The loss of large wood affects rocky mountain trout populations

Western U.S. rivers are currently influenced by legacy effects of reduced large wood (LW) loading and retention that has substantially reduced in‐stream habitat complexity. Large wood is typically associated with streams in undisturbed old‐growth forest and in the correct geomorphic context can drastically alter stream and valley habitat complexity. Streams with LW are typically multichannel and depositional, while streams lacking LW, due to relatively recent wildfire or logging (<200 years ago), are usually single channelled and erosional. We compared population biomass and individual growth rates of Brook Trout Salvelinus fontinalis in streams across a gradient of wood volumes. At both the square metre and valley length scales, standing stock biomass of aquatic invertebrates was the best predictor of trout biomass. However, at the valley scale, the number of pools was important in predicting trout biomass in combination with standing stock biomass of aquatic invertebrates. Individual growth rates of age‐1 Brook Trout were negatively affected by increasing density; however, growth rates for the largest and smallest individuals at each site were unaffected by density. Our results suggest the pool habitat created by LW acts synergistically with prey availability to dramatically increase trout populations. However, in streams lacking LW, negative effects of detrimental land use practices have persisted >100 years, suggesting that recovering lost animal production in mountain stream networks will only occur at decadal to century time scales.     

Effect of fine wood on juvenile brown trout behaviour in experimental stream channels

In‐stream wood can increase shelter availability and prey abundance for stream‐living fish such as brown trout, Salmo trutta, but the input of wood to streams has decreased in recent years due to harvesting of riparian vegetation. During the last decades, fine wood (FW) has been increasingly used for biofuel, and the input of FW to streams may therefore decrease. Although effects of in‐stream FW have not been studied as extensively as those of large wood (LW), it is probably important as shelter for small‐sized trout. In a laboratory stream experiment, we tested the behavioural response of young‐of‐the‐year wild brown trout to three densities of FW, with trout tested alone and in groups of four. Video recordings were used to measure the proportion of time allocated to sheltering, cruising and foraging, as well as the number of aggressive interactions and prey attacks. Cruising activity increased with decreasing FW density and was higher in the four‐fish groups than when fish were alone. Foraging decreased and time spent sheltering in FW increased with increasing FW density. Our study shows that juvenile trout activity is higher in higher fish densities and that trout response to FW is related to FW density and differs from the response to LW as reported by others.     

Experimental provision of large woody debris in streams as a trout management technique

• 1. The natural stock of large woody debris (LWD) in the afforested Douglas River (Fermoy, Co. Cork) is very low relative to old‐growth forests, which seems to arise from deficiency both of supply and retention. Woody debris is important to the ecology and physical structure of forest streams, so its abundance is relevant to aquatic conservation and the maintenance and size of salmonid fish stocks.
• 2. The physical characteristics and fish stocks of 16 contiguous segments of two 200 m stream reaches were surveyed in spring 1998 prior to the installation of 12 partially spanning debris structures on four of the segments. This study investigated the effect of debris structures on the heterogeneity of flow and substratum, and the distribution of brown trout (Salmo trutta), and assessed the potential use of woody debris manipulation as a tool in the management of forest streams.
• 3. Surveys of stream habitat conditions over a 2 year period following the installation of woody debris showed a change in stream architecture. This created more suitable habitat for trout through development of additional pools in which beds of fine sediment developed, and constraining the main current, increasing the amount of eddies and slack water areas.
• 4. There were significant increases in trout density and biomass in the debris segments relative to control segments without debris dams 1 and 2 years after debris addition, although trout condition was not modified by the addition of LWD. These results suggest that the addition of woody debris offers a positive and practical management technique for enhancing fish in plantation forest streams.

Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Broad‐scale habitat classification variables predict maximum local abundance for native but not non‐native trout in New York streams

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Effects of reduced summer flows on the brook trout population and temperatures of a groundwater‐influenced stream

Withdrawal of water from streams and groundwater is increasing in Midwestern North America and is a potential threat to coldwater fishes. We examined the effects of summer water withdrawals on brook trout Salvelinus fontinalis populations and water warming rates by diverting 50–90% of summer baseflow from a 602‐m treatment zone (TZ) in a groundwater‐influenced Michigan stream during 1991–1998. We compared density of brook trout in fall, and spring‐to‐fall growth and survival of brook trout, between the TZ and an adjacent reference zone (RZ) whose flows were not altered. Flow reductions had no significant effects on the density of brook trout in fall or spring‐to‐fall survival of brook trout. However, spring‐to‐fall growth of brook trout in the TZ declined significantly when 75% flow reductions occurred. Cold upstream temperatures and the relatively short study reach kept thermal habitat conditions excellent for brook trout in the TZ throughout the dewatering experiments. These findings suggest that brook trout can tolerate some seasonal loss of physical habitat if temperature conditions remain suitable. In summer 1999, we experimentally assessed the influence of flow reduction on the warming rate through the TZ by diverting from 0% to 90% of flow around the TZ in 3‐ or 4‐day trials on a randomised schedule. Average daily temperature increased exponentially as stream flows declined from normal summer levels. Our findings suggest the risk of trout habitat loss from dewatering is potentially large and proportional to the magnitude of withdrawal, especially as thermal conditions approach critical levels for trout.     

Seasonal movement,growth and survival of brook trout in sympatry with brown trout in Midwestern US streams

Seasonal patterns in growth, survival and movement of brook trout Salvelinus fontinalis were monitored in two southeastern Minnesota streams divided into study reaches based on brown trout Salmo trutta abundance. We estimated survival and movement while testing for effects of stream reach and time using a multistrata Cormack–Jolly–Seber model in Program MARK. Multistrata models were analysed for three age groups (age‐0, age‐1 and age‐2+) to estimate apparent survival, capture probability and movement. Survival varied by time period, but not brown trout abundance and was lower during flood events. Age‐0 brook trout emigrated from reaches with low brown trout abundance, whereas adult brook trout emigrated from downstream brown trout‐dominated reaches. Growth was highest in spring and summer and did not differ across streams or reaches for the youngest age classes. For age‐2+ brook trout, however, growth was lower in reaches where brown trout were abundant. Interspecific interactions can be age or size dependent; our results show evidence for adult interactions, but not for age‐0. Our results suggest that brook trout can be limited by both environmental and brown trout interactions that can vary by season and life stage.     

Country‐wide analysis of large wood as a driver of fish abundance in Swedish streams: Which species benefit and where?

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Restoration of dead wood in Basque stream channels: effects on brown trout population

Abstract – Streams in the Aiako Harria Natural Park (Basque Country, Spain) have excellent water quality, but are physically impoverished after centuries of snagging. In an attempt to restore channel complexity and ecosystem functioning, especially in‐channel retention of sediments and organic matter, large woody debris (LWD) was introduced into four mountain streams (channel width 3–13 m) following a before/after, control/impact (BACI) design. Logs were introduced by means of hand‐held machinery and located uncabled, mimicking the natural amount and disposition of LWD in streams. Floods disrupted most of the structures at the large stream, but caused little damage to those in the small ones. Only minnow and brown trout inhabit in the area. Before wood addition, trout densities were fairly high in the small streams, low in the large one, where recruitment seemed very poor. In the small tributaries, trout populations showed a strong imbalance towards young fish, adults being only found in the spawning season. Wood addition produced some interesting trends in trout, although statistical significance was low as a result of large environmental variability. Fish densities showed small changes, but biomass increased, especially in the spawning season. Also, there was a trend towards more aged 2⁺ or larger, thus suggesting wood addition improved adult habitat. Although restoring LWD is extremely unusual in Spain, the changes in physical habitat and the trends in fish populations detected in the present project suggest it is worth making more experiments, at least in safe settings where there is no risk of flooding or damaging properties.     

How deforestation drives stream habitat changes and the functional structure of fish assemblages in different tropical regions

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Stream bed organic carbon and biotic integrity

• 1. Allochthonous carbon is the basis of the detrital food web in low‐order, warmwater stream ecosystems, and stream‐bed sediments typically function as carbon reservoirs. Many of the same factors that govern carbon input and storage to streams (e.g. riparian vegetation, large wood, heterogeneous boundaries) have also been identified as key attributes of stream fish habitat.
• 2. Effects of channel incision on sand‐bed stream carbon reservoirs and indices of biological integrity (IBIs) based on fish collections were examined for four streams exhibiting a range of incisement in northern Mississippi. Observed mean C concentrations (mass percentage) ranged from 0.24±0.36% for a non‐incised stream to only 0.01±0.02% for a severely incised channel, and were not correlated with large wood (LW) density, perhaps because LW density at one site was elevated by a habitat rehabilitation project and at another site by accelerated inputs from incision‐related riparian tree fall. Fish IBI was positively correlated with bed C (r=0.70, p=0.003), and IBIs for reference streams were more than 50% greater than those computed for the most severely degraded sites.
• 3. More testing is needed to determine the efficacy of stream bed C as an indicator, but its importance to warmwater stream ecosystems, and the importance of covarying physical and hydrologic conditions seems evident.

Published in 2008 by John Wiley & Sons, Ltd.     

Use of first‐order tributaries by brown trout (Salmo trutta) as nursery habitat in a cold water stream network

Many investigators have examined the importance of suitable in‐stream habitat and flow regime to salmonid fishes. However, there is much less known about the use of small (<5 l·s^?1 discharge) first‐order streams within a larger stream network by salmonids. The purpose of this study was to evaluate the use of small headwater streams by juvenile brown trout Salmo trutta in the Emmons Creek stream network in Wisconsin, USA, and to determine whether abundance was related to habitat variables in these streams. Fishes in eight spring‐fed first‐order streams were sampled during a 7‐month period using a backpack electroshocker and measured for total length. Habitat variables assessed included stream discharge, water velocity, sediment composition and the abundance of cover items (woody debris and macrophytes). Densities of YOY trout ranged from 0 to 1 per m² over the course of the study and differed among first‐order streams. Stepwise multiple regression revealed discharge to be negatively associated with trout density in spring but not in summer. All other habitat variables were not significantly related to trout density. Our results demonstrate the viability of small first‐order streams as nursery habitat for brown trout and support the inclusion of headwater streams in conservation and stream restoration efforts.     

Distribution modelling to guide stream fish conservation: an example using the mountain sucker in the Black Hills National Forest,USA

• 1. Conservation biologists need tools that can utilize existing data to identify areas with the appropriate habitat for species of conservation concern. Regression models that predict suitable habitat from geospatial data are such a tool. Multiple logistic regression models developed from existing geospatial data were used to identify large‐scale stream characteristics associated with the occurrence of mountain suckers (Catostomus platyrhynchus), a species of conservation concern, in the Black Hills National Forest, South Dakota and Wyoming, USA.
• 2. Stream permanence, stream slope, stream order, and elevation interacted in complex ways to influence the occurrence of mountain suckers. Mountain suckers were more likely to be present in perennial streams, and in larger, higher gradient streams at higher elevations but in smaller, lower gradient streams at lower elevations.
• 3. Applying the logistic regression model to all streams provided a way to identify streams in the Black Hills National Forest most likely to have mountain suckers present. These types of models and predictions can be used to prioritize areas that should be surveyed to locate additional populations, identify stream segments within catchments for population monitoring, aid managers in assessing whether proposed forest management will potentially have impacts on fish populations, and identify streams most suitable for stream rehabilitation and conservation or translocation efforts.
• 4. When the effect of large brown trout (Salmo trutta) was added to the best model of abiotic factors, it had a negative effect on the occurrence of mountain suckers. Negative effects of brown trout on the mountain sucker suggest that management of recreational trout fisheries needs to be balanced with mountain sucker conservation in the Black Hills. However, more spatially explicit information on brown trout abundance would allow managers to understand where the two species interact and where recreational fisheries need to be balanced with fish conservation.

Copyright © 2008 John Wiley & Sons, Ltd.     

Projected impacts of climate change on stream salmonids with implications for resilience‐based management

The sustainability of freshwater fisheries is increasingly affected by climate warming, habitat alteration, invasive species and other drivers of global change. The State of Michigan, USA, contains ecologically, socioeconomically valuable coldwater stream salmonid fisheries that are highly susceptible to these ecological alterations. Thus, there is a need for future management approaches that promote resilient stream ecosystems that absorb change amidst disturbances. Fisheries professionals in Michigan are responding to this need by designing a comprehensive management plan for stream brook charr (Salvelinus fontinalis), brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) populations. To assist in developing such a plan, we used stream‐specific regression models to forecast thermal habitat suitability in streams throughout Michigan from 2006 to 2056 under different predicted climate change scenarios. As baseflow index (i.e., relative groundwater input) increased, stream thermal sensitivity (i.e., relative susceptibility to temperature change) decreased. Thus, the magnitude of temperature warming and frequency of thermal habitat degradation were lowest in streams with the highest baseflow indices. Thermal habitats were most suitable in rainbow trout streams as this species has a wider temperature range for growth (12.0–22.5 °C) compared to brook charr (11.0–20.5 °C) and brown trout (12.0–20.0 °C). Our study promotes resilience‐based salmonid management by providing a methodology for stream temperature and thermal habitat suitability prediction. Fisheries professionals can use this approach to protect coldwater habitats and drivers of stream cooling and ultimately conserve resilient salmonid populations amidst global change.     

Are native brook charr and introduced rainbow trout differentially adapted to upstream and downstream reaches?

Abstract— Due to species introductions, brook charr (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) occur together in many North American streams and typically exhibit a pattern of distribution in which brook charr numerically dominate headwaters and rainbow trout dominate downstream reaches. It has been suggested that 1) the two species compete or 2) the two species do not compete because they are differentially adapted to environmental conditions found in upstream and downstream zones. We assessed whether there were differences in growth and macrohabitat (pool, run and riffle) selection of brook charr and rainbow trout in upper, middle and lower stream zones of a small Pennsylvania stream. Brook charr and rainbow trout placed in replicate paired enclosures set in upstream and downstream reaches showed no significant differences in growth and survival rates upstream, but brook charr had significantly greater growth rates than rainbow trout downstream. Enclosed fish and free-ranging fish both had negative growth rates during the summer. Enclosed fish lost significantly less weight than free-ranging fish. Instantaneous growth rates of free-ranging adult brook charr and rainbow trout from May to August were negative for both species in all stream zones. Underwater observations of adult brook charr and rainbow trout showed both species occurred significantly more often in pool macrohabitats than expected on the basis of macrohabitat availability, except for rainbow trout in the upstream zone. The proportion of pool macrohabitat was not significantly different among stream zones. Brook charr do not appear to be better adapted to upstream environments in Powdermill Run based on growth, survival and macrohabitat selection during summer. Negative biotic interactions acting along with differential environmental adaptations may explain the pattern of distribution of brook charr and rainbow trout in streams, but long-term transplant experiments with additional life stages will be necessary to examine this hypothesis.     

Spawning salmon increase brook trout movements in a Lake Michigan tributary

Abstract – Salmon and trout have been introduced to many ecosystems worldwide, yet the ecological impacts of salmonid introductions remain poorly understood. We investigated the effects of introduced Pacific salmon on stream‐resident fish in the Great Lakes basin by monitoring the movements of passive integrative transponder (PIT)‐tagged brook trout in a Lake Michigan tributary receiving a salmon spawning run and in a nearby stream not receiving salmon. Coincident with the September arrival of salmon, an estimated 52% of resident brook trout moved >200 m, while no such movements were detected in the nonsalmon stream. After 3–4 days, however, brook trout movement patterns became similar in the two streams, suggesting that salmon effects on brook trout movements were short‐lived. Movements in the salmon stream were predominantly upstream in direction, with 50% of the tagged brook trout travelling from below to above the stationary PIT tag readers during the study. Declining water temperatures with the onset of fall coincided with low levels of brook trout movement at both sites, likely due to relocation for spawning. Increased brook trout movements coincident with the arrival of salmon spawners could be driven by a combination of interference competition and egg consumption. Using a bioenergetics model, we estimated that the energy lost to movement was likely compensated by the energy obtained from consuming salmon eggs. We conclude that salmon spawners can substantially increase brook trout movement, but in our study, these effects were short‐lived and likely negligible from a bioenergetics perspective.     

Relationships between trout habitat use and woody debris in two southern New England streams

Abstract – In-stream habitat was measured and trout density was estimated in Merrick Brook (105 habitat units) and the Tankerhoosen River (135 habitat units), Connecticut to determine relationships between habitat use of brook trout Salvelinus fontinalis and brown trout Salmo trutta and woody debris. In each habitat unit, woody debris was inventoried, and length, width, depth, area, width : depth ratio and undercut bank area were estimated. Trout abundance was estimated by snorkeling. Multiple regression was used to test relationships between trout density and principal components describing habitat unit variables. In Merrick Brook, habitat unit size and shape explained most of the variability in density of brook trout (<130 and ≥130 mm) and brown trout (<150 mm) among habitat units, although principle components describing large woody debris or fine woody debris contributed significantly to variations in density of brook trout (≥130 mm) and brown trout (<150 and ≥150 mm). In the Tankerhoosen River, fine woody debris explained most of the variability in density of brook trout (<130 and ≥130 mm), followed by habitat unit size and shape. Both large woody debris and fine woody debris contributed significantly to variations in density of brown trout (≥150 mm). These results suggest that woody debris is an important component of wild trout habitat above that provided by habitat unit shape and size alone.     

Estimating size‐specific brook trout abundance in continuously sampled headwater streams using Bayesian mixed models with zero inflation and overdispersion

Abstract – We examined habitat factors related to reach‐scale brook trout Salvelinus fontinalis counts of four size classes in two headwater stream networks within two contrasting summers in Connecticut, USA. Two study stream networks (7.7 and 4.4 km) were surveyed in a spatially continuous manner in their entirety, and a set of Bayesian generalised linear mixed models was compared. Trout abundance was best described by a zero‐inflated overdispersed Poisson model. The effect of habitat covariates was not always consistent among size classes and years. There were nonlinear relationships between trout counts and stream temperature in both years. Colder reaches harboured higher trout counts in the warmer summer of 2008, but this pattern was not observed in the cooler and very wet summer of 2009. Amount of pool habitat was nearly consistently important across size classes and years, and counts of the largest size class were correlated positively with maximum depth and negatively with stream gradient. Spatial mapping of trout distributions showed that reaches with high trout counts may differ among size classes, particularly between the smallest and largest size classes, suggesting that movement may allow the largest trout to exploit spatially patchy habitats in these small headwaters.     

Effects of food supply and stream physical characteristics on habitat use of a stream‐dwelling fish

Identifying the environmental factors that affect freshwater fish can be crucial for their conservation and management. Despite the widespread investigation of relationships between fish habitat use and environmental variables, there is a paucity of knowledge on how abiotic and biotic factors jointly influence stream fish habitat use. Three New Zealand South Island streams were selected to investigate the habitat preference of a stream‐dwelling galaxiid, banded kokopu (Galaxias fasciatus). Fish abundance in several permanent pools was determined by spotlighting at night once a month from June 2008 to May 2009. Drifting invertebrates and key physical features of each pool were measured at the time of fish sampling. An information‐theoretic approach (AIC) indicated that the most parsimonious candidate model to predict banded kokopu biomass was the one that included pool area, undercut banks, water velocity, overhanging vegetation, invertebrate drift density and an interaction term between invertebrate drift density and water velocity. Banded kokopu biomass was positively related to pool area, undercut banks, overhanging vegetation and invertebrate drift density. Our study suggests that fish resource use patterns need to be understood in the context of multiple interacting ecological factors, including prey abundance.     

Instream and catchment characteristics affecting the occurrence and population density of brown trout, Salmo trutta L., in forest brooks of a boreal river basin

The occurrence and density of ≥ 1+ brown trout, Salmo trutta L., and their relationship with prevailing instream and catchment characteristics were studied in 50 small forest streams, partially dredged for forest ditching. The occurrence of trout at a stream site was largely determined by the abundance of pools, size of upper catchment and water pH. Moreover, at sites where trout occurred, the abundance of pools was lower at dredged locations than at those in a natural state. In riffles in a natural state, there was a positive relationship between trout density and three instream variables: the abundance of stream pools, cascades and instream vegetation, while an inverse relationship was found with the abundance of substratum of 2–10 cm in diameter. Of the catchment variables, correspondingly, the proportion of forest in the upper catchment was positively related and the proportion of peatland negatively related to trout density. No significant regression model could be fitted for dredged riffles. The possibility of enhancing trout populations in dredged riffles is discussed.