Movements of adult Atlantic salmon in relation to a hydroelectric dam and fish ladder

The movements of adult Atlantic salmon were recorded as they approached, entered and ascended the pool-and-orifice fish ladder at Pitlochry Dam, Scotland. Thirty-nine returning salmon were captured in the River Tummel by rod-and-line angling, radio-tagged and released near where they were caught. The subsequent movements of each fish were then monitored. An electronic fish counter collected additional data on movements of untagged fish past a fixed point in the ladder. Of the 39 fish that were radio-tagged, 29 individuals were recorded approaching and ascended the ladder. The remaining fish either did not approach the dam (three fish), approached the dam after detailed tracking had ended (two fish), were recaptured by anglers (three fish), or the radio tags failed (two fish). Salmon released earlier in the year delayed longer before first approaching the dam. Delays between first approaching the dam and ascent of the ladder were greater for fish that approached the dam earlier. The majority of salmon visited the ladder entrance more than once (maximum 10 visits) before ascending. Having entered, all but four salmon ascended the fish ladder successfully on their first attempt. The four individuals that failed to do so succeeded on their second attempt. The rate at which salmon ascended the ladder was related directly to temperature. The shortest ascent time of a radio-tagged salmon was 5·25 h. Movements of eight of 11 tagged fish through the ladder ceased with the onset of darkness but continued on the following morning. No radio-tagged fish entered the ladder at temperatures below 9) C. Similarly, few untagged fish were recorded ascending the ladder by the electronic fish counter at water temperatures below 8·5) C. Records from the fish counter indicated that 92% of upstream movements were made during daylight.     

Vertical distribution of Atlantic salmon (Salmo salar) as a function of illumination

The vertical distribution of cultured Atlantic salmon larger than 1 kg was monitored by hydroacoustics in both a shallow (6 m) and a deep (20 m) cage. Surface light levels and fish distribution were registered for three periods throughout the year. The two fish groups were fed calculated rations twice a day. During the winter and spring (including vernal equinox), the fish were distributed at around 5 m depth in both cages when not feeding. Around summer solstice, the fish preferred deeper waters when not feeding. The light dependency of the vertical migration was pronounced, and indicates clearly that even a 20 m deep cage is not deep enough to meet the depth requirements of large Atlantic salmon during summer.     

Deep-diving of Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis>) during their marine feeding migrations

Data from seven data storage tags recovered from Atlantic salmon marked as smolts were analyzed for depth movements and patterns of deep diving during the marine migration. The salmon mostly stayed at the surface and showed diurnal activity especially from autumn until spring. During the first months at sea the salmon stayed at shallower depths (<100 m). The salmon took short deep dives (>100 m), that were rare or absent during the first summer at sea but increased in frequency and duration especially in late winter. The maximum depth of the dives varied from 419 to 1187 m. Most of dives were short, (<5 h) but could last up to 33 h. The duration of dives increased in late winter until spring and the overall depth and maximum depth per dive increased exponentially over time. The initiation of the dives was more common in evenings and at night, suggesting nocturnal diving. We hypothesized that deep diving is related to feeding of salmon as mesopelagic fish can be important food for salmon during winter.     

Factors affecting river entry of adult Atlantic salmon in a small river

In this study, effects of stock origin, fish size, water flow and temperature on time of river ascent of adult Atlantic salmon Salmo salar were tested. Brood stocks were collected in eight Norwegian rivers situated between 59 and 69° N. The fish were reared to smolts, individually tagged and released in the River Imsa, south-west Norway (59° N). Adults from all stocks approached the Norwegian coast concurrently, but Atlantic salmon ≥70 cm in natural tip length entered coastal water slightly earlier during summer than smaller fish. Atlantic salmon <70 cm, however, ascended the river significantly earlier and at lower water flow and higher water temperature than larger fish. Although largest in size, the fish from the northern populations (62–69° N) ascended the River Imsa almost 1 month earlier than those from the south (59–60° N). They seemed less restricted by the environmental factors than the fish originating from the more southern rivers. There was no apparent trend among years in time of river ascent. Maximum ascent per day occurred at water discharges between 12·5 and 15 m³ s⁻¹ and at water temperatures between 10 and 12·5° C. There was a significant positive correlation between water flow and river ascent during the first part of the upstream run from July to September with best correlation for September, when multiple regression analysis indicated that water temperature had an additional positive effect. Stock origin, fish size and water discharge were important variables influencing the upstream migration of Atlantic salmon in small rivers.     

Changes in swimming depths of Atlantic salmon Salmo salar post‐smolts relative to light intensity

Eight hatchery‐reared Atlantic salmon Salmo salar post‐smolts, implanted with acoustic depth sensing transmitters and manually tracked for 5–12 h in the Hardangerfjord (Norway), spent most of their time (49–99%) at 1–3 m depth during the day, whereas four of seven fish tracked were found close (<0·5 m) to the surface at night, with a strong negative cross‐correlation between general swimming depth and surface light intensity. Hence, the actual swimming depth of post‐smolts during their early marine migration may depend on the light conditions, although the individual variation in vertical movement pattern was large. No cross‐correlations were found between light intensity and swimming depth during daytime periods with rapid changes in light intensity, indicating that other factors than light intensity were important in initiating the irregular dives that were recorded down to 6·5 m depth.     

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.     

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.     

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.     

Effect of origin, sex and sea age of Atlantic salmon on their recapture rate after river ascent

The recapture rate of Atlantic salmon (Salmo salar L.) after river ascent was examined by the trapping and tagging of ascending spawners in the lower reaches of the Simojoki River, which flows into the northern Baltic Sea. In 1997 and 1998, altogether 825 Carlin‐tagged salmon were released to continue their upstream migration. Of these, 800 could be sexed and categorized as reared (91%) or wild (9%) salmon. In 1997, most of the ascending salmon were multi‐sea‐winter (MSW) fish, whereas in 1998 almost all were one‐sea‐winter (1SW) male grilse due to the late trapping season. About 10% of all tagged fish were recaptured, two‐thirds of which were caught in the river before their descent to the sea. There was no difference in the recapture rate between salmon of wild (8.5%) or reared (9.5%) origin, or between females (11.6%) and males (9.3%). Generalized linear models for data from 1997 showed that the recapture rate increased with length and age of females, but that the opposite was true for males. River fishing did not seem to remove proportionally more early ascending salmon than fish that ascended later.     

Previously spawned Atlantic salmon ascend a large subarctic river earlier than their maiden counterparts

Spawning migration timing of maiden Atlantic salmon Salmo salar and previous spawners was analysed in the catches in 1989–2004 in the large subarctic River Teno in the northernmost parts of Finland and Norway. The hypothesis was that the migration timing of previous spawners and their maiden counterparts is similar, with the migration timing similar between sexes. In most cases, however, previous spawners were observed to migrate into the River Teno and its tributaries earlier than their maiden counterparts. The difference in run timing was especially evident between maiden one-sea-winter (1SW) Atlantic salmon and the corresponding group of previous spawners [1S1, 1 year at sea (1) followed by first spawning (S) and reconditioning period of 1 year (1) at sea and second spawning run] for both sexes in the River Teno and in its two tributaries. The same was also evident between 2SW maiden and 2S1 previous spawning female Atlantic salmon in the River Teno. Females showed earlier spawning migration than males both in previous spawners and maiden Atlantic salmon. Different maiden sea-age classes also showed differences in run timing as multi-sea-winter fish (2–4SW) ascended earlier than 1SW fish but the timing of 1S1 and 2S1 previous spawning females coincided. The results suggest that run timing of Atlantic salmon may not be strictly genetically fixed as previous spawners ascend earlier than they did on their first spawning migration as maiden fish, and indicated that the closeness of the reconditioning area of postspawners to the river of origin resulted in an early ascent. Run timing of different sea-age groups has major management implications if the populations are heavily exploited with numerous fishing methods in different periods of the fishing season, as in the River Teno system.     

Migration speeds and orientation of Atlantic salmon and sea trout post-smolts in a Norwegian fjord system

We recorded the observed and actual swimming speeds of Atlantic salmon and sea trout post-smolts in a Norwegian fjord system, and initiated studies on the orientation mechanisms of the post-smolts. We tracked Atlantic salmon and sea trout with acoustic transmitters for up to 14 h after release. The actual swimming speed and direction of a fish relative to the ground is the vector sum of the observed movements of the fish and the movements of the water. We determined actual swimming speeds and directions of the post-smolts, which reflect their real swimming capacities and orientation, by corrections for the speed and direction of the water current. The post-smolts were actively swimming. The observed direction of movement was dependent on the actual movement of the fish and not the water current. Water currents were not systematically used as an orientation cue either in Atlantic salmon or sea trout, as the actual movements were random compared to the direction of the water current. The actual movement of sea trout were in all compass directions, with no systematic pattern. The Atlantic salmon also moved in all compass directions, but with the lowest frequency of actual movement towards the fjord.     

Fish may fight rather than feed in a novel environment: metabolic rate and feeding motivation in juvenile Atlantic salmon

This study tested the hypothesis that juvenile Atlantic salmon Salmo salar with a high resting metabolic rate and probability of dominance will also have a correspondingly higher feeding motivation in a novel environment to offset their greater costs of maintenance. The opposite was found to be the case: Atlantic salmon with a high standard metabolic rate had a slightly but significantly lower feeding motivation. It is hypothesized that Atlantic salmon with higher maintenance costs opt to be more aggressive at the expense of the costly activities associated with feeding, since elevated aggression in a new habitat is a more successful strategy for acquiring a feeding territory (and hence sustaining food intake in the long-term) than a high feeding motivation alone.     

Adaptive winter survival strategies: defended energy levels in juvenile Atlantic salmon along a latitudinal gradient

Current knowledge suggests that patterns of energy storage and depletion in animals are governed by behavioural trade-offs between risks associated with feeding and future energy demands. However, the length of adverse periods varies over geographical or climatic gradients. To explore the potential for genotypic sources of variation in behavioural trade-offs, we compared the winter energy-depletion patterns among 13 wild populations of juvenile Atlantic salmon (Salmo salar L.) along a latitudinal gradient (58–70°N) and performed common-environment experiments of energy-state-dependent feeding. In the wild, winter lipid-depletion rates were lower for northern than for southern populations. The variation in spring lipid levels among the population was lower than autumn variation, with storage lipid levels clustered close to critical limits for survival. In semi-natural stream channels with natural food supply, hatchery-reared fish originating from northern populations showed a positive scaling of feeding activity with decreasing energy levels, whereas southern populations did not. In conclusion, juvenile Atlantic salmon from northern populations defend their energy levels more strongly than fish from southern populations. Adaptive variation in feeding activity appears important for this difference. Thus, the present study shows a link between geographical patterns in storage energy trajectories and adaptive differences in state-dependent feeding motivation.     

Seasonal changes in hypoosmoregulatory ability in landlocked and anadromous populations of Arctic charr,Salvelinus alpinus,and Atlantic salmon,Salmo salar

Synopsis Seasonal changes in hypoosmoregulatory ability were compared in landlocked and anadromous strains of Arctic charr and Atlantic salmon. Seawater adaptability was assessed using periodic 48 h seawater challenge tests with 25. seawater. The landlocked strains of Arctic charr, two from northern Sweden and one from Southern Norway, displayed similar seasonal changes in seawater adaptability as the anadromous strain. Seawater tolerance increased during spring and remained high until the end of July — early August after which it declined. The two strains of Atlantic salmon displayed different seasonal patterns in hypoosmoregulatory ability. The anadromous strain showed a pronounced seasonal pattern with maximal seawater adaptability in early June. In contrast, seawater tolerance in the landlocked strain improved steadily during spring and remained high until late autumn. During the period of enhanced seawater tolerance, hypoosmoregulatory ability increased significantly with body size in both Arctic charr and anadromous Atlantic salmon. The minimum size at which fish were able to regulate plasma sodium following seawater transfer at a level comparable to freshwater levels (<170 mmol I^–1) differed significantly between anadromous Atlantic salmon (ca. 14 cm) and Arctic charr (ca. 22 cm). The results show that seasonal changes in hypoosmoregulatory ability are present in both Atlantic salmon and Arctic charr, and that these physiological traits are retained in the corresponding landlocked strains. However, the seasonal pattern of seawater adaptability as well as the minimum size at which seawater tolerance occurs differs between the two species.     

Summer temperature variation and implications for juvenile Atlantic salmon

Temperature is important to fish in determining their geographic distribution. For cool- and cold-water fish, thermal regimes are especially critical at the southern end of a species’ range. Although temperature is an easy variable to measure, biological interpretation is difficult. Thus, how to determine what temperatures are meaningful to fish in the field is a challenge. Herein, we used the Connecticut River as a model system and Atlantic salmon (Salmo salar) as a model species with which to assess the effects of summer temperatures on the density of age 0 parr. Specifically, we asked: (1) What are the spatial and temporal temperature patterns in the Connecticut River during summer? (2) What metrics might detect effects of high temperatures? and (3) How is temperature variability related to density of Atlantic salmon during their first summer? Although the most southern site was the warmest, some northern sites were also warm, and some southern sites were moderately cool. This suggests localized, within basin variation in temperature. Daily and hourly means showed extreme values not apparent in the seasonal means. We observed significant relationships between age 0 parr density and days at potentially stressful, warm temperatures (≥23°C). Based on these results, we propose that useful field reference points need to incorporate the synergistic effect of other stressors that fish encounter in the field as well as the complexity associated with cycling temperatures and thermal refuges. Understanding the effects of temperature may aid conservation efforts for Atlantic salmon in the Connecticut River and other North Atlantic systems.     

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.     

Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0+ year Atlantic salmon and brown trout and 1+ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1+ and 2+ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1+ and 2+ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2+ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0+ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.     

Accelerated recovery of Atlantic salmon (Salmo salar) from effects of crowding by swimming

The effects of post-crowding swimming velocity (0, 0.35, and 0.70 m/s) and recovery time (1.5, 6, and 12 h) on physiological recovery and processing quality parameters of adult Atlantic salmon (Salmo salar) were determined. Atlantic salmon crowded to a density similar to that of a commercial slaughter process (>200 kg/m(3), 40 min) were transferred to a swimming chamber for recovery treatment. Osmolality and concentrations of cortisol, glucose and lactate in blood plasma were used as physiological stress indicators, whereas image analyses of extent and duration of rigor contraction, and fillet gaping were used as measures of processing quality. Crowded salmon had a 5.8-fold higher plasma cortisol concentration than control salmon (P<0.05). The elevated plasma cortisol concentration was reduced by increasing the swimming velocity, and had returned to control levels after 6 h recovery at high water velocity. Similar effects of swimming velocity were observed for plasma osmolality and lactate concentration. A lower plasma glucose concentration was present in crowded than in control fish (P<0.05), although a typical post-stress elevation in plasma glucose was observed after the recovery treatments. Lower muscle pH was found in crowded compared with control salmon (P<0.05), but muscle pH returned to control levels after 6 h recovery at intermediate and high swimming velocities and after 12 h in the low velocity group. Crowding caused an early onset of rigor mortis contraction. However, subjecting crowded salmon to active swimming for 6 h before slaughter delayed the onset of rigor mortis contraction from 2.5 to 7.5 h post mortem. The extent of rigor mortis contraction was also affected by crowding and post-stress swimming activity (P<0.05), and the largest degree of contraction was found in crowded salmon. In conclusion, active swimming accelerated the return of plasma cortisol, hydromineral balance, and the energy metabolism of adult Atlantic salmon to pre-stress levels. Moreover, an active swimming period delayed the onset of rigor mortis contraction, which has a positive technological implication for the salmon processing industry.     

Postprandial intestinal blood flow, metabolic rates, and exercise in Chinook salmon (Oncorhynchus tshawytscha)

Following a relatively large meal (2% body mass of dry pellets), intestinal blood flow in chinook salmon (Oncorhynchus tshawytscha) increased significantly, up to 81%, between 14 and 29 h postprandially. Also, 15 h postprandially, oxygen consumption (M(2)) was elevated by 128% compared with a measurement of routine M(2) made after 1 wk of fasting. The postprandial increase in MO(2) (the heat increment) was 33 micromol O(2) min(-1) kg(-1). Because intestinal blood flow is known to decrease during swimming activity in fish, we therefore tested the hypothesis that swimming fish would have to make a trade-off between maximum swimming activity and digestive activity by comparing the swimming performance and metabolic rates of fed and fasted chinook salmon. As expected, MO(2) increased exponentially with swimming velocity in both fed and fasted fish. Moreover, the heat increment was irreducible during swimming, such that MO(2) remained approximately 39 micromol O(2) min(-1) kg(-1) higher in fed fish than in fasted fish at all comparable swimming speeds. However, maximum M dot o2 was unaffected by feeding and was identical in both fed and fasted fish (approximately 250 micromol O(2) min(-1) kg(-1)), and, as a result, the critical swimming speed (U(crit)) was 9% lower in the fed fish. Three days after the fish were fed and digestion was completed, MO(2) and U(crit) were not significantly different from those measured in fasted fish. The ability of salmonids to maintain feeding metabolism during prolonged swimming performance is discussed, and it is suggested that reduced swimming performance may be due to postprandial sparing of intestinal blood to support digestion, thereby limiting the allocation of blood flow to locomotory muscles.