Summer habitat use and feeding of juvenile Arctic charr,Salvelinus alpinus,in the Canadian High Arctic

Abstract – Owing to limited knowledge of the habitat use and diet of juvenile Arctic charr from the High Arctic, particularly young‐of‐the‐year (YOY), we assembled data obtained from samples taken in and around Lake Hazen, Nunavut, Canada, to assess juvenile habitat use and feeding. Juvenile charr demonstrated a preference for stream environments, particularly those fed by warm upstream ponds. Charr occupying both stream and nearshore lake habitats were found to feed similarly, with chironomids occurring most frequently in diets. Some older stream‐dwelling charr preyed on smaller, younger Arctic charr. Preferred stream occupancy is likely mediated by physical barriers created mainly by water velocity, and by distance from the lake, lake‐ice dynamics, low water depth, and turbidity. Water velocities resulted in stream habitat segregation by size, with YOY mainly found in low‐velocity pools and back eddies adjacent to stream banks, but not in water velocities >0.1 m·s^?1. Greatest charr densities in streams were found in small, shallow, slow‐flowing side channels, which are highly susceptible to drought. Under predicted climate change scenarios, streams fed by small ponds will be susceptible to intermittent flow conditions, which could result in increased competition among juvenile charr for the remaining stream habitats. In addition, glacier‐fed streams are likely to experience increased flow conditions that will exacerbate physical barriers created by water velocity and further reduce the availability of preferred stream habitat.     

The influence of predation risk on habitat selection and food intake by Arctic charr, Salvelinus alpinus (L.)

Abstract— Large piscivorous fish are assumed to affect habitat selection and food intake of prey fish. To study the effects of cannibalistic Arctic charr, Salvelinus alpinus (L.), on smaller stunted charr, we sampled the prey fish in littoral and pelagic habitats using gill nets, before and shortly after the release of large charr in a small lake (0.52 km²). In the habitats where the risk of predation was highest, the catch per unit effort de creased from 13.3 to 4.8 fish per 100 m² of gillnet after release of pred ators. The large decrease in numbers of charr < 18 cm corresponded with the predicted vulnerable prey sizes, according to a model based on the size distribution of predators. The occurrence of planktivorous fish and weight-specific food intake decreased in the high risk habitat and remained unaffected in the low risk habitats. Changes in the food intake of prey fish could not be explained in terms of fish length, indicating that prey fish changed diet when the risk of predation was high.     

Habitat use, size and age structure in sympatric brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) stocks: resistance of populations to change following harvest

Abstract– Habitat use and population dynamics in brown trout Salmo trutta and Arctic charr Salvelinus alpinus were studied in an oligotrophic lake over a period of 10 years. Previous studies showed that the species segregated by habitat during summer. While brown trout occupied the surface water down to a depth of 10 m, Arctic charr were found deeper with a maximum occurrence at depth 10–15 m. Following the removal of a large number of intermediate sized fish in 1988–89, habitat segregation between the species broke down and Arctic charr were found in upper waters, while brown trout descended to deeper waters. The following year, both species were most frequently found in surface waters at depths of 0–5 m. During the last four years, the species reestablished their original habitat segregation despite another removal experiment of intermediate-sized fish in 1992–1994. The removal of fish resulted in an increased proportion of large (≥ 25 cm) fish in both species. Furthermore, the charr stock responded by reduced abundance and increased size-at-age. The results revealed plasticity and strong resistance to harvest populations of brown trout and Arctic charr. This is probably due to internal mechanisms of intraspecific competition within each population, which result in differential mortality among size classes.     

The effects of wood on stream habitat and native fish assemblages in New Zealand

Historic deforestation has deprived many river systems of their natural wood loadings. To study the effects of the loss of wood from waterways, a field trial was conducted in three small forested streams in New Zealand. The objectives were to (i) examine differences in fish assemblages among wooded pools (where wood provided cover), open pools and riffles and (ii) measure the effects of wood removal on channel morphology and fish assemblages. In the first part of the study, no significant differences were found in total fish density among the three habitats. However, total fish biomass was significantly higher in wooded pools (64% of total fish biomass) compared with open pools and riffles. Mean density and biomass of banded kokopu (Galaxias fasciatus) and mean biomass of longfin eel (Anguilla dieffenbachii) were highest in wooded pools, whereas the density and biomass of bluegill bully (Gobiomorphus hubbsi) and torrentfish (Cheimarrichthys fosteri) were highest in riffles. In the second part of the study, wood was removed from a 200‐m section (treatment) in each stream, significantly reducing pool area and increasing the proportion of channel area and length in riffles. At the habitat scale, banded kokopu and large longfin eel were the two species mostly affected by wood removal. At the reach scale, banded kokopu biomass was significantly lower in the treatment sections. Although wooded pools were a small portion of total habitat, they provided important habitat for two of New Zealand's larger native fish taxa.     

The effect of planting grass filter strips on fishes in agricultural headwater streams

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Distinct fluvial and adfluvial migration patterns of a relict charr, Salvelinus confluentus, stock in a mountainous watershed, Idaho, USA

Abstract –  Sixty-five large (>385 mm fork length) bull trout ( Salvelinus confluentus ), a threatened relict charr (Family Salmonidae), were captured in the upper East Fork South Fork Salmon River (EFSFSR), Idaho, USA and implanted with radio tags to investigate their spatial and temporal movements and distribution throughout the South Fork Salmon River (SFSR) basin and beyond. All radio-tagged fish were migratory. Most fish displayed a fluvial migration pattern. They typically overwintered in the larger rivers downriver of the EFSFSR (SFSR and the Salmon River further downstream), migrated upriver to the EFSFSR in June and further upriver into small (<2 m wide) tributaries to spawn in August and September. Both consecutive-year and nonconsecutive-year spawners were found. A typical migration distance between the overwintering habitat and the spawning habitat was 100 km. A minor fraction (<10%) of the fish displayed an adfluvial life history pattern, overwintering in a small (2 ha) 60-year-old flooded mine pit in the EFSFSR upstream of the spawning tributaries. The stock exhibited distinct site fidelity for spawning and overwintering. Similar fluvial and adfluvial migration patterns have been reported for bull trout in the region as well as for other charr species worldwide. Effective management of this and other migratory charr stocks will require protection of a wide range of habitats, from large rivers to the smallest tributaries.     

The influence of land cover composition and groundwater on thermal habitat availability for brook charr (Salvelinus fontinalis) populations in the United States of America

Abstract – Brook charr (Salvelinus fontinalis) is a sentinel fish species that requires clean, cold water habitats generally resulting from landscapes that allow for surface water flows devoid of sediment and contaminants and high groundwater discharge of cold water. As such, brook charr are impacted by land cover changes that alter stream temperature regimes. We evaluated brook charr populations across their eastern and midwestern range in the United States with reference to thermal habitat availability in relationship to land cover and per cent baseflow. We found that while forest cover does protect brook charr thermal habitat, high levels of groundwater discharge can allow for increased levels of agriculture within a watershed by keeping the water cold in spite of warm ambient summer temperatures. Our study concludes that with enhanced communication among land, water and fisheries managers, society can provide for sustainable stream salmonid populations despite increased threats on cold water resources.     

Growth variability of brook charr (Salvelinus fontinalis) in coastal maine

Brook charr inhabit a wide variety of habitats and exhibit considerable growth variation within and among populations. We used biannual mark–recapture electrofishing surveys and PIT tags at two streams in coastal Maine to examine spatial and temporal variability in brook charr growth. We documented considerable stream‐to‐stream and year‐to‐year variation in the growth of coastal brook charr. Based on 2892 recaptures between 2006 and 2010, we developed a suite of linear mixed models to examine variation in individual growth rates at Stanley Brook as a function of environmental variables. We distilled physical habitat data from surveys in 2010 into principle components for entry into the growth model. Growth was related primarily to an interaction between season and body length, mean water temperature, and instream location. Growth rates were highest in the summer, and smaller individuals grew more rapidly than larger conspecifics in the same stream. Individuals using habitats closer to the head of tide grew faster than those in upstream locations. Physical habitat variation was not an important control of individual growth variation within Stanley Brook. We suggest local variability in growth rates should be considered when developing management strategies for coastal populations.     

Food composition, habitat use and growth of stocked and native Arctic charr, Salvelinus alpinus, in Lake Muddusjärvi, Finland

Habitat use, growth and food composition of native and stocked Arctic charr, Salvelinus alpinus (L.), were studied in the subarctic Lake Muddusjärvi, northern Finland, to investigate reasons for poor stocking success. Samples were collected with pelagic and epibenthic gill nets. Stocked and native charr occurred in similar epibethic habitats, whereas pelagic habitat was avoided. Native charr grew fast after shifting to piscivory. Growth rate of stocked charr was slow because only a small proportion of stocked fish became piscivorous during the first year after stocking. During the first lake year, stocked charr divided into slow-growing planktivores and fast-growing piscivores. Piscivorous stocked and native charr consumed only whitefish, Coregonus lavaretus (L.), as their prey. Small-sized (<10 cm) whitefish were preferred when shifting to piscivory.     

Does embeddedness affect growth performance in juvenile salmonids? An experimental study in brown trout,Salmo trutta L.

Abstract –  The effect of an embedded substratum on emigration and growth in juvenile brown trout was investigated in an artificial stream with sand added to produce sections of embedded or nonembedded substratum. Fish were allowed to leave the sections and were caught daily in a downstream trap. After catching and counting, fish were put back in their original section in order to keep the same amount of fish. Captures were high only on the first days after fish release. During the first 6 days after fish release, downstream-moving fish were more numerous in the embedded than in the nonembedded sections. The embedded substratum significantly decreased the final mean body weight and condition factor and increased heterogeneity in fish size. We suggest that a decrease in the habitat carrying capacity for juveniles could be the main factor explaining this result.     

Determining the consistency of thermal habitat segregation within and among Arctic charr morphotypes in Gander Lake,Newfoundland

Abstract – Otolith carbon and oxygen isotope data obtained from distinct genetic and ecological groups of lacustrine Arctic charr, Salvelinus alpinus L., from Gander Lake, Newfoundland, were used to examine hypotheses regarding the consistency of differential habitat use among the groups. Results indicated thermal habitat separation by group, with small ‘pale’ individuals consistently remaining in cooler profundal habitats and larger ‘dark’ individuals more frequently occupying warmer upper water column habitats. Theoretical measures of resource separation and competition indicated lower thermal habitat overlap among the forms and greater within‐form competition. Depth at capture data indicated more varied short‐term use of available lake habitats by ‘dark’ form Arctic charr, possibly as a result of cannibalistic foraging on profundal ‘pale’ form fish. Nominal capture depth data only partially explained observed variation in the mean temperature of occupied thermal habitat, suggesting that capture depth can only be used as a rough index of thermal habitat use. Provided that sufficient thermal gradients exist in the environments being studied, otolith oxygen isotopes provide a useful means of establishing the significance of niche differentiation among individuals.     

Residency and movement of stream-dwelling Japanese charr, Salvelinus leucomaenis, in a central Japanese mountain stream

Abstract – The residency and movement of stream-dwelling adult (2+ and older) Japanese charr, Salvelinus leucomaenis , were studied by mark and recapture experiments in the Jadani Stream, a headwater tributary of the Tedori River, central Japan, from 1986 to 1989. Of the marked fish, 31.3–58.3% were recaptured in the same pools where they had been caught during the study periods of summer (June–August), autumn (August–November) and winter–spring (November to the next June) and no seasonal movement was observed. The mean distances that the fish moved during the study periods ranged from 139.0 to 502.3 m and many movements longer than 1000 m were observed. Between the resident and the moved fish, there was little difference in body length, growth rate or sex. For the fish that had been resident in the same pools and riffles at previous recaptures, most fish were recaptures in the same locations at the next recapture. Conversely, for the fish that moved previously, most fish were recaptured in different locations from previous sites at the next recapture. These results suggest that Japanese charr exhibit relatively high residency throughout the year, but many fish moved longer distance. The results also suggest the presence of static and mobile components in the charr population.     

How fast is too fast? Water velocity differentially affects growth of four Gila River,native cyprinids

Understanding trade-offs associated with occupying various aquatic habitats provides a mechanistic understanding of habitat needs that can be used to evaluate the consequences of habitat loss or alteration. We used instream enclosures and field observations to identify how velocity affects the growth rates of four native species in the upper Gila River basin: longfin dace (Agosia chrysogaster) and speckled dace (Rhinichthys osculus), two species of no conservation concern, and loach minnow (Tiaroga cobitis) and spikedace (Meda fulgida), two federally endangered species. Elevated velocity was predicted to increase food delivery through drift or stimulation of benthic primary production. Energetic costs of high-velocity habitat were predicted to vary with morphology and behaviour and be lowest for speckled dace and loach minnow because they are adapted to occupy interstitial spaces of the substrate in riffles. Spikedace and longfin dace should perform best in moderate velocities, where the trade-off between exposure to drifting macroinvertebrates outweighs the energetic costs of maintaining position in the water column. Growth rates of loach minnow and speckled dace increased in higher velocities, but contrary to our initial predictions, spikedace growth rates also increased in high-velocity habitats while longfin dace grew fastest in low-velocity habitats; similar to the locations these species occupied based on field observations. These results indicate that for spikedace, the increased abundance of drifting macroinvetebrates in high-velocity habitats outweighs the energy expenditure, but for longfin dace the energetic costs of occupying moderate to high-velocity habitats outweigh the benefit to increased food availability. Our experiment provides a mechanistic understanding of habitat requirements across species and may inform predictions on how modifications or restoration of riverine ecosystems influence native fish diversity.     

Land cover,riparian zones and instream habitat influence stream fish assemblages in the eastern Amazon

The Amazon rainforest has experienced rapid land‐use changes over the last few decades, including extensive deforestation that can affect riparian habitats and streams. The aim of this study was to assess responses of stream fish assemblages to deforestation and land cover change in the eastern Amazon. We expected that percentage of forest in the catchment is correlated with local habitat complexity, which in turn determines fish assemblage composition and structure. We sampled 71 streams in areas with different land uses and tested for relationships between stream fish assemblages and local habitat and landscape variables while controlling for the effect of intersite distance. Fish assemblage composition and structure were correlated with forest coverage, but local habitat variables explained more of the variation in both assemblage composition and structure than landscape variables. Intersite distance contributed to variance explained by local habitat and landscape variables, and the percentage of variance explained by the unique contribution of local habitat was approximately equivalent to the shared variance explained by all three factors in the model. In these streams of the eastern Amazon, fish assemblages were most strongly influenced by features of instream and riparian habitats, yet indirect effects of deforestation on fish assemblage composition and structure were observed even though intact riparian zones were present at most sites. Long‐term monitoring of the hydrographic basin, instream habitat and aquatic fauna is needed to test for potential legacy effects and time lags, as well as assess species responses to continuing deforestation and land‐use changes in the Amazon.     

Habitat use and foraging success of 0+ pike (Esox lucius L.) in experimental ponds related to prey fish, water transparency and light intensity

Abstract  – This study investigated the habitat use of 0+ pike (9–17 cm) in relation to two different water transparency regimes (clear water/chlorophyll water), two different light regimes (day/night) and the presence/absence of prey using 16 m² experimental ponds. Pike could freely choose between two structured habitats (a simple structured and a complex structured), an interface habitat (between the structured habitats and open water) and an open water habitat. Foraging success of the pike in relation to water transparency was investigated by comparing mean condition (Fultons K ) of the pike as well as the number of surviving prey fish. Habitat use was influenced by the presence/absence of prey and varied between waters with different transparency. The presence of prey intensified the use of structural habitats of 0+ pike in both clear and chlorophyll waters. A preference for complex habitats was found in clear water and was presumably related to foraging. The pike in chlorophyll water, in contrast, appeared more evenly distributed among all habitats, as illustrated by a more intensive use of open water in chlorophyll water compared to the clear water. No detectable impact of water transparency on the foraging success of 0+ pike was found.     

Spatiotemporal patterns of mountain whitefish (Prosopium williamsoni) in response to a restoration of longitudinal connectivity

To examine the role of longitudinal connectivity on the spatial and temporal dynamics of mountain whitefish (Prosopium williamsoni), we quantified movement and population dynamics following installation of the Landsburg Dam fishway, Cedar River, WA, USA. Mountain whitefish is widely distributed, poorly studied and not the focus of restoration. Before the fishway, mountain whitefish were not observed above the dam. Here, we focus on snorkel counts collected at reach and mesohabitat (e.g. pools) scales over 11 summers on the 20‐km above‐dam segment following restoration. A camera within the ladder provided number, size and movement timing, thereby informing on behaviour and recolonisation. Segment‐scale abundance increased following fish passage reaching an asymptote in 7 years, and mountain whitefish were detected throughout the main stem in 10 years. Annual movement through the ladder increased over time and was positively correlated with instream abundance and discharge, but negatively correlated with water temperature. About 60% of fish movements occurred in spring and early summer, potentially for foraging opportunities. Reach‐scale abundance peaked between 7 and 10 km from the dam; deep, cool (~10.6 to 11.6°C) conditions characterised these reaches. At the mesohabitat scale, mountain whitefish detection increased with depth and velocity after accounting for distance from the dam. Our results show how restoring longitudinal connectivity allowed this nontarget species to colonise newly available habitat. Their response supports the critical roles of longitudinal connectivity and environmental conditions, that manifest at different spatial scales, in dictating how freshwater fish respond to habitat disturbance.     

Is the winter period a severe bottleneck of anadromous riverine Arctic charr parr?

Abstract –  Winter growth and survival of wild individually tagged juvenile Arctic charr (1+ age) from a riverine anadromous stock, were studied in a small ice-covered (∼6 months) ground water brook (temperature ∼1 °C) connected to Skibotnelva in subarctic northern Norway. The overall winter survival was estimated to be 68% from late October 2005 to early May 2006. The recaptured charr were not significantly different in initial length or weight compared with the nonrecaptured fish suggesting low size-dependent mortality. The majority (98%) of the recaptured charr showed significant increase in size, with a mean increment of 62% from their initial bodyweight and 12% of the charr parr had more than doubled their weight. The mean specific growth rate was calculated to be slightly lower (0.27) than estimated values from a growth model (∼0.35). In addition, the condition factor increased significantly during the field experiment. These results are the first individual growth data on riverine anadromous Arctic charr parr under natural winter conditions, and indicate that charr can grow relatively fast during periods with low temperature and also that ground water brooks can be good over-wintering habitat for juvenile Arctic charr. These results suggest that the winter period is perhaps a less severe bottleneck than previously recognised for the cold-adapted Arctic charr.     

The influence of instream cover and predation risk on microhabitat selection of stone loach Barbatula barbatula (L.)

Abstract— The microhabitat selection of stone loach ( Barbatula barbatula ) in relation to instream cover and predation risk was investigated in artificial streams. In one experiment, stone loach were presented different combinations of shelter structures, two at a time, that provided visual isolation, flow refuge, both, or none. In all cases, visual isolation shelters were selected by the fish, with little use of clear shelters and no apparent selection of flow refuges. In a second experiment, diel activity patterns and habitat selection of stone loach were measured when visual isolation structures were placed in the riffle only, pool only, both riffle and pool or in no habitat. Stone loach were marked with passive integrated transponder tags and a sensor plate was placed between the riffle and pool habitats to measure diel activity patterns. Habitat use was measured by releasing drop gates between the two habitats at the end of each trial. More stone loach used the pool than the riffle in all treatments, but use of the riffle increased when cover was present only there. Furthermore, stone loach were most active between 2100 and 0300. To determine the effect of predation threat on habitat use by stone loach, one adult brown trout ( Salmo trutta ) was added to each pool. The presence of trout caused stone loach to move into the riffle, especially the smaller loach. Overall, the study showed that stone loach used cover in a visual isolation context, preferring deeper water when there was no predation risk, but made more use of shallower water in the presence of brown trout.     

Temporary pools provide stability to fish assemblages in Amazon headwater streams

Temporal variations in the environment may function as disturbances that affect population processes and change the structure and composition trajectories of communities. The presence of refuges may buffer such variations and also support the recovery of communities after disturbances. In this study, we tested the effect of temporary pools as seasonal refuges for the ichthyofauna in upland forest streams in the Brazilian Amazon. We sampled environmental characteristics and fish in five occasions over one seasonal cycle in eight stream stretches and their adjacent temporary pools in Reserva Ducke, Manaus. We recorded 27 fish species, 13 of which occurred in both habitats and represented 81% of the total fish abundance in the stream–pool system (3695 records of fish, 54% made in pools). During rainy periods, fish abundance in pools was higher than in the channels. Total number of records varied inversely among habitats over time with a reduction in the pools and an increase in the channels as the pools gradually desiccated. The number of species exhibited a similar pattern. When data from pools and channels were pooled, representing the entire flood zone, fluctuations in all the measured variables along the time were smoothed. The variability in fish assemblage composition was associated with variability in the volume of water in the entire system. These results indicate that temporary pools are an important habitat component of upland forest streams, providing resources and spatial refuges for the fish during rainy periods, and regulating the stability of these communities.     

Effects of habitat and internal prey subsidies on juvenile coho salmon growth: implications for stream productive capacity

Abstract –  To evaluate the effects of habitat, foraging strategy (drift vs. limnetic feeding) and internal prey subsidies (downstream transport of invertebrate drift between habitats) on fish production, we measured the growth of juvenile coho salmon confined to enclosures in flowing (pond inlets and outlets) or standing water (centre of pond) habitats in a constructed river side-channel. The effects of habitat and foraging strategy on fish growth were mediated primarily through habitat effects on prey abundance. Invertebrate drift biomass was nearly an order of magnitude higher at pond inlets relative to outlets. Drift-feeding coho in inlet enclosures grew 50% faster than drift-feeding coho at pond outlets or limnetic feeding coho in the centre of ponds, suggesting that elevated drift at inlets was sufficient to account for higher inlet growth rates. Forty per cent of prey biomass in stomachs was terrestrial in origin. These results indicate that, in addition to dependence on external terrestrial subsidies, streams with alternating slow and fast water (i.e., pool-riffle) sequences are also characterised by internal prey subsidies based on transport of drifting invertebrates from refuge habitats (high velocity riffles) to habitats more suitable for drift-feeding predators (e.g., pools), which may result in higher maximum fish growth in systems where internal subsidises are large. Restoration of small streams to maximise productive capacity for pool-rearing salmonids will require a better understanding of the length and interspersion of habitats that maximises both internal prey subsidies and available rearing habitat for juvenile salmon.