Effects of Environmental Hypoxia Associated with the Annual Flood Pulse on the Distribution of Larval Sunfish and Shad in the Atchafalaya River Basin, Louisiana

Abstract

The Atchafalaya River basin (ARB) is a hardwood floodplain swamp associated with the Atchafalaya River in Louisiana. When the ARB is inundated by the river's annual spring flood pulse, the decomposition of organic matter causes hundreds of hectares to become hypoxic (dissolved oxygen (DO) ≤ 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 ARB. Because for most fishes in the ARB reproduction occurs on the floodplain during the flood pulse, hypoxic conditions may severely impact larval fish abundances. We examined the relationships between larval fish abundance and hypoxic conditions in the ARB during 1994 and 1995. Of the 5,389 larval fishes from 14 taxa that were collected, sunfish Lepomis spp. (73.9%) and shad Dorosoma spp. (19.1%) dominated the assemblage during both years. Overall, there was a strong positive relationship between DO level and the presence of larval sunfish and shad. Higher DO levels were most strongly associated with the presence of larval sunfish in 1994 but with the presence of larval shad in 1995. These abundance patterns appear to be related to differences in ARB inundation during the 1994 and 1995 flood pulses. The mean monthly 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 after local conditions (e.g., turbulence and increasing primary production) caused hypoxic areas to become normoxic (DO > 2.0 mg/L). These abundance patterns suggest that larvae were present in adjacent littoral macrophytes that served as DO refugia. Because widespread hypoxia limits the nursery potential of large areas of the inundated floodplain, the reproductive success of many ARB fishes would benefit from a water management plan designed to increase the exchange of water between the main channel and backwater areas.