Rearing conditions influence refuge use among over-wintering Atlantic salmon juveniles

At low population densities, both hatchery- and wild-reared Atlantic salmon Salmo salar , which were progeny of a common genetic stock, occupied sub-gravel shelters alone. However, at high population densities, hatchery-reared fish occurred within shelters in significantly higher numbers than wild-reared fish. There was a significant effect of time after introduction to the experimental arenas on the proportion of hatchery-reared fish sheltering. These results show that rearing environment influences the behaviour of Atlantic salmon parr during winter and may also affect the carrying capacity of their environment.     

The disruption of dominance hierarchies by a non-native species: an individual-based analysis

We studied the effects of the exotic rainbow trout (Oncorhynchus mykiss) on the performance and the dominance hierarchy of native Atlantic salmon (Salmo salar) at the group and individual level using laboratory and semi-natural experiments. At the group level, we compared the effects of interspecific and intraspecific competition (substitutive and additive design) on behavioural responses and growth of young-of-the-year Atlantic salmon. At the individual level, the same design was used to evaluate: (1) the temporal consistency of behavioural responses, dominance hierarchy and growth rate of Atlantic salmon; (2) the pattern of correlations between behaviours; and (3) the relationship between individual growth rate and behaviour. In the laboratory, group-level analyses revealed a weak but similar effect of rainbow trout and intraspecific competition on the behaviour and growth of Atlantic salmon. In contrast, individual-based analyses demonstrated that rainbow trout (but not intraspecific competition) strongly affected behavioural strategy, dominance hierarchy and growth trajectory of individual Atlantic salmon. Specifically, behaviours, dominance status and growth rate of salmon were temporally consistent in the intraspecific environment, while these patterns were disrupted when rainbow trout were present. Similarly, we found that rainbow trout strongly affected behavioural correlations and the relationships between individual growth rate and behaviour. The semi-natural experiments confirmed these results as interspecific competition affected relationships between individual growth rate of salmon, initial weight and activity index. Overall, individual-based analyses highlighted important mechanisms that were concealed at the group level, and that may be crucial to understand ecological and evolutionary consequences of exotic species. Moreover, these results demonstrated that competition with an exotic species disrupts the hierarchical relationship among native individuals and may therefore represent a potential for a shift in selective pressure.     

Winter behaviour of juvenile Atlantic salmon Salmo salar L. in experimental stream channels: effect of substratum size and full ice cover on spatial distribution and activity pattern

Activity and choice of areas offering different cover (substratum or surface ice) for juvenile Atlantic salmon Salmo salar were studied in experimental stream channels during winter. Channels were completely ice covered between December and March. During this period, the ice thickness increased from 50 to 300 mm after which 50% of the ice was experimentally removed and followed by c. 2·5-fold increase in discharge to simulate the effects of spring flood. Large substrata provided preferred habitats but areas with small substratum sizes were also used when full surface ice provided above-stream cover and the stream discharge was relatively low. The fish remained nocturnal throughout the study but the level of day activity significantly increased as the surface ice became thicker. Maximum movement distance during a 24 h period and homing-at-dawn behaviour remained at a constant level throughout the main winter, but significantly changed during the simulated spring flood (mean ± s . e . maximum extent of movements within 24 h increased from 1·1 ± 0·1 to 3·0 ± 0·5 m; homing behaviour decreased from the highest level of 89·3 to 34·6% during spring flood). Overwinter survival was high (92·9%). Relative mass increase during the study ranged from –8·3 to 28·5%, and 84% of the juvenile Atlantic salmon gained mass. The highest rates of mass increase were associated with frequent movements between areas of different substratum size. The results indicate that during winter: (1) Atlantic salmon parr preferred large substratum cover compared with surface ice cover at the fish densities studied here, (2) juvenile Atlantic salmon were predominantly nocturnal but diurnal activity increased as surface ice became thicker and (3) increase in water discharge during spring altered the behaviour of juvenile Atlantic salmon and may have caused additional habitat shifts.     

Effects of food availability on temporal activity patterns and growth of Atlantic salmon

1. Patterns of sheltering and activity are of fundamental importance in the ecology of animals and in determining interactions among predators and prey. Balancing decreased mortality risk when sheltering with increased feeding rate when exposed is believed to be a key determinant of diel patterns of sheltering in many animals. 2. Despite lower foraging efficiency at night than during the day, Atlantic salmon Salmo salar parr are nocturnal during winter and at low summer temperatures. Nocturnal activity also occurs at warm water temperatures during summer, but little is known about the functional significance of this behaviour. 3. This study aimed to determine: (1) the preferred activity and shelter pattern of Atlantic salmon parr during warm summer months, and (2) their response to variations in food availability when balancing growth rate (G) and mortality risk (M), as expressed through time out of shelter. We differentiated among four potential responses to reduced food availability: (1) no response; (2) G decreases but M remains constant; (3) G remains constant but M increases; and (4) G decreases and M increases. 4. Time exposed from shelter was inversely related to food availability. Fish subject to high food availability were significantly less active during the day than those with restricted rations. However, food availability had no significant effect on the extent to which fish were active at night. There was no evidence of variation in growth rate with food availability. 5. Salmon were predominantly nocturnal at high ration levels, consistent with their previously reported behaviour during winter. Rather than switching to diurnal behaviour at high temperatures per se, as previously was supposed, it appears that the fish are diurnal only to the extent needed to sustain a growth rate, and this extent depends on food availability. 6. Atlantic salmon parr modulate the amount of time they are active rather than growth when responding to variations in food availability over an order of magnitude.     

Competition for food between an introduced crayfish and two fishes endemic to the Colorado River basin

Synopsis Crayfish are not native to the Colorado River basin (CRB), however they are now established in portions of the mainstem and in many tributaries. I used density manipulation experiments in a laboratory setting to determine intra- and interspecific competition for food between Orconectes virilis, an aggressive polytrophic crayfish now common in the CRB, and two native fishes: Gila chub, Gila intermedia, and flannelmouth sucker, Catostomus latipinnis. I tested each fish species in separate trials. Growth of Gila chub decreased when animal densities increased, however they were more affected by intraspecific competition than by crayfish presence. In contrast, growth of flannelmouth suckers was more affected by crayfish than by intraspecific competition. Crayfish growth was not significantly altered by presence of either fish. Crayfish thus reduced fish growth by competition for food, but the effect differed markedly between the two species. An erratum to this article can be found at .     

Effects of discharge and local density on the growth of juvenile Atlantic salmon Salmo salar

The study explored the combined effects of density, physical habitat and different discharge levels on the growth of juvenile Atlantic salmon Salmo salar in artificial streams, by manipulating flow during both summer and winter conditions. Growth was high during all four summer trials and increased linearly with discharge and mean velocity. Differences in fish densities (fish m^?3) due to differences in stream volume explained a similar proportion of the variation in mean growth among discharge treatments. Within streams, the fish aggregated in areas of larger sediment size, where shelters were probably abundant, while growth decreased with increasing densities. Fish appeared to favour the availability of shelter over maximization of growth. Mean growth was negative during all winter trials and did not vary among discharge treatments. These results suggest that increased fish densities are a major cause of reduced summer growth at low discharge, and that habitat‐mediated density differences explain the majority of the growth variation across habitat conditions both during summer and winter.     

Density-dependent refuge use among over-wintering wild Atlantic salmon juveniles

Sheltering behaviour of wild juvenile Atlantic salmon in an indoor stream was found to be density dependent; the proportion of fish sheltering decreased significantly with increasing population density. The mean number of fish occupying refugia was ≤1.5 fish per refuge even at very high densities (potentially 5 fish per shelter). These results suggest that shelter availability has potentially important consequences for the carrying capacity for natural populations of salmon in streams.     

Abnormal development of gonads of dwarf females and low survival of their offspring as the cause of rarity of resident populations of atlantic salmon (Salmo salar L.)

Fecundity and mass of egg, as well as survival of offspring, of hatchery-reared small (less than 406 g) females of Atlantic salmon are determined. The absolute fecundity and mass of egg of artificially reared fish are sufficiently less than that of anadromous spawners. Small females are characterized by delayed spawning; histological analysis has demonstrated significant asynchrony in oocyte development of some specimens. Survival of offspring of dwarf females at the early stages of development is lower than of anadromous females. Our results agree completely with the published data. It is suggested that dwarf females are not able to withstand intraspecific and interspecific competition because of low reproductive ability. This explains rarity of natural populations of dwarf (landlocked) forms of Atlantic salmon.     

Allozyme heterozygosity, date of first feeding and life history strategy in Atlantic salmon

The relations between allozyme heterozygosity, relative date of first feeding and life history strategy in juvenile Atlantic salmon Salmo salar were examined using eggs obtained from a 400 family cross (20 male × 20 female adult Atlantic salmon). Multilocus heterozygosity, through its positive associations with the timing of first feeding and growth rate, was correlated with life history strategy in juvenile Atlantic salmon, albeit under genotype × environmental (temperature, food availability) regulation. Under hatchery conditions, a 10 day difference was observed in the relative date of first feeding between early and late first feeding Atlantic salmon. Early first feeding Atlantic salmon exhibited a significantly higher mean heterozygosity, grew faster at ambient water temperature (April to November) and a significantly higher proportion adopted the early freshwater maturation (age 0+ years, male fish) or early migrant (age 1+ years, mainly female fish) strategies compared to late first feeding Atlantic salmon. Elevated water temperatures over the winter (December to April, >10·5° C) provided additional growth opportunity allowing previously mature male parr (mainly early first feeders) and lower modal group parr (mainly late first feeders) to adopt the early migrant strategy by the following spring.     

Shelter availability affects behaviour, size-dependent and mean growth of juvenile Atlantic salmon

1. Anthropogenic disturbances of the physical habitat and corresponding effects on fish performance are key issues in stream conservation and restoration. Reduced habitat complexity because of increased sediment loadings and canalization is of particular importance, but it is not clear to what extent fish populations are influenced directly by changes in the physical environment, or indirectly through changes in the biotic environment affecting the food availability. 2. Here, we test for the direct effect of habitat complexity on the performance (growth) of juvenile Atlantic salmon by manipulating shelter availability (interstitial spaces in the substrate) across 20 semi‐natural stream channels without altering the substrate composition, and stocking each channel with a common density of fish. A simple method for measuring salmonid shelters using flexible PVC tubes was developed and tested. Daytime sheltering behaviour and growth rates were compared across the channels differing in shelter availability. 3. Measured shelter availability was strongly negatively correlated with observed number of fish not finding shelters and mass loss rates of the fish (growth performance) increased with decreasing number of measured shelters. Number and mean depth of interstitial spaces explained up to 68% and 24% of the among‐channel variation in sheltering behaviour and growth performance, respectively. Furthermore, negative effects of shelter reduction increased with fish body size. Thus, changes in habitat structure may even influence the size selection gradients. 4. Shelter availability is an easily measured variable, possibly affecting the population demographics and long‐term evolutionary processes, and is therefore a key habitat factor to be considered in stream restoration and habitat classification.     

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.     

The survival of semi‐wild, wild and hatchery‐reared Atlantic salmon smolts of the Simojoki River in the Baltic Sea

The recapture rate and survival of hatchery‐reared Atlantic salmon Salmo salar stocked as 1 year‐old parr (semi‐wild) with that of hatchery‐reared Atlantic salmon stocked as 2 year‐old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986–1988 and 1992. The recapture rate of the semi‐wild groups varied from 1·0 to 13·1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1·7–17·0%). The recapture rate of the semi‐wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery‐reared groups stocked as smolts (1·3–6·3%). The survival of semi‐wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery‐reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery‐reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.     

Competition and facilitation within and between a snail and a mayfly larva and the effect on the grazing process

We studied the competitive effects within and between two taxonomically distant freshwater herbivores, a snail and a mayfly, common in Swedish lakes, Lymnaea peregra and Cloeon dipterum, respectively, and their effect on grazing in a laboratory experiment. The experimental set-up consisted of 2-l aquaria, each containing a periphyton covered tile. Intra- and interspecific effects were tested by increasing the density of one species at a time in four different treatments, (1) snails (intraspecific treatment), (2) mayflies (intraspecific treatment), (3) mixed-snails (interspecific treatments, snails kept constant) and (4) mixed-mayflies (interspecific treatments, mayflies kept constant). Intraspecific competition affected both snails and mayflies negatively, i.e. increasing mortality with increasing con-specific density. Furthermore, there was a decrease in snail growth with increasing snail density. In the mixed-species treatments both species changed their microhabitat use indicating interspecific competition. Despite this, we also found a positive effect of mayfly density on snail growth, most likely due to indirect commensalism. No density-dependent effect of grazing on periphyton was found, probably due to interference competition between grazers. However, there was a significant difference in periphyton biomass, due to species composition of grazers. Irrespective of their densities, if they co-existed, the two grazer species decreased the periphyton biomass significantly compared with both single-species treatments. We considered this as a joint action of facilitation and interaction. Our results suggest that competition can be an important structuring factor in macroinvertebrate communities and that species composition can be significant for ecosystem processes within lentic environments.     

The contrasting effects of short-term climate change on the early recruitment of tree species

While interspecific competition is prevalent in natural systems, we do not yet understand how it can influence an individual’s phenotype within its lifetime and how this might affect performance. Morphology and swimming performance are two important fitness-related traits in fishes. Both traits are essential in acquiring and defending resources as well as avoiding predation. Here, we examined if interspecific competition could induce changes in morphology and affect the swimming performance of two strains of juvenile Atlantic salmon (Salmo salar). We imposed competitive scenarios on the fish using artificial streams containing different combinations of four interspecific competitors. Exposure to interspecific competitors induced morphological changes over time, through the development of deeper bodies, whereas controls free of interspecific competitors developed more fusiform body shapes. Furthermore, swimming performance was correlated to fusiform morphologies and was weaker for Atlantic salmon in competitive scenarios vs. controls. This implies that interspecific competition has direct effects on these fitness-related traits in Atlantic salmon. To the best of our knowledge, this is the first time that morphology, an important fitness-related trait linked to swimming performance, has been shown to be negatively impacted through interactions with an interspecific competitor.     

Temporal variation in abundance, return rate and life histories of previously spawned Atlantic salmon in a large subarctic river

The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.     

Pollination outcomes reveal negative density‐dependence coupled with interspecific facilitation among plants

Pollination is thought to be under positive density‐dependence, destabilising plant coexistence by conferring fitness disadvantages to rare species. Such disadvantage is exacerbated by interspecific competition but can be mitigated by facilitation and intraspecific competition. However, pollinator scarcity should enhance intraspecific plant competition and impose disadvantage on common over rare species (negative density‐dependence, NDD). We assessed pollination proxies (visitation rate, pollen receipt, pollen tubes) in a generalised plant community and related them to conspecific and heterospecific density, expecting NDD and interspecific facilitation due to the natural pollinator scarcity. Contrary to usual expectations, all proxies indicated strong intraspecific competition for common plants. Moreover interspecific facilitation prevailed and was stronger for rare than for common plants. Both NDD and interspecific facilitation were modulated by specialisation, floral display and pollinator group. The combination of intraspecific competition and interspecific facilitation fosters plant coexistence, suggesting that pollination can be a niche axis maintaining plant diversity.     

Seasonal changes in the body composition of young riverine Atlantic salmon and brown trout

The body composition of protein and fat in Atlantic salmon Salmo salar and brown trout Salmo trutta before and after winter was investigated in a temperate, small river, normally ice covered from the middle of November until the end of March. Fat, protein and specific energy declined greatly in winter but were replenished rapidly in spring. Rates of decline were slower for the smallest fish, which also had the lowest specific content of fat, protein, and energy, while the differences in absolute amounts were greatest for the largest fish. The mean specific fat content was reduced by 45–70% during winter, relative to the pre-winter period (September). Mean daily reductions in specific enegy of the larger size groups of brown trout (3·7 × 10⁻³ kJ g⁻¹ day⁻¹) were almost half of the corresponding values for the largest Atlantic salmon (6·3 × 10⁻³ kJ g⁻¹ day⁻¹) during winter. A minor reduction in protein content was found during winter, with mean reductions of 6–10% in comparison to those in September. During spring the fat content was replenished rapidly, particularly for the smallest salmon fry (a threefold increase from April to June). Fat content in the larger salmon and trout increased by about 1·8 times. Based on estimated metabolic rates, digested energy during wintertime may contribute about two-thirds of the brown trout fry's energy demand. For Atlantic salmon, the corresponding value is about 50%. The winter period put considerable stress on the young salmonids living in lotic environments, in particular for the smallest fry with the lowest energy content before winter and the largest losses during winter. This should make the fry more vulnerable to adverse abiotic and biotic factors.     

Microsatellite markers in common carp (Cyprinus carpio L.)

Microsatellite markers of the poly (CA) type in common carp ( Cyprinus carpio L.) are described. Clones containing a (CA) repeat were isolated from a common carp genomic library and sequenced. The number of repeats found was high compared to mammals but comparable with other teleost fishes. Classification of the repeats (perfect, imperfect and compound) are compared with the Atlantic cod ( Gadus morhua L.), rainbow trout ( Oncorhynchus mykiss ), and Atlantic salmon ( Salmo salar L.). A total of 41 primer sets were designed and tested for polymorphism on a test panel of eight animals (derived from outbred lines, inbred lines and gynogenetic clones). Thirty-two markers were found to be polymorphic. The heterozygosity in the outbred animals was 60·4%, 51·1% in the inbred animals and 0% in the gynogenetic clones. The average number of alleles among the eight animals was 4·7 per marker. Six markers (18·8%) gave an additional polymorphic amplification product besides the polymorphic amplification product in the expected size range. The possibility that these loci are tetraploid is discussed. The polymorphic loci described for common carp will be valuable as genetic markers for use in population, breeding, and evolutionary studies.     

The effect of habitat heterogeneity on the population density of juvenile Atlantic salmon Salmo salar L.

In each of eight sites in 2 years in Catamaran Brook and the Little Southwest Miramichi River, in New Brunswick, Canada, 36 boulders (median diameter = 0·20 m) were added to a 2 × 3 m quadrat in one treatment, all boulders were removed in another treatment, and one quadrat was left as a control. As predicted, adding boulders increased the density of salmonid fishes, primarily juvenile Atlantic salmon Salmo salar , by 2·8-fold, but had no significant effect on non-territorial fishes, primarily cyprinids and catostomids. Moreover, the effect of adding boulders was greatest for age 0+ year Atlantic salmon, intermediate for age 1+ year Atlantic salmon and had no effect on age 2+ year Atlantic salmon. The results suggest that adding boulders is an effective short-term technique for increasing the density of stream-dwelling salmonids.