Enhanced growth and feeding rate in Atlantic salmon parr occupying a lacustrine habitat in the River Utsjoki, northern Scandinavia

The length, weight and stomach fullness index of salmon parr from a lake were higher than in a fluvial habitat on the same river in northern Finland, but no differences in diet composition were detected between the habitats in August. The most important food items of the parr in both habitats were insect pupae and adults. Some lake parr also fed on fish.     

Growth of Atlantic Salmon parr in fluvial and lacustrine habits

The growth of naturally recruited Atlantic salmon Salmo salar parr in fluvial and lacustrine habitats was investigated in three northern Norwegian watercourses. Salmon parr were sampled in autumn with small mesh gillnets in lakes, and by electrofishing in streams. In three northern Norwegian watercourses, lacustrine Atlantic salmon Salmo salar parr were larger in length and weight at age than fluvial parr. Back-calculated lengths indicated that lacustrine salmon parr were recruited from the fastest growing part of the populations and they had larger growth (length) increments between age intervals than fluvial parr. There was no consistent pattern in growth comparisons between parr from the inlet and outlet streams. Temperature regime alone could not explain the growth difference observed between parr from the different habitats.     

Comparative proximate body composition of Atlantic salmon with emphasis on parr from fluvial and lacustrine habitats

The nutritional status of Atlantic salmon Salmo salar parr was assessed by examining how proximate body composition and energy content (kJ g⁻¹) were associated with the habitat rearing origin of parr utilizing fluvial or lacustrine areas. Comparisons were made among sampling sites within the two habitats, as well as between the two major habitat types applying both fixed effect and random effect models. Mean protein concentration varied from 13·87 to 15·67% among fluvial sites, and from 14·96 to 16·38% among lacustrine sampling locations. Mean fat concentration ranged from 1·71 to 3·32% and from 2·22 to 4·29% among fluvial and lacustrine locations, respectively, with energy concentration, on average, c . 11% higher in lacustrine parr. Interpretation of results of comparisons between habitats was dependent upon the statistical model used. Benefits associated with lacustrine rearing, such as greater body size and nutritional status, have implications for overwintering survival in fresh water as well as subsequent survival during initial sea entrance as smolts.     

Habitat selection by juvenile Atlantic salmon: the interaction between physical habitat and abundance

The effect of physical river habitat variables on the distribution of juvenile Atlantic salmon Salmo salar L. in the Rivière de la Trinité, Québec, Canada, was examined using generalized additive modelling. A survey of Atlantic salmon fry and parr densities and habitat variables (flow velocity, water column depth and substratum size) was conducted in the summer months from 1984 to 1992. Clear patterns of habitat use existed: specific ranges of habitat variables were selected, with parr preferring greater velocities, depths and substratum sizes than fry. There was a large variation, however, in juvenile densities for given velocities, depths or substratum sizes, with this variation being greatest in optimal habitats. On examination of an individual year, interaction between the variables was found to explain some of the variation. On a year‐to‐year basis the juvenile Atlantic salmon population was found to exhibit an 'Ideal Free Distribution', which resulted in greatest variation in optimal habitats with year‐to‐year changes in population abundance.     

Body and scale growth of wild Atlantic salmon smolts during seaward emigration

The relationship between scale and body growth for emigrating Atlantic salmon, Salmo salar, smolts was previously not understood and therefore was examined in this study using mark-recapture techniques. The size of smolts at time of recapture was significantly greater than when marked (P = 0.0002). The growth in length of smolts emigrating 5 km over an average of 20 days was 7.7 ± 6.1 mm per day. Instantaneous somatic growth (G _body) ranged from 7.0 × 10⁻⁴ to 5.1 × 10⁻³ (mean = 2.7 × 10⁻³ ± 1.3 × 10⁻³). The mean number of plus growth circuli present per scale was significantly greater for smolts when recaptured compared to when marked (P = 0.0014). The instantaneous growth rate of scales (G _scale) ranged from 1.4 × 10⁻³ to 11.5 × 10⁻³ (mean = 6.6 × 10⁻³ ± 4.3 × 10⁻³). The relationship between body size and scale radius showed positive allometry rather than isometry. The relationship of G _scale with G _body showed positive allometry indicating that scales grew at a slightly faster rate than the body during the emigratory period.     

Experimental investigation of the effects of temperature and feeding regime on scale growth in Atlantic salmon Salmo salar post-smolts

Salmo salar post-smolts were reared in seawater under controlled laboratory conditions for 12 weeks. The fish were exposed to three constant temperature treatments (15, 10.5 and 6°C) and four feeding treatments (constant feeding, food withheld for 7 days, food withheld for 14 days and food withheld intermittently for four periods of 7 days). Scale growth was proportional to fish growth across all treatments, justifying the use of scale measurements as a proxy for growth during the early marine phase. The rate of circuli deposition was dependant on temperature and feeding regime and was generally proportional to fish growth but with some decoupling of the relationship at 15°C. Deposition rates varied from 4.8 days per circulus at 15°C (constant feeding) to 15.1 days per circulus at 6°C (interrupted feeding). Cumulative degree day (^°D) was a better predictor of circuli number than age, although the rate of circuli deposition ^°D⁻¹ was significantly lower at 6°C compared with 15 and 10.5°C. Inter-circuli distances were highly variable and did not reflect growth rate; tightly packed circuli occurred during periods without food when growth was depressed, but also during periods of rapid growth at 15°C. The results further current understanding of scale growth properties and can inform investigations of declining marine growth in S. salar based on interpretations of scale growth patterns.     

Influence of habitat structure and fish density on Atlantic salmon Salmo salar L. territorial behaviour

A laboratory stream experiment was performed to test the influence of young‐of‐ the‐year Atlantic salmon Salmo Salar density on habitat selection ( i.e . pools and riffles) and the combined effect of habitat and density on territoriality. Pools were preferred at low and medium fish density. At higher density, the saturation of the riffles led to an equal fish repartition in both habitats. Aggressive interactions increased according to fish density, and the level of aggression was significantly higher in the pools than in the riffles.     

Territory size and distribution of newly-emerged Atlantic salmon (Salmo salar)

Newly emerged Atlantic salmon (Salmo salar) were observed from May to August 1981 at six isolated redds in Washington County, Maine, USA. Territorial size and distribution were measured. At the end of the emergence period (12 to 28 May), fish maintained positions (stations) at redds where water velocity did not exceed 52 cm s^–1 By 12 June, most salmon (80–96%) had moved off the redds of origin and had established territories 1 to 5 m from the redd. The area defended increased substantially after mid-June, but territorial aggression diminished by 15 July, and the fry dispersed downstream. All fish observed were territorial, and the percentage of time during which stations were held decreased from 89 in mid-May to as low as 40% in mid-June.Cooperators are the Maine Department of Inland Fisheries and Wildlife, University of Maine, U.S. Fish and Wildlife Service, and the Wildlife Management Institute     

A synopsis of ‘hydropeaking’ studies on the response of juvenile Atlantic salmon to experimental flow alteration

In insular Newfoundland, Canada, studies were conducted from 1999 to 2003 on the effects of ‘simulated’ hydropeaking power generation on juvenile Atlantic salmon (Salmo salar). In 1999, Atlantic salmon parr were released into an experimental reach below a hydroelectric facility and flow was manipulated over a range of discharge (1.0–4.2 m³ s⁻¹) during a series of ‘experiments’ simulating hydropeaking in both summer and fall. Fish were implanted with radio transmitters, manually tracked, precisely located (±1 m), habitat selection evaluated, and movement response determined. Experiments were continued in 2002 and 2003 to contrast response of salmon between summer and winter, the magnitude of flow changes were greater (0.7–5.2 m³ s⁻¹) and changes were made more rapidly (instantly). As discharge was increased, velocity and depth use by parr increased, and fish adapted behaviourally by increased contact with the substrate. Salmon parr also exhibited two distinct movement patterns in the summer and fall of 1999 studies; high site fidelity or considerable movement during trials. Salmon were more mobile during both static and dynamic flow conditions and throughout the diel cycle in the summer of 2002 experiment, and 2 fish were stranded and died, the only time this happened in the four series of experiments. Within each experiment generally there were no differences between movements at static high and low flows for day and night movements, with one exception, and night time movements were always greatest, again with one exception. During dynamic flow changes, within each experiment, distances moved during down ramping and up ramping were not significantly different except in the summer of 1999. Overall, comparing between experiments for up and down ramping events, distances moved in the summer of 2002 were statistically higher than for all other experiments. Not surprisingly, the home ranges of fish in the summer of 2002 were also the greatest while the smallest home ranges were in the winter of 2003. Results suggest hydropeaking regimes may be energetically costly potentially affecting over-winter survival which is related to energy reserves obtained during summer. Collectively these studies provide comprehensive information on the response of juvenile Atlantic salmon parr to hydropeaking, on both diel and seasonal scales, and will assist hydro producers and regulators design and operate hydropeaking regimes to minimize ecological impact. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Interhabitat and instream movements of large Atlantic salmon parr in a Newfoundland watershed in winter

Radio‐telemetry was used to investigate movement of large, mainly mature male (80%) Atlantic salmon Salmo salar parr in Stoney River, Newfoundland during early winter (November; water temperature 6·0 ± 0·1° C) and mid‐winter (January to February; 0·8 ± 0·0° C). Site fidelity of parr in early winter was low. Parr moved between fluvial and lacustrine habitats and were active throughout the diel cycle. Parr caught in fluvial habitats in mid‐winter were smaller and younger than parr caught in early winter. Site fidelity of parr in mid‐winter was greater than in early winter. Parr in mid‐winter moved between fluvial and adjacent small lacustrine habitats, but avoided a larger pond inhabited by large piscivorous fishes. Instream movement rates in mid‐winter were lower than in early winter and occurred primarily during hours of darkness (dawn, dusk and night). Fluvial habitats were relatively stable and ice‐free throughout the study periods. These results suggested that large Atlantic salmon parr utilize a variety of habitats and remain active throughout the winter, even under stable environmental conditions.     

Endurance swimming of diploid and triploid Atlantic salmon

When groups of diploid (mean ±  s . e . fork length, L _F) 33·0 ± 1·4 cm and triploid (35·3 ± 0·5 cm) Atlantic salmon Salmo salar were forced to swim at controlled speeds in a carefully monitored 10 m diameter 'annular' tank no significant difference was found between the maximum sustained swimming speeds ( U _ms, maintainable for 200 min) where the fish swam at the limit of their aerobic capability. Diploids achieved 2·99 body lengths per second (bl s⁻¹)(0·96 m s⁻¹) and triploids sustained 2·91 bl s⁻¹(1·02 m s⁻¹). The selection of fish for the trials was based on their ability to swim with a moving pattern projected from a gantry rotating at the radius of the tank and the selection procedure did not prove to be significant by ploidy. A significant difference was found between the anaerobic capabilities of the fish measured as endurance times at their prolonged swimming speeds. During the course of the experimentation the voluntary swimming speed selected by the fish increased and the schooling behaviour improved. The effect of the curvature of the tank on the fish speeds was calculated (removing the curved effect of the tank increased the speed in either ploidy by 5·5%). Implications of the endurance times and speeds are discussed with reference to the aquaculture of triploid Atlantic salmon.     

Feeding and prey-selection of wild Atlantic salmon post-smolts

The diet of post-smolt Atlantic salmon Salmo salar caught in the Trondheimsfjord and Frohavet in central Norway, based on stomach contents analysis, showed a gradual change during migration from the river to the estuary, fjord and coastal areas. Post-smolts caught in the estuary had eaten intertidal gammarid amphipods, while post-smolts caught further seawards preyed upon available marine prey such as Calanus spp., adult euphausiids and fish larvae. The frequency of adult insects was high in all post-smolt stomachs. The gradual change in diet suggested that feeding conditions in the early marine phase were important for post-smolt survival and growth. With the exception of the copepods, there was no overall similarity between species composition of the plankton samples and the stomach contents. Although the hypothesis that the post-smolts are opportunistic feeders cannot be rejected, the composition of the stomach contents suggests a possible selectivity of advantageous prey.     

Genetic changes in an Atlantic salmon population resulting from escaped juvenile farm salmon

The study was undertaken on three adjacent rivers in NW Ireland, on one of which an Atlantic salmon Salmo salar freshwater juvenile rearing unit is situated. Two markers which distinguished farm and wild populations were used. An Ava II-B RFLP in the ND1 region of mtDNA was at a frequency of 0.58 in the farm strain but absent in the wild populations. Allele E at minisatellite locus Ssa- A45/2/l was at a frequency of 0.91 in farm samples, but at a maximum of 0.41 in the populations in the two rivers adjacent to the one with the juvenile rearing unit. The farm strain showed a significant reduction in mean heterozygosity (0.281 ± 0.057), over three minisatellite loci examined, compared to wild samples (0.532 ± 0.063). The occurrence of farm genotypes and the independent occurrence of mtDNA and minisatellite markers in several parr samples from the river indicated that escaped juvenile salmon completed their life cycle, bred and interbred with native fish, upon their return to the river. Escaped fish homed accurately, as adults, to the site of escape, i.e. the area adjacent to the hatchery outflow in the upstream part of the river. Breeding of males in the lower part of the river was also indicated but this could have been due to mature male parr which had moved downstream. The return of adults of farm origin to the river to breed was indicated by the presence of the Ava II-B haplotype in adults netted in the estuary.     

Smolt size in relation to age at first maturity of Atlantic salmon (Salmo salar): the role of lacustrine habitat

Back-calculated growth and size of Atlantic salmon smolts were compared in two groups of river systems in Newfoundland. One group consisted of rivers dominated by lacustrine habitats while the other had rivers characterized by fluvial habitats. Back-calculated length at each age and smolt size was significantly higher for the rivers dominated by lacustrine habitats. Associated with this was a lower proportion of maiden large salmon in adult returns. These findings are discussed in relation to density in fresh water, environmental conditions at sea, and life-history strategy.     

Testing predictions of the critical period for survival concept using experiments with stocked Atlantic salmon

Two separate field experiments were performed in the U.S.A. and Norway with stocked Atlantic salmon Salmo salar . In the Norwegian experiment, the offspring of early‐spawning fish which had larger eggs and emerged a few days before offspring of later spawning fish had consistently higher survival rates. In the U.S.A. experiment, stream sections with higher proportions of favourable foraging locations during the critical period (the transition from dependence on maternally‐derived yolk reserves to independent feeding) had lower loss rates of fish stocked as unfed fry. These results provide support for the critical period concept (CPC) in Atlantic salmon, underscores the utility of a manipulative approach to achieve further advances in knowledge of Atlantic salmon ecology and provide additional guidance to management and restoration. A mechanistic, conceptual model for density dependence is presented to identify important knowledge gaps that remain to further evaluate the importance of the CPC for Atlantic salmon population regulation.     

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.     

Seasonal variation in density of juvenile Atlantic salmon in fluvial shoreline habitats of a large subarctic river

The densities of Atlantic salmon fry (0+ years) and parr (1+ years and older) in shoreline habitats of the large River Teno watercourse and its tributary, the River Utsjoki generally fluctuated considerably, showing an increase from early summer towards late August and a subsequent decline towards autumn. The seasonal pattern of variation in density was more distinct for parr than for fry. In the period between late July and early September, parr density followed a sinusoidal curve, being highest in late August and lowest in early August and in September. Fry density had a weaker seasonal profile than parr, being highest in late August and in early September. Frequency distributions of the parr age groups (1+, 2+ and 3+ years) were mainly independent of the sampling month.     

Monthly indices of the post-smolt growth of Atlantic salmon from the Drammen River, Norway

Indices of the monthly post-smolt growth of three maturity groups of Atlantic salmon Salmo salar from the Drammen River in Norway were examined. Scale samples were collected from 1983 to 2003, providing a retrospective growth history for 2854 individuals. Overall post-smolt growth of all three maturity groups decreased throughout the study period. When the post-smolt year was segmented into 8 monthly indices, there was a significant decline in growth during the fourth and fifth month, typically months of maximum growth. The pre-fishery abundance of one and two sea-winter fish from Norway also showed a decline throughout the study period and was positively correlated to the post-smolt growth of fish returning to the Drammen River. Freshwater growth was negatively correlated to the pre-fishery abundance, thus indicating that factors affecting post-smolt growth during the fourth and fifth months at sea may be responsible for declining return numbers in Norway.     

Summer stream habitat partitioning by sympatric Arctic charr, Atlantic salmon and brown trout in two sub-arctic rivers

Summer habitat use by sympatric Arctic charr Salvelinus alpinus, young Atlantic salmon Salmo salar and brown trout Salmo trutta was studied by two methods, direct underwater observation and electrofishing, across a range of habitats in two sub-arctic rivers. More Arctic charr and fewer Atlantic salmon parr were observed by electrofishing in comparison to direct underwater observation, perhaps suggesting a more cryptic behaviour by Arctic charr. The three species segregated in habitat use. Arctic charr, as found by direct underwater observation, most frequently used slow (mean ±s .d . water velocity 7·2 ± 16·6 cm s⁻¹) or often stillwater and deep habitats (mean ±s .d . depth 170·1 ± 72·1 cm). The most frequently used mesohabitat type was a pool. Young Atlantic salmon favoured the faster flowing areas (mean ±s .d . water velocity 44·0 ± 16·8 cm s⁻¹ and depth 57·1 ± 19·0 cm), while brown trout occupied intermediate habitats (mean ±s .d . water velocity 33·1 ± 18·6 cm s⁻¹ and depth 50·2 ± 18·0 cm). Niche overlap was considerable. The Arctic charr observed were on average larger (total length) than Atlantic salmon and brown trout (mean ±s .d . 21·9 ± 8·0, 10·2 ± 3·1 and 13·4 ± 4·5 cm). Similar habitat segregation between Atlantic salmon and brown trout was found by electrofishing, but more fishes were observed in shallower habitats. Electrofishing suggested that Arctic charr occupied habitats similar to brown trout. These results, however, are biased because electrofishing was inefficient in the slow-deep habitat favoured by Arctic charr. Habitat use changed between day and night in a similar way for all three species. At night, fishes held positions closer to the bottom than in the day and were more often observed in shallower stream areas mostly with lower water velocities and finer substrata. The observed habitat segregation is probably the result of interference competition, but the influence of innate selective differences needs more study.