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.     

Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Many studies have experimentally addressed the effects of a particular predator species on prey behavior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquariusremigis) coexists with many potentially dangerous predators, two of which are the green sunfish (Lepomiscyanellus) and the fishing spider (Dolomedesvittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fishing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (riffles, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating activity occurred in pools with both predators present. Pools with either spiders or fish alone suffered 15–20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative effects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators. Received: 26 August 1996 / Accepted: 12 June 1998     

Foraging ecology of the Great Crested Grebe (Podiceps cristatus L.) at a mesotrophic-eutrophic reservoir

The foraging ecology of the Great Crested Grebe wasstudied at Dobczyce Reservoir in southernPoland. The mean diving frequency of grebes withoutyoung was 19.3 h^–1, while grebes with young dived onaverage 32.5 h^–1. The mean number of apparent prey caughtby grebes without young was 0.5 fish h^–1. Birds with young caught a greater number, on average4.5 fish h^–1. Probably, birds fishing forthemselves ate much prey underwater. The total lengthof the fish consumed by adult grebes was on average10.0 cm. Adults ate larger fish in the breeding seasonthan in the autumn migration period. Young birds werefed fish about 8.3 cm long. More than 50% of preywere bleak. The mean success of dives of birds withoutyoung was 3.3% and for individuals with young 14.8%.A comparison of the weight of the daily food demandwith the weight of fish brought to the surface showed that it was 70% lower than the dailyfood demand. The poor relation between prey size anddiving success shows that grebes can eat both small andlarge fish below the water surface.     

Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of foraging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor changing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhynchus clarki clarki, when searching for food at the pool scale by artificially increasing prey availability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was monitored from stream bank platforms under unmanipulated control conditions and under seven experimental sets of conditions involving different combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10–0.25 m s⁻¹) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not “perfect searchers” that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish’s normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about habitat conditions at the reach scale.     

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.     

Ontogenetic changes in foraging tactics of the piscivorous cornetfish <Emphasis Type="Italic">Fistularia commersonii</Emphasis>

Foraging behaviors of the piscivorous cornetfish Fistularia commersonii were observed at shallow reefs in Kuchierabu-jima Island, southern Japan. This fish foraged on two types of prey fishes: one was reef fish that typically dwell on or near substrata (e.g., Tripterygiidae and Labridae), and the other was pelagic fish that shoal in the water column (e.g., Clupeidae and Carangidae). The prey sizes, prey types and foraging behaviors changed as the predator size increased. Prey sizes were largely limited by gape size of the cornetfish, and small predators consumed small prey. The small cornetfish (10–30 cm in total length) fed only on reef fish captured after stalking (where the fish slowly approaches the prey and then suddenly attacks). The stalking was done either solitarily or in foraging association with conspecifics. Large fish (30–120 cm) fed on both types of fishes by stalking and/or chasing (where the fish chases the prey using its high mobility and attacks), either solitarily or in foraging association with con- or heterospecifics. Thus, chasing was only performed by the large cornetfish against pelagic prey fish in associative foraging with other con- and heterospecific predators. As their body sizes increased, F. commersonii began to show a diversification of foraging behaviors, which was strongly related not only to the habitat types and anti-predatory behaviors of the prey fishes but also to associative foraging with con- or heterospecifics, which improves their foraging success.     

Comparison of Antipredator Responses of Two Related Water Striders to a Common Predator

This study compares the antipredator responses of two congeneric water striders that live in different habitats yet share a common predator. Aquarius nebularis and A. remigis are stream-dwelling species of water striders found in close association in central Kentucky. Aquarius nebularis typically inhabit third- and fourth-order streams that support diverse and complex fish communities; A. remigis inhabit adjacent first- and second-order tributaries that tend to have either no fish or relatively few fish patchily distributed. The ecological conditions for these two species are distinctly different, yet the green sunfish (Lepomis cyanellus) is a surface-feeding predator and a potential risk to both species. In experimental pools with sunfish, an initial fish strike caused A. remigis to move to the edge of the pool and become inactive. For the rest of the observation period, A. remigis reduced behaviour patterns that attract predators such as skating on open water, mating and male–male aggression. The proportion of A. remigis missing from sunfish pools was 5 times higher than in pools lacking fish, suggesting that 20% of water striders were killed by predation. In contrast, an initial fish strike caused A. nebularis to exhibit a burst of activity as individuals erratically skated over the open water. They then resumed the less active pre-attack behaviours a short time later. Aquarius nebularis showed none of the antipredator behaviours typical for A. remigis. Furthermore, the proportion of A. nebularis missing from sunfish pools did not differ from that in pools without fish. Although antipredator behaviour has been examined for relatively few species of water striders, this study suggests that the behaviour of the most commonly studied species, A. remigis, may not be indicative for water striders in general., Corresponding author: James J. Krupa, School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225, U.S.A. E-mail: bio149@pop.uky.edu bio149@pop.uky.edu     

High altitude mountain streams as a possible refuge habitat for the catfish <Emphasis Type="Italic">Amphilius uranoscopus</Emphasis>

Amphilius uranoscopus is a catfish species, restricted to rivers and streams in east, southern and central Africa. It is likely to be displaced due to both competition and predation by exotic trout and other introduced fish. In high altitude mountain streams it can be the only species occurring, which means that this habitat may act as a refuge for this species. Ecosystems like this are threatened by habitat alteration and are therefore in need of protection. The abiotic environment, population structure, behavior and feeding biology of Amphilius uranoscopus were studied in a small, high-altitude perennial tributary of the Limpopo River in the Soutpansberg mountain range, Limpopo Province, South Africa, during 2005–2006. Here A. uranoscopus showed nocturnal behavior. It used dark hollow crevices in rapids as shelters during the daytime. The rapids are characterized by a high flow rate, high dissolved oxygen content and coarse riverbed substrate consisting mainly of boulders without fallen leaves. In contrast to the adults, juveniles found shelter among the fallen leaves in pools. At night, A. uranoscopus moved out of the rapids into the open water of the pools. The main food of A. uranoscopus consisted of macroinvertebrates, mainly Trichoptera larvae. Amounts of algae and detritus in its diet were negligible. A. uranoscopus foraged mainly in rapids and on rock surfaces, spending less time foraging between the fallen leaves at the bottom of pools, in open water, at the surfaces of bottom and bank and near the water surface. At the water surface, it also fed opportunistically on terrestrial insects that dropped into the stream, like flying termites. A. uranoscopus was the top predator of the stream system and reached a high density (0.71 fish m⁻²). There was no competition from other fish species at the study site as they were lacking. A. uranoscopus showed a striking ability to climb and cling on to vertical substrates to conquer waterfalls and very shallow streams that sometimes become dry, using its fins, adhesive body and protruding mouth teeth.     

Spatial and temporal patterns in the fish assemblages of individual pools in a midwestern stream (U.S.A.)

Synopsis The composition and consistency of fish assemblages in 14 adjacent pools (6–120 m long) of a clear-water, limestone and gravel creek in midwestern U.S.A. were quantified in eight snorkeling surveys over 19 months, to establish a baseline of natural variation in the system at this scale. The fauna of the stream was dominated numerically by minnows (Cyprinidae), sunfish and black bass (Centrarchidae), and topminnows (Fundulidae). The pool fish fauna of the total 1 km reach (including all 14 pools) was highly consistent throughout the study, despite two major floods. Assemblages in individual pools generally were consistent, but there was more variation within pools than at the scale of the entire reach. Throughout the study, most individual pools remained within discrete subsets of the total occupied multivariate space in a principal components analysis based on fish species abundances. Sunfishes (Lepomis spp.) and bass (Micropterus spp.) were more consistent in their distribution among pools than were minnows (Cyprinidae) or a topminnow (Fundulus). There were 25 significant correlations in occurrence of species pairs among stream pools, out of 91 possible comparisons of the 14 most abundant taxa in the reach. Many pools contained assemblages either dominated by large centrarchids or by abundant cyprinids and juvenile centrarchids, but intermediate assemblages also were observed. The dynamics of distribution of fish species and fish assemblages among individual stream pools are likely influenced by a combination of species-specific behaviors and habitat selection, predator constraints on use of individual pools by small fishes, riffles as size-selective barriers to fish movements between pools, dispersal of young-of-the-year fishes, and abiotic phenomena like floods. Individual stream pools appear to be discrete habitat units for fishes, and do represent an appropriate scale for biologically meaningful studies of fish assemblages or their effects on streams.Department of Zoology, University of Oklahoma     

Intertidal recruitment and feeding in relation to optimal utilization of nursery areas by juvenile English sole (Parophrys vetulus: Pleuronectidae)

Synopsis Juvenile Parophrys vetulus 19–102 mm were collected on an intertidal flat in Humboldt Bay during 1976–1977. Recently metamorphosed fish that fed almost exclusively on harpacticoid copepods and other epibenthic crustaceans were found in large numbers during spring and early summer. A feeding transition occurred among fish 50–65 mm in length, and infaunal polychaetes were the dominant prey of fish greater than 65 mm. Increases in the feeding niche width and average size of prey items, as well as a decrease in the number of prey items per stomach, accompanied growth.The disappearance of English sole from intertidal areas in early fall at an average size of 82 mm and their subsequent residence in subtidal channels until a size of 140 mm suggest a size-depth segregation within the nursery ground. The advantages of such a distribution are discussed in terms of optimal foraging and reduction of intra- and interspecific competition.     

Why do Foraging Stream Salmonids Move During Summer?

We hypothesize that foraging stream salmonids move during summer because (1) they monitor habitat conditions at a reach scale (100s of m), and (2) dominant fish move when conditions in their present foraging location become sub-optimal relative to conditions at other locations in the reach. To test these ideas, we quantified temporal variation in foraging habitat quality between late spring and early fall in a reach of a small Rocky Mountain brook charr, Salvelinus fontinalis, stream, predicted optimal-foraging fish distributions within the reach, and experimentally manipulated access to foraging sites and measured fish responses. Our results show that high-quality foraging sites were located at certain places in the reach during one period, but at different places during others, consistent with the hypothesis that fish movement is required if dominant fish are to occupy high-quality foraging sites throughout summer. The optimal foraging model was able to predict foraging locations within study pools, but not the exact location of individual fish within the pools or the reach. However, empirical evidence suggests that fish were distributed in order to maximize energy intake at the reach scale. Finally, dominant fish excluded from their preferred foraging location either left the pools (three of six cases), or began to occupy focal points of the next largest fish which, in turn, exited the pool (two of six cases). If habitat selection was occurring only within habitat units, then large fish, when excluded from their preferred locations, would select the next best locations within the pool. Taken together, these results suggest that charr use summertime movements to both monitor habitat conditions at a large spatial scale, and to gain access to optimal foraging locations even as conditions change temporally.     

Time and energy constraints and the evolution of sexual size dimorphism — to eat or to mate?

Summary We present an empirical test of the Ghiselin—Reiss small-male hypothesis for the evolution of sexual size dimorphism (SSD). In mating systems dominated by scramble competition, where male reproductive success is a function of encounter rate with females, small males may be favoured when food is limiting because they require lower absolute amounts of food. Given a trade-off between time and energy devoted to foraging and to mate acquisition, small males should be able to devote more time to the latter. If at the same time larger females are favoured, this mechanism will contribute to the evolution of SSD and may be the major determinant of the female-biased SSDs that characterize most animal taxa. We tested this hypothesis using the water strider,Aquarius remigis (Heteroptera: Gerridae), a scramble competitor which mates many times over a prolonged mating season and which shows female-biased SSD. Laboratory experiments demonstrated that foraging success and giving up times (GUTs) are lower for males than for females during the reproductive season and that male water striders flexibly alter their time budgets under conditions of energy limitation. Controlled feeding experiments showed that male and female longevity, female fecundity and male mating success are positively related to food availability. As predicted, male body size is negatively correlated with several indices of male fitness (longevity, number of mating attempts and mating success), while female body size is positively correlated with longevity. These results are consistent with the hypothesis that scramble competition for mates favours small males in this species and provides empirical support for the Ghiselin—Reiss small-male hypothesis.     

Comparison of allopatric cutthroat trout stocks with those sympatric with coho salmon and sculpins in small streams

Synopsis Juvenile stocks of allopatric (upstream of barrier falls) cutthroat troutSalmo clarki and those sympatric (downstream of barrier falls) with coho salmonOncorhynchus kisutch and sculpinsCottus spp., were sampled during the late summer period of low flows in six small coastal streams in British Columbia. The objective was to obtain and compare information on pattern of habitat use and fish size distribution of these two trout types. In most instances, density (n m^–2; g m^–2) of cutthroat trout was considerably greater in pools and glides in the allopatric than in the sympatric stocks. The sympatric salmonids were dominated by juvenile coho salmon in pools and cutthroat trout in riffles. Sympatric cutthroat trout constituted from 7 to 45 % of the salmonids present in pools and from 50 to 90% in riffles. Glides were areas of intermediate densities for both salmonids, although coho salmon was the more abundant species in most instances. The density of sculpins was high in all three habitat types, and frequently it exceeded that of coho salmon and cutthroat trout combined. Sympatric cutthroat trout consisted primarily of underyearling fish, whereas allopatric cutthroat trout consisted primarily of two or more age classes with a large proportion of them living in pools. When tested in a laboratory stream both types of cutthroat trout had similar habitat preferences and agonistic behaviours, with the exception that allopatric trout made greater use of cover and defended pools more intensely than sympatric trout when the flow was increased. The results of this study provide insight of potential impact of coho salmon juvenile transplants into stream segments supporting allopatric cutthroat trout.     

Habitat overlap between predatory benthic fish and their invertebrate prey in streams: the relative influence of spatial and temporal factors on predation risk

1. The spatial heterogeneity of ecosystems as well as temporal activity patterns of organisms can have far‐reaching effects on predator–prey relationships. We hypothesised that spatiotemporal constraints in mesohabitat use by benthic fish predators would reduce habitat overlap with benthic invertebrates and lead to mesohabitat‐specific predation risks. 2. We analysed the spatiotemporal activity patterns of two small‐bodied benthivorous fishes, gudgeon (Gobio gobio) and stone loach (Barbatula barbatula), and of benthic invertebrates in a small temperate stream during three 24‐h field experiments. By applying a novel method of field video observation, we monitored the spatiotemporal foraging behaviour of the fish in their natural environment. A parallel analysis of invertebrate mesohabitat use by means of small area Hess sampling allowed a direct estimation of habitat overlap at a pool–riffle scale. 3. Gudgeon showed a dominant spatial activity pattern preferring pools at all times of day, whereas stone loach used both mesohabitats but with a distinct temporal (nocturnal) activity pattern. The patterns of residence were not identical with those of active foraging. Invertebrate community composition differed significantly between mesohabitats but not between times of day. More than half of the total dissimilarity between pools and riffles was accounted for by six invertebrate taxa. Five of these were subject to higher fish predation in pools than in riffles. The total prey consumption of the two fish species together in pools was about three times as high as in riffles. Trophic niche breadth of stone loach and thus its predation range was broader than that of gudgeon. 4. These results indicate that the potential predation risk for stream invertebrates depends on the combination of spatial and temporal patterns of both predator and prey. Given the distinct differences in predation risk found between pools and riffles, we conclude that spatial heterogeneity at the mesohabitat scale can influence mechanisms and consequences of selective predation. We also suggest that the analysis of spatiotemporal predator–prey relationships should not be based on the premise that the main residence habitat and active foraging habitat of a predator are identical.     

Size-specific social interactions and foraging styles in a shallow water population of mutton snapper,Lutjanus analis (Pisces: Lutjanidae), in the central Bahamas

Synopsis Field observations quantified the effects of fish size and time of day on activity patterns, intraspecific encounters, and foraging styles in mutton snapper,Lutjanus analis, during the spring and winter of 1991. Fish ranged in size from 15 to 65 cm fork length (FL), and were associated with an artificial patch reef system located on a shallow seagrass meadow in the Exuma Cays, Bahamas. The most common, non-resting diurnal activities were intraspecific chasing and displacing, and feeding. Intraspecific displacing was significantly higher during midday compared to morning or evening. The highest proportion of intraspecific encounters (combined chasing and displacing events) occurred among medium (25–35 cm FL) and large (> 35 cm FL) fish. The few large fish observed (13% of population) initiated the same proportion of encounters as the predominant (50%) medium fish. The remaining (37%) small fish (> 25 cm FL) were the least aggressive. Dark barred and dark nape color patterns were associated with displacing and chasing, respectively. Fish exhibited considerable variability in feeding behavior, Proportionally fewer fish fed during midday compared to morning or evening, although small fish fed proportionally more often than medium or large fish despite time of day. Picking was the primary feeding mode and was observed during all times of day. Winnowing was observed during midday and evening, whereas midwater strikes were confined to morning and evening. Small fish displayed proportionally higher picking and midwater strikes during morning and evening, respectively, compared to medium or large fish. However, large fish winnowed proportionally more often than small or medium fish during evening. Dark barred color patterns were associated with feeding on the substrate, whereas no color changes occurred during midwater strikes. Our results indicate thatL. analis forms dominance hierarchies and that high variability in foraging styles, according to fish size and time of day, may be a means to reduce intraspecific competition.     

Reduction of Reactive Distance and Foraging Success in Smallmouth Bass, Micropterus dolomieu, Exposed to Elevated Turbidity Levels

We determined how turbidity affected the reactive distance and foraging success of smallmouth bass, Micropterus dolomieu. Smallmouth bass reactive distance decreased exponentially with increasing turbidity, from 65cm in clear water to 10cm at the highest turbidity. Turbidity significantly decreased the probability of a fish reacting to a prey item, but did not influence foraging success following reaction to the prey. Elevated turbidity may reduce stream fish foraging efficiency and decrease prey consumption.     

Overlapping territory of the benthophagous cichlid fish, Lobochilotes labiatus, in Lake Tanganyika

Synopsis Feeding territories do not usually overlap in the same fish species. We studied overlapping territories and feeding behaviours of a benthophagous cichlid fish Lobochilotes labiatus Boulenger in a coastal area of Lake Tanganyika. Forty-four individuals of both sexes, 6–31 cm in total length, maintained territories in a 21 m × 25 m area. These could be regarded as feeding territories. The territories of similar-sized owners (usually different by less than 5 cm) did not overlap each other, but those of different-sized owners overlapped widely. Aggressive interactions were rarely observed between fish distinctly different in body size. Both large and small fish took benthic animals of the same kinds and of similar size. In contrast, the size of foraging sites (crevices) was different between them: the large fish foraged at large crevices and small fish at small crevices. This partitioning of foraging sites may enable the overlap of territories in this benthivore.     

Nile perch,Lates niloticus,predation on the freshwater prawn,Caridina nilotica,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis The percentage of Nile perch, Lates niloticus, containing the prawn, Caridina nilotica, declined from 60–85%, for fish between 5 and 40 cm total length, to less than 10% for fish larger than 80 cm in length. The maximum number of prawns eaten by a Nile perch increased steadily, from 10 prawns for a fish in the 5–9.9 cm length group, to 913 prawns in a fish in the 60–69.9 cm length group, and then fell sharply for larger fish. The mean number of Caridina eaten by fish that were foraging on this animal followed a similar trend. Gill raker spacing increased in direct proportion to fish length and this may prevent fish larger than 70 cm in length from being effective prawn predators.     

The consequences of flooding for the dispersal and fate of poeciliid fish in an intermittent tropical stream

Summary We studied the rainy season dispersal of the fish Poecilia gillii (Poeciliidae) from pools in a steepgradient, intermittent stream in Santa Rosa National Park, Costa Rica. The stream consisted of about 20 pools separated by dry streambed except during two floods and subsequent brief periods of flow. Individually recognizable tags permitted mark-recapture estimates of population size and information on individual movements. The first flood was very severe, with pools losing an average of 75% of their populations (range 12–99%). Most of the lost fish died by becoming trapped in desiccating pools. Males and juveniles were more likely to be lost than were females. Population loss was negatively related to pool volume and positively related to streambed slope. In addition, population loss was positively related to preflood population size when the effects of pool volume and slope were removed indicating that pools with higher densities lost more fish. Of the fish recaptured after the flood, the average proportion found in the pool in which they had been tagged varied from 16%–96%, depending on the area of the stream. Of fish that moved, 92% went downstream. The second flood was less severe though stream flow lasted as long, and there was little effect on the pool populations. Involuntary flushing during the flood and voluntary departure apparently interacted to produce the observed patterns.     

The effects of trout on water striders in stream pools

Summary Observations made in Rattlesnake Creek, Santa Barbara County, California, U.S.A. indicate that rainbow trout (Salmo gairdneri) affect the microdistribution of adult water striders (Gerris remigis). Gerrids occupied all areas of stream pools lacking trout but stayed in margins of pools containing trout. To test the hypothesis that the presence of trout controlled the distribution of water striders within pools, trout were removed from some pools and transferred to pools lacking trout. Undistrubed pools with and without trout served as controls for the trout transferrals. The results supported the hypothesis; within one week, gerrids moved to the edge of pools where trout were added and to the middle of pools where trout were removed. Time budgets and gut analyses suggest that local patterns of gerrid distribution were maintained by harrassment of gerrids by trout. At times trout had significant effects on components of gerrid fitness.