River bed construction: impact and habitat restoration for juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

The River Soya, Norway, was canalized for agricultural purposes. In order to compensate for damage to the Atlantic salmon, salmo salar L. and brown trout, Salmo trutta L. populations, different weirs were built. The aims of this study were firstly to analyse the effects of weirs covering the river bank and entire river bottom with blasted stones and secondly, to analyse the effects of sediments transported by freshets on the downstream salmon and trout populations after canalization. Restoration of the river bottom with blasted stones provided salmon with more substrate spaces. Densities of trout increased after the river bank was covered with stones. Sediments transported downstream from the canalized river stretch decreased the densities of juvenile salmon and trout.     

Comparison of population dynamics of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a small tributary of the River Scorff (Brittany, France)

The population dynamics of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) were compared in a small tributary of the River Scorff (Brittany, France) from spawning time to the beginning of the third growing season. The spawning and fry emergence of the two species took place at approximately the same time. In the first autumn, the densityof 0+ juveniles and settling rate from the egg stage were much higher in trout than in salmon. The emigration rate from 0+ population was much higher in trout than in salmon. The size of resident and migrating fish was always smaller in salmon than in trout, whatever the age. The low level of salmon production in the brook, compared with trout, was the result of low survival from egg to 0+ stage in autumn, combined with the small proportion of juveniles migrating after the first growing season. This was not compensated by a high number of migrants the next year. The role of physical habitat, inter-and intraspecific competition, predation and migration dependence on size and early sexual maturity is discussed. Indications are that small tributaries of the type studied are of great value for recruitment in trout but not very productive for juvenile salmon.     

The effect of competition among three salmonids on dominance and growth during the juvenile life stage

Although non‐native species can sometimes threaten the value of ecosystem services, their presence can contribute to the benefits derived from the environment. In the Great Lakes, non‐native brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) support substantial recreational fisheries. With current efforts underway to restore once‐native Atlantic salmon (Salmo salar) to Lake Ontario, there is some concern that Atlantic salmon will impede non‐native contributions to the recreational fishery because Atlantic salmon exhibit niche overlap with brown trout and rainbow trout, particularly during the juvenile life stage. We therefore examined competition and growth of juvenile Atlantic salmon, brown trout and rainbow trout in semi‐natural streams. We found that brown trout were the most dominant and had the greatest growth rate regardless of what other species were present. Rainbow trout were more dominant than Atlantic salmon and consumed the most food of the three species. However, in the presence of brown trout, rainbow trout fed less frequently and exhibited negative growth as compared to when the rainbow trout were present with only Atlantic salmon. These data suggest that, outside of density‐dependent effects, Atlantic salmon will not impact stream production of brown trout and rainbow trout.     

Linking habitat characteristics with juvenile density to quantify Salmo salar and Salmo trutta smolt production in the river Sävarån,Sweden

Abstract Habitat mapping along 85 km of river was related to juvenile (15 years of electric fishing) and smolt (3 years of screw‐trapping) abundance data to estimate salmon, Salmo salar L., and sea trout, Salmo trutta L., smolt production in the River Sävarån, northern Sweden. Spawning site selection by radio‐tagged salmon (n = 12) and sea trout (n = 4) was also assessed. Fifty‐one hectares of potential spawning and nursery habitat was found in the main stem river, representing 25% of the total river area. These areas were estimated to yield 1300–7580 salmon and 630–3540 sea trout smolts based on juvenile densities, equating with 3 years of screw‐trap data (2990–5080 salmon and 680–2520 trout smolts, respectively). A hypothetical maximum production of about 19 900 salmon smolts was predicted for the river at a density of 40, 0+ salmon 100 m^?2. Tracking adults during the spawning period identified optimal and potential reproductive areas.     

Selective segregation in intraspecific competition between juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Interactive segregation has been suggested as the ruling competition mechanism determining niche and niche segregation between juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). Results from allopatry–sympatry observations of habitat use in both nature and in experiments were contrary to predictions derived from the interactive segregation hypothesis. Habitat use parameters under natural conditions such as distance to shore for Atlantic salmon parr were nearly identical in allopatric (mean ± SD; 3.2 ± 1.4 m) and sympatric (3.3 ± 1.4 m) situations. Occupied water depths largely reflected available water, but water depths <15 cm were avoided by salmon parr. Under experimental conditions, habitat use of allopatric salmon was density independent and salmon size had only minor effects, with smaller fish being more likely to occur in the shallow. Habitat use of salmon in sympatry with trout did not differ from allopatric salmon habitat use, and only salmon size had minor effects on depth choice – occurrence of trout or fish density had no effect. Allopatric trout was in general more frequent in the shallow habitat than salmon. Habitat use of sympatric trout was affected by the occurrence of salmon and trout size, resulting in a higher use of the shallow habitats for small trout. To conclude, selective segregation has a dominant role in salmon habitat use (not affected by trout occurrence), whereas a mixed situation occurs in trout habitat use with elements of interactive segregation when competing with Atlantic salmon (affected by salmon occurrence).     

Lake-use by juvenile Atlantic salmon (Salmo salar L.) and other salmonids in northern Norway

Abstract– The utilization of lakes, and inlet and outlet streams by juvenile Atlantic salmon ( Salmo salar L.), brown trout ( Salmo trutta L.) and Arctic charr ( Salvelinus alpinus (L.), were investigated in 16 watercourses northern Norway, all known to inhabit salmon stocks. In lakes, fish were caught by small mesh size gill nets, while in rivers fish were caught electrofishing. In the shallow littoral (0-3 m depth) there were juvenile salmon in 15 of 19 investigated lakes, juvenile trout in 17 and juvenile charr in seven. Trout dominated significantly in numbers in the shallow littoral of seven lakes, while salmon and charr dominated in three lakes each. When trout and salmon were frequent in the shallow littoral, charr was usually not present in this habitat, but were found in the profundal zone in most of the lakes. Atlantic salmon parr utilized both shallow and deep lakes, and used both stones and macrophytic vegetation as shelter. The utilization of lakes by salmon parr seemed to be closely related to utilization of small inlet streams for spawning. In most inlet and outlet streams salmon dominated over trout in numbers, while charr were absent. This is the first documentation of lake-use by naturally occurring salmon parr in Scandinavia.     

Linking watershed and microhabitat characteristics: effects on production of Atlantic salmonids (Salmo salar and Salmo trutta)

The spatial scale of environmental factors influencing population dynamics ranges from microhabitat to continental or even global scales. Integration of multiple spatial scales is important in order to understand links between environmental variation and population processes. In the present study, we investigate how multiscale drivers influence the production of stream‐rearing Atlantic salmonids (Atlantic salmon, Salmo salar L. and brown trout, Salmo trutta L.) measured in terms of abundance. Variation in juvenile production was studied using data from single‐pass electrofishing surveys (measured as biomass per m²) from nine rivers. These data were combined with habitat data ranging from an important in‐stream microhabitat variable (shelter availability) to properties of the catchment. Variation in productivity within and among rivers was affected by both properties of in‐stream habitat and catchment properties. Shelter availability and the proportion of the catchment consisting of cultivated land and lakes influenced biomass positively, while catchment area had the opposite effect. For a different set of rivers (N = 20), river gradient and catchment area were shown to positively affect the amount of shelter. Finally, the variables identified in the two preceding analysis were included in the analysis of population productivity using catch statistics from 160 rivers. The proportion of cultivated land and lakes, estimated shelter availability were found to have positive effects. In addition, temperature had a positive effect, while river width had a negative effect. This study shows that combining multiple‐scale environmental factors can explain a substantial proportion of variation in population productivity among and within the populations of Atlantic salmonids.     

Ecology and movement of juvenile salmonids in beaver-influenced and beaver-free tributaries in the Trøndelag province of Norway

There is concern that expanding beaver (Castor fiber) populations will negatively impact the important economic, recreational and ecological resources of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) populations in Europe. We studied how beaver dams influenced habitat, food resources, growth and movement of juvenile Atlantic salmon and trout on three paired beaver-dammed and beaver-free (control) tributaries of important salmon rivers in central Norway. Lotic reaches of beaver-dammed and control sites were similar in habitat and benthic prey abundance, and ponds were small (<3,000 m²). Though few juvenile salmonids were detected in ponds, trout and salmon were present in habitats below and above ponds (comprising 9%–31% and 0%–57% of the fish collected respectively). Trout dominated control sites (79%–99%), but the greatest proportion of Atlantic salmon were upstream of beaver ponds (0%–57%). Growth rates were highly variable, with no differences in growth between lotic reaches of beaver-dammed and control sites. The condition and densities of juvenile salmon and trout were similar in lotic reaches of beaver-dammed and control sites, though one beaver-dammed site with fine sediment had very few juvenile salmonids. Beaver dams did not block the movement of juvenile salmonids or their ability to use upstream habitats. However, the degree of repeated movements and the overall proportion of fish moving varied between beaver-dammed and control sites. The small scale of habitat alteration and the fact that fish were able to move past dams makes it unlikely that beaver dams negatively impact the juvenile stage of salmon or trout populations.     

Direct and indirect effects of riparian canopy on juvenile Atlantic salmon,Salmo salar,and brown trout,Salmo trutta,in south‐west Ireland

Abstract Dense riparian tree canopy is generally found to have a negative effect on salmonid populations. Canopy can affect fish both directly via trophic impacts and its effects as cover, and indirectly via its effects on the distribution of instream vegetation. This study examined the impact of riparian canopy on the ecology of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in south‐west Ireland. Riparian canopy reduced the density, length and gut contents of juvenile salmon, but not brown trout. The negative relationship between canopy cover and fish size and feeding was strongly influenced by the abundance of instream macrophytes, which in turn varied as a function of catchment water chemistry. As a management strategy, the reduction of dense riparian canopy is unlikely to have the same effect on juvenile salmonids on all streams within an ecoregion because of differences in catchment‐wide factors.     

Effects of intra‐ and interspecific competition and hydropeaking on growth of juvenile Atlantic salmon (Salmo salar)

The effects of hydropeaking and intra‐ and interspecific competition on the growth performance (growth in length, mass and lipid content) of juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta were studied in six experimental channels (three experiencing hydropeaking and three controls with a stable discharge of water). Changes in the water‐covered area in the hydropeaking channels were small to avoid fish stranding. Each channel was divided into three similar‐sized sections and stocked with either low or high density of Atlantic salmon, or a mix of Atlantic salmon and brown trout, with the density of the latter equalling the high‐density treatment of Atlantic salmon. A marked effect of competition was visible as salmon in the low‐density treatment were significantly larger (27–33%) and had a higher mass (30–38%) than salmon in both the high‐density salmon treatment and the high‐density salmon and trout treatment. Hydropeaking had only minor and insignificant effects on the growth performance: overall final length, mass and body lipid content in the salmon experiencing hydropeaking differed by ?9%, ?7% and +2% compared with controls. Furthermore, there was no indication that the competitive regime influenced hydropeaking effects. The increase in both intra‐ and interspecific competition among the juvenile salmon had a pronounced and significant effect on growth. Our study adds to the growing evidence that energetic consequences of hydropeaking are likely to be small for Atlantic salmon and that stress and mortality associated with stranding represent the main source of population impact.     

Stomach Flushing of Salmonids: A Simple and Effective Technique for the Removal of the Stomach Contents

A stomach flushing technique is described which has been used for over a year, both in the laboratory and in the field. It is reliable, quick and relatively easy to perform and has been applied to juvenile Atlantic salmon (Salmo salar L.) and brown trout (S. trutta L.). The only limitation on its use was that fish <4 cm in length were considered too small to flush. Experiments carried out show that the technique removes 98.9% of the stomach contents from the fish and has very little effect on subsequent survival (99.3%) and condition. It is suggested that this technique is an improvement on previous designs.     

Competitive interactions among multiple non‐native salmonids and two populations of Atlantic salmon

Competitive interactions with non‐native species can have negative impacts on the conservation of native species, resulting in chronic stress and reduced survival. Here, juvenile Atlantic salmon (Salmo salar) from two allopatric populations (Sebago and LaHave) that are being used for reintroduction into Lake Ontario were placed into semi‐natural stream tanks with four non‐native salmonid competitors that are established in Ontario streams: brown trout (S. trutta), rainbow trout (Oncorhynchus mykiss), Chinook salmon (O. tshawytscha) and coho salmon (O. kisutch). Brown trout and rainbow trout reduced the survival and fitness‐related traits of Atlantic salmon, whereas Chinook salmon and coho salmon had no impact on these traits. These data support theories on ecological niche overlap and link differences in observed aggression levels with competitive outcomes. Measurements of circulating hormones indicated that the Atlantic salmon were not chronically stressed nor had a change in social status at the 10‐month time point in the semi‐natural stream tanks. Additionally, the Sebago population was better able to coexist with the non‐native salmonids than the LaHave population. Certain populations of Atlantic salmon may thus be more suitable for some environments of the juvenile stream phase for the reintroduction into Lake Ontario.     

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.     

Above parr: Lowland river habitat characteristics associated with higher juvenile Atlantic salmon (Salmo salar) and brown trout (S. trutta) densities

Understanding juvenile salmonid habitat requirements is critical for their effective management, but little is known about these requirements in lowland rivers, which include important but unique salmonid habitats. We compared the relative influence of in-stream Ranunculus cover, water depth, prey abundance, distance upstream and two previously unexplored factors (water velocity heterogeneity and site colonisation potential) on summer densities of juvenile Atlantic salmon and brown trout. We applied electrofishing, habitat surveys and macroinvertebrate kick sampling, and calculated the site colonisation potential from salmon redd surveys across 18–22 sites in a lowland river in 2015–2017. Due to a recruitment crash in 2016, models including and excluding this unusual year were explored. Excluding 2016 data, juvenile salmon densities showed a positive association with Ranunculus cover and numbers of nearby upstream redds, and a negative association with distance upstream from the tidal limit. Trout densities were positively associated with velocity heterogeneity, indicating a potential indirect influence of Ranunculus mediated by water velocity. When including 2016, year had the largest effect on densities of both species, highlighting the impact of the recruitment failure. These findings uncover interspecific differences in the habitat requirements of juvenile salmonids in lowland rivers. Velocity heterogeneity and site colonisation potential had high explanatory power, highlighting that they should be considered in future studies of habitat use. These findings demonstrate that temporal replication and recruitment dynamics are important considerations when exploring species–habitat associations. We discuss potential management implications and argue that Ranunculus cover could be an important management tool in conservation of lowland salmonids.     

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.     

Resource partitioning between lake-dwelling Atlantic salmon (Salmo salar L.) parr, brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.))

Abstract – Resource partitioning between Atlantic salmon parr, brown trout and Arctic charr was studied throughout the ice-free season in a north Norwegian lake. Juvenile salmon and trout (≤160 mm) utilized the littoral zone and juvenile charr the profundal, while adult trout and charr (>160 mm) were found in both. Juvenile salmon and trout had a similar diet, although trichopteran larvae were more important for the trout and chironomid pupae and three-spined sticklebacks for the salmon parr. Small salmon and trout parr (≤120 mm) had a higher diet overlap than larger parr (121–160 mm). The feeding habits of adult trout were similar to that of juvenile trout, but the former took larger prey items. At the population level, both salmon and trout were generalistic feeders with a broad diet, but at the individual level, both species had specialized on a single or a few prey categories. Juvenile charr were segregated from salmon and trout in both habitat and food utilization; they had a narrow diet consisting of chironomids and zooplankton, possibly reflecting their confinement to the profundal habitat which have a low diversity of potential prey. Larger charr also took zoobenthos and sticklebacks in the littoral zone. ^Note     

Effect of sudden increase in discharge in a large river on newly emerged Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) fry

Abstract– Downstrcam displacement of salmonid fry due to flow increase, from 12–15 m³/_s to >100 m³_s, was documented in the river Suldalslåen, Western Norway. In May only fry of brown trout ( Sulmo trutta ) occurred in the drift, while from the beginning of June only newly emerged Atlantic salmon ( Sulmo sular ) were found. The maximum number of Atlantic salmon fry drifting during a single day was estimated to be 17 000 individual. Their density in the drift was higher during the night than during the day, and their appearance in the drift coincided with the predicted period of emergence. Total brown trout numbers in the drift were estimated to vary between 4000 and 16 000 per day. Fry displaced downstream from the lowermost part of the river were lost from the population. The total losses were estimated to be between 75 000 and 100 000 Atlantic salmon fry which represents between 5.6 and 11.1%) of juvenile mortality during the first year of life.     

Seawater tolerance in Atlantic salmon, Salmo salar L., brown trout, Salmo trutta L., and S. salar?×?S. trutta hybrids smolt

High levels of hybridization between Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) have been reported in the Gyrodactylus salaris infected Rivers Vefsna and Driva in Norway. The survival and behaviour during the sea phase of such hybrids is unknown. The reported work documents ionoregulatory status after 24 h seawater challenge tests (24hSW) and gill Na⁺/K⁺-ATPase (NKA) activity of migrating wild smolts of Atlantic salmon, brown trout and hybrids at two sampling dates during the 2006 smolt run in River Driva. Salmon, trout and hybrids contributed to 27, 52 and 21% of the catches, respectively. The large contribution of hybrids suggests both a high hybridization rate and a high survival rate from fry to smolt. Both salmon and hybrids had a well-developed seawater tolerance at the time of downstream migration, revealed by small ionoregulatory effects and no or low mortality rates during the 24hSW tests. The trout were not fully adapted to seawater, and high mortality rates were observed (71 and 92%) during the 24hSW tests. The NKA activity was not significantly different between salmon and hybrids. Most of the hybrids were physiologically capable of direct entry to full strength seawater. The incomplete seawater tolerance in trout compared to salmon corresponds well with differences in life-history patterns between these two species. The life history strategy of the hybrids during the sea phase is not known, and further investigations on the marine behaviour and survival is needed to evaluate the role of hybrids in the risk of spreading G. salaris to nearby river systems.     

Homing behaviour of juvenile coho salmon (Oncorhynchus kisutch) within an off-channel habitat

Abstract – Although homing behaviour has been observed in juvenile Atlantic salmon, brown trout and resident cutthroat trout, this behaviour has not been well studied in juvenile Pacific salmon. We examined the site fidelity and homing behaviour of juvenile coho salmon ( Oncorhynchus kisutch ) by marking and relocating them within an off-channel habitat. Over 80% of displaced fish returned to the area from which they were originally collected. The proportion of fish that returned to the original location did not vary significantly among three sampling dates. However, we found that this proportion decreased over time in a brackish lagoon when we statistically analysed the data reported by Day (1966) . Our results suggest that juvenile coho salmon exhibit strong site fidelity and are able to return to their home ranges after displacement. These behaviours are likely to be important for the winter survival of juvenile coho salmon.     

A survey of salmon gill poxvirus (SGPV) in wild salmonids in Norway

In 2016, the Norwegian health monitoring programme for wild salmonids conducted a real‐time PCR‐based screening for salmon gill poxvirus (SGPV) in anadromous Arctic char (Salvelinus alpinus L.), anadromous and non‐anadromous Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.). SGPV was widely distributed in wild Atlantic salmon returning from marine migration. In addition, characteristic gill lesions, including apoptosis, were detected in this species. A low amount of SGPV DNA, as indicated by high Ct‐values, was detected in anadromous trout, but only in fish cohabiting with SGPV‐positive salmon. SGPV was not detected in trout and salmon from non‐anadromous water courses, and thus seems to be primarily linked to the marine environment. This could indicate that trout are not a natural host for the virus. SGPV was not detected in Arctic char but, due to a low sample size, these results are inconclusive. The use of freshwater from anadromous water sources may constitute a risk of introducing SGPV to aquaculture facilities. Moreover, SGPV‐infected Atlantic salmon farms will hold considerable potential for virus propagation and spillback to wild populations. This interaction should therefore be further investigated.