Early predictors of life-history events: the link between first feeding date, dominance and seaward migration in Atlantic salmon, Salmo salar L.

Previous studies have suggested that the earliest fry to emerge from a salmonid redd may have an advantage in the subsequent competition for feeding sites, partly through a 'prior residence' effect. Here we examined whether there was any relationship between the relative date of first feeding and subsequent dominance status and growth in a sibling group of Atlantic salmon, Salmo salar L., fry. Earlier-feeding fry were dominant over their later-feeding siblings (controlling for prior residence), despite not being any larger. However, these early fish soon established and then maintained a size advantage. This led to an increased probability of early-feeding fish migrating to sea at age 1 year (rather than 2 or more). Thus a difference of less than 1 week in the relative timing of first feeding can translate into a year's difference in the timing of migration.     

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.     

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., down a release ladder. 2. Effect of fish developmental strategy on speed and pattern of movement

The behaviour of different types of Atlantic salmon smolt migrating down a release ladder were compared. Immature S2 smolts were faster through a release ladder, and their journey times less variable than S1 and mature male S2 fish. Journey time variability was less among stripped than among unstripped mature male smolts. The mean residence time of passive objects (oranges) in each pool of the ladder was correlated with pool surface area, whilst that of fish was correlated with pool volume. The general pattern of movement of smolts through the ladder was consistent with the hypothesis of non-directed passive displacement of fish near the water surface. In fact, fish spent most of their time nearer the bottom of the pools with the result that their migration speed was slower than that of passive objects. Pools towards the top of the ladder had the highest volume and surface area and therefore offered the greatest hindrance to passive displacement. Immature S2 smolts may have been more active in seeking a passage through this barrier. In 1986 and 1987 no significant change in ladder travel time or fish condition factor was detected over the period May to July. In 1986 fish condition factors and variance in ladder travel time increased significantly in August. Immature S2 smolt migration may have been less nocturnal. Mature male smolts were affected more by high daytime water temperatures than immature S2 and S1 smolts.     

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.     

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.     

Effects of light level and growth history on attack distances of visually foraging juvenile salmon in experimental tanks

The effects of light level, developmental pathway, and previous growth history on the foraging attack distances of juvenile Atlantic salmon Salmo salar were examined in circular rearing tanks. Former manipulation of growth rates had no significant influence on distances moved to intercept food items despite the fact that it caused substantial differences in post-treatment growth. Attack distances of fish that were entering a state of overwinter dormancy (lower modal group; LMG) were shorter than those of actively feeding (and growing) fish (upper modal group; UMG). These differences were explained generally by differences in body size between the two groups, suggesting that actual effort per attack was unassociated with growth requirements. Significant differences between growing and non-growing fish in attack distances could contribute to the variation in growth rates through their effect on feeding rates, but were unlikely to have affected energetic costs. This may be due to the fact that attack distances were consistently short throughout the study period as is also evident from the pattern of change between night and daytime. Whereas in the first experiment (daylight v . twilight) fish moved further to reach food during the day, in the second (daylight v . overcast night) nocturnal attack distances matched (LMG fish) or exceeded (UMG fish) diurnal attack distances. Thus diurnal attack distances were probably minimized in the second experiment. These results are interpreted within a framework of overwintering strategies.     

Diel feeding rhythms and daily food consumption of juvenile Atlantic salmon in the River Alta, northern Norway

Atlantic salmon fry (0+) sampled from the River Alta exhibited only minor differences in stomach content weights and feeding rates throughout diel periods, but feeding rates were generally lowest at night. In contrast, salmon parr (1+ to 3+) had large diel fluctuations in stomach content weight, with the largest weights usually being recorded during the night and early morning. Accordingly, their feeding rates were highest at night. This nocturnal feeding pattern was consistent throughout all sampling occasions, and appeared to reflect a persistent feeding periodicity in the salmon parr. The daily food consumption rates of both fry and parr were highest during midsummer and decreased towards autumn.     

Feeding chronology of Atlantic salmon parr in subarctic rivers: consistency of nocturnal feeding

During the ice-free season in three subarctic rivers in northern Norway and Finland, Atlantic salmon parr Salmo salar exhibited significant diel fluctuations in stomach content weights in August and September, the largest weights being recorded consistently at 0300 hours. Accordingly, the feeding rate of the parr was highest during the night and lowest in daytime, whereas during the other periods of the ice-free season, no pronounced diel fluctuations in food intake occurred. The consistent nocturnal feeding periodicity in early autumn in subarctic rivers may be associated with benthic feeding and an increased availability of benthic prey at night.     

Allozyme and mtDNA divergence between Atlantic salmon populations in North Wales

Allozyme variation differentiates between Atlantic salmon Salmo salar populations in North Wales to a greater extent than mtDNA diversity.     

Influence of sexual maturity on feeding, growth and energy stores of wild Atlantic salmon parr

Maturing Salmo salar parr had significantly lower short-term energy stores (indicated by hepato-somatic index) in August, and significantly lower energy stores and growth rate (indicated by RNA : DNA) in September than immature parr captured from the same stream sites on the same date. There were no significant differences in gut fullness or protein concentrations, suggesting that up to early September the main energetic consequences of maturation were a reduction in allocations to growth and short-term energy storage, but not a mobilization of long-term stored energy in the form of proteins. These are the first observations of relative food intake and energy storage for maturing parr under natural conditions, and also the first to assess growth effects on wild fish before completion of the maturation process.     

The effect of domestication on some life history traits of sea trout and Atlantic salmon

During the period 1968–1991, certain morphological traits of Atlantic salmon Salmo salar and sea trout S. trutta have been recorded regularly at the hatchery at Älvkarleby, central Sweden. Total body length, weight (for females both before and after stripping), number of eggs, egg size and date of ovulation. A smaller data set for fish marked and released as smolts, providing information about total body length of released smolts, time spent at sea and body size of the recovered adults, was also available for analysis. According to theory and empirical data, the process of artificial breeding results in an evolutionary divergence of the cultured strain from the wild phenotypic norm. The reason for such a divergence is that both natural and sexual selection pressures are altered or relaxed during the process of artificial breeding, as well as random genetic processes, such as founder effects and in- and outbreeding. A path analysis of both species and sexes revealed that the size of the released smolts had increased during the study period. Time spent in sea has decreased for both female and male sea trout, but not for Atlantic salmon. Adult body size for female and male trout have increased as well as female trout condition factor. The increase found in egg size of both species was greater for the salmon than for the trout, indicating that female salmon invest more in egg size with increasing body size. These results support the view that domestication probably has a significant and selective impact on the life history traits of the two salmonid species studied.     

The feeding response of diploid and triploid Atlantic salmon and brook trout

Pair and group experiments were conducted to determine whether differences exist in feeding success between juvenile diploid and triploid salmonids in a competitive situation. In the pair experiments, 22 pairs (one diploid and one triploid) of size-matched Quebec-strain brook trout Salvelinus fontinalis (7·2–46·3 g) were fed an unlimited number of pellets three times a day for 5 days. Dominance was assigned to the fish which ate the most pellets within each pair. In the group experiments, groups of three diploid and three triploid size-matched fish were fed a restricted ration three times a day for 5 days. Hierarchical rank within the group was assigned based on the number of pellets consumed by each fish. The group experiment was repeated 10 times with Atlantic salmon Salmo salar (5·1–62·7 g), Quebec-strain brook trout (11·8-110·8 g), and large UNB-strain brook trout (18·2–33·0 g), and 12 times with smaller UNB-strain brook trout (0·6–2·0 g). A statistically significant difference in rank between ploidies was found only for the smaller UNB-strain brook trout in the group experiments, with diploids dominant over triploids. This suggests that there may be a difference in competitive feeding success between diploid and triploid brook trout early in development, but that this difference diminishes as the fish grow.     

Growth rate correlations across life-stages in female Atlantic salmon

Within smolt ages there was a strong negative correlation between pre- and post-smolt growth in wild three-sea-winter female Atlantic salmon Salmo salar . This indicates that trait values providing, or resulting from, increased growth in one environment may cause growth to decrease in the other, potentially constraining growth rate evolution.     

The feeding behaviour of juvenile Atlantic salmon in relation to turbulent flow

The feeding behaviour of juvenile Atlantic salmon Salmo salar in the Sainte‐Marguerite River, Quebec, Canada, varied with the characteristics of turbulent flow. Simulations indicated that juveniles would decrease their swimming costs during attacks by 19·8% in low and by 31·1% in high turbulent conditions by initiating movements in low‐speed flow events. The real swimming costs did not differ from the swimming costs estimated for a situation where fish initiate their movements at randomly selected flow velocities. The juvenile Atlantic salmon did not seem to prefer low‐speed flow events when initiating their movements. The proportion of time used for movements by fish decreased with an increase in the mean and the s . d . of the flow velocity.     

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.     

Evidence of freshwater feeding by adult salmon in the Tana River, northern Norway

In the River Laksjohka, a tributary of the Tana River (northern Norway), stomachs from 45 out of 133 adult Atlantic salmon Salmo salar contained food items of terrestrial and freshwater origin. In the neighbouring River Maskejohka, none of 18 sampled stomachs contained any food items.     

The influence of life-history strategy on lipid metabolism in overwintering juvenile Atlantic salmon

From November to May, the lipid mass in the viscera and carcass of juvenile Atlantic salmon Salmo salar that were undergoing smolt transformation prior to seaward migration ('early migrants') were significantly greater than those of their siblings that would delay migration for at least a further year. During winter (November-February), the depletion of lipid associated with the viscera was significantly greater in early migrants, whilst lipid depletion in the remaining carcass was greater in delayed migrants. Early migrants continued to deplete both lipid compartments in spring (February-May), whereas delayed migrants depleted visceral lipid but replenished carcass lipid over the same period. Fatty acid accumulation rates (a measure of storage lipid synthesis rates) were two to six times greater in visceral than in carcass lipid throughout the study, suggesting that lipid turnover is much more rapid in the viscera. There were no differences in fatty acid accumulation rates between migrant groups in November, despite the much lower food consumption rate of delayed migrants at that time, suggesting that these fish allocated a larger proportion of their nutritional resources to lipid synthesis. In the carcass lipid of early migrants, and in both the visceral and carcass lipid of delayed migrants, the fatty acid accumulation rate was negatively correlated with lipid mass. Fatty acid accumulation rates increased from November to February in both visceral and carcass lipid in the two migrant groups. The fatty acid accumulation rate in carcass lipid was significantly higher in delayed migrants than in early migrants in February, but not in May. These results support the hypothesis that life history strategies involving rapid growth will result in a relatively low allocation of resources to lipid reserves.     

Social dominance, prior residence and the acquisition of profitable feeding sites in juvenile Atlantic salmon

Twelve groups of 10 Atlantic salmon Salmo salar parr were screened for dominance in small tanks with a single localized food source using a serial removal method and the top, middle and bottom two fish in each group were classified as high, medium and low social status, respectively. These 72 ranked fish were weighed and formed into four groups or waves, each consisting of six fish of each status category. The fish were given dye marks according to status and wave and were then introduced into an artificial stream in four waves over a 12-day period. Patterns of settlement/emigration were recorded over a 2-month period, at the end of which the fish were removed from the stream, identified and reweighed, and daily specific growth rates calculated. Fifty-one fish established feeding stations in the flume, the remainder emigrating via a downstream trap. The average growth rate over the 2-month period was 0·87% day⁻¹. Probability of settlement and growth rates following settlement were significantly higher in fish from the earlier waves. In contrast, salmon that were dominant in the initial trial were less likely than subordinate fish to settle in the experimental stream and showed lower postsettlement growth rates. The possibility that fish have individually consistent behavioural profiles that promote success in different competitive environments is discussed with reference to published literature for other groups. A small-scale trial at the end of the main experiment confirmed the prior residence effect and demonstrated that 1 day was sufficient for such an effect to be evident.     

Otolith and somatic growth rates in Atlantic salmon parr, Salmo salar L: evidence against coupling

It is often assumed that otolith growth is in some way dependent on somatic growth (i.e. that the two processes are coupled). We examined the relationships between sagitta radius and fork length in 0+ Atlantic salmon parr that would subsequently smolt aged 1 + (UMG fish) or 2+ (LMG fish). Repeated measurements of fork lengths of individually marked parr, taken over a 211-day period from first feeding, were compared to sagitta radii on the same measuring dates (obtained by analysis of daily increments). The results showed that there was a linear relationship between fork length and otolith radius in UMG parr. However, this was not true for LMG parr. These fish enter a state of natural anorexia in their first autumn (despite excess food), but their otoliths continued to grow at the same rate despite the virtual cessation of somatic growth; they had therefore developed disproportionately large otoliths by the end of the study period. The relative growth rates of soma and otoliths first changed in LMG fish in late July/early August; this is the most precise estimate yet obtained of the timing of divergence in the developmental pathways of UMG and LMG parr. The rate of sagitta accretion was consistently lower in LMG parr, possibly indicating a lower metabolic rate in these fish. The results are discussed in relation to previous theories of the relationship between otolith and somatic growth.     

Demographic and genetic description of Greenland's only indigenous Atlantic salmon Salmo salar population

A survey of the Kapisillit River system was conducted in 2005 and 2012 to study the only indigenous Atlantic salmon Salmo salar population in Greenland. Little is known about its characteristics or its relationship with other S. salar populations across the species range. Juvenile S. salar were captured in all stations surveyed within the lower river with the highest densities lower in the river and decreasing densities with increasing distance from the river mouth. Captured juveniles ranged from 0+ to 7+ years old and the predominant smolt age was between 4 and 6 years. Median length of 0+ and 1+ juveniles in August–September was 38.8 and 70.4 mm, respectively. The proportion of mature male parr increased from 4% for 1+ year old fish to 95% for fish greater than 2 years old. Genetic analysis using 96 single nucleotide polymorphisms (SNP) revealed a high degree of genetic similarity between collections, extremely low genetic diversity and low estimates of effective population size (N_e = 28.7; 95% CI = 19.7–42.4). Genetic comparison to range-wide S. salar populations demonstrated that the Kapisillit River S. salar is an outgroup of the eastern Atlantic stock complex, which is consistent with the hypothesised colonisation from the east. River morphology and the absence of glacier runoff are hypothesised to be the main reasons for the relatively high river temperatures supporting this self-sustaining population of S. salar. Given its uniqueness and persistence, this population represents an important part of range-wide biodiversity of S. salar.