Abiotic Attributes Surrounding Alluvial Islands Generate Critical Fish Habitat

Identifying the appropriate scale at which habitat is biologically relevant to riverine fishes in large, sand-dominated rivers is a challenge. Alluvial islands are important to several of these fishes throughout the central USA, but there is a paucity of information on island habitat features that restoration efforts should try to replicate. We determined the physical characteristics of two island complexes in the middle Mississippi River that facilitate the settlement and survival of age-0 shovelnose sturgeon Scaphirhynchus platorynchus at relatively large (mean 39,000m²) and small (mean 320m²) scales. Depth (m), flow rate (ms^-1), substrate (sand, rock, silt) and vegetation were quantified at these two scales using hydroacoustic techniques (split-beam sonar and acoustic Doppler current profiler). Abiotic attributes in the surrounding littoral zone of the island complexes were highly correlated but differed depending on location. At the coarse spatial scale, vegetation was positively related to shovelnose sturgeon abundance. At the fine spatial scale, age-0 shovelnose sturgeon were restricted to flow rates<0.89ms^-1, with abundance peaking at about 0.40ms^-1. However, heterogeneity in depth and flow was important, and sturgeon abundance peaked at intermediate variability in these two abiotic attributes. A computer-generated model of the habitat surrounding islands suggests that these habitats are diverse and may provide flow refugia and foraging patches for shovelnose sturgeon. We submit the results presented here that can contribute to a hierarchical model for island restoration in large rivers.