Stream channel experiments on downstream movement of recently emerged trout, Salmo trutta L., and salmon, S. salar L.—11. Effects of constant and changing velocities and of day and night upon dispersal rate

Four experimental stream channels were used to study instantaneous downstream dispersal rates of young trout and salmon relative to day and night, constant low and high water velocities and changes of velocity from high to low and vice versa.
At high and low velocity for trout and at high velocity for salmon, the dispersal rate was higher by night than by day. Changes of velocity during daytime from high to low or low to high for trout and from high to low for salmon, were associated with much higher dispersal rates than were constant velocities.     

Stream channel experiments on downstream movement of recently emerged trout, Salmo trutta L. and salmon, S. salar L.—I. Effect of four different water velocity treatments upon dispersal rate

Four experimental stream channels were used to study instantaneous downstream dispersal rates of young trout, Salmo trutta L., and salmon, S. salur L ., relative to four different water velocities.
Young salmon showed a high rate of dispersal at a low velocity of 7.5 cm s⁻¹ and lower rates at higher velocities of 25 to 70cm s⁻¹. Trout showed their lowest rate at 25cm s⁻¹ with a slightly higher rate at 7.5 cm s⁻¹ and increasingly higher rates at velocities in excess of 25 cm s⁻¹. These results are consistent with field observations on the velocity preferences of young trout and salmon.     

Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Based on data from Norwegian streams with sympatric populations of Atlantic salmon and brown trout, it is suggested that temporal segregation is the main mechanism segregating Atlantic salmon and brown trout during spawning. Peak spawning of trout was about 15 days earlier than that of salmon. Physical factors, such as water depth, water velocity and distance from the river banks segregate spawning sites of salmon and trout poorly. Gravel sizes of the redds of salmon and trout were significantly different, though with a considerable overlap, and mean egg depth of salmon and trout were 0.18 and 0.12 m, respectively, probably attributable to the different size of spawners of salmon and trout. None of the temporal or spatial parameters analysed segregate spawners of salmon and trout completely. Species determination of eggs and alevins from the redds showed no interspecific superimposition of redds. It is, therefore, concluded that low survival of hybrids after hatching does not explain the low frequency of hybrids observed in sympatric populations of salmon and trout.     

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.     

A preliminary investigation into the vulnerability of young trout (Salmo trutta L.) and Atlantic salmon (S. salar L.) to downstream displacement by high water velocities

The downstream movement of young trout and salmon in relation to water velocity was studied in simulated river channels. The results are interpreted as showing that these young salmonids pass through a short period when they are very vulnerable to downstream displacement by flow. Behavioural differences between the two species are considered with the influence on fish movement of changing rate of water velocity, light and temperature.     

The influence of water velocity on the downstream movement of alevins and fry of brown trout, Salmo trutta L.

Trout eggs were planted in four experimental stream channels, each channel being run at a different but constant discharge. Survival of the eggs to hatching was low and apparently unrelated to surface water velocity. However, movement of young trout out of channels was affected by water velocity, the higher the velocity the greater the proportion of trout that were lost. Virtually all fry moved out of a channel with a mean surface water velocity of 0.73 m s⁻¹. The rate of loss was not constant over the experiment but increased as the young trout entered the free-feeding stage. At the end of the experiment loss of fry, after abrupt increases in discharge, was demonstrated in the lower velocity channels.     

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.     

Sperm motility characteristics of wild Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta m. trutta L.) as a basis for milt selection

The aim of the study was to determine the sperm motility parameters in wild Atlantic salmon and sea trout to define criteria important for selection of milt for controlled fertilisation. Parameters for these species were determined in the fish migrating into north‐western rivers of Poland at spawning time. Eight motility parameters percentage of motile sperm (MOT), curvilinear velocity (VCL), average path velocity (VAP), straight line velocity (VSL), linearity (LIN), straightness (STR), amplitude of lateral head displacement (ALH), beat cross frequency (BCF) and motility duration were subjected to computer‐assisted sperm analysis (CASA). Milt of most individuals studied representing both salmon and trout showed spermatozoa density of 12–22 × 10⁹ ml^?1 and a high percentage of motile sperm (>70%). In general, spermatozoa swim progressively with slightly curved trajectories (mean STR = 70%, LIN = 65%) and velocity VCL of 180 μm s^?1 (salmon) and 190 μm s^?1 (trout), at 10 s post‐activation. Such sperm is easily accessible in the wild populations of salmon and sea trout and is recommended for use in reproduction trials. The spermatozoa of sea trout seem to show a greater tendency to follow curvilinear trajectories than those of salmon, both in the beginning and the final phase of motion. In the first phase of motility, the values and time dependencies of the motility parameters were similar in both species. In the end phase of movement differences in LIN and BCF time dependencies were found in the samples representing the two species. In salmon the linearity and beat cross frequency remained stable in this phase, contrary to the patterns in sea trout for which LIN decreased while BCF increased in the end period of movement. Durations of movement were similar in both species (ranges of 20–40 s).     

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean L_T 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (L_T 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles <7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.     

Downstream migration and critical water velocities in stream channels for fry of four salmonid species

Fry of brown trout, Atlantic salmon, brook trout and lake trout were tested for downstream migration and critical velocities with a method of stepwise increasing water velocities. Each velocity was tested for 15 min before increase to the next step. Critical velocities for fry entering the free-feeding stage, defined as the stage when the fry has resorbed its yolk sac and will have to ascend from the bottom gravel to catch food, were between 0.10 and 0.25 m s⁻¹, varying among individuals and depending on species and water temperature. Downstream displacement started at lower velocities. Lake trout had the lowest critical velocity. Temperature influenced swimming performance considerably. On average, a 7°C increase in temperature resulted in a 0.05 m s⁻¹ increase in critical velocity. The fry actively searched out the low-velocity niches in the channels. Flow-sensivity gradually decreases with fry development; when the fry had reached a length of 40–50 mm they were able to tolerate water velocities higher than 0.50 m s⁻¹.     

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.     

Seasonal and Diel Changes in Behaviour,Microhabitat use and Preferences by Young Pool-dwelling Atlantic Salmon,Salmo salar,and Brown Trout,Salmo trutta

There was a pronounced decline in activity of young pool-dwelling Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, as the water temperatures dropped in the autumn and early winter, and the fish switched from a predominantly diurnal towards a nocturnal activity pattern. Such a switch in activity pattern has previously been observed in young brown trout, but the present study is the first documentation for juvenile Atlantic salmon under natural conditions. Juvenile fish fed actively even when water temperatures were below 0°C, although foraging behaviour at near-freezing temperatures was recorded exclusively during night surveys. This indicates that other proximate factors, in addition to water temperature, affect the activity of young salmon and trout in rivers. Trout kept feeding positions significantly higher above bottom than salmon in August and September, but both species reduced the height above bottom at the onset of winter, possibly due to reduced swimming performance and lowered food availability in the upper part of the water column.     

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.     

Growth, survival and production of juvenile salmon and trout in a Scottish stream, 1966–75

Investigations of the growth, survival and production of young salmon Salmo salar , brown trout and sea trout S. trutta in sections of a stream in Scotland were made during 1966–75. At the end of the growing season, in autumn, the size of the 0+ salmon ranged from a mean weight of 1.12 g in 1966 to 2.82 g in 1973, and the size of the 0+ trout ranged from a mean of 2.20 g in 1966 to 3.56 g (68.0 mm) in 1974. Growth rates of 0+ salmon between July to September were similar from year to year, as was the case with the 0+ trout. The greater size attained in their first year by trout, resulted from the longer feeding season, provided by earlier emergence of fry and ability to continue growing in colder weather in autumn. The lengths attained by 0+ salmon and 0+ trout in September were related to the population densities of 0+ salmon and the number of days above 0° C from 1 December. There was no discernible relationship between lengths of 0+ trout and the population densities of 0+ trout. Salmon and trout lost weight during the winter, which was made up by April. The densities of 0+ salmon in June varied between 2–12m ^–2. Rates of decrease of the population densities in their first year were related to their densities at the beginning of the season, and, more closely, to the densities of salmon and trout combined. At the end of the second year's growth there were between 0.06 and 0.25 salmon m ^–2. Size of the trout populations varied less from year to year than those of salmon. The life of a year class of salmon and trout could be divided into several stages characterized by different rates of decrease of the population.     

Segregation by species and size classes of rainbow trout,Salmo gairdneri,and Sacramento sucker,Catostomus occidentalis,in three California streams

Synopsis The hypothesis that Sacramento suckers, Catostomus occidentalis, compete with rainbow trout, Salmo gairdneri, for space in streams was examined by measuring microhabitat utilization of both species in three California streams. Two streams were similar in most respects except one contained only trout and one contained trout and a large population of suckers. The third stream, formed by the union of the first two, contained trout and a small population of suckers. The species overlapped in five of the six microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. However, the species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the third stream and the other two was attributed to the larger size of the third stream. Both sucker and trout showed a similar within-species segregation of size classes - fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. These results suggest that competition for space between trout and suckers was not a major factor regulating microhabitat utilization of trout, although the possibility that larger suckers may displace small trout needs further study.     

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.     

Influence of hydrologic attributes on brown trout recruitment in low-latitude range margins

Factors controlling brown trout Salmo trutta recruitment in Mediterranean areas are largely unknown, despite the relevance this may have for fisheries management. The effect of hydrological variability on survival of young brown trout was studied during seven consecutive years in five resident populations from the southern range of the species distribution. Recruit density at the end of summer varied markedly among year-classes and rivers during the study period. Previous work showed that egg density the previous fall did not account for more than 50% of the observed variation in recruitment density. Thus, we expected that climatic patterns, as determinants of discharge and water temperature, would play a role in the control of young trout abundance. We tested this by analyzing the effects of flow variation and predictability on young trout survival during the spawning to emergence and the summer drought periods. Both hatching and emergence times and length of hatching and emergence periods were similar between years within each river but varied considerably among populations, due to differences in water temperature. Interannual variation in flow attributes during spawning to emergence and summer drought affected juvenile survival in all populations, once the effect of endogenous factors was removed. Survival rate was significantly related to the timing, magnitude and duration of extreme water conditions, and to the rate of change in discharge during hatching and emergence times in most rivers. The magnitude and duration of low flows during summer drought appeared to be a critical factor for survival of young trout. Our findings suggest that density-independent factors, i.e., hydrological variability, play a central role in the population dynamics of brown trout in populations from low-latitude range margins. Reported effects of hydrologic attributes on trout survival are likely to be increasingly important if, as predicted, climate change leads to greater extremes and variability of flow regimes.     

Susceptibility of rainbow trout Oncorhynchus mykiss,Atlantic salmon Salmo salar and coho salmon Oncorhynchus kisutch to experimental infection with sea lice Lepeophtheirus salmonis

Physiological, immunological and biochemical parameters of blood and mucus, as well as skin histology, were compared in 3 salmonid species (rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon O. kisutch) following experimental infection with sea lice Lepeophtheirus salmonis. The 3 salmonid species were cohabited in order to standardize initial infection conditions. Lice density was significantly reduced on coho salmon within 7 to 14 d, while lice persisted in higher numbers on rainbow trout and Atlantic salmon. Lice matured more slowly on coho salmon than on the other 2 species, and maturation was slightly slower on rainbow trout than on Atlantic salmon. Head kidney macrophages from infected Atlantic salmon had diminished respiratory burst and phagocytic capacity at 14 and 21 d post-infection (dpi), while infected rainbow trout macrophages had reduced respiratory burst and phagocytic capacities at 21 dpi, compared to controls. The slower development of lice, coupled with delayed suppression of immune parameters, suggests that rainbow trout are slightly more resistant to lice than Atlantic salmon. Infected rainbow trout and Atlantic salmon showed increases in mucus lysozyme activities at 1 dpi, which decreased over the rest of the study. Mucus lysozyme activities of infected rainbow trout, however, remained higher than controls over the entire period. Coho salmon lysozyme activities did not increase in infected fish until 21 dpi. Mucus alkaline phosphatase levels were also higher in infected Atlantic salmon compared to controls at 3 and 21 dpi. Low molecular weight (LMW) proteases increased in infected rainbow trout and Atlantic salmon between 14 and 21 dpi. Histological analysis of the outer epithelium revealed mucus cell hypertrophy in rainbow trout and Atlantic salmon following infection. Plasma cortisol, glucose, electrolyte and protein concentrations and hematocrit all remained within physiological limits for each species, with no differences occurring between infected and control fish. Our results demonstrate that significant differences in mucus biochemistry and numbers of L. salmonis occur between these species.     

Otolith shape discriminates between juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Otoliths of Atlantic salmon, Salmo salar L., are more slender than the otoliths of brown trout, Salmo trutta L. Discriminant analysis on otolith measurements of juvenile Atlantic salmon and brown trout from four river systems revealed a discriminant function which distinguished more than 94% of the cases. This function was tested by using data from a fifth river with cohabiting Atlantic salmon and brown trout: all Atlantic salmon and 91 % of the brown trout were correctly classified.     

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.