Complex predator-prey interactions and predator intimidation among crayfish,piscivorous fish,and small benthic fish

Summary Predator-prey interactions were studied among a small prey fish (the johnny darter Etheostoma nigrum) and two predators (crayfish Orconectes rusticus and smallmouth bass Micropterus dolomieui) with complementary foraging behaviors. When only smallmouth bass were present, darters reduced activity to 6% of control rates and spent most of the time hiding under tile shelters. When only crayfish were present, darter activity and shelter-use were similar to controls. When both crayfish and bass were present, an interaction occurred. Darters, normally inactive in the presence of bass, were often forced to move by approaching crayfish and thus activity increased to 19% of control rates. Also, darters were often evicted from shelters by intruding crayfish. Thus, crayfish increased the vulnerability of small fish to bass by evicting them from shelters and causing increased activity. Conversely, bass increased the vulnerability of small fish to crayfish by forcing these fish to seek cover under shelters occupied by crayfish. Intimidation effects of bass on darters last for some time. After a 30-min exposure to bass, darters showed reduced activity and increased shelter use lasting at least 24 h after the bass was removed. Thus predators, throught intimidation, can influence prey behavior even though the predators are no longer present.The Unit is sponsored jointly by the United States Fish and Wildlife Service, the Ohio Department of Natural Resources, and The Ohio State University     

Fish size and habitat depth relationships in headwater streams

Summary Surveys of 262 pools in 3 small streams in eastern Tennessee demonstrated a strong positive relationship between pool depth and the size of the largest fish within a pool (P<0.001). Similarly, the largest colonizers of newly-created deep pools were larger than the colonizers of shallow pools. We explored the role of predation risk in contributing to the bigger fish — deeper habitat pattern, which has been noted by others, by conducting five manipulative field experiments in two streams. Three experiments used stoneroller minnows (Campostoma anomalum); one used creek chubs (Semotilus atromaculatus); and one used striped shiners (Notropis chrysocephalus). The stoneroller experiments showed that survival of fish approximately 100 mm in total length (TL) was much lower in shallow pools (10 cm deep) than in deep (40 cm maximum) pools (19% versus 80% survival over 12 d in one experiment) and added cover markedly increased stoneroller survival in shallow pools (from 49% to 96% in an 11-d experiment). The creek chub experiment showed that, as for stonerollers, pool depth markedly influenced survival: the chubs survived an average of 4.9 d in shallow pools and >10.8 d in deep pools. In the striped shiner experiment in shallow artificial streamside troughs, no individuals 75–100 mm TL survived as long as 13 d, where-as smaller (20–25 mm) fish had 100% survival over 13 d. The results of the experiments show that predation risk from wading/diving animals (e.g., herons and raccoons) is much higher for larger fishes in shallow water than for these fishes in deeper water or for smaller fish in shallow water. We discuss the role of predation risk from two sources (piscivorous fish, which are more effective in deeper habitats, and diving/wading predators, which are more effective in shallow habitats) in contributing to the bigger fish — deeper habitat pattern in streams.     

Effects of Predation Risk on Habitat Selection by Water Column Fish, Benthic Fish and Crayfish in Stream Pools

Predation risk can affect habitat selection by water column stream fish and crayfish, but little is known regarding effects of predation risk on habitat selection by benthic fish or assemblages of fish and crayfish. I used comparative studies and manipulative field experiments to determine whether, (1) habitat selection by stream fish and crayfish is affected by predation risk, and (2) benthic fish, water column fish, and crayfish differ in their habitat selection and response to predation risk. Snorkeling was used to observe fish and crayfish in, (1) unmanipulated stream pools with and without large smallmouth bass predators (Micropterus dolomieui >200 mm total length, TL) and (2) manipulated stream pools before and after addition of a single large smallmouth bass, to determine if prey size and presence of large fish predators affected habitat selection. Observations of microhabitat use were compared with microhabitat availability to determine microhabitat selection. Small fish (60–100 mm TL, except darters that were 30–100 mm TL) and crayfish (40–100 mm rostrum to telson length; TL) had significantly reduced densities in pools with large bass, whereas densities of large fish and crayfish (> 100 mm TL) did not differ significantly between pools with and without large bass. Small orangethroat darters (Etheostoma spectabile), northern crayfish (Orconectes virilis), and creek chubs (Semotilus atromaculatus) showed significantly greater densities in pools without large bass. The presence of large smallmouth bass did not significantly affect depths selected by fish and crayfish, except minnows, which were found significantly more often at medium depths when bass were present. Small minnows and large and small crayfish showed the greatest response to additions of bass to stream pools by moving away from bass locations and into shallow water. Small darters and sunfish showed an intermediate response, whereas large minnows showed no significant response to bass additions. Response to predation risk was dependent on prey size and species, with preferred prey, crayfish and small minnows, showing the greatest response. Small benthic fish, such as darters, are intermediate between small water column fish and crayfish and large water column fish in their risk of predation from large smallmouth bass.     

Indirect food web interactions increase growth of an algivorous stream fish

1. We tested the hypothesis that indirect food web interactions between some common, invertivorous fishes and their prey would positively affect growth of an algivorous fish species. Specifically, we predicted that orangethroat darter (Etheostoma spectabile) would increase periphyton biomass via a top‐down pathway, indirectly enhancing growth of the algivorous central stoneroller minnow (Campostoma anomalum). Moreover, we predicted that sand shiner (Notropis stramineus) would increase periphyton biomass via a bottom‐up pathway and indirectly enhance growth of the stoneroller minnow. 2. In an 83‐day experiment in large, outdoor, stream mesocosms, we stocked two fish species per mesocosm (stoneroller and either darter or shiner), estimated the effects of the invertivorous and grazing fishes on periphyton biomass and estimated growth of the algivorous fish. 3. The darter consumed grazing invertebrates, indirectly increasing periphyton biomass. The shiner consumed terrestrial insects as predicted, but it did not affect periphyton biomass. 4. In support of our hypothesis, the darter indirectly enhanced stoneroller growth. As predicted, stonerollers consumed the increased periphyton in streams with darters, resulting in greater growth, condition and gut fullness compared to streams without darters. No indirect interaction was observed between stonerollers and shiners. 5. Our study suggests that some invertivorous fish species can positively affect growth of algivorous fishes through indirect food web interactions. Thus, in stream communities, it is possible that the loss of a single, invertivorous fish taxon could have negative consequences on algivorous fish populations via the removal of positive indirect food web interactions.     

Habitat associations of fish assemblages in the Cache River, Illinois

Fish and habitat were sampled by state agencies at 48 stations throughout the Cache River watershed, Illinois between 1992 and 2009. Two distinct fish assemblages were identified, one primarily found in the lower mainstem Cache River and a second found throughout tributaries and the upper mainstem Cache River. Using a canonical correspondence analysis, the distribution of fish species was largely explained by substrate, land use, drainage area and local habitat features. Creek chub, central stoneroller, fringed darter and fantail darter are species found to be positively associated with gravel substrate and forest. In contrast, black buffalo, gizzard shad, smallmouth buffalo, freshwater drum and bigmouth buffalo were positively associated with drainage area, silt, channel width and row crops. Cobble appears to be rare habitat associated with fringed darter, freckled madtom and fantail darter. Results suggest that substrate, land use and local habitat features influence fish assemblages within the Cache River watershed. This information contributes to both understanding aquatic community structure in a highly altered yet diverse watershed as well as management activities within the Cache River watershed.     

The adaptive significance of behavioural ontogeny in some. centrarchid fishes

Synopsis Studies on the ontogeny of behaviour in fish have seldom considered the adaptive significance of the order of appearance of the behaviours. Results of laboratory studies and field observations on the ontogeny of feeding, predator-avoidance, and agonistic behaviour in largemouth bass, Micropterus salmoides, rock bass, Ambloplites rupestris, and pumpkinseed, Lepomis gibbosus, young indicate that the order of appearance of the various behavioural systems enhances the survival of the young. In the laboratory, all species spend significantly more time feeding during the first weeks of free-swimming than the later weeks. During a corresponding period in the field the young are either occupying an offshore area low in predators (rock bass, pumpkinseed) or are being guarded by a parent (largemouth bass); thus the risk of predation is low. When the young are in a predator-rich environment (inshore) both the predator-avoidance response and agonistic behaviour are well developed. Agonistic behaviour is the last to appear and may serve to disperse the young. Dispersal may relate to the feeding mode of the various species and may also reduce the probability of predation. It is apparent that the sequential onsets of the behavioural systems are in concert with ecological events and selective pressures confronting centarchid young.     

The effects of turbidity and vegetation on the risk of juvenile salmonids, Oncorhynchus spp., to predation by adult cutthroat trout, O. clarkii

Synopsis During their seaward migration, juvenile salmonids encounter structural and visual cover which varies between and within watersheds. In this study, the effects of two types of cover (turbidity and artificial vegetation) on the predation mortality of juvenile salmonids exposed to fish piscivores was investigated in outdoor concrete ponds. During experiments, adult coastal cutthroat trout, Oncorhynchus clarkii clarkii, were allowed to feed on juvenile salmonid prey — chinook salmon, O. tshawytscha, chum salmon, O. keta, sockeye salmon, O. nerka, and cutthroat trout — in separate trials. Daily instantaneous per capita predation rate was determined for each turbidity and vegetation treatment, within each trial. Mean predation rates varied between 1% and 76% daily. In the presence of cover, mean daily predation rates were 10–75% lower than those in controls (no vegetation and clear water), depending on prey species. Predation rates were significantly lower in the presence of vegetation cover and did not covary with prey size or species. The effects of turbidity were generally not significant and were not additive with the effects of vegetation. However, turbidity appeared to significantly reduce the effectiveness of vegetation as cover for juvenile chinook and sockeye salmon. We suggest that these two forms of cover do not affect risk of predation by fish piscivores to juvenile salmonids via the same mechanism.     

The roles of predation,competition, and exploitation in the trophic dynamics of a warmwater stream: a model synthesis,analysis, and application

We developed a trophic dynamic model of key populations and processes in the New River, West Virginia, to identify the mechanisms most responsible for maintaining food web structure. Key populations were represented by thirteen model components and were aquatic insects; age-1 and age-2 crayfish (three species); age-1 and age-2 hellgrammites (Corydalus cornutus larvae); non-game fishes; age-0, age-1, and adult smallmouth bass (Micropterus dolomieu); age-0, age-1, and adult rock bass (Ambloplites rupestris); and age-0, age-1 to age-3, and adult flathead catfish (Pylodictis olivaris). In this system, crayfish and hellgrammites are harvested to provide bait for the recreational fishery that extensively exploits the three predatory fish species. Predation and intraspecific regulation were represented with nonlinear algorithms, and linear terms represented fishery harvests. Interspecific competition among components occurred through predation on shared prey. Error analysis of the model suggested that predation was the most important mechanism in maintaining system structure (the disposition of biomass among system components). Further, the trophic relation between each component and its prey accounted for 34–64% of the variability in food web structure, whereas predation on each component explained 1–24% of food web structure variability. Therefore, so-called ‘bottom-up’ effects were more influential than ‘top-down’ effects. Interspecific competition and intraspecific regulation had secondary roles in maintaining New River food web structure, although intraspecific regulation was most important to aquatic insects, which were not predatory in our model. Both forms of competition are probably tempered by extensive predation and exploitation in the New River system. Exploitation was a secondary structuring agent to adult smallmouth bass, which experience a high rate of harvest in the New River.     

Behavior of fish predators and their prey: habitat choice between open water and dense vegetation

Synopsis Behavior of largemouth bass, Micropterus salmoides, and northern pike, Esox lucius, foraging on fathead minnows, Pimephales promelas, or bluegills, Lepomis macrochirus, was quantified in pools with 50% cover (half the pool had artificial stems at a density of 1000 stems m⁻²). Both predators spent most of their time in the vegetation. Largemouth bass searched for bluegills and ambushed minnows, whereas the relatively immobile northern pike ambushed all prey. Minnows were closer to predators and were captured more frequently than bluegills. Even when minnows dispersed, they moved continually and eventually wandered within striking distance of a predator. Bluegills dispersed in the cover with predators. Bass captured the few bluegills that strayed into the open and pike captured those that approached too closely in the cover. The ability of predators to capture prey while residing in habitats containing patches of dense cover may explain their residence in areas often considered to be poor ones for foraging. The unit is sponsored jointly by the United States Fish and Wildlife Service, Ohio Department of NaturalResources, The Ohio State University, and the Wildlife Management Institute     

The environmental basis of diet variation in pumpkinseed sunfish, Lepomis gibbosus, and largemouth bass, Micropterus salmoides, along an Iberian river basin

The diet composition of largemouth bass, Micropterus salmoides, and pumpkinseed sunfish, Lepomis gibbosus, and its relation to environmental factors was studied in intermittent watercourses of the lower Guadiana basin (southern Iberia) during the particularly dry summer of 1994. Overall, both species took food items similar to those found in their North American and acclimatised ranges, with pumpkinseed consuming invertebrates (chiefly Chironomidae) and bass preying on invertebrates (chiefly Micronecta meridionalis) and fish (chiefly L. gibbosus and Gambusia holbrooki). Both bass and pumpkinseed were opportunistic feeders, feeding on the most frequent and abundant prey. However, Diptera larvae were apparently preferred by pumpkinseed and avoided by bass, while the contrary occurred with respect to Heteroptera. The two main fish prey of bass were eaten on the basis of random encounter. The relative abundance of macro-prey (i.e. fish and Atyephira desmarestii) were the principal environmental variables constraining bass dietary variation along the basin. On the contrary, pumpkinseed diet variation was mainly related to pumpkinseed size and the presence of piscivorous bass nearby, although habitat size and cyprinid abundance were also influential. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grazer identity changes the spatial distribution of cascading trophic effects in stream pools

Non-lethal effects of predators on prey behavior can mediate trophic cascades, but the extent of effects depends on habitat characteristics and risk sensitivity of prey. Furthermore, predation risk for stream organisms varies along the depth gradient and strongly influences their behavior. Grazing minnows (Campostoma anomalum) and crayfish (Orconectes virilis) are both prey for largemouth bass (Micropterus salmoides) in streams, but differ in their predator-avoidance behavior. This study contrasts the effects and mechanisms of non-lethal trophic cascades on the spatial distribution of filamentous green algae among stream pools and along a depth gradient within pools. Presence/absence of a largemouth bass was crossed with four combinations of the two grazer species (0 grazers, 30 minnows, 30 crayfish, and 15 each) in outdoor, experimental streams. Grazer densities were maintained by restocking. I used geostatistics to quantify spatial patterns of predator and grazer habitat use, height of filamentous algae in the water column, and spatial covariation of water depth with algal height, and depth with grazer habitat use. In streams with only minnows, bass were sedentary, and hid within tall algae in a single "bass pool". In pools with grazed algae, bass actively pursued prey within and among pools and used deeper water. This set up a hierarchy of risk to grazers along the depth gradient from bass in deep water to potential risk from terrestrial predators in shallow water. Thus, minnows were more sensitive than crayfish to predation risk from bass, but less sensitive than crayfish to risk from terrestrial predators. Minnows mediated cascades at the scale of whole pools by avoiding "bass pools", but only if crayfish were absent. Crayfish avoided potential interactions both with terrestrial predators and bass by grazing and burrowing in deeper water at night (when bass were inactive), and by hiding in burrows during daytime. Crayfish without burrows avoided bass and crayfish defending burrows by using shallow edges of pools as corridors, but did not graze there. Thus, crayfish-mediated cascades were limited to pool edges. Effects of grazer identity may extend to other consumers via modification of risk for biota that use filamentous algae as either foraging or refuge habitat.     

Effects of turbidity and an invasive waterweed on predation by introduced largemouth bass

Anthropogenic activities lead to changes in characteristics of aquatic ecosystems, including alteration of turbidity and addition of invasive species. In this study, we tested how changes in turbidity and the recent invasion of an aquatic macrophyte, Egeria densa, may have changed the predation pressure by introduced largemouth bass on juvenile striped bass and delta smelt, two species that have seen a drastic decline in recent decades in the Sacramento-San Joaquin Delta. In a series of mesocosm experiments, we showed that increases in vegetation density decreased the predation success of largemouth bass. When placed in an environment with both open water and vegetated areas, and given a choice to forage on prey associated with either of these habitats, largemouth bass preyed mainly on open water species as opposed to vegetation-associated species, such as juvenile largemouth bass, bluegill or red swamp crayfish. Finally, we showed that turbidity served as cover to open water species and increased the survival of delta smelt, an endemic species at risk. We also found that such open water prey tend not to seek refuge in the vegetation cover, even in the presence of an imminent predation threat. These results provide the beginning of a mechanistic framework to explain how decreases in turbidity and increases in vegetation cover correlate with a decline of open water species in the Sacramento-San Joaquin Delta.     

Ecological and evolutionary consequences of early ontogenies of darters (Etheostomatini)

Synopsis The ecological classification of fishes into reproductive guilds is based on the premises that (1) reproductive styles and early ontogeny are closely related, and (2) both are correlated with the ecology of a species. A comparison of early ontogenies of logperch (Percina caprodes), rainbow darter (Etheostoma caeruleum), and fantail darter (E. flabellare) confirmed these premises, and provided possible explanations for diversity within the Etheostomatini. Young logperch have limited vitelline circulation, hatch while still poorly developed, and therefore must drift from oxygen rich lotic habitats to lentic habitats where small planktonic prey are available. Young rainbow and fantail darters have extensive vitelline plexuses, are well developed at transition to first feeding, and begin feeding on aquatic insects. Thus there is no necessity for a drift interval. As a result, the latter species are adapted for stream life. Interspecific differences in reproductive styles and early ontogenies may have contributed to speciation of darters by allowing partitioning of breeding sites and food resources for young. In addition, reduced drift dispersal and small stream habitation may have indirectly contributed to speciation by reducing genetic exchange among populations.     

An in situ study of rock bass (Ambloplites rupestris) physiology: effect of season and mercury contamination

Selected physiological and biochemical variables were examined in rock bass, Ambloplites rupestris, which were collected on five different sampling dates from an area of chronic mercury contamination and a reference site on the South River, Virginia.The onset of spawning represented the most significant seasonal influence in the physiological profile of the fish, with elevations in hematocrit, hemoglobin, plasma protein, and plasma glucose. Sex-related differences in plasma calcium, liver glycogen and liver ascorbic acid were also unique to the period. Female rock bass had significantly higher levels of liver glutathione than did males on all but one of the sampling dates, although the cause of this difference is not clear.Rock bass from the mercury contaminated site had an average muscle mercury concentration of 1.37 mg Hg g^–1, and an average liver mercury concentration of 2.86 mg Hg g^–1. These levels were approximately an order of magnitude greater than those found in the tissues of the reference fish which averaged 0.165 and 0.101 mg Hg g^–1 in muscle and liver respectively. In July 1987, mercury concentrations in the liver of both reference and contaminated fish increased significantly, possibly the result of greater uptake of the metal through increased feeding or changes in the mercury level of selected prey items. Rock bass collected from the two sites in July also had significantly different levels of liver glutathione: reference fish exhibited an elevation and contaminated fish a depression. When fish from the two sampling stations received a 96-hr exposure to 150 µg HgCl₂ in the laboratory, both groups exhibited elevated liver mercury and decreased liver glutathione. Mercury levels in the gall bladders of the exposed fish were also elevated, suggesting that glutathione may have been lost through excretion with the metal in the bile.On the whole, physiological differences between the two groups of rock bass were limited, indicating that exposure to the mercury is not having a significant impact on the rock bass from the contaminated area. This is further supported by field examination of the fish and comparison of condition indices from rock bass previously taken from the same two stations.Those factors which significantly altered the physiology of the rock bass were unique to certain times of the year, indicating that the most appropriate sampling approach in future studies is one which examines a number of variables over a range of environmental conditions.     

Predation rates of nemertean predators: the case of a rocky shore hoplonemertean feeding on amphipods

Estimates of the predation rates of benthic nemerteans are often based on observations of single individuals, and consequently they may not be representative for all members of a population of these predators. Herein we conducted controlled and repeatable laboratory experiments on the predation rate of the hoplonemertean Amphiporus nelsoni Sánchez 1973, which is common at exposed rocky shores along the central Chilean coast. During the austral fall (April, May 2000), nemerteans were observed in relatively high numbers crawling in the intertidal zone during early morning or late-afternoon low tides. When these nemerteans were offered living amphipods held by a forceps, they immediately attacked the amphipods and fed on them. In the laboratory experiments, nemerteans preferred the amphipod Hyale maroubrae Stebbing, 1899, which is also very common in the natural habitat of A. nelsoni. The nemerteans preyed to a higher extent on small males and non-ovigerous females than would have been expected from their abundance. We suggest that these (non-reproductive) stages of H. maroubrae are very mobile and therefore have a high likelihood of encounters with nemerteans. Predation rates reached maxima when nemerteans were provided prey densities of four or more of their preferred prey species, H. maroubrae, furthermore indicating that encounter rates with prey may affect predation rates. In long-term laboratory experiments, A. nelsoni consumed more amphipods during low tide conditions than during high tide conditions. Many nemerteans in the field prefer particular environmental conditions (e.g. nocturnal low tides), which restricts the time available for successful feeding. In the long-term experiment, predation rates of A. nelsoni never exceeded 0.5 amphipods nemertean^–1 d^–1. Maximum feeding events were 3 or 4 amphipods nemertean^–1 d^–1, but this only occurred during 10 out of a possible 2634 occasions. Nemerteans that had consumed 3 or 4 amphipods during 1 day, consumed substantially less prey during the following days. Towards the end of the long-term experiment, average predation rates decreased to 0.2 amphipods nemertean^–1 d^–1, corresponding to predation rates reported for other nemertean species (0.1–0.3 prey items nemertean^–1 d^–1). We suggest that predation rates from laboratory experiments represent maximum estimates that may not be directly transferable to field populations. Additionally, low predator–prey encounter rates with preferred prey in the field may further limit the predation impact of nemertean predators in natural habitats.     

Habitat use by bluegill in laboratory pools: where is the refuge when macrophytes are sparse and alternative prey are present?

Synopsis In northern clear-water systems, bluegill, Lepomis macrochira, select habitats based on relative reward rates and risk of predation. However, because freshwater systems to the south are often turbid, and their resident fish species different, bluegill habitat use patterns may differ from those found in northern clear-water systems. Here I examine the effects of elimination of the inshore macrophyte refuge (an indirect effect of turbidity), and decreased pelagic predation rates on bluegill due to the presence of gizzard shad, Dorosoma cepedianum, in laboratory pools. Bluegill used depths 20 cm more often with largemouth bass, Micropterus salmoides, than without them; the presence of a preferred alternate prey, gizzard shad, did not affect this behavior. Though bluegill did not alter their habitat use in response to bass activity, they did school more as predator activity increased. Even in the presence of a preferred alternate prey without macrophytes, bluegill continue to use shallow littoral areas to avoid predation.This Unit is sponsored jointly by The United States Fish and Wildlife Service, Ohio Department of Natural Resources, The Ohio State University, and the Wildlife Management Institute.     

The short-term effects of perch predation on a zooplankton prey community

An enclosure experiment was performed in 1981 to determine if predation by perch (Perca flavescens) affected the density, depth distribution or size structure of members of the crustacean zooplankton community in Lac Choiniere, Quebec. Perch predation reduces the density of all but the smallest zooplankton in enclosures, and the relative rate of predation is influenced by both size and depth distribution of prey species. More individuals and proportionally more large individuals are found in deep water when perch are present. These results are discussed in relation to theories that size-selective predation alters numerous characteristics of zooplankton prey community structure.     

Performance as a fish predator of largemouth bass [<Emphasis Type="Italic">Micropterus salmoides</Emphasis> (Lacepède)] invading Japanese freshwaters: a review

This review focuses on how predator performance of the invasive largemouth bass [Micropterus salmoides (Lacepède)] has been, or will be, formed in Japanese freshwaters. Predation impacts of largemouth bass on fish communities appear pervasive in both Japanese as well as North American freshwaters. Factors affecting performance as a piscivorous predator are (1) light intensity and water clarity, (2) oxygen depletion, (3) prey size and gape size, (4) behavioral refuge of prey, (5) weed beds as refuge for prey fish, (6) interaction with bluegill. Size and behavioral refuges requirements are so rigorous that they may have evolved only in some North American prey fish species like bluegill; therefore, most Japanese native fish species are unlikely to be equipped with such refuges. However, refuge habitats like aquatic weed beds could develop in Japanese freshwaters, allowing prey fish species to survive under predation pressure. The density, architecture, and species composition of aquatic plants may affect their suitability as refuges. Studies in Japanese waters have suggested that the presence of rich aquatic vegetation or invasive bluegill in bass-introduced waters have suppressed the predation impact of largemouth bass on fish communities. In addition to these environmental factors, original genotypic and phenotypic traits of the introduced largemouth bass, and hybridization between different lineages of largemouth bass or with Florida bass [Micropterus floridanus (Lesueur)] may be involved in further adaptation of invasive largemouth bass to Japanese freshwaters.     

Spatiotemporal variability of stream habitat and movement of three species of fish

Relationships between environmental variability and movement are poorly understood, due to both their complexity and the limited ecological scope of most movement studies. We studied movements of fantail (Etheostoma flabellare), riverweed (E. podostemone), and Roanoke darters (Percina roanoka) through two stream systems during two summers. We then related movement to variability in measured habitat attributes using logistic regression and exploratory data plots. We indexed habitat conditions at both microhabitat (i.e., patches of uniform depth, velocity, and substrate) and mesohabitat (i.e., riffle and pool channel units) spatial scales, and determined how local habitat conditions were affected by landscape spatial (i.e., longitudinal position, land use) and temporal contexts. Most spatial variability in habitat conditions and fish movement was unexplained by a site’s location on the landscape. Exceptions were microhabitat diversity, which was greater in the less-disturbed watershed, and riffle isolation and predator density in pools, which were greater at more-downstream sites. Habitat conditions and movement also exhibited only minor temporal variability, but the relative influences of habitat attributes on movement were quite variable over time. During the first year, movements of fantail and riverweed darters were triggered predominantly by loss of shallow microhabitats; whereas, during the second year, microhabitat diversity was more strongly related (though in opposite directions) to movement of these two species. Roanoke darters did not move in response to microhabitat-scale variables, presumably because of the species’ preference for deeper microhabitats that changed little over time. Conversely, movement of all species appeared to be constrained by riffle isolation and predator density in pools, two mesohabitat-scale attributes. Relationships between environmental variability and movement depended on both the spatiotemporal scale of consideration and the ecology of the species. Future studies that integrate across scales, taxa, and life-histories are likely to provide greater insight into movement ecology than will traditional, single-season, single-species approaches. The Virginia Cooperative Fish and Wildlife Research Unit is jointly sponsored by the US Geological Survey, Virginia Polytechnic Institute and State University, Virginia Department of Game and Inland Fisheries, and Wildlife Management Institute.     

Predator-prey interactions in size-structured fish communities: implications of prey growth

Predator-prey interactions among size-structured populations may be strongly influenced by factors which affect growth rates of prey. I examined the importance of prey growth in the interaction between large-mouth bass (Micropterus salmoides) and their prey, bluegill (Lepomis macrochirus), by analyzing diets and growth rates of bass in a set of seven lakes in south-central Wisconsin. Sizes of bluegill consumed by bass changed dramatically across a gradient of bluegill growth, which resulted in differing patterns of bass growth. In lakes with slow bluegill growth, small bass fed on the youngest bluegill cohort, but large bass were capable of feeding on several age classes. Consequently, bass growth rates were strongly size-dependent; small bass ate small prey and had low growth, but growth rates increased substantially with size as bass ate progressively larger prey. When bluegill had high growth rates, they quickly reached a size refuge from predation and bass of all sizes were restricted to feeding on the youngest/smallest prey. In these lakes, bass growth rates were more uniform across bass sizes. Because growth rates influence population size-distributions, variation in bluegill growth can have strong effects on the structure of bass populations. These effects could potentially feed back to further influence the interaction between predator and prey.