We used radiotelemetry to determine diel and seasonal movements, habitat use, and home range behavior of 37 adult spotted gars Lepisosteus oculatus in the Atchafalaya River basin, Louisiana. The Atchafalaya River exhibits a distinct spring flood pulse each year, inundating a 3,640-km 2 floodplain that includes a complex network of canals, bayous, and lakes. During nonflood months, diel locations of 27 fish recorded at 2-h intervals (median ϭ 24 locations/fish) indicated that most spotted gars were shoreline oriented (P Ͻ 0.0001), preferred submerged branches as cover, and avoided areas of exposed bank (P Ͻ 0.0001). Median spotted gar movement rates were higher during summer (40.4 m/h) than fall-winter (15.1 m/h), and during both seasons, rate of movement and percentage of home range used were significantly greater at night than during dawn, day, or dusk periods (P Ͻ 0.03). Increased nocturnal activity appeared to be related to feeding periodicity, as a substantially greater proportion (70%) of spotted gars collected at night (N ϭ 30) contained food compared with dawn, day, and dusk samples (29%; N ϭ 90). Seasonal movement studies (twice-weekly locations; N ϭ 37 fish) indicated that spotted gar activity increased as water temperature and river stage rose in the spring, resulting in greater median movement rates (130.1 m/d) and home ranges (265.1 ha) than during fall-winter (35.8 m/d; 6.2 ha) and summer (34.6 m/d; 10.5 ha). Spotted gar home ranges established during the flood pulse typically included large areas of inundated floodplain, which provided extensive areas of suitable spawning and nursery habitat.

Growth, mortality, fecundity, egg diameter, and age at maturity were determined from samples of 270 paddlefish Polyodon spathula collected from the Atchafalaya River basin and Lake Pontchartrain, Louisiana, during a commercial harvest moratorium from 1987 to 1989. Early growth of paddlefish determined from back-calculation of annular measurements on dentary cross sections was high relative to paddlefish populations in the upper Mississippi River drainage; lengths of age-1 fish ranged from 411 to 455 mm in eye-fork length. Natural mortality of Louisiana paddlefish was high (26-48%), and data suggested a reduction in age at maturity (100% females mature at age 10), lower fecundity (average fecundity, 9,500 eggs/kg body weight), and larger eggs (average diameter, 2.67 mm) relative to other paddlefish populations. Changes in reproductive life history characteristics may reflect rapid growth and high natural mortality rates. As the postmoratorium commercial fishery for Louisiana paddlefish develops, conservative harvest regulations should be promulgated until the effects of harvest on paddlefish stock dynamics can be determined.

Low levels of dissolved oxygen (DO) are potential stressors of fishes and invertebrates in freshwater and marine systems. Because of the incredibly nutrient-rich and warm nature of the Atchafalaya River Basin (ARB), microbial decomposition in backwater areas with limited flow often results in potentially stressful, if not lethal, DO levels during and after the annual flood pulse. As water temperatures increase and flood levels fall, DO levels often stratify, restricting fish habitat. In order to better guide development of water management projects, we investigated the relationship between DO and DO stratification (DOD; surface DO -bottom DO) and a suite of environmental variables potentially linked to DO and DOD. Based on continuous monitoring data collected from 2000 to 2007, we used a variable selection procedure to construct statistical generalized linear models to describe the relationship between DO levels and DOD and several potentially important predictor variables at three spatial scales: the entire sampled area of the ARB; individual water management units (WMUs) or subunits (WMSs); and individual sampling sites. We conducted these analyses using models that assume the normal distribution and compared them to models that incorporate alternate error terms based on other error term distributions. Goodness of fit statistics suggested that our analyses best described the relationship between DO and structuring variables at the WMU/WMS scale. At this scale, water temperature, river stage height, current velocity, and to a lesser extent, water depth appeared to be the most important measured variables that influence DO levels. We suggest that management decisions at the WMU and WMS scales will offer the greatest potential benefits for improvement of DO conditions in the ARB.

No abstract

1990. Physiological responses of largemouth bass, Micaopaerus salrnoides, exposed to salinity. Can. 8 . Fish. Aquat. Sci. 47: 2358-2363 Plasma osmotic and electrolyte concentrations as well as branchial Na'/K+ and Mg* + ATPase activities were determined in the field for largemouth bass, Micrspterus sa%moides, from a brackish marsh and freshwater lake in southcentral Louisiana. Laboratory experiments were conducted to evaluate plasma chemistry and gill ATPase activities of largemouth bass from both locations exposed to 0, 4, 8, and 12% salinity. No significant differences in physiological responses were detected between marsh and freshwater largemouth bass exposed to 0, 4, or 1 2%0. Exposure to 1 2% salinity resulted in osmotic stress in largemouth bass from both locations. At 8%0, marsh largemouth bass had significantly higher plasma solutes and Dower gill ATPase activities than freshwater fish. Different physiological responses by marsh and freshwater largemouth bass during exposure to 8%0 salinity indicated that marsh largemouth bass have adapted to environments of variable sali nity by reducing active ion transport and tolerating elevated plasma ion levels. La pression osmotique et les concentrations des 6lectrolytes dans le plasma de mGme que I'activite de la Na*/K+ ATPase et de la Mg' + ATPase dans les branchies ont 4t6 determinees chez des achigans 3 grande bouche, Micropterus salrnsides, provenant d'un marais d'eau saurnatre et d'un lac d'eau douce du centre-sud de la Louisiane. La chimie du plasma et I'activite des ATPases au niveau des branchies ont 6t6 6tudiees chez tes deux groupes d'achigans qui ont 6te expapst% des salinit6s de 8, 4, 8 et 12%0. Aucune difgrence significative n'a 6t6 obsewee dans les r6ponses physiologiques des achigans de marais et d'eau d o u c~ pour les salinites de 0, 4 et 12%~. A 12%, les achigans des deux provenances ont subi un stress osmotique. A 8%0, les achigans de marais montraient des concentrations de solut6s plasmatiques seeperieures et des activites branchiales ATPasiques inferieures A celles des achigans d'eau douce. Les r6ponses physiologiques differentes observees chez ces deux groupes d'achigan lors de leur exposition 2 une salinite de 8 %~ montrent que les achigans de marais se sont adapt& aux variations de salinite de leur milieu en reduisant le transport actif des ions et en developpant une tolerance aux concentrations elevees d'ions dans leur plasma.

No 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.

We sampled ichthyoplankton, zooplankton, and juvenile fish during May, August, and November 1988 from 15 excavated floodplain ponds along the Mississippi River. In May, densities of larval shad Dorosoma spp. ranged from 1.1 to 87.8/m3 and densities of larval sunfish Lepomis spp. ranged from 0.3 to 19.3/m3 among ponds. Ponds with long (>1,800 m), sinuous shorelines, high volumes (>55,000 m3 ), and variable depth had high ichthyoplankton densities (>50/m3); smaller, low‐volume ponds frequently had low ichthyoplankton densities (<10/m3). Ponds that sustained high densities of larval sunfish during May had high numbers of young‐ofthe‐year sunfish in August and November, which suggests that ponds with high ichthyoplankton densities ultimately contributed more fish to river populations. Pond morphometry probably did not affect food availability, because zooplankton densities were not correlated with ichthyoplankton densities and larval threadfin shad D. petenense grew faster in ponds with high shad densities. Morphometry may have regulated access of spawning adults to ponds or affected larval fish growth and survival in ponds. Future excavations should adhere to guidelines used by the U.S. Army Corps of Engineers to ensure that ponds function as viable fish nursery areas.