Aquatic and terrestrial invertebrate drift in southern Appalachian Mountain streams: implications for trout food resources

1. We characterised aquatic and terrestrial invertebrate drift in six south‐western North Carolina streams and their implications for trout production. Streams of this region typically have low standing stock and production of trout because of low benthic productivity. However, little is known about the contribution of terrestrial invertebrates entering drift, the factors that affect these inputs (including season, diel period and riparian cover type), or the energetic contribution of drift to trout. 2. Eight sites were sampled in streams with four riparian cover types. Drift samples were collected at sunrise, midday and sunset; and in spring, early summer, late summer and autumn. The importance of drift for trout production was assessed using literature estimates of annual benthic production in the southern Appalachians, ecotrophic coefficients and food conversion efficiencies. 3. Abundance and biomass of terrestrial invertebrate inputs and drifting aquatic larvae were typically highest in spring and early summer. Aquatic larval abundances were greater than terrestrial invertebrates during these seasons and terrestrial invertebrate biomass was greater than aquatic larval biomass in the autumn. Drift rates of aquatic larval abundance and biomass were greatest at sunset. Inputs of terrestrial invertebrate biomass were greater than aquatic larvae at midday. Terrestrial invertebrate abundances were highest in streams with open canopies and streams adjacent to pasture with limited forest canopy. 4. We estimate the combination of benthic invertebrate production and terrestrial invertebrate inputs can support 3.3–18.2 g (wet weight) m⁻² year⁻¹ of trout, which is generally lower than values considered productive [10.0–30.0 g (wet weight) m⁻² year⁻¹]. 5. Our results indicate terrestrial invertebrates can be an important energy source for trout in these streams, but trout production is still low. Any management activities that attempt to increase trout production should assess trout food resources and ensure their availability.     

Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0+ year Atlantic salmon and brown trout and 1+ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1+ and 2+ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1+ and 2+ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2+ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0+ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.     

The food of brown and rainbow trout (Salmo trutta and S. gairdneri) in relation to the abundance of drifting invertebrates in a mountain stream

Summary The diet of Salmo gairdneri and S. trutta in a Pyrenean stream was very similar, and was also similar to the percentage composition of the drift but not the benthos. There was a good correlation between diel changes in the amount of food (both numbers and biomass) in the stomachs and diel changes in the abundance of drifting invertebrates. The major feeding period was in the early hours of the night when the trout fed chiefly on benthic invertebrates in the drift. This was the only feeding period in experiments 1 and 2 (mean water temperatures 4.7 and 7.3° C) but in experiment 3 (10.8° C), there was a second feeding period in the day when terrestrial invertebrates and emerging aquatic insects formed a large proportion of the diet.Neither species was consuming a greater amount of food than the other. The weight of food consumed/trout/day increased with water temperature, and was close to the daily food requirements for resting metabolism in experiment 1. for twice resting metabolism (active metabolism) in experiment 2, and for four times resting metabolism in experiment 3. Therefore the energy of the second meal in experiment 3 was available for growth.The effect of temperature, on rates of gastric evacuation was the chief factor which determined the number of meals/day; the availability of food organisms in the drift determined the time of feeding; and the requirements for metabolism (affected by temperature and body weight) determined the amount of energy left for growth.     

Effects of fish predation and habitat type on stream benthic communities

The influence of habitat on interactions between a fish predator (brown trout Salmo trutta) and a benthic invertebrate community was studied in nine field enclosures (8 ×3 m) in a creek in southern Sweden. Three habitat treatments were tested, a shallow sandy habitat, a deep habitat containing a mixture of large and small cobbles and a moderately deep habitat with large cobbles. The one month-long experiment showed that there were no major differences in the abundance and biomass of the benthic macroinvertebrate fauna among these habitats as no functional groups of invertebrates and only a few taxa differed between treatments. Invertebrate drift rates decreased over time, which was probably related to seasonal changes in invertebrate life cycles or to effects of predation independent of habitat type, as there was no difference between treatments.     

Selective foraging on terrestrial invertebrates by rainbow trout in a forested headwater stream in northern Japan

The important contribution of terrestrial invertebrates to the energy budget of drift-foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m^–2 day^–1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m^–2 day^–1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.     

Impacts of introduced brown and rainbow trout on benthic invertebrate communities in shallow New Zealand lakes

1. Brown and rainbow trout have been introduced to many inland waters in New Zealand, but research on the impacts on native communities has focused mainly on streams. The purpose of this study was to compare the benthic communities of trout and troutless lakes. Based on previous studies in North America and Europe, we predicted that the benthic biomass, and especially the abundance of large invertebrates, would be lower in lakes with trout as compared to those without. We surveyed the invertebrate fauna of 43 shallow, high‐elevation lakes (26 with and 17 without trout) in four geographic clusters on the central South Island and then conducted a detailed quantitative study of invertebrate biomass and community structure in 12 of these lakes. 2. Benthic community composition and diversity of lakes with and without trout were nearly identical and biomass was as high or higher in the lakes with as without trout. There was no evidence that trout have caused local extinctions of benthic invertebrates. Although the proportional abundance of large‐bodied aquatic was slightly lower in lakes with than without trout, the abundance of several groups of large‐bodied benthic taxa (dragonflies, caddisflies and water bugs) did not differ. 3. Our findings are in contrast to those in North American and Europe where trout introductions into previously troutless lakes have led to declines in the abundance of benthic invertebrates, especially large‐bodied taxa. We propose that the modest effects of trout in New Zealand could be explained by (i) the high areal extent of submergent vegetation that acts as a benthic refuge, (ii) low intensity of trout predation on benthic communities and/or (iii) characteristics of the benthic invertebrates that make them relatively invulnerable to fish predation. 4. Regardless of the relative importance of these hypotheses, our results emphasise that the same invertebrates occurred in all of the lakes, regardless of size, elevation and presence of trout, suggesting habitat generalists dominate the benthic fauna in shallow New Zealand lakes.     

Seasonal and diel patterns of invertebrate drift in different alpine stream types

1. We examined the seasonal and diel patterns of invertebrate drift in relation to seston and various habitat characteristics in two each of four different kinds of alpine streams [rhithral (snow‐fed) lake outlets, rhithral streams, kryal (glacial‐fed) lake outlets and kryal streams]. Samples were collected at four times of the day (dawn, midday, dusk and midnight) during three seasons (spring, summer and autumn). 2. Habitat characteristics differed mainly between rhithral and kryal sites, with the latter having higher discharge and turbidity, lower water temperature, and higher concentrations of ammonium, and particulate and soluble reactive phosphorus. Seasonality in habitat characteristics was most pronounced for kryal streams with autumn samples being more similar to rhithral sites. 3. The concentration of seston was lowest in the glacial‐influenced lake outlets and slightly higher in the stream sites; no seasonal or diel patterns were evident. 4. The density of drifting invertebrates averaged less than 100 m^?3 and was lowest (<10 m^?3) at three of the four kryal sites. Taxon richness and diversity were lowest at rhithral lake outlets. Chironomidae dominated the drift as well as benthic communities and <30% of benthic taxa identified were found in the drift. 5. Drifting invertebrates showed no consistent seasonal pattern. However, density tended to be highest in spring at rhithral sites and in autumn at kryal sites. No diel periodicity in drift density was found at any site and the lack of diel pattern may be a general feature of high altitude streams. 6. Glacially influenced habitat parameters were a major factor affecting drift in these alpine streams, whereas no clear differences were observed between streams and lake outlets. Our findings indicate that invertebrate drift in alpine streams is primarily influenced by abiotic factors, and therefore, substantially differs from patterns observed at lower altitude.     

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.     

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.     

Effects of headwater impoundment and channelization on invertebrate drift

The construction of a flood control impoundment on Twitty's Creek added large numbers of organisms of limnetic origin to the stream ecosystem. However, the number of limnetic organisms per unit volume of water decreased rapidly as the distance downstream from the reservoir increased and, during most sampling periods, made up an insignificant portion of the total drift biomass at 7.2 km downstream. Factors favoring the extended downstream drift of limnetic organisms were high stream discharge and low water temperature.Several taxa of benthic organisms had much lower drift rates in the station immediately below the dam than at other stations and several taxa commonly taken at other stations were not captured immediately below the reservoir outfall. One possible explanation is that these organisms may have longer drift recruitment distances than the distance from the reservoir outfall to the sample location.A comparison of drift densities of organisms of benthic origin and benthic standing crop densities in channeled and unchanneled streams revealed that drift densities were higher in channeled streams than in unchanneled streams for most taxa of invertebrates. In addition, channeled streams appeared to have lower benthic standing crops than unchanneled streams for most taxa of invertebrates.In stream sections impacted by either channelization or the Twitty Lake outfall, the energy dynamics of the stream ecosystems were altered by increased density of drifting invertebrates. From the standpoint of increasing food availability to the fish fauna of the stream, these changes would appear to benefit drift feeding species and negatively impact bottom feeding species.     

Diurnal bottom feeding of predator fish strengthens trophic cascades to benthic algae in experimental flow-through pools

Mechanisms that determine the strength of trophic cascades from fish to benthic algae via algivorous invertebrates in stream communities have not been clarified. Using seven fish species, we tested the hypothesis that the interspecific variation of predatory behavior of fishes affects the strength of trophic cascades in experimental streams. One or two species of fish were introduced into flow-through pools of 2.5 m² and the abundances of benthic invertebrates and algae were monitored. Pike gudgeon, a diurnal benthic feeder, triggered a strong trophic cascade but masu salmon, a diurnal drift feeder, did not have a cascading effect. Japanese dace, which is both a diurnal benthic and drift feeder, increased the algal biomass, but the nocturnal benthic feeder cut-tailed bullhead had little cascading effect. The diurnal benthic feeder silver crucian carp also had a cascading effect, but no trophic cascade was triggered either by Asian pond loach or by Japanese common catfish, both of which are nocturnal benthic feeders. Thus, diurnal benthic fish exerted a stronger cascading effect than diurnal drift feeders or nocturnal fish. The combination of two fish species enhanced the per-capita strength of trophic cascades, probably because one of the two species, the benthic feeder, preyed on more invertebrates than in the single-species pools.     

Incorporating invertebrate predators into theory regarding the timing of invertebrate drift

Theory concerning the timing of lotic invertebrate drift suggests that daytime-feeding fish cause invertebrates to restrict their drift behavior to the nighttime. However, there is growing evidence that the nighttime foraging of invertebrate predators also contributes to the nocturnal timing of drift, though it is unclear whether the nocturnal behavior of invertebrate predators is innate or proximately caused by fish. In two experiments, one conducted in a fish-bearing stream and a second in a fishless stream, we compared the drift patterns of Baetidae (Ephemeroptera) from channels with and without benthic invertebrate predators. We tested whether invertebrate predators affect the timing of drift, either as a proximate cause of nocturnal drift in the fishless stream (diel periodicity) or as a proximate cause of a pre-dawn peak in drift in the fish-bearing stream (nocturnal periodicity). In the fish-bearing stream experiment, a pre-dawn increase of baetid drift occurred independently of invertebrate predators, indicating that invertebrate predators were not the proximate cause of nocturnal periodicity in the stream. In the fishless stream experiment, invertebrate predators caused more baetid drift at night than during the day, indicating that invertebrate predators caused the nocturnal drift pattern we observed in the stream, and that invertebrate predators can influence drift timing independently of fish. Therefore, we suggest that both visually feeding fish and nocturnally foraging benthic predators, when present, affect the timing of invertebrate drift; visually feeding fish by reducing daytime drift, and benthic predators by increasing nighttime drift.     

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.     

Diel variation in feeding rate and prey composition of herring and mackerel in the southern Gulf of St Lawrence

Diel feeding patterns of herring Clupea harengus and mackerel Scomber scombrus in the southern Gulf of St Lawrence were examined based on samples obtained by midwater trawling between 19 and 26 June 2001. Within 3 h time periods, stomach contents tended to be more similar between fish from the same tow than between fish from different tows. Thus, in contrast to previous diet studies, which have used individual fish stomachs as independent observations, tow was used as the experimental unit in statistical analyses in this study. Diel patterns in stomach fullness were identified using generalized additive models. Two peaks in stomach fullness occurred for herring, one in the morning and the other in the evening. Mackerel showed an increase in feeding intensity throughout the day with a peak in mid‐afternoon. The diel changes in stomach contents suggested rapid gastric evacuation rates for both species, especially for herring. The estimate of the instantaneous evacuation rate for herring was twice that for mackerel. Calanus copepods (mainly C. hyperboreus ), fishes (mainly capelin Mallotus villosus ) and euphausiids were the main prey found in the stomachs of both species. Calanus copepods dominated the diet of herring regardless of time period. They also dominated the diet of mackerel during the late afternoon, evening and night while fishes and euphausiids were dominant during the morning and early afternoon. These diel patterns emphasize the need for sampling throughout the day and night in order to estimate ration and diet composition for bioenergetic and ecosystem models.     

Diel variation in habitat use by planktivores in field enclosure experiments: the effect of submerged macrophytes and predation

The importance of submerged macrophytes and predation risk for habitat use by 0+ perch ( Perca fluviatilis ) and 0+ roach ( Rutilus rutilus ) was investigated in triplicate 78-m² field enclosures with and without macrophytes in the middle. During three experimental runs, habitat use by fish were monitored every 6 h with Breder traps. Each period included 2 days of fish monitoring before stocking with piscivorous perch, and 2 days after. Predation risk significantly changed habitat use by 0+ perch in the morning, midday and evening, but not at night. By comparing with the unvegetated controls, we found a refuging effect of macrophytes in the morning. Under predation risk there was significant diel variation in habitat use by 0+ perch, suggesting a migration from the open water habitat at night into the macrophytes in the morning. Roach continued to use open water even after predators were stocked, but responded like perch by reducing overall activity.     

An experimental study of ontogenetic and seasonal changes in the temperature preferences of unfed and fed brown trout,Salmo trutta

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Disentangling the stream community impacts of <Emphasis Type="Italic">Didymosphenia geminata</Emphasis>: How are higher trophic levels affected?

Human activities frequently result in either intentional or unintentional introductions of species to new locations, and freshwater environments worldwide are particularly vulnerable to species invasions. An introduced freshwater diatom, Didymosphenia geminata, was first discovered in New Zealand in 2004 but there was limited research available to predict the drivers of D. geminata biomass and how biomass variability might influence higher trophic levels (e.g. invertebrates and fish). We examined the effect of D. geminata biomass on benthic invertebrates, invertebrate drift and fish communities in 20 rivers in New Zealand with variable hydrology, physical habitat and water chemistry. Variation in D. geminata biomass was best explained by a model that showed D. geminata biomass increased with time since the last flow event exceeding three times the median annual discharge and decreasing concentration of dissolved reactive phosphorus. Analyses of biotic responses showed that high D. geminata biomass did not affect either invertebrate or fish diversity but altered the structure of benthic communities, changed the composition of drifting invertebrate communities and reduced fish biomass by 90 %, particularly trout. A partial least squares path model was used to disentangle both direct and indirect effects of D. geminata on fish communities and showed D. geminata had a significant negative direct effect on fish communities. This is the first study to show how the potential effects of the introduced diatom D. geminata can impact fish communities and has shown that D. geminata impacts fish both directly and indirectly through changes in their invertebrate prey community.     

Forest-stream linkages: effects of terrestrial invertebrate input and light on diet and growth of brown trout (Salmo trutta) in a boreal forest stream

Subsidies of energy and material from the riparian zone have large impacts on recipient stream habitats. Human-induced changes, such as deforestation, may profoundly affect these pathways. However, the strength of individual factors on stream ecosystems is poorly understood since the factors involved often interact in complex ways. We isolated two of these factors, manipulating the flux of terrestrial input and the intensity of light in a 2×2 factorial design, where we followed the growth and diet of two size-classes of brown trout (Salmo trutta) and the development of periphyton, grazer macroinvertebrates, terrestrial invertebrate inputs, and drift in twelve 20 m long enclosed stream reaches in a five-month-long experiment in a boreal coniferous forest stream. We found that light intensity, which was artificially increased 2.5 times above ambient levels, had an effect on grazer density, but no detectable effect on chlorophyll a biomass. We also found a seasonal effect on the amount of drift and that the reduction of terrestrial prey input, accomplished by covering enclosures with transparent plastic, had a negative impact on the amount of terrestrial invertebrates in the drift. Further, trout growth was strongly seasonal and followed the same pattern as drift biomass, and the reduction of terrestrial prey input had a negative effect on trout growth. Diet analysis was consistent with growth differences, showing that trout in open enclosures consumed relatively more terrestrial prey in summer than trout living in covered enclosures. We also predicted ontogenetic differences in the diet and growth of old and young trout, where we expected old fish to be more affected by the terrestrial prey reduction, but we found little evidence of ontogenetic differences. Overall, our results showed that reduced terrestrial prey inputs, as would be expected from forest harvesting, shaped differences in the growth and diet of the top predator, brown trout.     

Short-term study testing the resilience of an estuarine bivalve to macroalgal mats

Increased mortality of juvenile Atlantic salmon (Salmo salar L.), related to lowered levels of stored energy following the loss of ice cover during winter, has been observed after hydropower development in the subarctic River Alta, northern Norway. Drift samples were compared to examine if drift densities, and thus drift prey availabilities for juvenile salmon, were lower in the ice-free than the ice-covered area. In addition, juvenile salmon stomach contents were compared to benthos and drift in the ice-free area to examine salmon winter feeding habitat. Zooplankton, originating from the reservoir, dominated drift at the ice-free site but had lower densities at the downstream ice-covered site. Excluding zooplankton, Chironomidae comprised most of the remaining drift at both the ice-free and ice-covered site, followed by Ephemeroptera, Plecoptera and Simuliidae. No Trichoptera were found in the drift samples. There was no consistent diel periodicity in drift. Benthos was dominated by Chironomidae, followed by Ephemeroptera, Plecoptera and Trichoptera. Other invertebrates occurred in low numbers. Juvenile salmon demonstrated size-selective feeding and fed mainly on Ephemeroptera, followed by Trichoptera and Plecoptera. No zooplankton and few Chironomidae were found in the stomach samples. Stomach content was more similar to benthos than to drift, indicating a larger extent of benthic than drift feeding. No evidence was found for the hypothesis that lack of ice cover reduced the invertebrate drift or caused diel periodicity in the drift. Differences in drift between areas with and without ice could not account for the observed differences in mortality of juvenile salmon during the winter in these areas.     

Ontogenetic shifts in drift periodicity and benthic dispersal in elmid beetles

1. There is a paucity of information on ontogenetic changes in the dispersal of benthic invertebrates, which is an important aspect of their ecology. This study quantifies ontogenetic changes in diel periodicity in drift, and in upstream–downstream dispersal on the substratum for Elmis aenea, Oulimnius tuberculatus, Esolus parallelepipedus and Limnius volkmari (Coleoptera: Elmidae). Three drift nets were emptied every 3 h over 24 h in each month (October 1965–December 1968) at two contrasting sites: one in a deep section with abundant macrophytes, the other in a shallow stony riffle. Comparisons of periodicity between life‐stages of the same species were limited to months when numbers in the drift were highest. Dispersal was evaluated in six experimental stream channels, placed above the stream, with initial numbers of each life‐stage varying from 20 to 80. 2. Drift numbers were always highest at night with few or no animals in the day samples. Ontogenetic shifts in diel periodicity were similar for all four species. Drift catches were similar throughout the night for the early and intermediate larval instars and for mature adults, but were highest in the early hours of the night with a gradual decline thereafter for later larval instars and immature adults. These patterns were unaffected by a severe spate, even though drift numbers increased considerably. 3. Dispersal was density‐independent; the number of dispersing animals was a constant proportion of the initial number for each life‐stage. The relationship between dispersal distance and the number of animals travelling that distance was well described by an inverse power function. Median and maximum distances (m day^?1) were estimated for each life‐stage. 4. Ontogenetic shifts in dispersal in the stream channels matched those shown in diel drift periodicity. For all four species, the later larval instars and immature adults showed little movement in either direction, whereas early and intermediate larval instars and mature adults dispersed predominantly upstream, adults travelling further than any other life‐stage. 5. Ontogenetic shifts in diel drift periodicity and dispersal were related to seasonal changes in drift density and critical periods in the life cycle. Such shifts have not been quantified in other stream invertebrates, but should be considered when evaluating the role of dispersal in their population dynamics and their colonization ability.