Fish nursery habitat function of the main channel, tributaries, and oxbow lakes of a river-floodplain system

Rivers are increasingly disconnected from their floodplain due to dams, levees, and other forms of river modifications. However, in some instances tributaries can provide a corridor for fish movement between the main river channel and the floodplain. In comparison to oxbow lakes, low-order tributaries are a relatively understudied feature of the floodplain, and direct comparisons of the fish reproductive function of tributaries and oxbow lakes are scarce. A research project was undertaken to investigate the reproductive value of floodplain tributaries by (i) characterizing and comparing larval and juvenile fish assemblages in the main river channel and two permanently inundated floodplain habitats (oxbow lakes and tributaries) and (ii) examining how connectivity to the main channel influences young-of-year fish assemblages among river-floodplain habitats and within tributary habitats (upstream and downstream of semi-permeable barriers). Mini-fyke nets and larval fish light traps were used to collect juvenile and larval fish monthly from April to July 2012. 12,925 larval and juvenile fish representing 15 families and 45 taxa were collected during the study period. Centrarchidae and Cyprinidae were the two most abundant taxa collected in all four habitats (main river channel, lower tributary, upper tributary, and oxbow lake. Family level fish assemblage analysis using light trap data found distinct differences between main river channel and floodplain young of year fish assemblages (tributary and oxbow lake habitats). Additional juvenile fish assemblage analysis with combined gears and main channel excluded found distinct fish assemblages between tributary and oxbow lake habitats, but not between upper and lower tributary habitats. Connectivity appeared to influence juvenile fish assemblages between upper and lower tributary habitats; upper and lower Caney Creek had a unique fish assemblage and grouped separate from their appropriate habitat groupings. Caney Creek was the least fragmented tributary, retaining the highest connection to the main channel and several other floodplain habitats (e.g., lakes and wetlands). Similar to other studies the main channel had a distinct fish assemblage compared to floodplain habitats, but not due to the presence of rheophilic taxa commonly driving these differences. The main channel had higher reproductive function for small-bodied cyprinids and centrarchids in this study relative to studies in larger river systems. Connectivity and habitat harshness (i.e., high temperature and decreasing water levels) were the primary factors driving differences between tributary habitats and oxbow lakes. Differences between floodplain habitats might have been amplified by extreme drought conditions during the study period, but the results of this study further highlight the importance of maintaining connectivity to the main river channel from a conservation perspective. Lower tributaries, which are permanently connected to the main river channel, had higher total number of taxa and more unique taxon than all other habitats, likely due to multiple sources of fish production ( in situ reproduction and fish exchange between adjacent habitats). Overall, tributary habitats were shown to be important fish reproductive and nursery areas and were more important than oxbow lakes in terms of total abundance and species usage. Oxbow lakes in this study were in the later stages of succession and had characteristics known to be disadvantageous for fish reproduction and nursery habitat (disconnected and shallow). Additionally, within-tributary connectivity appeared to homogenize fish communities in upper and lower Caney Creek and facilitated habitat usage of large-bodied main channel fish not found in high numbers in other tributaries. Tributaries in general were found to be important and further research will need to be conducted to understand how floodplain connectivity influences differences in low-order tributary fish reproduction and nursery function observed in this study and if tributaries are important at larger spatial scales.