Competition between Larval Fishes in Reservoirs: The Role of Relative Timing of Appearance

Funding for this project was provided by National Science Foundation grants DEB 9407859 and DEB 9107173 to R.A.S and Federal Aid in Sport Fish Restoration, project F-69-P, administered jointly by the U. S. Fish and Wildlife Service and the Ohio Division of Wildlife. A Presidential Fellowship from The Ohio State University supported J.E.G. during part of this research.In small, hypereutrophic reservoirs (100 mg total phosphorus/L), larval gizzard shad Dorosoma cepedianum and threadfin shad D. petenense (henceforth, shad) reach high densities in the limnetic zone, virtually eliminate zooplankton, and perhaps compromise success of other planktivorous larvae, such as bluegill Lepomis macrochirus. Because relative timing of appearance of shad and bluegills probably influences their relative success, we quantified densities of fish larvae and zooplankton during spring through summer in three reservoirs across 8 years (1987–1994), and we conducted three hatchery experiments with varying larval appearance times and gizzard shad densities in plastic bags (1 m3). When shad were abundant in reservoirs, bluegill abundance often peaked either at the same time (36% of reservoirs and years combined) or after (40% of reservoirs and years combined) shad peaks. When gizzard shad were placed in bags 2 weeks before bluegills (N = 1 experiment), they depleted zooplankton, reducing growth (~0.075 g · g-1· d-1) but not survival of bluegills. In experiments (N = 2) in which both species were added simultaneously, zooplankton declined only slightly with gizzard shad, and there was little effect on bluegill growth (~0.21 g · g-1· d-1) and survival; in general, gizzard shad growth declined with time and increasing gizzard shad density. Based on experiments, bluegill success should vary among reservoirs and years as a function of their appearance relative to gizzard shad. In reservoirs, zooplankton availability and bluegill abundances were consistently low during years when gizzard shad dominated reservoir fish assemblages. Because gizzard shad probably reduce bluegill success in hypereutrophic Ohio reservoirs, management strategies that reduce gizzard shad should improve bluegill success

Lateral Exchange of Larval Fish between a Restored Backwater and a Large River in the East-Central USA

Exchange of larval fish between a river\u27s main channel and its floodplain backwaters is compromised by sedimentation. Restoration projects to reduce sediment loading are being implemented in large rivers of the midwestern United States to curb backwater habitat loss and restore backwater-to-river connectivity. During 2004 and 2005, drift nets were set bidirectionally (with and against the flow) within a constructed channel between the Illinois River and an adjacent, 1,100-ha restored backwater, Swan Lake, to investigate the interplay between life history strategies and lateral drift on a diel and seasonal basis. Ambient larval density and species composition within the river and backwater also were quantified. Drift was positively correlated with water velocity in the main stem during 2004, and an estimated 32.3 × 106 larvae drifted at the surface of the channel into Swan Lake. In the absence of a flood in 2005, the density and composition of the larval fish assemblage in Swan Lake and the Illinois River appeared to drive larval drift timing, magnitude, and composition. Swan Lake\u27s restoration has maintained some river connectivity and lateral drift functionality for resident fish, but its functionality compared with natural, connected river–backwater systems remains unknown

From Star Charts to Stoneflies: Detecting Relationships in Continuous Bivariate Data

Within many ecological systems, relationships between controlling factors and associated response variables are complex. In many cases, the response should vary little when the controlling factor exerts strong effects. Conversely, when the effect of the controlling factor is weak or absent, the response may vary greatly with effects of other factors. Correlation or regression analyses often may not be appropriate for testing these relationships, because variance of the response changes with values of the controlling factor. We suggest using a technique from the astronomy literature, a two-dimensional Kolmogorov- Smirnov (2DKS) test, to detect relationships in bivariate data with these patterns of variance. This technique successfully identified simulated bivariate data composed of paired independent values as having nonsignificant relationships and simulated bivariate data in which mean and variance of y was constrained at high levels of x as having significant relationships. Using these simulations and examples from aquatic and terrestrial systems, we demonstrate that the 2DKS is a robust test for detecting nonrandom patterns in bivariate distributions that commonly arise in many ecological systems.Support for this research was provided to E. A. Marschall by National Science Foundation (NSF) grant DEB 9410327, to J. E. Garvey and R. A. Wright by NSF grant DEB 9407859 and Federal Aid in Sport Fish Restoration Project F-69-P (administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife, to R. A. Stein, The Ohio State University), and to J. E. Garvey by a Presidential Fellowship from The Ohio State University

Assessing How Fish Predation and Interspecific Prey Competition Influence a Crayfish Assemblage

In northern Wisconsin lakes. the introduced crayfish 0rconectes rusticus is replacing 0. propinquus, a previous invader, and 0. virilis,a native crayfish. Herein, we explore how fish predation and competition interact to drive this change in crayfish species composition. In outside pools, we conducted selective predation experiments exposing crayfish to largemouth bass. Micropterus salmoides, to quantify patterns of crayfish vulnerability. To determine how interactions among crayfish influence susceptibility. we quantified shelter use and behavioral interactions among crayfish in aquaria and outside pools. At equal size, 0. virilis was more susceptible to fish predation than either of the invaders. 0. rusticus and 0. propinquus; the two invaders were equally susceptible to predation. However, sizes of these crayfish in the field are 0. virilis > 0. rusticus > 0. propinquus. Because fish predators prefer small crayfish. at unequal size, small 0. propinquus were more vulnerable to predation than large 0. rusticus. Thus, 0. rusticus can replace 0. propinquus due to natural size differences. Although 0. virilis grows larger than the invaders, it was more susceptible even when 3 mm larger. We hypothesized that 0. virilis, although large, participated in behaviors that increased its risk to predation. When provided with unlimited shelters, all three species increased refuge use under predatory risk. When shelters were limiting and fish present. 0. virilis was excluded from shelters by invaders. 0. virilis also participated in risky behaviors, such as increased activity and swimming. Both agonistic interactions with congeners and approaches by largemouth bass increased risky behaviors in 0. virilis. In addition, 0. virilis was innately less aggressive than invaders. Given these behaviors, 0. virilis was consumed at high rates and would be eventually replaced in lakes. In northern Wisconsin lakes, fish predation and crayfish-crayfish competition interact to influence crayfish replacements. Based on our results, largemouth bass predation modifies the outcome of interference competition among the three crayfishes and, in turn, competitive interactions among the crayfishes influence susceptibility to fish predation. We predict that 0. virilis should suffer high mortality to fish predation in the presence, rather than in the absence, of the two invading species. Our results support the hypothesis that, in areas of sympatry where predators are selective and prey species compete, predation and competition interact to determine community structure.This project was supported through NSF grant BSR 8907691 to R. A. Stein in collaboration with David Lodge and Ken Brown

Evaluating How Local- and Regional-Scale Processes Interact to Regulate Growth of Age-0 Largemouth Bass

Regional- and local-scale processes may interact to influence early growth and survival, thereby governing cohort strength. During summer through fall 1994–1996, we assessed how precipitation (a regional-scale process) and prey availability (a local-scale process) influenced piscivory and growth of age-0 largemouth bass Micropterus salmoides in five Ohio reservoirs (190–1,145 ha). We expected early growth to vary with the abundance and relative sizes of age- 0 gizzard shad Dorosoma cepedianum. We collected age-0 largemouth bass and prey fishes every 3 weeks in each reservoir. In 1994, May precipitation was low (total = 4 cm), resulting in low mean daily reservoir discharge (x¯5 reservoirs = 3.6 m3/s). In four reservoirs, stable water levels may have led to successful largemouth bass reproduction and perhaps an early hatch. As such, age-0 largemouth bass in these systems were abundant, consumed gizzard shad, and reached large sizes by fall (15.3 g). In 1995 and 1996, high precipitation (total > 12 cm) and high reservoir discharge [x¯5 reservoirs = 13.8 m3/s (1995), 28.8 m3/s (1996)] in some reservoirs in May likely reduced largemouth bass abundances. Growth during these years was density dependent across reservoirs. When age-0 largemouth bass abundance was low, nonshad prey fish were consumed, and mean fall sizes were similar to those in 1994 (12.0 g). Conversely, fall weights (4.5–7.4 g) declined in reservoirs with increasing largemouth bass density. Surveying May precipitation in Ohio across 48 years revealed that conditions like those in 1994 occurred less than 15% of the time. Because gizzard shad should rarely be available and other prey fish species probably are limited, density-dependent processes should often regulate early piscivory, growth, and potentially, cohort strength in these systems.This research was funded by National Science Foundation grant DEB 9407859 to RAS and Federal Aid in Sport Fish Restoration project F-69-P, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife. A Postdoctoral Fellowship and Presidential Fellowship from The Ohio State University supported RAW and JEG, respectively, during part of this work

Predicting How Winter Affects Energetics of Age-0 Largemouth Bass: How Do Current Models Fare?

During the first winter of life, loss of energy reserves as a function of low feeding activity and scarce prey may contribute to high mortality of age-0 largemouth bass Micropterus salmoides. To explore how two current bioenergetics models predict winter energy depletion, we quantified growth and consumption by age-0 largemouth bass from Alabama, Ohio, and Wisconsin fed maintenance rations in 55-L aquaria in three simulated winters mimicking temperatures and photoperiods at low temperate latitudes (Alabama; 33N), middle latitudes (Ohio; 40N), and high temperate latitudes (Wisconsin; 46N).We compared observed growth in aquaria with that predicted by putting observed consumption into both models. During winter 1995–1996, we validated one of the models with a separate pool experiment (5,800-L) in which age-0 largemouth bass were fed either at 0.5 X or 1.5 X maintenance ration. In aquaria, energy density of the largemouth bass declined in the high- and middle- but not in the low-latitude winter. Though error was slight in the low- and middle-latitude winters for one of the models, both models underestimated growth in the high-latitude winter. To fit the model to the data, the function that estimates weight-specific resting metabolism had to be reduced by about 16%. In pools, where we predicted consumption from observed growth, the model adequately predicted consumption by largemouth bass fed 1.5 X maintenance, but overestimated consumption by 0.5 X maintenance individuals. Current bioenergetics models perform poorly at the cold temperatures (<6C), photoperiods, and low prey abundances typical of high-latitude lakes, likely because metabolic costs are overestimated.This research was funded by National Science Foundation grant DEB 9407859 to R.A.S. and Federal Aid in Sport Fish Restoration Project F-69-P, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife. A University PostDoctoral Fellowship and a Presidential Fellowship from The Ohio State University supported R.A.W. and J.E.G., respectively, during part of this work