Differential behavioural responses of mayflies from streams with and without fish to trout odour

1. In streams, mayflies (Order Ephemeroptera) are at risk from fish feeding visually in the water column. The effect of fish odour on the behaviour of Baetis bicaudatus from a fishless stream and a trout stream was investigated in four large oval tanks supplied with water from the fishless stream.
2. For each mayfly population, mayfly positioning on the substratum and movement in the water column (drift) were measured during the day and night, over 3 days. Brook trout ( Salvelinus fontinalis ) odour was added to two tanks to test the effect of a threat from fish.
3. Throughout the experiment more mayflies from the trout stream were observed on the substratum surface and in the water column during the night than the day, but the magnitude of night drift was less in tanks with fish odour.
4. Baetis from the fishless stream also displayed a nocturnal periodicity in drift and positioning, but their night-time drift was not affected by the presence of fish odour. On the first day of the experiment, however, more mayflies were observed on the substratum surface and drifting in tanks without fish odour during the day.
5. Sensitivity to fish odour may enable mayflies to alter their behaviour according to the risk of predation from fish.     

Differential behavioural responses of mayflies from streams with and without fish to trout odour

1. In streams, mayflies (Order Ephemeroptera) are at risk from fish feeding visually in the water column. The effect of fish odour on the behaviour of Baetis bicaudatus from a fishless stream and a trout stream was investigated in four large oval tanks supplied with water from the fishless stream.
2. For each mayfly population, mayfly positioning on the substratum and movement in the water column (drift) were measured during the day and night, over 3 days. Brook trout ( Salvelinus fontinalis ) odour was added to two tanks to test the effect of a threat from fish.
3. Throughout the experiment more mayflies from the trout stream were observed on the substratum surface and in the water column during the night than the day, but the magnitude of night drift was less in tanks with fish odour.
4. Baetis from the fishless stream also displayed a nocturnal periodicity in drift and positioning, but their night-time drift was not affected by the presence of fish odour. On the first day of the experiment, however, more mayflies were observed on the substratum surface and drifting in tanks without fish odour during the day.
5. Sensitivity to fish odour may enable mayflies to alter their behaviour according to the risk of predation from fish.     

Effects of trout on the diel periodicity of drifting in baetid mayflies

Some benthic invertebrates in streams make frequent, short journeys downstream in the water column (=drifting). In most streams there are larger numbers of invertebrates in the drift at night than during the day. We tested the hypothesis that nocturnal drifting is a response to avoid predation from fish that feed in the water column during the day. We surveyed diel patterns of drifting by nymphs of the mayfly Baetis coelestis in several streams containing (n=5) and lacking (n=7) populations of rainbow trout, Oncorhynchus mykiss. Drifting was more nocturnal in the presence of trout (85% of daily drift occurred at night) than in their absence (50% of daily drift occurred at night). This shift in periodicity is due to reduced daytime drifting in streams with trout, because at a given nighttime drift density, the daytime drift density of B. coelestis was lower in streams occupied by trout than in troutless streams. Large size classes of B. coelestis were underrepresented in the daytime drift in trout streams compared to nighttime drift in trout streams, and to both day and night drift in troutless streams. Differences in daytime drift density between streams with and without trout were the result of differences in mayfly drift behaviour among streams because predation rates by trout were too low to significantly reduce densities of drifting B. coelestis. We tested for rapid (over 3 days) phenotypic responses to trout presence by adding trout in cages to three of the troutless streams. Nighttime drifting was unaffected by the addition of trout, but daytime drift densities were reduced by 28% below cages containing trout relative to control cages (lacking trout) placed upstream. Drift responses were measured 15 m downstream of the cages suggesting that mayflies detected trout using chemical cues. Overall, these data support the hypothesis that infrequent daytime drifting is an avoidance response to fish that feed in the water column during the day. Avoidance is more pronounced in large individuals and is, at least partially, a phenotypic response mediated by chemical cues.     

Nested species subsets, gaps, and discrepancy

Chemical cues from fish can alter the behaviour of stream invertebrates in experimental tanks but their effect in natural streams has received little attention. By adding brook trout (Salvelinus fontinalis) odour to a trout stream in the Rocky Mountains of Colorado, USA, we tested whether changes in the concentration of chemical cues from visually feeding predatory fish would alter the drift of mayfly nymphs (Ephemeroptera). Stream water was piped from stream-side tanks with (odour) and without (control) three brook trout to two locations in the stream 3.5 m upstream of drift nets at six replicate sites. Five-minute drift samples were collected downstream from odour and control pipes before, during and after the release of water from the tanks into the stream during both the day and night. Almost all drift occurred at night and consisted predominantly of Baetis bicaudatus nymphs. The odour manipulation had no measurable effect on Baetis drift during the day but statistical power was low. During the night, however, the drift of large (>0.65 mm head capsule width, HCW) Baetis nymphs decreased significantly during the odour addition compared to control drift. In contrast, the drift of small nymphs (≤0.65 mm HCW) increased both during and after the odour addition in comparison to control drift. Since the stream contains brook trout (0.04–0.18 m⁻²), and water from the stream (presumably containing fish odour) altered the behaviour of fishless-stream Baetis nymphs in another experiment, we conclude that the changes in Baetis drift density were a response to an increase in the concentration of fish odour in the stream. Furthermore, we were able to detect the effect within 5 min. of odour addition, indicating that mayfly behavioural response to trout odour was rapid. These results suggest that mayflies can distinguish different concentrations of trout odour in natural streams and that the response is size-specific, according to the relative risk of predation of large and small Baetis. Received: 12 May 1998 / Accepted: 23 October 1998     

Fitness and community consequences of avoiding multiple predators

We investigated the fitness and community consequences of behavioural interactions with multiple predators in a four-trophic-level system. We conducted an experiment in oval flow-through artificial-stream tanks to examine the single and interactive sublethal effects of brook trout and stoneflies on the size at emergence of Baetis bicaudatus (Ephemeroptera: Baetidae), and the cascading trophic effects on algal biomass, the food resource of the mayflies. No predation was allowed in the experiment, so that all effects were mediated through predator modifications of prey behaviour. We reared trout stream Baetis larvae from just before egg development until emergence in tanks with four treatments: (1) water from a holding tank with two brook trout (trout odour), (2) no trout odour + eight stoneflies with glued mouthparts, (3) trout odour + stoneflies and (4) no trout odour or stoneflies. We ended the experiment after 3 weeks when ten male and ten female subimagos had emerged from each tank, measured the size of ten male and ten female mature nymphs (with black wing pads), and collected algal samples from rocks at six locations in each tank. To determine the mechanism responsible for sublethal and cascading effects on lower trophic levels we made day and night observations of mayfly behaviour for the first 6 days by counting mayflies drifting in the water column and visible on natural substrata in the artificial streams. Trout odour and stoneflies similarly reduced the size of male and female Baetis emerging from artificial streams, with non-additive effects of both predators. While smaller females are less fecund, a fitness cost of small male size has not been determined. The mechanism causing sublethal effects on Baetis differed between predators. While trout stream Baetis retained their nocturnal periodicity in all treatments, stoneflies increased drift dispersal of mayflies at night, and trout suppressed night-time feeding and drift of mayflies. Stoneflies had less effect on Baetis behaviour when fish odour was present. Thus, we attribute the non-additivity of effects of fish and stoneflies on mayfly growth to an interaction modification whereby trout odour reduced the impact of stoneflies on Baetis behaviour. Since stonefly activity was also reduced in the presence of fish odour, this modification may be attributed to the effect of fish odour on stonefly behaviour. Only stoneflies delayed Baetis emergence, suggesting that stoneflies had a greater sublethal effect on Baetis fitness than did trout. Delayed emergence may reduce Baetis fitness by increasing risks of predation and parasitism on larvae, and increasing competition for mates or oviposition sites among adults. Finally, algal biomass was higher in tanks with both predators than in the other three treatments. These data implicate a behavioural trophic cascade because predators were not allowed to consume prey. Therefore, differences in algal biomass were attributed to predator-induced changes in mayfly behaviour. Our study demonstrates the importance of considering multiple predators when measuring direct sublethal effects of predators on prey fitness and indirect effects on lower trophic levels. Identification of an interaction modification illustrates the value of obtaining detailed information on behavioural mechanisms as an aid to understanding the complex interactions occurring among components of ecological communities. Received: 20 March 1997 / Accepted: 29 September 1997     

Predator effects on prey population dynamics in open systems

Animal population dynamics in open systems are affected not only by agents of mortality and the influence of species interactions on behavior and life histories, but also by dispersal and recruitment. We used an extensive data set to compare natural loss rates of two mayfly species that co-occur in high-elevation streams varying in predation risk, and experience different abiotic conditions during larval development. Our goals were to generate hypotheses relating predation to variation in prey population dynamics and to evaluate alternative mechanisms to explain such variation. While neither loss rates nor abundance of the species that develops during snowmelt (Baetis bicaudatus) varied systematically with fish, loss rates of the species that develops during baseflow (Baetis B) were higher in streams containing brook trout than streams without fish; and surprisingly, larvae of this species were most abundant in trout streams. This counter-intuitive pattern could not be explained by a trophic cascade, because densities of intermediate predators (stoneflies) did not differ between fish and fishless streams and predation by trout on stoneflies was negligible. A statistical model estimated that higher recruitment and accelerated development enables Baetis B to maintain larger populations in trout streams despite higher mortality from predation. Experimental estimates suggested that predation by trout potentially accounts for natural losses of Baetis B, but not Baetis bicaudatus. Predation by stoneflies on Baetis is negligible in fish streams, but could make an important contribution to observed losses of both species in fishless streams. Non-predatory sources of loss were higher for B. bicaudatus in trout streams, and for Baetis B in fishless streams. We conclude that predation alone cannot explain variation in population dynamics of either species; and the relative importance of predation is species- and environment-specific compared to non-predatory losses, such as other agents of mortality and non-consumptive effects of predators. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nocturnal drift of mayfly nymphs as a post-contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom-feeding, nocturnal predators, has rarely been tested.
2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout ( Salmo trutta , a drift-feeding fish), one with alpine bullhead ( Cottus poecilopus , a benthic fish), one with both species, and two fishless streams.
3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non-dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish.
4. In laboratory experiments, bullheads were night-active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre-contact response) and benthic fish (a post-contact response). In streams with bottom-feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.     

Cascading effects of predatory fish on the composition of benthic algae in high‐altitude streams

Cascading effects of predators can affect ecosystem properties by changing plant biomass, distribution and assemblage composition. Using data from field surveys and whole‐stream experiments we tested the hypothesis that predatory trout change assemblage composition of benthic algae in high‐elevation streams mediated by grazer behavior. Field surveys revealed that the taxonomic composition of algal assemblages differed significantly between streams that contained trout and those that were fishless; but comparisons of palatable versus unpalatable algal taxa between fish and fishless streams were equivocal because of high natural variability. Therefore, we tested for a behavioral (non‐consumptive) trophic cascade experimentally by adding brook trout chemical cues to six naturally fishless streams for 25 days and compared responses of grazers and algae to six reference streams without fish cues added. Algal response variables included rates of change in the abundance of three physiognomic categories, from most palatable (attached erect and prostrate diatoms) to least palatable (non‐diatoms), as determined from food selectivity analyses of the most common grazers (mayflies and caddisflies). Fish cues did not affect the mean densities or changes in densities of total grazers or any individual grazer species. However, in streams where fish cues were added, rates of accrual of attached erect diatoms, which was the preferred algal type for the grazer most vulnerable to trout predation (Baetis), were higher and their densities increased significantly faster with increasing densities of this grazer species than in reference streams. Results of his experiment support the hypothesis that predator induced suppression of grazer foraging behavior, rather than cascading effects of top predators on grazer density, may contribute to variation in the composition of algal assemblages among streams by allowing proliferation of most palatable algal species.     

Interactions between fish,grazing invertebrates and algae in a New Zealand stream: a trophic cascade mediated by fish-induced changes to grazer behaviour?

Experiments in laboratory stream channels compared the behaviour of Deleatidium mayfly nymphs in the absence of fish with that in the presence of either native common river galaxias (Galaxias vulgaris Stokell) or introduced brown trout (Salmo trutta L.). Galaxias present similar predation risks to prey during day and night but are more active at night. Whereas, trout present a higher predation risk during the day. Deleatidium maintained a fixed nocturnal drift periodicity that is characteristic of streams containing visually feeding fish regardless of the nature of the predation regime presented in the laboratory. However, the number on the substratum surface, and therefore able to graze algae, was lower when fish were present than when they were absent. The number was lower during the day in the presence of trout, when they present the highest predation risk, and lower during the night compared to the day in trials with galaxias when galaxias activity disturbs Deleatidium from the substratum. Increases in the probability of Deleatidium leaving a patch, reductions in the proportion of mayflies on high quality patches and reductions in the distance travelled from refuge also reflected variations in the predation regime. Similar differences in positioning were observed under the same predation regimes in in situ channels in the Shag River and these were associated with differences in algal biomass. Algal ash-free dry mass (AFDM) and chlorophyll a (chl a) were higher on the tops of cobbles when fish were present. Fish also affected the biomass and the distribution of algae on cobbles as AFDM and chl a were higher on the sides of cobbles from channels with trout compared to those with galaxias. Changes in grazing behaviour, caused by predator avoidance, are likely to have been responsible for differences in algal biomass because no significant differences were detected between treatments in the biomass of Deleatidium or of total invertebrates.     

Night — day drift patterns and the size of larvae of two aquatic insects

A test of Allan's (1978) hypothesis about differential drift abundance of mayflies of the genus Baetis between night and day, and the size of larvae was performed at a mountain stream in Idaho. Palisades Creek, Idaho, contains a different species of mayfly, B. tricaudatus, and vertebrate predator, Salmo clarki, than Cement Creek, Colorado (B. bicaudatus, and brook trout, Salvelinus fontinalis). Consequently it was not known if B. tricaudatus would exhibit a similar pattern as its congener in Cement Creek, with large instars tending to avoid daylight drift, as found by Allan (1978). However, similar results were observed in the present study. It appears that the earlier hypothesis may have generality for geographically distinct streams with a different vertebrate predator and mayfly prey. The existence of a similar pattern for chironomid larvae was also tested, however, no such pattern existed. This discrepency between taxa may be due to differential predation, or to inherent differences in drift abilities.     

Trout reverse the effect of water temperature on the foraging of a mayfly

Climate change is likely to increase the metabolisms of ectothermic animals living below their thermal optimum. While ectothermic top predators may compensate by increasing foraging, ectothermic prey may be unable to increase foraging because of increased predation risk from ectothermic predators. We examined how the diurnal drift behavior (i.e., the downstream movement associated with foraging) of the mayfly Baetis, an ectothermic herbivore, responds to changing temperature in the implied presence and absence of trout, an ectothermic predator. In an experiment replicated at the catchment scale, water temperature and trout presence strongly interacted to affect the diurnal drift of Baetis from artificial channels lacking periphyton over a water temperature range of 4.2–14.8 °C. In fishless streams, daytime drift increased with increasing water temperature, likely because of increased metabolic demand for food. However, in trout-bearing streams, daytime drift decreased with increasing water temperature. Our interpretation is that the perceived threat of trout rose with increasing water temperature, causing mayflies to reduce foraging despite heightened metabolic demand. These results suggest that anticipated increases in stream temperature due to climate change may further escalate divergence in structure and process between fishless and trout-bearing streams. Similar dynamics may occur in other ecosystems with ectothermic predators and prey living below their thermal optima.     

Reduction of predation risk under the cover of darkness: Avoidance responses of mayfly larvae to a benthic fish

Summary Mayfly larvae of Paraleptophlebia heteronea (McDunnough) had two antipredator responses to a nocturnal fish predator (Rhinichthys cataractae (Valenciennes)): flight into the drift and retreat into interstitial crevices. Drift rates of Paraleptophlebia abruptly increased by 30 fold when fish were actively foraging in the laboratory streams but, even before fish were removed, drift began returning to control levels because larvae settled to the substrate and moved to areas of low risk beneath stones. This drifting response was used as an immediate escape behavior which likely decreases risk of capture from predators which forage actively at night. Surprisingly, drift most often occurred before contact between predator and prey, and we suggest that in darkness this mayfly may use hydrodynamic pressure waves for predator detection, rather than chemical cues, since fish forage in an upstream direction. Although drifting may represent a cost to mayfly larvae in terms of relocation to a new foraging area with unknown food resources, the immediate mortality risk probably out-weighs the importance of staying within a profitable food patch because larvae can survive starvation for at least 2 d. In addition to drifting, mayflies retreated from upper, exposed substrate surfaces to concealed interstitial crevices immediately after a predator encounter, or subsequent to resettlement on the substrate after predator-induced drift. A latency period was associated with this response and mayflies remained in these concealed locations for at least 3 h after dace foraging ceased. Because this mayfly feeds at night and food levels are significantly lower in field refugia under stones, relative to exposed stone surfaces, predator avoidance activity may limit foraging time and, ultimately, reduce the food intake of this stream mayfly.     

Facilitation and interference among three predators affect their consumption of a stream-dwelling mayfly

1. We experimentally tested if a multiplicative risk model accurately predicted the consumption of a common mayfly at risk of predation from three predator species in New Zealand streams. Deviations between model predictions and experimental observations were interpreted as indicators of ecologically important interactions between predators. 2. The predators included a drift‐feeding fish [brown trout (T), Salmo trutta], a benthivorous fish [galaxiid (G), koaro, Galaxias brevipennis] and a benthic predatory stonefly (S; Stenoperla sp.) with Deleatidium sp. mayflies as prey. Eight treatments with all predator species combinations and a predator‐free control were used. Experiments were performed in aquaria with cobbles as predator refuges for mayflies and we measured the proportion of prey consumed after 6 h for both day and night trials. 3. Trout consumed a higher proportion of prey than other predators. For the two predator treatments we found less than expected prey consumption in the galaxiid + trout treatment (G + T) for both day and night trials, whereas a higher than expected proportion of prey was consumed during night time in the stonefly + trout (S + T) treatment. 4. The results indicate interference (G + T) and facilitation (S + T) between predators depending on predator identity and time of day. Thus, to make accurate predictions of interspecific interactions, it is necessary to consider the ecology of individual species and how differences influence the direction and magnitude of interactions.     

Are populations of mayflies living in adjacent fish and fishless streams genetically differentiated?

1. Conspecific populations living in habitats with different risks of predation often show phenotypic variation in defensive traits. Traits of two species of mayflies (Baetidae: Baetis bicaudatus and Baetis sp. nov.) differ between populations living in fish and fishless streams in a high altitude drainage basin in western Colorado, U.S.A. We tested for genetic differentiation between mayfly populations in these two habitat types, assuming that lack of genetic differentiation would be consistent with the hypothesis that those traits are phenotypically plastic. 2. Previous work has shown that larvae of both species behave differently and undergo different developmental pathways in adjacent fish and fishless streams. These phenotypic differences in behaviour and development have been induced experimentally, suggesting that populations from fishless streams have the genetic capability to respond to fish. 3. During summer 2001 we collected Baetis larvae from several fish and fishless streams, and from fish and fishless sections of the same streams. We used allozymes and a fragment of the cytochrome oxidase subunit 1 mitochondrial gene to examine genetic variation of Baetis individuals within and among streams. 4. Results showed that genetic variation exists among populations of the same species of Baetis from different streams, but none of that variation was associated with the presence or absence of fish. These data confirm that populations of Baetis living in fish and fishless streams are not genetically distinct, and are consistent with the hypothesis that traits associated with environments of different risk are phenotypically plastic.     

Predator detection and avoidance by lotic mayfly nymphs of different size

We studied antipredatory responses of lotic mayfly (Baetis) nymphs in a factorial experiment with four levels of fish presence: (1) a freely foraging fish (the European minnow,Phoxinus phoxinus), (2) a constrained fish, (3) water from a fish stream, (4) water from a fishless stream. LargeBaetis nymphs drifted mainly during night-time in treatments involving either the chemical or actual presence of fish, whereas no diel periodicity was observed when the water was not conditioned with fish odour. The response was strongest when the fish was uncaged, which suggests that visual or hydrodynamic cues are needed in addition to chemical ones for an accurate assessment of predation risk. Fish presence had no effect on the drift rates of small nymphs. Instead, they increased their refuge use in the presence of a live fish. Chemical cues alone did not have any effect on the refuge use of any of theBaetis size classes. Our results indicate active drift entry by mayfly nymphs. Because predation pressure is spatially and temporally variable, nymphs must sample the environment in order to locate predator-free areas or areas with low predation risk. Drifting should be the most energy-saving way to do this. To avoid the risk from visually feeding fish, large individuals can sample safely (i.e. enter drift) only at night-time, while the small ones can also do this safely during the day. We suggest that, contrary to some earlier assumptions, mayfly drift is not a fixed prey response. Instead,Baetis nymphs are able to assess the prevailing predation pressure, and they adjust their foraging behaviour accordingly.     

Fish predation affects the structure of a benthic community

1. We conducted an experimental study of predation by benthivorous fish on a natural community of stream invertebrates using a reach‐scale approach. Over a 2‐year period (experimental phase), the benthic invertebrate community of a stretch containing two species of benthivorous fish was compared with a fishless stretch. Thereafter, all fish were removed and benthic community structure was analysed again to account for natural differences between the two stretches (reference phase). 2. Benthivorous fish at the moderate densities investigated did not affect total benthic biomass or density, but did alter species composition. In addition, the fish effect differed between pool and riffle habitats, with larger effects in the pools indicating a habitat‐specific predation effect. In the reference phase, when all fish were removed from the stream, the difference between the two stretches was reduced. 3. The benthivorous fish reduced the densities of four taxa (Pisidium sp., Dugesia gonocephala, Gammarus pulex, Limoniidae), representing 29% of total biomass. It is possible that density reductions of other species were masked by prey migration despite the relatively large spatial scale. Indeed, higher drift activity in the upstream fishless stretch could have increased the density of Baetis rhodani in the fish stretch, as indicated by the results of a drift model. 4. Our results provide insights into stream food web ecology because fish predation showed effects even in a natural system where habitat complexity was high, environmental factors were highly variable and many predator and prey species interacted and because benthivorous fish were the focus, whereas the majority of previous predation experiments in streams have used drift‐feeding trout.     

Diel foraging in relation to available prey in an Adirondack Mountain stream fish community

The diets of the fish community of Trucka Brook, a small stream located in the central Adirondack Mountains in northern New York, were examined in relation to the bottom fauna and invertebrate drift. Measures of overlap were calculated between the diets of each fish species examined, brook trout (Salvelinus fontinalis), blacknose dace (Rhinichthys atratulus), creek chub (Semotilus atromaculatus) and pearl dace (Semotilus margarita). Overlap was also examined between the fish diets and bottom and drift samples. Blacknose dace, pearl dace and brook trout had the most similar diets which were closely associated with the benthos. Creek chub had the most distinctive diets which did not compare well with any other fish species during either diurnal or nocturnal periods. The mayfly nymph Litobranchia recurvata was the most abundant bottom invertebrate and was the major prey of benthic feeding fishes. The invertebrate drift did not compare favorably with any of the fishes' diets because of the predominance of large cased limnephilid larvae (primarily Psychoglypha sp.) which were not readily consumed by fish.     

Benthivorous fish reduce stream invertebrate drift in a large-scale field experiment

Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or to escape after encountering a predator. While the control of the diurnal patterns of invertebrate drift activity by fish predators has received considerable interest, it remains unclear whether benthivorous fish reduce or increase drift activity. We performed a large-scale field experiment in a second-order stream to test if invertebrate drift was controlled by two benthivorous fish species (gudgeon Gobio gobio and stone loach Barbatula barbatula). An almost fishless reference reach was compared with a reach stocked with gudgeon and loach, and density and structure of the invertebrate communities in the benthos and in the drift were quantified in both reaches. The presence of gudgeon and stone loach reduced the nocturnal drift of larvae of the mayfly Baetis rhodani significantly, in contrast to the findings of most previous studies that fish predators induced higher night-time drift. Both drift density and relative drift activity of B. rhodani were lower at the fish reach during the study period that spanned 3 years. Total invertebrate drift was not reduced, by contrast, possibly due to differences in vulnerability to predation or mobility between the common invertebrate taxa. For instance, Chironomidae only showed a slight reduction in drift activity at the fish reach, and Oligochaeta showed no reduction at all. Although benthic community composition was similar at both reaches, drift composition differed significantly between reaches, implying that these differences were caused by behavioural changes of the invertebrates rather than by preferential fish consumption. The direction and intensity of changes in the drift activity of stream invertebrates in response to the presence of benthivorous fish may depend on the extent to which invertebrate taxa can control their drifting behaviour (i.e. active versus passive drift). We conclude that invertebrate drift is not always a mechanism of active escape from fish predators in natural streams, especially when benthos-feeding fish are present.     

Top-down and bottom-up influences on populations of a stream detritivore

1. Knowledge of the influence of predatory fish in detritus‐based stream food webs is poor. We tested whether larval abundance of the New Zealand leaf‐shredding caddisfly, Zelandopsyche ingens (family Oeconesidae), was affected by the presence of predatory brown trout, Salmo trutta and the abundance of their primary detrital resource (Nothofagus leaves). 2. The density of Z. ingens and the biomass of leaves were determined in seven fishless streams and four trout streams in the Cass region, central South Island, on four occasions spanning 5 years. 3. Physicochemical conditions were similar in trout and fishless streams, but ancova indicated that Z. ingens numbers were positively related to leaf biomass and that caddisfly numbers were significantly greater in fishless streams than trout streams for any given biomass of leaf. The cases of trout stream larvae were also heavier per unit length than those in fishless streams. 4. Our results provide evidence for both top‐down and bottom‐up influences on a detritus‐based stream food web. Although stream detritivores may benefit from a habitat that provides both food and a degree of protection from predators, top‐down effects of predators on detritivore population abundance were still important. Thus, detrital resource availability may determine maximum attainable population size, whereas predation is likely to reduce the population to a level below that.     

Diel activity patterns of the fish community in a temperate stream

The diel activity patterns of fishes in a temperate New Brunswick stream were studied during the summer over 5 years. Young‐of‐the year Atlantic salmon Salmo salar and blacknose dace Rhinichthys atratulus were more active during the day than at night, whereas lake chub Couesius plumbeus, brook trout Salvelinus fontinalis and adult white suckers Catostomus commersonii were more active at night than during the day. Because fishes were as likely to be nocturnal as diurnal, the data suggest that more night‐time sampling is needed to provide an unbiased view of fish community structure in temperate streams.