The effects of intra- and inter-specific competition on the survival and growth of stocked juvenile Atlantic salmon, Salmo solar L., and resident trout, Salmo trutta L., in an upland stream

The relative effects of inter- and intra-specific competition on the survival and growth of stocked salmon were investigated in an upland trout stream during summer and winter sampling periods. The stream was divided into two areas by an impassable fish barrier, and trout were removed from the upstream section prior to 2 years of salmon stocking. Salmon fry stocked into the cleared area survived more than twice as well and grew significantly larger than those stocked into the area containing trout and older salmon. Intra-specific competition from older salmon in the second year of stocking in the cleared area significantly reduced the survival and growth of the O+ salmon. However, these were still significantly larger and survived better than those in the control area where inter-specific competition from trout was maintained. Some immigration of trout to the cleared area occurred; these showed greatly enhanced growth rates compared to those in the control area, reflecting low intra-specific trout competition in the former. Inter-specific competition effects of older salmon on both trout fry growth and survival were also detected, although the latter did not become apparent until the winter. This is discussed in terms of the relative importance of biotic and abiotic regulating mechanisms. Evidence of allopatric niche segregation is also discussed, since salmon in the cleared area did not have a biomass equivalent to that in the area which also contained trout.     

The distribution of salmonids in upland streams in relation to depth and gradient

The distribution of various age classes of salmon and trout was assessed in upland streams by electrofishing. Water depths and site gradients were measured and correlated to fish densities. The fry of both species were significantly more abundant in shallow water; up to 75·3% of salmon fry and 72·2% of trout fry were captured in sites of mean depth < 20 cm. Older trout were found mainly in the deeper areas, with a maximum of 7·4% captured in sites < 20 cm mean depth. Yearling fish were found in all the depth-ranges sampled, but with a tendency for higher numbers in mid-range depths. There were similar correlations in the abundance of each age class with the actual areas of shallow, mid-range and deep water habitat available within sites. Correlations of fish density with gradient indicated that trout were limited in their distribution to areas of lower flow, whereas salmon were not. Since depth and gradient were significantly negatively correlated, there was an apparent preference of trout for slightly deeper habitats than the equivalent year classes of salmon. The observed habitat segregation is discussed in terms of competition and selection.     

Microhabitat use by brown trout, Salmo trutta L. and Atlantic salmon, S. salar L., in a stream: a comparative study of underwater and river bank observations

Two methods, visual observation from the river bank and visual observation underwater by diving, were compared for microhabitat studies in young brown trout and Atlantic salmon in a stream. A wide range of habitat conditions were surveyed. Each method yielded different results with respect to microhabitat use. River bank observations missed small fish under surface turbulence and in deeper waters. Underwater observations missed small fish in shallow areas.     

Seasonal and spatial microhabitat selection and segregation in young Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Norwegian river

Seasonal microhabitat selection by sympatric young Atlantic salmon and brown trout was studied by diving. Both species, especially Atlantic salmon, showed seasonal variation with respect to surface and mean water velocities and depth. This variation is partly attributed to varying water flows and water temperatures. In winter the fish sought shelter in the substratum. A spatial variation in habitat use along the river due to different habitat availabilities was observed. Both species occupied habitats within the ranges of the microhabitat variables, rather than selecting narrow optima. It is hypothesized that the genetic basis allows a certain range to the behavioural response. Microhabitat segregation between the two species was pronounced, with brown trout inhabiting the more slow-flowing and partly more shallow stream areas. Atlantic salmon tolerated a wider range of water velocities and depths. Habitat suitability curves were produced from both species. It is suggested that habitat suitability curves that are based on observations of fish occupancy of habitat at median or base flow may not be suitable in habitat simulation models, where available habitat is projected at substantially greater water flows.     

Population changes after two years of salmon (Salmo salar L.) stocking in upland trout (Salmo tmtta L.) streams

The survival of salmon stocked in upland trout streams in the presence of salmon parr was found to be only about half the value recorded when trout alone made up the resident stock. Changes in the trout population were also recorded following the two years of salmon stocking, and these suggested that the presence of salmon parr may also influence trout fry survival. The findings are discussed in the context of habitat competition and total stream holding capacity.     

Movements of brown trout, Salmo trutta, and juvenile Atlantic salmon, Salmo salar, in a coastal stream in northern Norway

Movements of resident brown trout (age 2+ to 9+ years) and young Atlantic salmon (age 1+), stocked as fry, were studied in July, August and September in a coastal stream in northern Norway. Between 85 and 89% of the brown trout were recaptured in the study area (346m, 1326m²) within 45m of their release point throughout the investigation period. Most specimens had moved less than 150m. Trout movements were related to local variation in density. Trout occupying those sections of stream with the lowest fish densities (5.3–10.9 fish 100m^?2) had a significantly lower movement rate than fish from sections with densities between 13.7 and 31.5 fish 100m^?2. Trout that moved from their marking section were significantly larger than specimens that did not leave their original site. There was a significant correlation between permanence of station each month and the mean water level that month. The majority of the trout (47%) were caught at undercut stream banks or at sites in the proximity of these. A decrease in water level during the study period resulted in a high loss (36%) of such habitat, probably forcing some individuals to move. The recapture data indicate that the trout population consists of one small (c. 15–20%) mobile, and one large sedentary component. Young salmon displayed high station permanence in July and August (93% and 96%). However, in the autumn they exhibited a significant downstream movement, and only 73% were recaptured within their release section. This movement was significantly higher for larger specimens, and is thought to occur because of a pre-winter change in habitat, initiated by a decline in stream temperature. In contrast to trout, salmon in sections containing the lowest densities (22.0–25.0 fish 100m^?2) did not show significantly lower movement rates when compared with salmon at higher densities (32.2–46.3 and 51.8–60.6 fish 100m^?2). The spatial distribution of young salmon indicated the formation of territorial mosaics over the stream bed, which are thought to reduce intraspecific competition.     

Influence of water depth on dispersion of juvenile salmonids, Salmo salar L. and S. trutta L., in a Scottish stream

The dispersion of salmon and trout of 0 +, 1 + and 2 + age classes was examined by electrofishing 12 sections of a stream in Perthshire, Scotland at the end of the growing season in September to October in 1972 and 1976. In 1972 sections ranged in surface area from 21 to 122 m² and had volumes of 5.0–17.3 m³ and when surveyed in 1976 surface areas were 15–106 m² and volumes 2.1–6.7 m³. In 1976 widths and areas of sections were 63–87% of the values for 1972, depths were 39–68% and volumes 28–52% of the earlier values. These differences were due to 1976 being drier and warmer. In both years, all sections contained 0 + and 1 + age classes of salmon and trout and some sections contained 2 + age salmon and trout. The total density of fish in a section ranged from 1.9 to 3.9 m⁻² in 1972 and from 3.72 to 6.08 m⁻² in 1976. There was an inverse relationship, significant in 1972 only, between the density of 0 + salmon and that of 1 + trout in the different sections. Densities of both 0 + and 1 + salmon in the sections were inversely correlated, and those of 0 + and 1 + trout were directly correlated, with area of water deeper than 10 cm.     

Production of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., within different sections of a small enriched Norwegian river

Growth, density and production of juvenile Atlantic salmon and brown trout were studied in three different sections of the Kvassheimsåna River in south-western Norway from 1979 to 1983. Section 1. in the upper part of the river, is located above a waterfall impassable for migratory salmonids and is surrounded by grazing land. Sections 2 and 3, in the middle and lower parts of the river, are influenced by agricultural activity. Total nitrogen concentration varied between 250 and 1000 μg l ^?1 in section 1 and 1500 and 2500 μg l^?1 in sections 2 and 3. Total phosphorus (Tot-P) concentrations also increased with decreasing altitude: 19–46 μg l^?1 in section I and 31–101 μg l ^?1 in sections 2 and 3. The number of 0 + salmon in sections 2 and 3 varied between 30.1 and 167.8 specimens 100 m ^?2, with means 90.2 and 95.2 specimens 100 m ^?2:, respectively; the density of 1 + salmon, with mean values of 16.3 and 51.0 specimens 100m^?2 was significantly correlated with the original fry density. The growth rate of 0+ salmon was not inversely related to cohort density, but was significantly so for 1 + salmon. Mean annual salmon production in section 2 was 1595 g 100 m^?2 year ¹, and in section 3 was 841 g 100m^?2 year ¹. A logarithmic function gave the best curve fit between salmon production and mean annual biomass. Thus, production levelled off for the highest values recorded in section 2, and perhaps approached the carrying capacity of the stream. A multiple regression analysis showed that yearly variation in 1 + salmon density was the single factor accounting for most of the total variability in production (60%). Variation in water temperature and nutrient content were not significantly related to variation in fish production. Densities of brown trout were low in all sections (<20 specimens 100m ^?2). Fry density was highest in section 3 and parr density in section 1. All age groups of sympatric brown trout grew significantly faster in sections 2 and 3 compared with allopatric brown trout in section 1.     

Effect of mink, Mustela vison Schreber, predation on cohorts of juvenile Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in three small streams

Two cohorts of Atlantic salmon parr and one of brown trout were studied in periods with and without the presence of mink, Mustela vison . In all localities a marked increase in mortality rate was observed during periods when mink were present. Mink were observed catching salmon parr, and approximately 10% of the parr had bite marks, especially on the tail fins. In the smallest stream with brown trout, the mortality rate was 0.80 during a few days with mink present; remnants of trout were found along the stream. The present study suggests that mink predation may be a major cause of mortality of salmonids in small streams. The results indicate that predation efficiency may vary with characteristics of the habitat, especially stream width and discharge, and fish density.     

An electrophoretic method for distinguishing the young fry of salmon Salmo salar (L.) from those of trout Salmo trutta (L.)

Young fry of salmon were distinguished from those of trout using electrophoresis of whole fry extracts on cellulose acetate sheets. The separation patterns were revealed by general protein staining. The method was found helpful in resolving the mixed salmon and trout fry of an upland stream.     

Effects of predation, trout density and discharge on habitat use by brown trout, Salmo trutta, in artificial streams

1. The effects of predation risk, fish density and discharge on habitat use by juvenile brown trout, Salmo trutta, in four artificial streams were studied. Each stream contained three habitats, riffles, runs and pools, the latter two each being further divided into shallow margins and deeper mid-regions. 2. The presence of northern pike, Esox Indus, caused trout to decrease use of pool midregions, where pike also occurred, and to increase use of other habitats. Increasing the number of trout caused trout to increase use of pools and the shallow margins of runs. Decreasing discharge reduced the area of the run and pool margins covered by water, thereby reducing use of these areas by trout. 3. Habitat selection indices for the different treatments were calculated. The data indicated that riffles and the mid-regions of runs were preferred habitats, whereas run margins and pools were inferior habitats used when intraspecific fish densities were high. 4. Despite density- and discharge-dependent habitat use by trout, the number of trout consumed by pike was independent of trout density and discharge. 5. The results reveal the flexibility of habitat use by trout and illustrate the potential danger of applying data on habitat use in one stream to others where habitat availability and bioric interactions may differ.     

Infestations by Gyrodactylus sp. of Atlantic salmon, Salmo salar L., in Norwegian rivers

Young Atlantic salmon, Salmo salar , brown trout, Salmo trutta , and Arctic char, Salvelinus alpinus , from eight rivers in North and Mid Norway were examined for Gyrodactylus. The fish were collected from 1975 to 1980. A Gyrodactylus salaris type was observed infecting salmon from six of these rivers. No trout or char were infected. A high frequency and intensity of infection of salmon were observed in all but one of the rivers surveyed. In the Saltdalselva, only one specimen of Gyrodactylus infecting one fish was observed. No subsequent mortality of salmon was observed in this river while there were signs of a high mortality of salmon in the other rivers. The salmon parr were more frequently attacked than the fry, the mortality of salmon seemed to have happened in most of the rivers one year after the first observations of Gyrodactylus were made. The mortality of salmon and the high infection rate of Gyrodactylus in these rivers appears unique, and as far as known to the authors there are no other described cases of mortality due to Gyodactylus in Atlantic salmon in natural waters. The reasons for the outbreak of Gyrodactylus in these rivers are not known. Two theories are discussed: one that the fish were weakened by environmental factors and the other that Gyrodactylus was introduced from some of the infected salmon hatcheries in Scandinavia.     

Prey availability and selective feeding of sea trout (Salmo trutta L., 1758) fry stocked in small forest streams

The food base and stomach contents of stocked sea trout (Salmo trutta) fry were determined in the first 4 months (April–August 2010) of life in two small lowland streams after resorption of the yolk sac. In each stream, 600 individual trout were released in a 200 m stocking‐section. The macrozoobenthos given as a food base for the fry were collected once a month from the streams using a bottom scraper. Three subsamples of macrozoobentos were considered as one sample from the stocking‐section. On the same day, 50 stocked fry were captured in each stream using electric fishing gear. Preferred food components were usually the taxa represented by number in a given stream in a particular month. Width size range of the prey in fish stomachs in May was from 0.10 to 1.47 mm, and from 0.11 to 3.78 mm in August. All food items found in both streams during the study months were also represented in intensity in the fish: Cyclopoida, and larvae of Baetidae and Chironomidae. Almost all Helodidae and Simuliidae larvae as well as the majority of Limoniidae and Nemouridae larvae were available as food each month.     

Survival and growth of salmon, Salmo salar (L.), planted in a Scottish stream

Salmon eggs and unfed fry were planted in reaches (total length 2.8 km, mean width 4 m) of a Scottish stream between 1971 and 1977 and their subsequent progress was studied by sampling 16 sections (areas 38–126 m²) of the stream. Brown trout are the only fish which spawn in the stream, waterfalls and a dam near its mouth preventing adult salmon and sea-trout passing upstream. There were no restraints on the downstream movement of fish except in 1977, when a fry trap was operated. In 1971 and 1974 boxes each containing 300 eggs were buried in groups of 3–6. In other years fry were evenly distributed at 3.6–29.3 m^?2. At the end of the first growing season, salmon occurred at decreasing population densities for a distance of 600 m below the plantings, but after two growing seasons there was little remaining indication of their pattern of dispersion when planted. Rates of survival between planting and the end of the growing season were 9.4–31%. Survival when eggs were planted (11.1–14.8%) was not affected by the numbers planted together at one point (900–1800) or the distance apart of groups of boxes (10–85 m). When fry were planted the instantaneous mortality rate (M) of the 0₊ salmon during their first growing season was related to the initial stocking density (D_p) by the formula M= 0.00637 + 0.00444 log₁₀D_p. Twenty-two to 88% of 0₊ salmon present at the end of the growing season were still surviving in the stream as 1₊ fish one year later. In 1973–1976 only a small number of 2₊ salmon occurred, the majority having migrated between the end of the second growing season and the following spring. There were more 2₊ salmon in 1977 and 1978 resulting from higher stocking densities in 1975 and 1976 and slower growth. Trout of several age classes were present but their population densities were never high (<0.6 m^?2). Salmon reached a greater size than trout by the end of the first growing season. Their mean weight (W^o, g) at this time was inversely related to their population density (D_o No. m^?2) and the biomass (B₁, g m^?2) of 1₊ salmon present, giving the relationship log₁₀w^o= 0.6584–0.0558 D₀-0.0352B₁. The mean weight of 1₊ salmon tended to be highest in sections where the 0₊ salmon had reached a relatively large size the previous year. When a reach of the stream was planted twice (11 and 30 May 1977) with salmon fry (total 13.9 m^?2) at the same stage of development, M during the first growing season was 0.0099 per day. This was less than that of fry in a control (M= 0.0107) where the stocking density was lower (6.8 m^?2) and also less than in previous years when single planting rates of approximately 14 m^?2 were used (M=0.0115). The double planting resulted in a wide range of lengths of 0₊ salmon in September and the highest biomass values encountered during all experiments.     

Home range of juvenile Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, in a Norwegian stream

SUMMARY. 1. The sizes of home ranges of juvenile Atlantic salmon (age 1 +) and brown trout (age 2+ to 9+) in a Norwegian coastal stream were estimated by local movements of batch-marked fish from 12.5 and 25 m long sections. Only recoveries made in the release section and in up-and downstream neighbouring sections were considered.
2. There was no significant difference in the average percentage of recaptures of salmon and trout between 12.5 and 25 m sections; a stream area of about 40–50 m² defines the size of home range for stocks of both species.
3. The fraction of brown trout recaptured in release sections increased with increasing fish densities, indicating a smaller home range under these conditions.     

An experiment on faster growth of salmon Salmo salar (L.) in a Scottish stream

The experiment was made in an attempt to modify the usual relationship in which young trout grow faster than young salmon in streams in which they occur together. A stretch of a trout stream was stocked with advanced salmon eggs, which produced fry earlier than the trout eggs laid naturally. The salmon grew faster than the trout and were longer than the trout at the end of the growing season. The mean length of 77.7 mm attained by the salmon is the largest known size reached by salmon in their first year when feeding on natural food supplies in streams in Scotland. Survival rate from egg planting to production of salmon of this mean length was high.     

The growth of juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Scottish river system subject to cooling-water discharge

The River Fiddich, a tributary of the R. Spey in north-east Scotland, is a spawning river for both Atlantic salmon and brown trout. Warm cooling water effluent is discharged from several distilleries at different points in the lower reaches and raises the temperature of the river 1–3°C above ambient for most of the year. Salmon and trout grow more rapidly in this region than further upstream, and juvenile salmon generally migrate a year earlier, as 2 + smolts. Available data were too few to determine whether there was a similar difference for trout. Similar studies on the R. Dullan, a tributary of the Fiddich, and on the Cromdale Burn in the same area, confirmed that the growth rate of fish is faster downstream from distillery discharge points. It is suggested that increased invertebrate production may influence the growth rate.     

Comparison of allopatric cutthroat trout stocks with those sympatric with coho salmon and sculpins in small streams

Synopsis Juvenile stocks of allopatric (upstream of barrier falls) cutthroat troutSalmo clarki and those sympatric (downstream of barrier falls) with coho salmonOncorhynchus kisutch and sculpinsCottus spp., were sampled during the late summer period of low flows in six small coastal streams in British Columbia. The objective was to obtain and compare information on pattern of habitat use and fish size distribution of these two trout types. In most instances, density (n m^–2; g m^–2) of cutthroat trout was considerably greater in pools and glides in the allopatric than in the sympatric stocks. The sympatric salmonids were dominated by juvenile coho salmon in pools and cutthroat trout in riffles. Sympatric cutthroat trout constituted from 7 to 45 % of the salmonids present in pools and from 50 to 90% in riffles. Glides were areas of intermediate densities for both salmonids, although coho salmon was the more abundant species in most instances. The density of sculpins was high in all three habitat types, and frequently it exceeded that of coho salmon and cutthroat trout combined. Sympatric cutthroat trout consisted primarily of underyearling fish, whereas allopatric cutthroat trout consisted primarily of two or more age classes with a large proportion of them living in pools. When tested in a laboratory stream both types of cutthroat trout had similar habitat preferences and agonistic behaviours, with the exception that allopatric trout made greater use of cover and defended pools more intensely than sympatric trout when the flow was increased. The results of this study provide insight of potential impact of coho salmon juvenile transplants into stream segments supporting allopatric cutthroat trout.     

Growth and survival of Atlantic salmon (Salmo salar) in Llyn Dwythwch, North Wales

SUMMARY. Atlantic salmon fry have been annually stocked into Llyn Dwythwch, North Wales, since 1969, in an attempt to increase the natural stocks of the area. The growth and survival of 1- and 2-year-old salmon were investigated, and compared with that of other lake-reared populations and also with salmon in the natural stream environment. Lake-reared salmon follow the same patterns of slow and rapid growth as found for river fish, but the growth rate was superior in the former. The variation in length – weight relationship with age and sex was investigated. Survival rates in general compared favourably with the survival in rivers, with high mortality rates of salmon in Llyn Dwythwch resulting from predation at spring stocking by the resident brown trout. This was later avoided by stocking larger fish in the autumn.     

Interspecific relationships between emerging Atlantic salmon, Salmo salar, and coho salmon, Oncorhynchus kisutch, juveniles

Interspecific relationships between Atlantic salmon and coho salmon were studied at early life stages in laboratory and semi-natural stream channels. During emergence, the survival and dispersal patterns were similar for the two species in single or mixed populations. Survival of Atlantic salmon fry was reduced in the presence of older coho fry. However, no predation was observed. Microdistribution differed between the two species, with Atlantic salmon fry more numerous in riffles when coho were present.
Coho juveniles had a pelagic and gregarious distribution, in contrast to the benthic behaviour of the Atlantic salmon. In laboratory streams, Atlantic salmon fry moved out or adopted a subordinate cryptic behaviour which allowed them to escape predation while negatively affecting their growth.