The detritivorous fish, gizzard shad (Dorosoma cepedianum), provides nutrients to phytoplankton in reservoirs by ingesting organic detritus associated with sediments and excreting substantial quantities of nutrients such as N and P in soluble forms that are highly available to phytoplankton, We estimated nutrient excretion by gizzard shad in a eutrophic reservoir (Acton Lake, Ohio) during April-October 1994 by measuring N and P excretion of fieldcaught fish (n = 135). Excretion rates were then extrapolated to nutrient release by the gizzard shad population using quadrat rotenone biomass estimates, electrofishing surveys, and historic seasonal trends. N and P excretion were positively correlated with fish wet mass and temperature, but mass-specific excretion declined with increasing fish mass. Lakewidc gizzard shad biomass in July 1994 was 417 kg ha -I, Our estimates of nutrient excretion by the gizzard shad population ranged from 0.487 to 0.769 pmol NH,-N liter-' d-l and 0.022 to 0.057 pmol soluble reactive phosphorus liter -I d .I, with the highest excretion occurring during mid-summer through early fall. The low N : P ratio at which gizzard shad excrete [mean molar N : P = 16.75 (kO.89 SE)] may alter phytoplankton community composition, favoring cyanobacteria. Our results indicate that nutrient excretion by detritivorous fish can be an important source of nutrients to open waters, especially when other sources of nutrients are reduced.

F . <~/ r i q \ .75(2). 1994. pp 532-547 i 1994 h) thc Ecologtcal Sorlrty ol Arncrlca ASSESSING HOW FISH PREDATION AND INTERSPECIFIC PREY COMPETITION INFLUENCE A CRAYFISH ASSEMBLAGE1.lI~sft.u~~t. In northern Wisconsin lakes. the introduced crayfish 0rc.otlrc.tcs r~lsticus is replacing 0. propiilqir~c.r. a previous invader. and 0. ~.irili.s, a native craqfish. Herein. \ve cxplore how fish predation and competition interact to drive this change in crayfish species composition. In outside pools. we conducted selective predation experiments exposing craqlish to largemouth bass. .21ic,t~optc,rus sultlro~dc.r, to quantify patterns of crayfish vulnerabilitq. T o deter-mine how interactions among crayfish influence susceptibility. we quantilied shelter usc and behavioral interactions among craqfish in aquaria and outside pools.At equal s~/ e . 0.~.rrilrs was more susceptible to fish predation than either ofthe invaders. 0. ru.\tic,~(.s and 0.~~ropttiy~rui: the two inbadcrs mere equally susceptible to predation. Howc\er. s i x s of these craqfish in the field are 0. 1.tr111.r > 0.~IISIICLIS> 0. prol1it1q1l1l.r. Because fish predators prefer sniall crayfish. at unequal size. small 0. p r o p r t q~~u s \Irere morc vulnerable to predation than large 0. rlc.rfirus. Thus. 0. rli.\t~c~ls replace 0.can ~r c~~~~t~y u~r s d u e to natural sile difftrences. Although 0.~Yrilisgrows larger than the invaders. ~t was more susceptible e\,en when 3 nini larger. We hqpothesized that 0. l,irili.s, although large. part~cipatcd in beha\ iors 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.~>irilis was excluded from shelters bq in\ ader-s. 0.~.trtlts also participated in risk! behaviors. such as increased activity and swimming. Both agonistic interactions with congeners and approaches by largemouth bass Increased risk! behaviors in 0.1.iri1t.s. In addition. 0. 1.1rili.r was innately less aggressive than in\.aders. G i \ e n these bcha\.iors. 0. 1.rt.111s was consumed at high rates and \vould be e\entuallq replaced in lakes.In northern Wisconsin lakes. lish predation and cra!fishxrayfish con~petition interact to Influence crayfish replacements. Based on our results. largemouth bass predation modifies the outcome of ~nterfercnce competition among the three crayfishes and. in turn. competitive Interactions among the crab fishes influence susceptibility to fish predation. We predict that 0. 1~rt.111.s should suffer high mortalit) to fish predation in the presence. rather than in the absence. of the two in\.ading species. Our results support the hypothesis that. in areas of sqmpatrq whcrc predators are \electi\e and pre! species compete. predation and competition interact to d e t c r m~n e community structure. et al. 1983). Clearlq. predation and competithat influence cornto dramat~call! influence communitq structure. .41-tion are interrelated n~e c h a n i s n~s though these two forccs can work indcpendentlq...

Global homogenization of biota is underway through worldwide introduction and establishment of nonindigenous (exotic) species. Freshwater ecologists should devote more attention to exotic species for two reasons. First, exotics provide an opportunity to test hypotheses about what characteristics of species or habitats are related to successful establishment or invasibility, respectively. Second, predicting which species will cause large ecological change is an important challenge for natural resource managers. Rigorous statistical relationships linking species characteristics to probability of establishment or of causing ecological impacts are needed. In addition, it is important to know how reliable different sorts of experiments are in guiding predictions. We address this issue with different spatial scales of experiments testing the impact of two predators on native snail assemblages in northern Wisconsin USA lakes: an exotic crayfish, the rusty crayfish (Orconectes rusticus); and a native fish predator, the pumpkinseed sunfish (Lepomis gibossus). For the crayfish, laboratory experiments, a field cage experiment, and a snapshot survey of 21 lakes gave consistent results: the crayfish reduced abundance and species richness of native snails. Laboratory and field experiments suggested that pumpkinseed sunfish should have a similar impact, but the lake survey suggested little impact. Unfortunately, no algorithms exist to guide scaling up from small‐scale experiments to the whole‐lake, long‐term management scale. To protect native biodiversity, management of freshwater exotic species should be targeted on lakes or drainages that are both vulnerable to colonization by an exotic, and that harbour endemic species. Management should focus on preventing introduction because eradication after establishment is usually not possible.

We evaluated the habitat use and movements of 50 adult bighead carp Hypophthalmichthys nobilis and 50 silver carp H. molitrix by means of ultrasonic telemetry during spring‐summer 2004 and 2005 to gain insight into the conditions that facilitate their establishment, persistence, and dispersal in the lower Illinois River (river kilometer 0‐130). Movement and habitat use were monitored with stationary receivers and boat‐mounted tracking. The relative availability of four macrohabitat categories (main channel, island side channel, channel border, and connected backwater) was quantified to determine selection; discriminant function analysis was used to evaluate changes in physical characteristics within each category. A flood pulse occurred in spring through early summer of 2004 but not 2005. Movement rates (km/week) of both species were positively correlated with flow but not with temperature. Including data from stationary receivers greatly increased estimates of daily movement. During low summer flow, both species typically selected channel borders and avoided the main channel and backwaters. Both species rarely occupied depths over 4 m, regardless of abiotic conditions. Flood pulses appear to trigger dispersal, while habitat use is only specific during low summer flow. Thus, movement prevention efforts (e.g., dispersal barriers) will require particular vigilance during late‐winter or spring flooding, and controlled removal (e.g., harvest) should be directed toward selected habitats during summer.

Ecologists increasingly recognize the need to understand how landscapes and food webs interact. Reservoir ecosystems are heavily subsidized by nutrients and detritus from surrounding watersheds, and often contain abundant populations of gizzard shad, an omnivorous fish that consumes plankton and detritus. Gizzard shad link terrestrial landscapes and pelagic reservoir food webs by consuming detritus, translocating nutrients from sediment detritus to the water column, and consuming zooplankton. The abundance of gizzard shad increases with watershed agriculturalization, most likely through a variety of mechanisms operating on larval and adult life stages. Gizzard shad have myriad effects on reservoirs, including impacts on nutrients, phytoplankton, zooplankton, and fish, and many of their effects vary with ecosystem productivity (i.e., watershed land use).Interactive feedbacks among watersheds, gizzard shad populations, and reservoir food webs operate to maintain dominance of gizzard shad in highly productive systems. Thus, effective stewardship of reservoir ecosystems must incorporate both watershed and food-web perspectives.

How large size affects overwinter growth and survival of age-0 fish may vary as a function of food, predation, and energetic condition. During two winters in Ohio, we assessed how these factors affected growth and survival of varying sizes of age-0 largemouth bass (Micropterus salmoides) by combining a field survey (N = 2 reservoirs) with multiscale experiments (reservoirs, ponds, outdoor pools). In our survey, more small (< 100 mm total length) individuals died by spring in one reservoir than in the other. Similarly, when we stocked two reservoirs with marked age-0 largemouth bass in fall, mortality of small individuals was higher in one system overwinter, potentially due to differences in predation intensity. In ponds during two winters, size-selective mortality of small largemouth bass occurred in only two of eight ponds, potentially as a function of cannibalism. Varying ration in pools (starved, 0.5× maintenance, or 1.5× maintenance) did not affect survival, even though starved individuals lost substantial wet weight and energy content. Only when predators were present did small individuals die at high rates, although energy depletion may have contributed to predatory mortality. To increase the probability of overwinter survival, managers should seek to improve first-summer growth, reduce winter predation, and increase winter forage.

The silver carp Hypophthalmichthys molitrix has spread throughout the Mississippi River drainage. During 2003, we determined its population status and potential impact in the middle Mississippi River (MMR), the conduit between the lower Mississippi River and the upper Mississippi, Missouri, and Illinois rivers. We quantified growth, age structure, fecundity, and diets of silver carp sampled with trammel nets and AC electrofishing in main‐channel areas. Mean length at age in the MMR exceeded that of populations in Asia by as much as 26%. Individuals were typically more than 1 year old and 230 mm total length, suggesting that small, young fish were absent. Individuals in this population matured earlier (age 2) than in the species' native range. Regardless of phytoplankton variation (using chlorophyll a as a surrogate) and zooplankton concentration at MMR sites, phytoplankton was consistently most abundant in diets. Silver carp are finding suitable resources within the MMR, allowing individuals to grow rapidly during early life, persist as adults, and successfully disperse upstream.

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