Managing the Flood Pulse for Optimal Fisheries Production in the Atchafalaya River Basin, Louisiana

The Atchafalaya River Basin (ARB) in south-central Louisiana comprises the largest contiguous river-floodplain swamp in North America.  Its winter-spring flood pulse is influenced directly by discharge from the Mississippi River.  The ARB acts as a distributary for the Mississippi River, diverting 30% of the combined flow of the Red River and Mississippi River to the Gulf of Mexico.  However, discharge and flood capacity of the Atchafalaya River is regulated by the U.S. Army Corps. of Engineers (USACE) by guide levees and by the Old River Control Structures near Simmesport, LA.  The purpose of our study was to assess the impact of flooding in the ARB on fisheries production at a basin-wide scale.  We modeled flood duration (days/year Butte LaRose gage height > 3.6 m) and magnitude (daily gage height at Butte LaRose) against fish relative abundances from long term (1987-2009) fishery-independent data collected by the Louisiana Department of Wildlife and Fisheries (LDWF) on largemouth bass (Micropterus salmoides), crappie (Pomoxis spp.), blue catfish (Ictalurus furcatus), buffalofish (Ictiobus spp.), and gizzard shad (Dorosoma cepedianum).  We also used percentage of age-1 and age-2 largemouth bass and crappie as indicators of recruitment, and we modeled annual (1999-2009) crayfish (Procambarus spp.) landings (1994-1996) from fishery-dependent trip ticket information, as well as monthly crayfish harvest from an LDWF creel survey.  When the Atchafalaya River at Butte LaRose is at flood stage (≥ 3.6 m) for approximately 124-157 days/year, then annual relative abundances of the recreationally and commercially important fishes are optimized.  Gizzard shad was negatively impacted by flood duration, as its relative abundance was optimized during dry years (< 10 days/year).  Annual recruitment of age-1 largemouth bass was associated positively with annual flow magnitude during the previous year, while age-2 largemouth bass and crappie were associated positively with flow magnitude two years prior to the fish sample.  Annual crayfish and gizzard shad landings (i.e., crayfish bait), as well as crayfish sacks/boat-month from the creel survey, were associated positively with flood magnitude and duration.  Our logistic model of Atchafalaya River discharge suggests that a minimum of 8,665 m³/s is required near its source for the lower ARB to reach the 3.6 m flood stage,  optimizing fisheries production.  The results of this study can assist the USACE in developing a flow management scheme for the Atchafalaya River that will benefit recreational and commercial fishers in the ARB, as well as the fisheries resources upon which they rely.