Effects of environmental hypoxia associated with the annual flood pulse onthe distribution of larval sunfish and shad in the Atchafalaya River Basin, Louisiana

The Atchafalaya River basin (ARB) is a hardwood floodplain swamp associated with the Atchafalaya River in Louisiana. When the ARE is inundated by the river's annual spring hood pulse, the decomposition of organic matter cause s hundreds of hectares to become hypoxic (dissolved oxygen [DO] less than o r equal to 2.0 mg/L). During this time hypoxic conditions are variable, but they often persist from April through June in more than 50% of the lower A RE. Because for most fishes in the ARB reproduction occurs on the floodplai n during the flood pulse, hypoxic conditions may severely impact larval fis h abundances. We examined the relationships between larval fish abundance a nd hypoxic conditions in the ARE during 1994 and 1995. Of the 5,389 larval fishes from 14 taxa that were collected, sunfish Lepomis spp. (73.9%) and s had Dorosoma spp. (19.1%) dominated the assemblage during both years. Overa ll, there was a strong positive relationship between DO level and the prese nce of larval sunfish and shad. Higher DO levels were most strongly associa ted with the presence of larval sunfish in 1994 but with the presence of la rval shad in 1995. These abundance patterns appear to be related to differe nces in ARB inundation during the 1994 and 1995 hood pulses. The mean month ly river stage for 1994 was consistent with a 34-year average, but the mean monthly river stage in 1995 was significantly lower in April and higher in June. These differences in timing and duration of the flood pulse appeared to have important consequences for the reproduction of sunfish and shad in the ARB. Both sunfish and shad larvae were collected almost immediately af ter local conditions (e.g., turbulence and increasing primary production) c aused hypoxic areas to become normoxic (DO > 2.0 mg/L). These abundance pat terns suggest that larvae were present in adjacent littoral macrophytes tha t served as DO refugia. Because widespread hypoxia limits the nursery poten tial of large areas of the inundated floodplain, the reproductive success o f many ARE fishes would benefit from a water management plan designed to in crease the exchange of water between the main channel and backwater areas.