Daily feeding rhythms in Atlantic salmon I: feeding and aggression in parr under ambient environmental conditions

Daily feeding patterns of tank-reared Atlantic salmon Salmo salar parr under ambient conditions and their relationship with patterns of aggression were investigated during spring and autumn. A single, early afternoon feeding peak was observed during spring but no consistent pattern emerged from the autumn trial (fish did tend to feed in synchrony, even though the population's feeding pattern varied between days). Aggression was highest at first light during both spring and autumn, reducing thereafter throughout the day. Feeding and aggression were out of phase during spring, with the level of feeding increasing as aggression decreased. It is suggested that the high levels of early morning aggression are associated with re-establishment of the social order that breaks down after a period of darkness; in the spring this may have inhibited feeding.     

Development of length–bimodality and smolting in wild stocks of Atlantic salmon, Salmo salar L., under different growth conditions

Growth dynamics of juvenile Atlantic salmon, Salmo salar L., from two sections of the Narcea River and one of the Esva River (Northern Spain) were examined in relation to the development of bimodality in their size–frequency distributions. Size–bimodality was clearer under intermediate growth (section A) than under relatively fast or slow growth. The proportion of fish entering the upper modal group increase with growth intensity. Composition of upper and lower modal groups became fixed prior to December, and at this time both groups separated on the 90–95 mm interval. Fish exhibiting smolt appearance in late March (larger than 130 mm) had already been upper group fish in December, while parr-like fish and those that remained in the river by May (potential 2-year-old smolts) had formed the lower modal group. Anadromous salmon catch in the Narcea River was mostly of previously 1-year-old smolts (97.6%), of which 94% were larger than 100 mm by their first winter. In the Esva River, slow growth of juveniles is consistent with a large proportion of 2-year-old smolts (47.9%) among anadromous salmon. Both juvenile samples and scale analysis of anadromous salmon indicate that 2-year-old smolts were larger than 1-year-olds. Early disappearance of the former (before March) is, at least, partially related to earlier migration of large fish, since sexual maturity of parr does not provide a complete explanation. The Narcea stock have a minimum length at smolting of about 130 mm and an optimum smolt size in the 155–175 mm interval. Mean smolt length did not vary although the winter length changed between years.     

Life-history patterns in a southern population of Atlantic salmon

Juvenile Atlantic salmon Salmo salar in the river Esva, Asturias, north-west Spain, developed a bimodal growth pattern during their first growing season. Segregation between the two modes was apparent by late autumn. All fish in the upper modal group (UMG) grew throughout the year and migrated downstream in the following spring (by April) at the age of 14 months. Some lower modal group (LMG) fish (56% of the main stem, 31% and 50% of the two tributaries) silvered like smolts and apparently migrated downstream the same spring, although c. 1·5 months after UMG fish. Larger LMG fish appeared more likely than smaller ones to migrate. LMG fish did not grow in winter, but they grew fast between March and May. Maturing male parr were detected first at 5 months old in July, and they occurred initially among the faster growers, >6·8 cm long. In spite of this, length of maturing males did not change from late summer and throughout the winter, whereas that of non-maturing UMG and LMG fishes continued to diverge from September onwards. These findings suggest that favourable conditions for growth (very early start of a long growing season) at the southern limit of the species' range may influence the life-history pattern of this population.     

Metabolic rate, growth and aggressiveness in three Atlantic salmon Salmo salar populations

Standard metabolic rate ( R _S), specific growth rate ( G ) and aggressiveness were investigated in three Finnish populations of Atlantic salmon Salmo salar (Neva, Saimaa and Teno), which were reared in identical hatchery conditions. The populations differed in their geographical origin and native habitat. There was a significant difference between populations in R _S: the southernmost Neva population had higher values in R _S than the northernmost Teno population. No difference was found in G or aggressiveness between the populations. G was found to have a significant positive association with aggressiveness and R _S among the three populations, however, these results were not statistically significant after correction for multiple tests. There was no significant association between R _S and aggressiveness. Higher metabolic rate of the most southern population Neva is suggested to be an adaptation to the more abundant food sources of the southern stream.     

Aggression and growth depression in juvenile Atlantic salmon: the consequences of individual variation in standard metabolic rate

In aquaculture, competitive interactions for food are a major source of growth rate variation, since they result in aggressive individuals acquiring a disproportionate amount of food and growing faster. Consequently, such competition increases the variance and skew of the size distribution of fish. In Atlantic salmon Salmo salar acquaculture, rearing is often initiated with juveniles of uniform size. However, the initial factors allowing fish to out-compete others of the same size is unclear. This study shows that individual differences in standard metabolic rate (SMR) may contribute to differences in aggression between juvenile Atlantic salmon. Fish were segregated into three holding tanks on the basis of differences in relative SMR aggression was highest in the tank containing high SMR fish, and lowest in a low SMR group. However, there were no significant differences in mean growth between the three treatments, although the skew of the size distribution was much less in the low SMR group than in the high SMR group. Therefore, although mean growth was not improved in the low SMR group by excluding high SMR fish, their growth subsequently showed less variability, possibly as a consequence of fewer despotic individuals gaining disproportionate access to food.     

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.     

Genetic divergence and interactions in the wild among native, farmed and hybrid Atlantic salmon

There is concern that the progeny resulting from the spawnings of escaped farmed Atlantic salmon may compete with and disrupt native salmon populations. This study compared, both in the hatchery and in the wild, fitness-related traits and examined interactions among farmed, native and hybrid 0+ parr derived from controlled crosses and reared under common conditions. The farmed salmon were seventh-generation fish from the principal commercial strain in Norway and native salmon were from the rivers Imsa and Lone, Norway. In the hatchery, farmed salmon were more aggressive than both native populations and tended to dominate them in pairwise contests. Farmed salmon were also more prone to risk, leaving cover sooner after a simulated predator attack, and had higher growth rates than native fish. Interbreeding between farmed and native fish generally resulted in intermediate expression of the above traits. There was, however, evidence of hybrid vigour in Lone/farmed crosses which were able to dominate both pure Lone and farmed parr in pairwise contests. In the wild, observations of habitat use and diet suggested that the populations compete for territory and food, and both farmed fish and hybrids expressed higher growth rates than native fish. Our results suggest that these innate differences in behaviour and growth, that probably are linked closely to fitness, will threaten native populations through competition and disruption of local adaptations.     

Seasonal growth of wild Atlantic salmon juveniles and implications on age at smoltification

The seasonal growth trajectories of wild Atlantic salmon Salmo salar juveniles by age group within the Margaree River, Canada, are described. Circuli counts from scales were used to infer growth rates at different ages and these were used to predict the proportions of age 2‐ and 3‐year old smolts from different portions of the watershed. In the wild Atlantic salmon juveniles from the Margaree River, there was no bimodality in fork length frequencies and no 1 year old smolts were produced. Water temperature differences during the growing season were insufficient to explain the differences in growth rates and size at age among the sites sampled. There was a positive association between the growth rate in the first year and the subsequent age at smoltification. In the Margaree River, differences in tributary specific growth rates and size at age were expected to produce important differences in the relative ages at smoltification.     

Social status and growth rates as determinants of site attachment in juvenile Atlantic salmon

This paper describes an experimental study of the effects of food supply, growth rates and social interactions on homing by juvenile Atlantic salmon Salmo salar in response to displacement. Groups of five fish were housed in a section of an artificial stream and given either rations allowing maximum growth (the rich condition) or 0·1 of this amount (the poor condition); daily specific growth rates were significantly higher in the rich condition. After a 6-day settlement period, the fish were captured, displaced downstream and their movements recorded over the next 3 h. Prior to displacement, the fish showed a high degree of site fidelity and high levels of aggression. Dominant fish and those with stronger site attachment grew faster prior to displacement, these effects being independent. Following displacement, 24% of all fish returned to their previously favoured site and stayed there, 23% returned home initially, but subsequently moved on, 5% settled in a new site and 49% failed to move. The distribution of responses was identical for the rich and poor conditions, but fish that homed were dominant and had grown faster during the pre-displacement period.     

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.     

Compensatory sea growth of male Atlantic salmon, Salmo salar L., which previously mature as parr

The proportion of mature male parr in 11 families of Atlantic salmon, Sulrno sulur, reared under similar conditions in fresh water varied from 0–43%. The mature males were smaller than their siblings in December as 1 + and in late March. After individual tagging and transfer to a sea cage in early April. the previously mature males grew faster than previously immature salmon during the next 6 months. This compensatory growth resulted in almost equal size between the two groups. The results are discussed in relation to the different life strategies of salmon.     

Daily feeding rhythms in Atlantic salmon II: size-related variation in feeding patterns of post-smolts under constant environmental conditions

Previous studies of feeding rhythms in post-smolt Atlantic salmon Salmo salar have produced variable results. Therefore, the present study was performed to clarify the source of these differences and to examine size-related variation in daily feeding patterns. A small group of hatchery reared post-smolts was kept in an indoor seawater tank under conditions of constant photoperiod and light intensity, and negligible diurnal variation in water temperature. Feeding responses were monitored by video camera throughout the day. The resultant feeding pattern for the population mirrored an earlier study, with the highest levels of feeding activity in the early morning and late evening. Differences between this and other studies may be due to variation in climate, season or life-history stage. Daily feeding patterns were size-dependent. The largest fish fed at a relatively constant level through the day, medium-sized fish fed in a pattern similar to that described above for the population and small fish fed most at first and last light. These differences were due probably to the effect of competitive interactions upon fish of different sizes.     

Size and age of maturity of Atlantic salmon correlate with the North Atlantic Oscillation Index (NAOI)

Sea‐age at maturity of Atlantic salmon Salmo salar decreased with increasing values of the seasonal NAOI from February to April. Body mass increment from smolts to adults of one‐sea‐winter Atlantic salmon increased with increasing NAOI in May at the time when the juveniles moved to sea.     

Effects of life history variation on size and growth in stream-dwelling Atlantic salmon

A large size variation amongst life histories for stream-dwelling Atlantic salmon Salmo salar was found and the relative effect of life histories on size varied over time. As early as December (age 0+ years), fish that later smolted at age 2+ years were significantly larger than fish that did not smolt at age 2+ years. In contrast, there were no mass differences at age 0+ years between fish that would mature or not at age 1+ years (October). The mass differences between smolts and non-smolts persisted until smolting, and differences between mature and immature fish first appeared in May (age 1+ years). Following September (age 1+ years), there was also a significant interaction between smolting and maturity. Previously mature and immature age 2+ year smolts were not significantly different in size, but immature age 2+ year non-smolts were much lighter than mature age 2+ year non-smolts. Based on mass differences, the apparent 'decision' to smolt occurred c . 5 months before (winter, age 0+ years) the decision to mature (late spring, age 1+ years). In addition to strong seasonal growth variation, sizes of freshwater Atlantic salmon were largely structured by the complex interaction between smolt-age and maturity.     

Patterns of growth and smolting in autumn migrants from a Scottish population of Atlantic salmon, Salmo salar L.

In late November 1990 salmon parr, Salmo salar L., from the Girnock Burn in northern Scotland were either caught on their feeding territories (n=25) or trapped during downstream migration (n= 18). They were then housed in a laboratory rearing tank and their food intake and growth rates were tracked, until their smolting status was ascertained in the following May. Female fish were predominant in both groups; although the range of ages was the same, the total age of migrants was 2+ while that of residents was 1+. In November, compared to resident fish of the same year class, migrants were larger, heavier and in better condition. Although growth rates dropped during the winter in both groups before increasing in spring, migrants ate more and consistently grew faster than residents. In seawater tolerance tests conducted in May, more residents than migrants failed to adapt. These results confirm the suggestion that autumn migrants smolt in the following spring and suggest that they represent the faster-growing component of their cohort.     

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.     

The effect of ultrasonic tags on the growth rates of Atlantic salmon, Salmo salar L., parr of varying size just prior to smolting

The attachment of dummy, ultrasonic, acoustic tags, of the Stirling Mk. VI design, to pre-smolt Atlantic salmon parr significantly affected fish growth rates. Fish less than 160 mm in length lost weight and showed no change in length. Fish over 160 mm in length put on weight, though at a rate significantly lower than that of untagged controls. For fish between 160 and 180mm in length, the increase in length was significantly smaller in tagged fish than in untagged controls. Over 180 mm in length, no difference could be detected in the length gains of tagged and untagged fish.
Smaller tagged fish gave up maintaining station and showed signs of distress at an earlier stage in the experiment than larger tagged fish. Tag-related mortality was inversely related to fish size. All but two of the control fish survived the experiment and continued to maintain station throughout.
The results suggest that the minimum length of Atlantic salmon parr on which this design of acoustic tag could be employed is 160 mm, and that caution is needed in the interpretation of track lengths longer than l0 daysof parr in the size range 160–180 mm.     

Marine post-smolt growth and age at maturity of Atlantic salmon

The annual variation in sea-age of maturation for a hatchery dependent stock of Atlantic salmon was compared to variation in post-smolt growth as evidenced by circuli spacing patterns. The proportion of returns of 1-seawinter (1 SW) and 2 SW salmon and the fraction of the smolt year class or cohort that maturated as 1 SW fish, were compared to seasonal growth indices determined from circuli spacing on the scales of smolt class survivors returning as 1 SW and 2 SW spawners. Using image processing techniques, we extracted inter-circuli distances from scales from 2244 recaptured fish. Spacing data for the first year at sea were collected and then expressed as seasonal growth indices for the spring period, when post-smolts first enter the ocean; the summer, when growth appears maximal; and winter, when growth appears to be at a minimum. In general, circuli spacings were wider for 1 SW than for the 2 SW returns of the same smolt cohort. The 1 SW fraction was significantly and positively correlated with late summer growth, suggesting that growth during this season is pivotal in determining the proportion of a smolt class that matures early.     

Effects of sexual maturation, castration, and androgen implants on growth in one- and two-year-old parr in a Baltic Atlantic salmon (Salmo salar L.) stock

The effects of maturation, castration, and androgen implants on growth in 1- and 2-year-old Baltic salmon ( Salmo salar L.) were studied in a stock originating from the Umeälven (Ume river) in northern Sweden. 1 + male parr that later matured were larger than non-maturing parr in the spring. Later in the summer, during the period of gonadal growth, the growth rate of maturing parr was depressed compared to that of non-maturing males. Implantation of capsules with 11-ketoandrostenedione (OA) or testosterone (T) in sexually immature 1-year-old fish, accelerated the specific growth rate in spring and early summer, particularly considering growth in weight, resulting in an increase in the condition factor. In contrast, the growth rate was depressed from late June onwards, the period when the gonads normally develop in 1 + parr. T, but not OA, also promoted sexual maturation. Castration or OA treatment did not affect total summer growth in 2-year-old salmon. This study suggests a causal relationship between androgens and the growth pattern observed in maturing 1 + male parr.