Physicochemical Effects on the Abundance and Distribution of Larval Fishes in the Atchafalaya River Basin, Louisiana
The Atchafalaya River Basin (ARB) experiences overbank inundations as springtime temperatures warm that create conditions favorable for bacterial respiration, which results in hypoxic conditions (dissolved oxygen [DO] ? 2.0 mg/L). Previous ARB research has shown that pelagic larval fish populations are impacted by hypoxic conditions, and that larvae may use macrophyte beds (e.g., hydrilla Hydrilla verticillata) as DO refugia. I examined the relationship between physicochemistry and the distribution and abundance or larval fishes associated with ARB macrophyte beds during 2001-2002. Larval fishes were collected with light traps at two normoxic and hypoxic sites, and at each site, surface and subsurface traps were placed in open channels, hydrilla beds, and the interface between the channel and macrophyte beds (border). Of the 1,718 larval fishes representing 10 families that were collected, centrarchids Lepomis spp. (45.5%) and catostomids Ictiobus spp. (25.7%) dominated the 2001 assemblage, whereas percids Etheostoma spp. (29.1%) and centrarchids (17.8%) were most abundant in 2002. River stages between years were different from each other (P < 0.001), and from the 42-year stage average (P < 0.001). Between-year differences in flood pulse timing and duration resulted in higher pH, temperature, and DO levels, and 2.5 times more larval fishes in 2001. Overall, larval fish abundances under hypoxic conditions were severely reduced during both years. Under normoxic conditions, larval fishes were collected from all habitats, but were typically in higher abundance in nearshore areas regardless of cover conditions (hydrilla vs. no cover). Larvae that could withstand hypoxic conditions (centrarchids, clupeids, cyprinids, and percids) were typically found in nearshore areas associated with hydrilla beds. Because of the increased abundance of predatory invertebrates in these areas, the use of hydrilla beds by fish larvae as oxygen refugia may provide optimal physiochemical conditions at an increased predation risk. Man-made alterations in the ARB have changed circulation patterns on the inundated floodplain, which contribute to seasonally widespread hypoxic conditions and reductions in larval fish abundance. Thus, any management plan that increases the historic connection between the mainstem Atchafalaya River and the floodplain, and improves floodplain circulation would benefit larval fish production in the ARB.
Advisor:James P. Geaghan; D. Allen Rutherford; William E. Kelso
School:Louisiana State University in Shreveport
School Location:USA - Louisiana
Source Type:Master's Thesis
Keywords:forestry wildlife fisheries
Date of Publication:07/10/2003