Use of a restored Central California floodplain by larvae of native and alien fishes

—We sampled larval fish in 1999 and 2001 on a restored floodplain along the lower Cosumnes River, California, from the onset of flooding to when the sites dried or when larval fish became rare. We collected more than 13,000 fish, of which prickly sculpin Cottus asper made up the majority (73%). Eleven species made up 99% of the catch. Three native fishes (prickly sculpin, Sacramento sucker Catostomus occidentalis, and splittail Pogonichthys macrolepidotus) and two alien species (common carp Cyprinus carpio and bigscale logperch Percina macrolepida) were associated with higher inundation and cool temperatures of early spring. In contrast, five alien taxa, sunfish Lepomis spp., largemouth bass Micropterus salmoides, crappie Pomoxis spp., golden shiner Notemigonus crysoleucas, and inland silverside Menidia beryllina, were associated with less inundation and warmer water temperatures. One native species, Sacramento blackfish Orthodon microlepidotus, was also associated with these conditions. Species did not show strong associations with habitat because of different spawning times of adults and expansion and contraction of flood waters. Most species could be found at all sites throughout flooded habitat, although river and floodplain spawning fishes usually dominated sites closest to levee breaches. Highest species richness was consistently found in two sloughs with permanent water because they both received drainage water from the floodplain and had a complement of resident species. Splittail, a floodplain spawner, was found primarily in association with submerged annual plants. Our results suggest that a natural hydrologic cycle in spring is important for providing flooding and cool temperatures important for many native larval fishes. Alien fishes are favored if low flows and higher temperatures prevail. Restoration of native fish populations that use floodplains for rearing should emphasize early (February–April) flooding followed by rapid draining to prevent alien fishes from becoming abundant. * Corresponding author: [email protected]