Increasing connectivity of Great Lakes tributaries: Interspecific and intraspecific effects on resident brook trout and brown trout populations

Understanding resident fish population responses to restored connectivity would enhance decision-making on dam removal and fish passage. Since such evaluations are limited in the Great Lakes region of North America, we compared abundance, survival, and growth of resident brook trout and brown trout between sets of Michigan streams where populations were or were not interacting with salmonid species that might be present if connectivity existed. We analysed data from 34 electrofishing index sites to compare resident trout populations between streams without versus with Great Lakes access (and migratory Pacific salmonids), and brook trout populations in Great Lakes inaccessible (land-locked) streams where brown trout were present versus absent. Great Lakes accessibility effects on fish density became increasingly positive for older age groups of brown trout while generally negative for all age classes of brook trout. Brown trout had consistently negative effects on brook trout density in land-locked streams. Increased connectivity had significant effects on annual survival for only one of seven trout age classes modelled, while intraspecific density-dependent effects on survival were significant in six models. Significant intraspecific effects on resident trout growth occurred for seven of eleven age classes examined. Negative interspecific effects of Great Lakes access on resident trout growth were most noticeable for age-0 and age-1 resident trout, age classes that likely compete with juvenile Pacific salmonids. Our findings provide a more robust understanding of how Great Lakes connectivity affects resident trout populations, highlighting negative influences of brown trout on brook trout and intraspecific density-dependent effects.     

Factors influencing growth of individual brown trout in three streams of the upper Midwestern United States

Growth rate variation of three age groups of brown trout, Salmo trutta L. (age‐0, 1 and 2, and 3+), was quantified from recaptured, individually tagged brown trout and related to season, stream reach, relative abundance, initial length and movement to examine factors influencing growth in length in three streams in the Midwestern United States. Total variation in growth was almost five times greater for age‐0 than for age‐3+ trout. Individual trout accounted for about 13% of total variation in age‐0 growth, season about 57%, and trout initial length and relative abundance combined another 2%. The 2006 age‐0 cohort had the fastest growth rates in their second spring and summer (2007) and slowest growth in their first winter (2006–2007). About 53% of total growth variation of age‐1 and age‐2 trout was accounted for by individual trout, season, initial length and stream reach. Predicted growth rates indicated strong effects of season and initial length. A significant interaction between these two factors indicated that, although smaller trout grew faster than larger trout, this length effect was most pronounced in spring and summer. About 35% of total growth variation of age‐3+ trout was accounted for by individual trout and season. Together, season and individual trout characteristics were identified as the most important factors influencing brown trout growth in these streams.     

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.     

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.     

Association of brook trout and Oncorhynchus spp. with large wood jams in a Lake Superior tributary in a northern old‐growth watershed

Wood in streams functions as fish habitat, but relationships between fish abundance (or size) and large wood in streams are not consistent. One possible reason for variable relationships between fish and wood in streams is that the association of fish with wood habitat may depend on ecological context such as large‐scale geomorphology. We studied the relationship between salmonid assemblages and large wood jams (LWJ) in four settings that differed geomorphically at the scale of the stream corridor along a tributary to Lake Superior in old‐growth conifer–hardwood forest in northern Michigan. The focal fish species of this study were brook trout (Salvelinus fontinalis), which were wild in the stream. Relocation efforts for coaster brook trout (an adfluvial life history variant of brook trout) were ongoing in the study stream. We measured fish abundance and length in pairs of pools of similar size and substrate, but varying in the presence of LWJ; this allowed us to evaluate associations of fish simply with the presence of LWJ rather than with other channel or flow‐shaping functions of LWJ. The length of Oncorhynchus spp. and young introduced brook trout was not strongly correlated with LWJ presence; however, the presence of LWJ in pools was positively correlated with larger wild brook trout. We also found that the correspondence of LWJ with the abundance of salmonids appears to be moderated by the presence of alternative habitat in this relatively natural, old‐growth forest stream.     

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.     

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

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Movement of brook trout (Salvelinus fontinalis) in four small subalpine streams in northern Colorado

We studied the movement of brook trout (Salvelinus fontinalis) in four small streams in northern Colorado using mark-recapture methods and weirs. The recapture rates of marked adult trout were low for all streams, and large numbers of unmarked adult trout, apparently immigrants, were found each year. Significantly more trout, immigrated into sections that were experimentally modified by installing low log dams, which increased depth, pool volume and the amount of overhead cover. The number of immigrant and resident trout was significantly related to the amount of cover in the sections. Resident trout were larger than immigrants in all streams in the last year of sampling. Most mobile brook trout moved upstream during summer on the two streams where weirs were operated, and upstream migrants were significantly larger than downstream migrants on both streams. We suggest that a high degree of movement may be an adaptive response by brook trout to the heterogeneous nature of small mountain streams.     

Exploring the relative influence of biotic interactions and environmental conditions on the abundance and distribution of exotic brown trout (Salmo trutta) in a high mountain stream

Abstract – In the Logan River, UT, USA, exotic brown trout demonstrate a strong allopatric distribution and occur at high densities at low‐elevation sites and in tributaries, and in low densities at native trout dominated, high‐elevation sites. Summer temperatures and discharge do not appear limiting for growth; adult growth rates were high overall and were greatest when fish were held experimentally at high elevation where they do not occur naturally. Brown trout are superior competitors; competition for space or food was stronger with their own con‐specifics than with other species. Evidence of density dependence was not observed at the juvenile life stage; no consistent relationships were detected between brown trout density and age‐1 condition or lagged, age‐0 weight (g). In contrast, adult brown trout demonstrated density‐dependent effects on condition and growth when reared experimentally. Field estimates of adult growth rates (g·day^?1), although variable, declined subtly with increasing density, and annual survival was significantly greater in the mainstem sites (mean = 52%) relative to a high‐density tributary site (mean = 22%). Annual predicted age‐0 brown trout growth potential was four‐times greater at the lowermost site, compared with the highest elevation site, although fish lost weight over winter months at all sites. While adult density dependence may influence population abundance at some sites, extreme spring–winter conditions may ultimately limit the upper elevational extent of brown trout in this system. With changing climatic conditions and the potential for habitat degradation in the future, these results have important implications for native fish conservation.     

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.     

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.     

Spatial and temporal variation in brook trout and brown trout proportional stock densities in Wisconsin streams

Abstract  Proper interpretation of measures used to describe fish populations requires knowledge of the measure's inherent spatial and temporal variation. Proportional stock density (PSD), the ratio of 'quality-length' fish to 'stock-length' fish multiplied by 100, is commonly used as a measure of population size structure; PSD values range from 0 to 100. Spatial and temporal variation in brook trout, Salvelinus fontinalis (Mitchill), and brown trout Salmo trutta L., PSD scores in Wisconsin are described and tested to determine if variation differed by stream order and ecoregion. Neither stream order nor ecoregion significantly affected variation of PSD scores. The mean standard deviation of PSD scores over time at a site was 12.49 for brook trout populations and 12.95 for brown trout populations. The mean standard deviation of PSD scores between sites in the same stream was 15.07 for brook trout populations and 12.50 for brown trout populations. Sampling frequency required to characterise a PSD score of a single population of trout in Wisconsin streams with a degree of precision equal to the amount of observed temporal variation is approximately 14 sites for brook trout and 20 sites for brown trout.     

Maternal variation in juvenile survival and growth of triploid hybrids between female rainbow trout and male brown trout and brook charr

Related sib‐groups of rainbow trout × brown trout and rainbow trout × brook charr triploid hybrids and monospecific diploid and triploid rainbow trout controls were obtained from a common set of rainbow trout dams. On the basis of hybrid juvenile performances, 10 sib‐groups were selected and the corresponding diploid rainbow trouts were raised up to adult stage. Females from each group of rainbow trout were used to produce a second generation of hybrid progeny, the performances of which were analysed for grandmaternal variation and relation with first‐generation relatives. Results showed that hybrid traits (alevin yield and weight, survival and growth of fingerlings) were strongly influenced by maternal origin, and could be correlated to those of rainbow trout controls, but that maternal abilities had a low rate of inheritance. It was concluded that little improvement can be expected through selective breeding within parental populations.     

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.     

Evidence of concealment behavior by adult rainbow trout and brook trout in winter

Abstract – There has been little investigation of the winter ecology of adult trout during winter, especially in regard to concealment behavior. We compared day vs night underwater counts of adult rainbow trout and brook trout from four streams. At water temperatures between 1°C and 9°C, daytime counts accounted for 44% and 16% of nighttime snorkeling counts for rainbow trout and brook trout adults, respectively. As winter progressed, nighttime counts declined more so for brook trout than rainbow trout, but the decline was not significant for either species. Nocturnalism of both species was higher in streams with colder water temperatures. We observed few fish within concealment structure; however, by electrofishing concealment habitat during the day, we captured 10 times more adult trout than we counted immediately beforehand by snorkeling. Adult trout were concealed in cobble-boulder substrate and woody debris during the day. ^Note     

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.     

Expert opinion on the status and stressors of brook trout,Salvelinus fontinalis,in Ontario

Ontario supports a vast fisheries resource with an abundance of lakes, rivers and streams. A landscape approach to management informed by a broad‐scale monitoring programme has been initiated to assess the status of fisheries within lakes. However, not all species are assessed by this programme, and there is no provincial monitoring of species inhabiting rivers and streams. As such, changes in the status of a species such as brook trout, Salvelinus fontinalis (Mitchill), could be entirely missed. Brook trout is a highly valued and sought after species by anglers within the province, but there are concerns the species is declining. Given the paucity of broad, empirical data, the status and trends of brook trout across the province have been based on expert opinion at multiple local scales. In 2016, a online questionnaire was sent to brook trout experts to determine status, stressors, management approaches and assess risks (magnitude and probability) to lake and river/stream populations in different geographic areas of Ontario. A Bayesian network was used to analyse responses and develop a risk assessment based on expert opinion for brook trout at multiple scales within the province.     

Developing bioelectrical impedance analysis methods for age‐0 brook trout

Year class strength of many fishes often is determined by survival through the first winter. Increased fat reserves improve survival and overall cohort success. Bioelectrical impedance analysis (BIA) methods are established for estimating proximate composition of adult brook trout, Salvelinus fontinalis (Mitchill), but none have been developed for early life stages. Small‐fish BIA would provide information about age‐0 percent dry weight, a proxy for fat reserves, allowing for better prediction of cohort success. The objective of this study was to develop BIA methods that provide reliable estimates of percent dry weight for age‐0 brook trout. BIA measurements were taken at seven anatomical locations from 48 to 115 mm fish. A model developed using BIA measures taken by subdermal needle electrodes precisely predicted percent dry weight (best model, RMSE = 1.03, R² = 0.86). Although lacking support, as determined by information theoretical analysis, BIA measured with non‐invasive external rod electrodes also precisely predicted percent dry weight (RMSE = 1.09, R² = 0.85). Models developed using two electrode locations performed better than models developed with only one location. For small brook trout, a dorsal to ventral pre‐dorsal fin electrode location should be used in conjunction with a dorsal total length location when measuring BIA to predict percent dry weight.     

Failure of predator conditioning: an experimental study of predator avoidance in brown trout (Salmo trutta)

Many studies have documented that hatchery‐reared salmonids generally have inferior survival after being stocked compared with wild conspecifics, hatchery and wild salmonids have been observed to differ in their antipredator responses. The response of brown trout (Salmo trutta) juveniles (0+) of differing backgrounds to a live predator was compared in two experiments. First, the antipredator behaviour of predator‐naïve hatchery‐reared brown trout and wild‐exposed brown trout were assessed in behavioural trials which lasted for eight days. Second, predator‐naïve and predator‐conditioned hatchery‐reared brown trout were assessed in identical behavioural trials. Brown trout were ‘predator‐conditioned’ by being held in a stream‐water aquarium with adult Atlantic salmon (Salmo salar) and adult brown trout for two days prior to behavioural trials. Predator‐conditioned hatchery‐reared brown trout spent more time in shelters in the trial aquaria than predator‐naïve hatchery‐reared fish, but did not differ in time spent in the predator‐free area. Predator conditioning may account for the increased time spent in the shelter, but does not appear to have affected time spent in the predator‐free area. However, even if significant alteration in behaviour can be noted in the laboratory, the response might not be appropriate in the wild.     

Brook trout (Salvelinus fontinalis) population dynamics and mottled sculpin (Cottus bairdi) occurrence in relation to acidic episodes in streams

We monitored water chemistry and populations of brook trout (Salvelinus fontinalis) and mottled sculpins (Cottus bairdi) in 5 streams and some tributaries to determine how the severity of acidic episodes (low pH and high Al) influenced density and distribution of these species. Among streams, Linn Run had the lowest pH (4.8) and highest concentrations of total dissolved Al (>400 μg-l^?1) during periods of high discharge. Densities of brook trout and mottled sculpins were low in Linn Run, but high in tributaries, where pH (6.5) and Al (30 μg-l^?1) remained well below lethal levels. Among the other streams, brook trout density was highest where the acidic episodes were least severe and the density of age-0 brook trout was lowest when precipitation during the early months of the year was above normal. Immigration of brook trout from downstream areas maintained populations in the stream segments subjected to severe episodes. Mottled sculpins were not found in streams with severe episodes.