Seasonal habitat use by brook trout,Salvelinus fontinalis (Mitchill), in a second-order stream

Abstract Seasonal habitat use by over-yearling and under-yearling brook trout, Salvelinus fontinalis (Mitchill), was examined in a second-order stream in north-central Pennsylvania, USA. The habitat occupied by brook trout and available habitat were determined in a 0.5-km stream reach during the spring, summer and autumn of 1989 and the spring and summer of 1990. Cover, depth, substrate and velocity were quantified from over 2000 observations of individual brook trout. Habitat used by under-yearling brook trout was more uniform between seasons and years than that used by over-yearling brook trout. Over-yearling brook trout occupied areas with more cover and greater depth than did under-yearling brook trout, suggesting ontogenetic shifts in these variables. Differences for velocity and substrate were not as great as those for cover and depth. The selection of areas with low water velocities governed trout habitat use in spring, whereas cover and depth were the most important habitat variables in summer and autumn. Principal component analysis showed that available habitat and trout habitat centroids diverged most in spring, indicating that habitat selection by brook trout may be greatest at this time.     

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.     

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.     

Influence of Physical Factors on Density of Stocked Brown Trout (Salmo trutta fario L.) in a Danish Lowland Stream

Physical factors and brown trout densities were studied in a small Danish lowland stream. The densities of brown trout larger than 15 cm were significantly correlated with gradient, mean depth, coefficient of variation in current velocity 7 cm above the bottom, the ratio between wetted perimeter and width, amount of overhanging banks and degree of macrophyte cover. Coefficient of variation in current velocity 7 cm above the bottom was the most important factor for brown trout density (r_s= 0.8364, 24 df, (P < 0.001)), which supports the idea of this value as a measure of stream complexity. A rather small relation between trout density and amount of overhanging bank cover (r_s= 0.4179, 24 df, (P < 0.050)), contrary to the closer relationships found in previous studies, is discussed as an effect of the self-shading capacity of this rather narrow and deep stream.     

Microhabitat use by three endemic Iberian cyprinids in Mediterranean rivers (Tagus River Basin, Spain)

Abstract  Microhabitat use by three endemic Iberian cyprinids, Barbus bocagei (Steindachner), Pseudochondrostoma polylepis (Steindachner) , and Squalius pyrenaicus (Günther) was studied in terms of depth, mean water column velocity, focal height, focal velocity, distance to shore and substrate. Data were obtained by snorkelling during spring and summer at nine sites of the Tagus River Basin, Spain. Habitat suitability criteria (HSC) were calculated, including fish position and focal velocity in the water column. Species comparison showed differences in depth and focal height (indicating a vertical segregation), and greater water velocities for Pseudochondrostoma . Size-class comparisons mainly showed differences in depth and focal height (correlated with fish size). The fish groups (3 species × 3 length classes) were assigned to microhabitat functional types. The results are essential for environmental flow assessments and allow 2- and 3-dimensional habitat simulations in Mediterranean rivers; they are also useful to define critical habitats for the conservation of native fish populations.     

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.     

Seasonal effects of macrophyte growth on rainbow trout habitat availability and selection in a low‐gradient,groundwater‐dominated river

Rainbow trout habitat use is often described in high‐gradient, runoff‐driven, heterotrophic streams where geomorphic features and overhanging riparian vegetation provide channel complexity and cover. However, many rainbow trout populations thrive in rivers with contrasting aquatic habitat. We describe rainbow trout habitat use in a low‐gradient, groundwater‐dominated tailwater river where river flow management and macrophyte growth and senescence largely govern available trout habitat. In the summers of 2013 and 2014, available aquatic habitat (depth, velocity, macrophyte cover, substrate size) was quantified, while individual trout location was determined by radio telemetry and linked to environmental variables. Detailed habitat surveys indicate that macrophyte cover increases throughout the summer and is a strong determinant of in‐stream habitat characteristics. Paired logistic regression shows that adult rainbow trout prefer greater depths. Water depth increases with macrophyte abundance at both reach and local scales as plants restrict flow, and available trout habitat is linked to this seasonal pattern. When macrophyte abundance is high, adult trout show secondary preference for localised areas of lower macrophyte cover but otherwise show no selectivity for macrophyte cover, velocity or substrate size. Results suggest that submerged aquatic plants increased the quantity and quality of rainbow trout habitat as a source of channel complexity and cover. Macrophytes may play a similar role in other low‐gradient streams and should not be overlooked by fisheries managers considering habitat suitability.     

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.     

Timing of brown trout spawning in Alpine rivers with special consideration of egg burial depth

Timing of spawning, habitat use and egg burial depths of brown trout were studied in seven Swiss (alpine and prealpine) rivers, which differed in size, altitude and flow regime. In general, we observed brown trout spawning activity between the end of October and the beginning of January. The spawning duration differed significantly, however, between rivers, ranging from 28 to 72 days. Analysis of environmental parameters for their influence on spawning activity revealed mean water temperature and altitude as the most explanatory variables. Detailed investigation of redd characteristics based on water velocity, water depth and substrate size clearly showed differences between positions on the redd. Brown trout in Alpine rivers preferred to use velocities of 30–40 cm·s^?1, water depths of 10–20 cm and substrates of 16–32 mm for spawning. It has to be noted, however, that recorded values cover almost the whole range of data on spawning habitats that has been reported in literature so far. A special focus of this study was on egg burial depths, which were surprisingly not found to differ significantly between the rivers despite their different flow regimes. Recorded egg burial depths were, however, found to be distinctly lower (mean burial depth: 3.8 cm) than reported by almost any study so far. We see this observation of low burial depths in Alpine rivers as useful in the context of scouring effects, especially when evaluating the influence of scouring on fish populations.     

Response of resident brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), to the stocking of hatchery-reared brown trout

Two strains of hatchery-reared adult brown trout, Salmo trutta L., [208–334 mm total length (TL); n =  591] were individually marked and released into a limestone stream. The estimated survival after one month (86%; n =  508) was comparable to that for resident brown trout and rainbow trout, Oncorhynchus mykiss (Walbaum), (89%; n =  771), but declined to 14% ( n =  83) after 8 months compared with 52% ( n =  451) for resident trout. The movement of resident trout out of stocked stretches was higher (14%) than from control sites (5%), but the population size in both individual sites and the overall study area were unaffected. The growth of resident brown trout was unaffected by stocking, but rainbow trout showed lower growth rates in stocked versus unstocked stretches both one and 8 months after stocking ( P <  0.002).     

Predation on wild and hatchery salmon by non‐native brown trout (Salmo trutta) in the Trinity River,California

Non‐native predators may interfere with conservation efforts for native species. For example, fisheries managers have recently become concerned that non‐native brown trout may impede efforts to restore native salmon and trout in California's Trinity River. However, the extent of brown trout predation on these species is unknown. We quantified brown trout predation on wild and hatchery‐produced salmon and trout in the Trinity River in 2015. We first estimated the total biomass of prey consumed annually by brown trout using a bioenergetics model and measurements of brown trout growth and abundance over a 64‐km study reach. Then, we used stable isotope analysis and gastric lavage to allocate total consumption to specific prey taxa. Although hatchery‐produced fish are primarily released in the spring, hatchery fish accounted for most of the annual consumption by large, piscivorous brown trout (>40 cm long). In all, the 1579 (95% CI 1,279–1,878) brown trout >20 cm long in the study reach ate 5,930 kg (95% CI 3,800–8,805 kg) of hatchery fish in 2015. Brown trout predation on hatchery fish was ca. 7% of the total biomass released from the hatchery. Brown trout only ate 924 kg (95% CI 60–3,526 kg) of wild fish in 2015, but this was potentially a large proportion of wild salmon production because wild fish were relatively small. As large brown trout rely heavily on hatchery‐produced fish, modifying hatchery practices to minimise predation may enhance survival of hatchery fish and potentially reduce the abundance of predatory brown trout.     

Brown trout fry move inshore at night: a choice of water depth or velocity?

Abstract –  The instream positions of brown trout fry differ between daylight and darkness. According to field and laboratory observations, recently emerged 0+ brown trout use shallow and slow-flowing areas close to the bank at night and tend to move off-shore during daylight. In laboratory channels, we tested whether the use of habitats close to the river bank could be attributed to a choice of either water depth or velocity. In two complementary experiments, emerging brown trout alevins were given the choice of using shallow-slow or deep-swift habitats (experiment 1), and deep-slow or shallow-swift habitats (experiment 2). At night, a persistent preference for the shallow habitats was displayed, regardless of velocity. It was concluded that swim-up brown trout fry respond to shallowness rather than ambient low water velocity when selecting habitats close to the bank at night. The behavioural significance of this result and implications for river management are discussed.     

Density of brown trout, Salmo trutta L., and Atlantic salmon, Salmo salar L., parr after point and scatter stocking of fry

Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., fry were point and scatter stocked in the early part of June at densities of 63–263 fry 100 m⁻² per species in the River Viantienjoki, a small river in northern Finland, and their population densities were assessed in late summer. Both species were always stocked together in similar quantities. Point stocking was used in the first 2 years and scatter stocking in the following 2 years. In point stocking, there was no correlation between the distance from the stocking sites (maximum = 250 m) and parr density in census sites ( r = −0.013 and 0.019 for brown trout and Atlantic salmon, respectively). The stocking density of fry did not influence parr density in August by either method or by species. Stocking density explained only from 11% to 23% of the parr survival depending on the species or stocking method. The mean densities of Atlantic salmon and brown trout parr did not differ significantly from each other at any fishing site ( P > 0.05). Both point and scatter stocking appear to be suitable methods for use in small rivers. The parr densities depend more on the other factors (e.g. habitat quality) than the stocking method, and the choice between methods could be based on the time and labour available.     

Effect of brown trout body size on post-stocking survival and pike predation

Abstract –  A total of 40 (20 age-3 + 20 age-4) radio-tagged hatchery-reared brown trout ( Salmo trutta L.) and 40 wild radio-tagged northern pike ( Esox lucius L.) were released into a regulated river. Age-3 brown trout were predicted and observed to be highly vulnerable to predation by pike (50% mortality), whereas age-4 brown trout were predicted and observed to enjoy an almost absolute size refuge from predation (5% mortality). Almost half of the fish from both age groups similarly emigrated and survived from the river within 3 days of the release. However, there was a considerable difference in survival between age groups for fish that remained in the river for a longer period. Of these, all except one age-3 brown trout were eaten by pike, whereas all but one age-4 fish survived predation.     

Movements by wild brown trout in a boreal river: response to habitat and flow contrasts

Abstract  Radio transmitters were implanted in wild brown trout, Salmo trutta L., in the River Måna at low summer water flows ( n  = 18), higher flow in summer ( n  =   20), and variable, peaking flows in autumn ( n  =   20), and tracked two to four times day and night for 4–5 weeks. Individuals were caught and released in a 4-km uniformly channelised section, and in a 4-km natural diverse river section. Substantial individual variation in home range and total movement (924–85 818 m² and 295–7014 m) suggested flexibility to adapt to local environmental conditions. Fish were stationary most of the time (median movement 0 m), but some individuals undertook few and apparently sporadic longer movements, sometimes involving shifts in home range. No consistent diurnal pattern in movements was found. Trout in the uniform habitat section appeared to have larger home ranges and moved more than trout in the natural section. Differences were, however, not statistically significant in most comparisons, due to large individual variation. Similarly, larger home ranges and movements between trials related to higher flow were found, but differences were generally not significant. No consistent effects of sudden, extreme peaking flows on area use or movements by the brown trout were observed.     

Sea-ranching of brown trout, Salmo trutta L.

Abstract  In total, 8211 1-year-old and 14839 2-year-old hatchery-reared brown trout, Salmo trutta L., from 11 stocks were released at the mouth of the River Imsa, southwestern Norway. The recapture rates and total estimated yield were higher for 2- than 1-year-old trout, although recapture rates varied between years of release and stocks. The recapture rate increased with mean individual weight at time of release. Total estimated yield from the individual groups of 1+ trout ranged from 2 to 20 kg per 1000 trout released and for 2+ trout between 11 and 250 kg per 1000 fish released. In all cases, yields were lower than the economic break-even yield. Most fish were recaptured the year of release (89.2% of 2+ and 76.2% of 1+ trout). Almost 31% of the recaptures were caught at sea and 69% in fresh water; 95.8% of the latter were taken in the River Imsa trap.     

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.     

Do instream habitat variables and the abundance of brown trout Salmo trutta (L.) affect the distribution and growth of stone loach,Barbatula barbatula (L.)?

Abstract –  Along a stream, we investigated whether the abundance of stone loach ( Barbatula barbatula , L.) was related to the presence of brown trout ( Salmo trutta , L.) and instream habitat variables. First, a field survey was carried out where different habitat variables and the densities of both species were quantified and subjected to principal components analysis. Then the abundance of stone loach was related to the scores of the retained axes (eigenvalues >1). The abundance of stone loach was positively correlated to substrate particle size, amount of shade, temperature, discharge and current velocity, but negatively correlated to brown trout abundance. Secondly, a month-long field enclosure experiment in a stream was performed to test for any negative effects of brown trout on stone loach growth. Four treatments were used: intraspecific competition (stone loach at double density), interspecific competition (stone loach + small trout), predation (stone loach + large trout) and a control (stone loach alone). The results showed that large trout tended to have negative effects on final stone loach biomass. The absence of a negative effect of large trout on resource density suggests that nonlethal effects rather than resource competition caused this trend.     

Some behavioural and ecological factors affecting distribution,biomass and production of juvenile Atlantic salmon

Observations were made in an experimental stream tank (total area 14.7 m²) on juvenile Atlantic salmon, Salmo salar L (parr), relating experimental observations to field observations, including the reported diurnal fasting behaviour of juvenile salmon at water temperatures <10 °C. Densities in the tank ranged from five to twenty parr, at water temperatures ranging from 4.6 °C to 15.8 °C. The wide channel of the stream tank, with mean water velocity of 18.8 cm·s^?1, was the preferred section, where territorial behaviour was observed. Biomass was regulated in the wide channel by territorial mosaics or by dominance hierarchies. Dominance hierarchies were reflected in coloration of the fish. Dominant salmon were generally in the wide channel. Densities of salmon parr (of mean fork length 10.2 cm) in the channel ranged from 0.84 m^?2 to 1.73 m^?2, with an average biomass of 14.2 g·m^?2. Growth was least at the 5.9 °C temperature treatment. In experiments at temperatures below 10 °C, feeding, dominance hierarchy and territorial behaviour were observed in daylight hours, contrary to the published literature. Interactions with other species may affect behaviour. Some observations were made on a closely related species, brown trout (Salmo trutta L.), a commonly cohabiting species in many systems. Trout displaced salmon from their preferred locations in the tank and were more aggressive than the salmon, reducing agonistic behaviour by the salmon. The commonest agonistic act shown by salmon was ‘charge’ and that by the trout was ‘approach’. Some field observations affecting behaviour and production are discussed.     

Effects of instream enhancement structures on brown trout, Salmo trutta L., habitat availability in a channelized boreal river: a PHABSIM approach

Stream channel morphology and hydraulic conditions were measured before and after channel modification and boulder structure placements in a channelized boreal river to determine whether more favourable rearing habitat for brown trout, Salmo trutta L., was created. The assessment was performed using physical habitat simulation (PHABSIM) procedures based on summer and winter habitat preferences of brown trout for depth, velocity and substrate. The results showed that the availability of potential physical trout habitat can be increased in the study river at simulated low and moderate flow conditions by reconstruction of the river bed and placing instream boulder structures. The resulting diversity of depth and velocity conditions created a spatially more complex microhabitat structure. Improved habitat conditions were able to sustain a larger trout population. Hydraulic habitat models, like the PHABSIM framework, seem to be a suitable procedure to evaluate the benefits of physical habitat enhancement.