net locations to capture fish as they immigrated into the lake through the structure. Catches were dominated by gizzard shad Dorosoma cepedianum and freshwater drum Aplodinotus grunniens. Adult freshwater drum, adult common carp Cyprinus carpio, and juvenile silver carp Hypophthalmichthys molitrix exhibited an affinity for the lower two-thirds of the water column. Lengths of freshwater drum increased with depth, particularly during the spring. Common carp catch rates were highest during spring, whereas those for silver carp were highest during fall and winter. Closure of the bottom two-thirds of the lower Swan Lake water control structure could reduce immigration of exotic and undesirable species into the backwater; their recruitment associated with the lake could be affected accordingly. The water control structure does not appear to have negatively affected lateral fish movement. However, habitat quality and availability probably determine seasonal patterns in backwater use. In systems where exotic species are abundant, lateral connectivity may need to be assessed from a cost-benefit perspective, specifically as it pertains to maintaining habitat availability for indigenous fishes versus limiting habitat availability for introduced species.