Competitive interactions between native and exotic salmonids: a combined field and laboratory demonstration

Abstract –  We studied the impact of two exotic salmonid species (brook trout, Salvelinus fontinalis and rainbow trout, Oncorhynchus mykiss ) on native brown trout ( Salmo trutta fario ) habitat, growth and survival. Habitat selection and vertical distribution between young-of-the-year of the three species were examined in a stream aquarium under different sympatric and allopatric combinations. In addition, similar species combinations were introduced in a Pyrenean mountain stream (southwest France) in order to extend laboratory results to growth and apparent survival. Both laboratory and field results indicated that rainbow trout significantly affected native brown trout habitat selection and apparent survival. On the contrary, brown trout habitat, growth and apparent survival were hardly affected by brook trout. These results support the idea that rainbow trout negatively influence native brown trout, and that competition could influence the outcome of fish biological invasions in freshwater ecosystems.     

A review of ecological models for brown trout: towards a new demogenetic model

Abstract – Ecological models for stream fish range in scale from individual fish to entire populations. They have been used to assess habitat quality and to predict the demographic and genetic responses to management or disturbance. In this paper, we conduct the first comprehensive review and synthesis of the vast body of modelling literature on the brown trout, Salmo trutta L., with the aim of developing the framework for a demogenetic model, i.e., a model integrating both population dynamics and genetics. We use a bibliometric literature review to identify two main categories of models: population ecology (including population dynamics and population genetics) and population distribution (including habitat–hydraulic and spatial distribution). We assess how these models have previously been applied to stream fish, particularly brown trout, and how recent models have begun to integrate them to address two key management and conservation questions: (i) How can we predict fish population responses to management intervention? and (ii) How is the genetic structure of fish populations influenced by landscape characteristics? Because salmonid populations tend to show watershed scale variation in both demographic and genetic traits, we propose that models combining demographic, genetic and spatial data are promising tools for improving their management and conservation. We conclude with a framework for an individual‐based, spatially explicit demogenetic model that we will apply to stream‐dwelling brown trout populations in the near future.     

Addition of structural complexity – contrasting effect on juvenile brown trout in a natural stream

In a field experiment, we examined the effects of structural complexity in the form of added artificial plastic plants and shredded plastic bags on growth and abundance of juvenile brown trout (Salmo trutta). Just after emergence, the added complexity had a positive effect on the density, biomass and condition factor of young‐of‐the‐year (0+) brown trout. This difference in density was not present six weeks later. In contrast, both young‐of‐the‐year and older brown trout generally tended to be larger in the simple habitat. Hence, our data suggest that increased complexity initially is beneficial for young‐of‐the‐year individuals probably due to lower risk of predation and increased densities of prey. However, as density increases in the complex environment, it may induce negative density‐dependent effects, here reflected in smaller sized fish in the complex environment. This might force fish to redistribute to habitats with lower densities of conspecifics as they grow larger. We propose that habitat complexity can increase survival of yearlings in early phases and thereby also affect the overall population structure of brown trout in natural streams.     

Seasonal movement patterns of Credit River brown trout (Salmo trutta)

Zimmer M, Schreer JF, Power M. Seasonal movement patterns of Credit River brown trout (Salmo trutta).
Ecology of Freshwater Fish 2010: 19: 290–299. © 2010 John Wiley & Sons A/S Abstract – Movement habits in riverine populations of brown trout vary among watersheds. Thus it is important to identify factors influencing differences in individual behaviour so as to improve the information resource base available for the design of river‐specific management strategies. Such information is particularly needed in the rapidly urbanising watersheds of eastern North America where relatively little is known about anthropogenic influences on brown trout populations. In this study, we examined the influence of water temperature on brown trout behaviour in the Credit River in south‐central Ontario, Canada with respect to seasonal movement patterns. Observed patterns of movement were also correlated with variations in river discharge and habitat quality. Forty‐three radio‐tagged, adult brown trout were tracked in a confined 39.8 km portion of the Credit River from 15 May 2002 to 28 July 2003. Fish were captured in three sections of the river that differed in distance downstream and habitat quality. Fish size had little impact on movement patterns. However, there was considerable variation in seasonal movement with upstream movements to summer positions, maintenance of summer positions, downstream and often extensive movements in fall. Also observed was maintenance of winter positions and repeated upstream movements in late spring‐early summer to previously used summer positions. The elaborate movement behaviour in the Credit River population was attributed to seasonal changes in thermal habitat quality. Fish tagged in less suitable thermal habitats moved significantly more than fish from more suitable thermal habitats.     

Density-dependent habitat selection and the ideal free distribution in marine fish spatial dynamics: considerations and cautions

Current methods and theory used in the study of the spatial dynamics of marine fish are problematic. Positive relationships between population abundance and occupied area are typically interpreted as evidence of density‐dependent habitat selection. However, both abundance and area may co‐vary with an un‐parameterized variable, such as a density‐independent effect. In addition, if density‐dependent habitat selection is present, population growth rates in optimal habitats would be expected to be lower than in marginal habitats. This same pattern can also evolve from a large‐scale, spatially autocorrelated change in a density‐independent factor. The theory underlying density‐dependent habitat selection, the ideal free distribution, can be tautological when no a priori information of how habitat suitability changes with density is known. In this case, an ideal free distribution can be defined for any pattern of habitat‐specific population growth rates. However, these problems are not insurmountable and solutions may be found by considering spatial variation in proxies of fitness and explicitly allowing for the relative importance of habitat selection (density dependent) and environmental (density independent) effects to vary with spatial scale.     

Modelling functional fish habitat connectivity in rivers: A case study for prioritizing restoration actions targeting brown trout

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Modelling potential spawning habitat of sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) in the Bay of Biscay

Large amplitude variations in recruitment of small pelagic fish result from interactions between a fluctuating environment and population dynamics processes such as spawning. The spatial extent and location of spawning, which is critical to the fate of eggs and larvae, can vary strongly from year to year, as a result of changing population structure and environmental conditions. Spawning habitat can be divided into ‘potential spawning habitat’, defined as habitat where the hydrographic conditions are suitable for spawning, ‘realized spawning habitat’, defined as habitat where spawning actually occurs, and ‘successful spawning habitat’, defined as habitat from where successful recruitment has resulted. Using biological data collected during the period 2000–2004, as well as hydrographic data, we investigate the role of environmental parameters in controlling the potential spawning habitat of anchovy and sardine in the Bay of Biscay. Anchovy potential spawning habitat appears to be primarily related to bottom temperature followed by surface temperature and mixed‐layer depth, whilst surface and bottom salinity appear to play a lesser role. The possible influence of hydrographic factors on the spawning habitat of sardine seems less clear than for anchovy. Modelled relationships between anchovy and sardine spawning are used to predict potential spawning habitat from hydrodynamical simulations. The results show that the seasonal patterns in spawning are well reproduced by the model, indicating that hydrographic changes may explain a large fraction of spawning spatial dynamics. Such models may prove useful in the context of forecasting potential impacts of future environmental changes on sardine and anchovy reproductive strategy in the north‐east Atlantic.     

Spatial niche variability for young Atlantic salmon (Salmo salar) and brown trout (S. trutta) in heterogeneous streams

Abstract– Habitat is important in determining stream carrying capacity and population density in young Atlantic salmon and brown trout. We review stream habitat selection studies and relate results to variable and interacting abiotic and biotic factors. The importance of spatial and temporal scales are often overlooked. Different physical variables may influence fish position choice at different spatial scales. Temporally variable water flows and temperatures are pervasive environmental factors in streams that affect behavior and habitat selection. The more frequently measured abiotic variables are water depth, water velocity (or stream gradient), substrate particle size, and cover. Summer daytime, feeding habitats of Atlantic salmon are size structured. Larger parr (>7 cm) have a wider spatial niche than small parr. Selected snout water velocities are consistently low (3–25 cm. s^?1). Mean (or surface) water velocities are in the preferred range of 30–50 cm. s^?1, and usually in combination with coarse substratum (16–256 mm). However, salmon parr demonstrate flexibility with respect to preferred water velocity, depending on fish size, intra- and interspecific competition, and predation risk. Water depth is less important, except in small streams. In large rivers and lakes a variety of water depths are used by salmon parr. Summer daytime, feeding habitat of brown trout is also characterized by a narrow selection of low snout water velocities. Habitat use is size-structured, which appears to be mainly a result of intraspecific competition. The small trout parr (<7 cm) are abundant in the shallow swift stream areas (<20–30 cm depths, 10–50 cm. s^?1 water velocities) with cobble substrates. The larger trout have increasingly strong preferences for deep-slow stream areas, in particular pools. Water depth is considered the most important habitat variable for brown trout. Spatial niche overlap is considerable where the two species are sympatric, although young Atlantic salmon tend to be distributed more in the faster flowing and shallow habitats compared with trout. Habitat use by salmon is restricted through interspecific competition with the more aggressive brown trout (interactive segregation). However, subtle innate differences in behavior at an early stage also indicate selective segregation. Seasonal changes in habitat use related to water temperatures occur in both species. In winter, they have a stronger preference for cover and shelter, and may seek shelter in the streambed and/or deeper water. At low temperatures (higher latitudes), there are also marked shifts in habitat use during day and night as the fish become nocturnal. Passive sheltering in the substrate or aggregating in deep-slow stream areas is the typical daytime behavior. While active at night, the fish move to more exposed holding positions primarily on but also above the substrate. Diurnal changes in habitat use take place also in summer; brown trout may utilize a wider spatial niche at night with more fish occupying the shallow-slow stream areas. Brown trout and young Atlantic salmon also exhibit a flexible response to variability in streamflows, wherein habitat selection may change considerably. Important topics in need of further research include: influence of spatial measurement scale, effects of temporal and spatial variability in habitat conditions on habitat selection, effects of interactive competition and trophic interactions (predation risk) on habitat selection, influence of extreme natural events on habitat selection use or suitability (floods, ice formation and jams, droughts), and individual variation in habitat use or behavior.     

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.     

Recaptures and resource use of native and non-native brown trout, Salmo trutta L., released in a Norwegian lake

Abstract Rate of recapture (gill netting), habitat use, and diet of three strains of stocked brown trout, Salmo trutta L., were compared with resident brown trout in a Norwegian lake. The strains originated from an alpine lake, from a boreal lake, and from the native brown trout population in the lake. Overall recapture rate was 5–8% for all strains. The low recapture rate could be due to the relatively small size at stocking; mean fish length varied between 13.1 and 14.5 cm with strain and stocking method. Two years after release, the frequency of the different strains decreased from about 12% in the first year to stabilize at about 1%. The alpine strain showed the highest overall recapture rate, whereas the native strain was recaptured at an intermediate rate. The overall recapture rate of scatter-planted brown trout was higher than that of spot-planted brown trout. Immediately after being stocked, introduced fish ate less and had a less-varied diet than resident trout; however, stocked fish adopted a natural diet within the first summer. The distribution of trout between the pelagic and the upper epibenthic habitat was similar for both the resident and the stocked brown trout. Results indicate that the habitat use of stocked brown trout is adaptive and becomes similar to that of indigenous fish.     

Habitat suitability analysis for lacustrine brown trout (Salmo trutta) in Lake Walchensee,Germany: implications for the conservation of an endangered flagship species

• 1. The lacustrine brown trout (Salmo trutta) is endangered and of high conservation importance. In the only spawning habitat of the population in the Bavarian Lake Walchensee, the River Obernach, a substantial decrease in spawning runs has been reported. In this study, the present ecological state of the spawning stream was analysed with the objective of identifying life‐stage specific limitations to successful recruitment attributable to deficiencies in (i) spawning migration, (ii) spawning habitat quality, and (iii) habitat quality for juveniles.
• 2. Structural stream analysis showed that discharge and several migration barriers — particularly near the river outlet into the lake — prevent successful spawning migrations at normal water levels. Migration barriers are probably the main limiting factor for reproduction of lacustrine brown trout, whereas structural variability of the Obernach meets the habitat requirements of both spawners and juveniles.
• 3. Spawning site quality was suitable for trout, as indicated by stream substratum texture and high exchange rates between free‐flowing water and the interstitial zone in physico‐chemical parameters (redox potential, dissolved oxygen, pH, temperature and conductivity).
• 4. Analyses of fish community structure revealed dominance of lithophilic species, in particular of riverine brown trout (Salmo trutta). Its density and intact demographic population structure suggest that spawning and juvenile habitat quality for salmonids is not limiting. Recapture of stocked lacustrine trout juveniles also indicates habitat suitability for the juvenile stage.
• 5. In conclusion, the results show that the methodology used in this study is suitable for the identification of life‐stage specific habitat deficiencies in lacustrine brown trout and other fish species. Availability of habitat data throughout the species' distribution range is a first crucial step for the development of an effective recovery plan. Copyright © 2009 John Wiley & Sons, Ltd.

     

Spatial scale effects on habitat associations of the Ashy Darter,Etheostoma cinereum,an imperiled fish in the southeast United States

Abstract – The ashy darter, Etheostoma cinereum, is an imperiled fish within the Cumberland and Tennessee drainages of the southeast United States. An understanding of habitat associations and the relationship of habitat use across multiple spatial scales are critical elements in its conservation. Our objectives were to quantify habitat associations at the stream reach and microhabitat scales for adult and juvenile darters, and to understand the linkage between the two scales based on gradients of habitat use. We focused our efforts within the Rockcastle River, Kentucky (Cumberland River drainage), because the watershed was known to contain a relatively large ashy darter population. Three hundred twenty‐two individuals were collected from 21 reaches. The species was restricted to the mainstem of the river and the lower reaches of the larger tributaries. The distribution and abundance of adult and juvenile darters differed significantly at each spatial scale, and both groups demonstrated non‐random use of the available habitat. Gradients of stream size and substrate size were identified as important factors. A threshold of environmental quality was determined based on the habitat use patterns among the two scales. Habitat use between the two scales was independent within the threshold, indicating that the specific quality of the microhabitats did not necessarily matter within a stream reach. However, beyond the threshold, a decrease of at least 48% in adult and juvenile darter abundance was seen, indicating that a sufficient network of suitable microhabitats is needed to support a good population of darters within a stream reach.     

A test for interaction between brown trout (Salmo trutta) and inanga (Galaxias maculatus) in an artificial stream

Abstract –  The interaction between brown trout ( Salmo trutta ; fork length (FL) range 255–390 mm) and inanga ( Galaxias maculatus ; FL range 55–115 mm) was tested during summer through autumn in an artificial stream consisting of a single run-riffle-pool sequence with a natural food supply. Each experimental trial lasted for 15 days, and consisted of two brown trout and 50 inanga collected fresh from a nearby stream, with each species given prior residence in four replicate tests, totalling eight trials in all. In addition, two control trials (each 10 days), with 50 inanga in each, were run. Brown trout almost exclusively occupied the pool, whereas inanga occupied all habitat types, although in different proportions, when tested with and without brown trout. The proportion of inanga in the pool was appreciably lower in the experimental trials with brown trout than in the control trials with no brown trout; prior residence had no significant effect on inanga habitat use. Mortality of inanga attributable to predation by brown trout ranged from 0 to 40% with a mean of 14.5 ± 4.7%. The results suggest that habitat use and survival of inanga populations in small streams can be adversely affected by brown 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.     

The effects of stream substratum composition on the emergence of salmonid fry

Sternecker K, Geist J. The effects of stream substratum composition on the emergence of salmonid fry.
Ecology of Freshwater Fish 2010: 19: 537–544. © 2010 John Wiley & Sons A/S Abstract – Salmonid fishes are target species for the conservation of freshwater habitats, but their natural reproduction is often insufficient. The emergence of fry is a crucial phase in the life cycle of salmonids and the stream substratum is the key habitat which regulates the emergence success. In this study, brown trout (Salmo trutta) and Danube salmon (Hucho hucho) eggs were exposed to different sediment textures and the emergence and the postemergence survival and growth were observed under constant water chemical conditions in the laboratory. In both species, textural effects on emergence rate, chronology of emergence, survival rate after emergence and growth after emergence were detected. Fine‐textured substratum (5–8 mm) formed a physical barrier to the posthatch migration of salmonids from the interstitial zone to the open water. The time period between the first and the last emerged fish was shorter in treatments with fine texture compared with coarse substratum. The survival rate was higher in treatments of coarser sediment. The effects of different textures on the growth of fry after emergence differed between brown trout and Danube salmon, which can be explained by different life history strategies. These results suggest that physical characteristics of substratum texture can have strong effects on salmonid emergence, and ultimately on the persistence of salmonid populations. They also suggest that biodiversity conservation in stream ecosystems can greatly benefit from an inclusion of the physical characteristics of the stream bed into catchment‐based management plans.     

Diurnal stream habitat use of juvenile Atlantic salmon, brown trout and rainbow trout in winter

Abstract  The diurnal winter habitat of three species of juvenile salmonids was examined in a tributary of Skaneateles Lake, NY to compare habitat differences among species and to determine if species/age classes were selecting specific habitats. A total of 792 observations were made on the depth, velocity, substrate and cover (amount and type) used by sympatric subyearling Atlantic salmon, subyearling brown trout and subyearling and yearling rainbow trout. Subyearling Atlantic salmon occurred in shallower areas with faster velocities and less cover than the other salmonid groups. Subyearling salmon was also the only group associated with substrate of a size larger than the average size substrate in the study reach during both winters. Subyearling brown trout exhibited a preference for vegetative cover. Compared with available habitat, yearling rainbow trout were the most selective in their habitat use. All salmonid groups were associated with more substrate cover in 2002 under high flow conditions. Differences in the winter habitat use of these salmonid groups have important management implications in terms of both habitat protection and habitat enhancement.     

Predation on native sculpin by exotic brown trout exceeds that by native cutthroat trout within a mountain watershed (Logan,UT, USA)

We explored potential negative effects of exotic brown trout (Salmo trutta) on native sculpin (Cottus sp.) on the Logan River, Utah, USA by (i) examining factors most strongly correlated with sculpin abundance (e.g., abiotic conditions or piscivory?), (ii) contrasting the extent of brown trout predation on sculpin with that by native cutthroat trout (Oncorhynchus clarkii utah) and (iii) estimating the number of sculpin consumed by brown trout along an elevational gradient using bioenergetics. Abundance of sculpin across reaches showed a strong (r ≥ 0.40) and significant (P < 0.05) correlation with physical variables describing width (positive) and gradient (negative), but not with abundance of piscivorous brown trout or cutthroat trout. In mainstem reaches containing sculpin, we found fish in 0% of age‐1, 10% of age‐2 and 33% of age‐3 and older brown trout diets. Approximately 81% of fish consumed by brown trout were sculpin. Despite a similar length–gape relationship for native cutthroat trout, we found only two fish (one sculpin and one unknown) in the diets of native cutthroat trout similar in size to age‐3 brown trout. Based on bioenergetics, we estimate that an average large (> 260 mm) brown trout consumes as many as 34 sculpin per year. Nevertheless, results suggest that sculpin abundance in this system is controlled by abiotic factors and not brown trout predation. Additional research is needed to better understand how piscivory influences brown trout invasion success, including in‐stream experiments exploring trophic dynamics and interactions between brown trout and native prey under different environmental conditions.     

Landscape features determine brown trout population structure and recruitment dynamics

Variation in brown trout (Salmo trutta L.) population recruitment and structure is related to migratory patterns, which should depend on ease of access to habitats providing increased opportunity for growth. We quantified the number of young of year (YOY) as a proportion of the total number of brown trout at 24 locations on 11 streams within the Taieri catchment, New Zealand, including back calculated growth rates and emergence dates from otoliths. Locations with high absolute and relative abundance of YOY fish were related to elevation and distance from the river mainstem (habitat used by migratory fish), fish density, and the interaction between invertebrate food biomass, distance and elevation. Hatch date and growth were not related to the proportion of YOY fish, though growth was negatively correlated to total fish density. We suggest landscape features play a large role in determining recruitment and population structure. Locations at lower elevations have a high YOY density, high competition and lower growth, likely prompting out‐migration. These conditions could be created by successful return migration and spawning of large fecund fish resulting in YOY densities exceeding the habitat carrying capacity. Environmental factors, such as food availability, also played a role in determining population structure. These results provide an example of how population structure and recruitment might be controlled by local conditions and access to high growth environments in wild populations of introduced brown trout across a catchment.     

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.