Article

Fast‐flowing areas affect the feeding activity of migrating Altantic salmon smolts in tributaries of a subarctic river

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Abstract

Feeding activity of migrating wild Atlantic salmon smolts was investigated in different types of tributaries of the River Teno, northernmost Finland. General feeding activity and the amount of surface prey in the stomachs were correlated inversely with proportion of habitats with high water velocity in the tributaries.

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... Smolt are, however, also clearly capable of drift -feeding, demonstrated through the occurrence of surface prey in the diet. Heinimaa & Erkinaro (1999) showed that feeding activity and the amount of surface prey in stomachs were inversely correlated with proportion of high water velocity habitats in different parts of the large Tana River system (northern Norway and Finland). Surface prey constituted 11 -88% of the smolt diet in Tana, while the proportion has typically been 15% or less in other northern salmon rivers, e.g. the Salatsa in Latvia (Mitans 1970 ), the Rickle å n in northern Sweden (S ö dergren & Ö sterdahl 1965 ) and the Orkla, central Norway (Garn å s & Hvidsten 1985 ). ...
Chapter
This review summarises the current state of knowledge on salmon freshwater feeding, emphasising the issues of what to eat, and when and where feeding activity occurs. It also provides a brief introduction to optimal foraging theory and drift - feeding models. We focus primarily on juvenile salmon freshwater feeding, as the adult salmon feed very little while in fresh water. The juveniles actively select prey items both in terms of prey types and sizes, with large - sized prey types usually being preferred. The diet composition, both in terms of taxa and average prey size, changes with fish size. Most of the diet consists of invertebrates, but there are also some records of juvenile salmon eating fish. In running water, the salmon can feed from invertebrates drifting either on the water surface or in the water column, or from benthic invertebrates on the streambed surface. In temperate areas, juveniles are predominantly daytime drift - feeders, whereas they spend more time feeding on the benthos at night in northern areas. The relative extent of benthic feeding also increases with decreasing light and temperature through the season. The highest feeding rates are observed during spring and early summer. However, although feeding rates decrease with falling water temperatures in the autumn, juveniles also feed during the winter. Thus, juvenile salmon are versatile fish, being able to feed successfully in different habitats ranging from small streams to large lakes and under different conditions (e.g. changing light levels and seasons).
... Active feeding of migrating smolts was also recorded in other Baltic and Atlantic rivers in the spring (e.g. Mitans 1970;Heinimaa & Erkinaro 1999). In the River Simojoki, migrating smolts commonly have aquatic insects in their stomachs, especially in late spring when benthic macroinvertebrates are also abundant in Finnish rivers (Haapala & Muotka 1998). ...
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Abstract  Seasonal differences in smolt traits and the post-smolt survival of wild Atlantic salmon, Salmo salar L., were investigated by smolt trapping and Carlin-tagging in the River Simojoki, northern Finland between 1991 and 2004. The annual trapping season was divided into two halves based on the median catch date. Smolt length was significantly higher during the first half of the season, while differences in smolt weight were typically small. Smolt age was always significantly higher during the first half of the season because older smolts tended to migrate earlier in the season. Many smolts migrating during the early season and almost all smolts migrating later had started their new growth, indicating that smolts grow in the spring before sea entry. The differences in recaptures between smolts tagged during the first and second halves were insignificant. Although variations in smolt traits and environmental conditions can produce inter-annual variation in post-smolt survival, their seasonal differences seem to be too small to have an effect.
... McCormick et al. 1998). Information on smolt feeding is lacking (but see Mitans 1970;Johnson et al. 1996;Heinimaa and Erkinaro 1999) and to what degree feeding of salmonid smolts during seaward migration may affect their survival during the migration and the initial time at sea has not been discussed. ...
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In general, hatchery salmonid smolts experience higher mortality during migration than wild smolts, which is suggested to be due to domestication effects and that hatchery fish lack experience of the natural environment. However, possible differences in feeding during smolt migration between hatchery and wild smolts have rarely been addressed. We compared the number of feeding smolts and stomach fullness among wild Atlantic salmon smolts, hatchery-reared smolts released as 1-year-old parr, and hatchery-reared smolts released as 2-year-old smolts during their descent to sea in River Tornionjoki. In addition, estimations of prey selection among the smolt groups were conducted. A high proportion of wild smolts and smolts stocked as parr actively fed during the smolt migration. A lower proportion of smolts stocked as smolts was feeding and their stomach fullness were much reduced in comparison with the two other groups. The study also indicated that the feeding of migrating smolts is selective rather than opportunistic. In conclusion, this study suggests that stocked 2-year-old smolts may enter sea with an inferior foraging behaviour and it is a possibility that this may contribute to the observed low post-smolt survival in the Baltic Sea. KeywordsStocking–Salmonid–Prey selection–Mortality–Sea survival
... Wild smolts have experience and adaptations for feeding in the natural environment, and might, therefore, stop periodically for feeding. Heinimaa & Erkinaro (1999) suggested that feeding activity, especially on surface prey, of wild S. salar smolts is correlated with the amount of pools and other slow flowing sections of the river, which are abundant in the lower part of the River Simojoki. Smolts are, thus, sometimes actively holding position in the current during their downstream migration and they can stay in low current areas for extended periods (Moser et al., 1991;Fängstam, 1993). ...
Article
The relative amount of muscle contraction regulating dihydropyridine and ryanodine receptors in the swimming muscles of trained reared Atlantic salmon Salmo salar smolts was compared with those of untrained and wild smolts. After an optimized 2 week training period, i.e. swimming with a velocity of 1·5 body lengths per second for 6 h per day, the level of both receptors was significantly higher in the muscles of trained S. salar than in the untrained ones before they were released into the natural environment. This difference persisted after downstream migration in the river. The highest level of receptors was observed in wild S. salar. Swimming performance was also higher in trained fish compared to untrained ones. Furthermore, swimming performance was positively associated with the level of receptors in both red and white muscle types. Downstream migration after release into the wild was significantly slower in trained smolts than in untrained fish. This indicates that trained smolts were most probably swimming harder against the current in the river than untrained smolts. The possible advantages for a slower migration in the river are discussed. This study shows that the prerequisites for effective contraction of the swimming muscles are better met in trained S. salar compared to untrained fish, and the muscles of trained smolts more closely resemble those of wild smolts. The results also imply that the capacity of untrained, reared smolts to swim against the current is not equal to that of their trained or wild counterparts which affects the downstream migration pattern of S. salar smolts.
... Active feeding of migrating smolts has also been recorded in other Baltic and Atlantic rivers in the spring (e.g. Mitans 1970, Garnås & Hvidsten 1985, Heinimaa & Erkinaro 1999). In the Simojoki river, migrating smolts commonly have aquatic insects in their stomachs, especially in late spring (E. ...
Article
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Long-term monitoring data collected from wild smolts of Atlantic salmon (Salmo salar) in the Simojoki river, northern Finland, were used in studying the relationships between the smolt size and age, smolt and postsmolt migration, environmental conditions and postsmolt survival. The onset of the smolt run was significantly dependent on the rising water temperature and decreasing discharge of the river in the spring. The mean length of smolts migrating early in the season was commonly higher and the mean age always older than among smolts migrating later. Many of the smolts migrating early in the season and almost all smolts migrating later had started their new growth in spring in the river before their sea entry. Among postsmolts, the time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river, being significantly shorter in years with warm than cold sea temperatures. After leaving the estuary, the postsmolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the geographical distribution of the tag recoveries coinciding with the warm thermal zone in spring in the coastal area. After arriving in the southern Gulf of Bothnia in late summer the postsmolts mostly migrated near the western coast, reaching the Baltic Main Basin in late autumn. Until the early 1990s there was only a weak positive association between smolt length and postsmolt survival. However, following a subsequent decrease in the mean smolt size, a significant positive dependence was observed between smolt size and the reported recapture rate of tagged salmon. The differences in recapture rates between smolts tagged during the first and second half of the annual migration season were insignificant, indicating that the seasonal variation in smolt size and age seem to be too small to affect survival. Among the climatic factors examined, the summer SST in the Gulf of Bothnia was most clearly related to the survival of the wild postsmolts. Postsmolt survival appeared to be highest in years when the SST in June in the Bothnian Bay varied between 9 and 12 ºC. In addition, the survival of wild postsmolts showed a significant positive dependence on the SST in July in the Bothnian Sea, but not on the abundance of the prey fish (0+ herring, Clupea harengus and sprat, Sprattus sprattus) in the Bothnian Sea and in the Baltic Main Basin. The results suggest, that if the incidence of extreme weather conditions were to increase due to climatic changes, it would probably reduce the postsmolt survival of wild salmon populations. For improving the performance of hatchery-reared smolts, it could be useful to examine opportunities to produce smolts that are in their smolt traits and abilities more similar to the wild smolts described in this thesis. Simojoen lohikannan pitkäaikaisseurannassa vaelluspoikasista eli smolteista kerättyä aineistoa käytettiin lohen (Salmo salar) villien smolttien koon ja iän, ympäristöolojen, saaliseläinten runsauden ja smolttien mereentulovuoden aikaisen eli ns. postsmolttivaiheen eloonjäännin välisten suhteiden tutkimiseksi. Smolttivaelluksen ajoittuminen keväällä riippui jokiveden lämpötilan noususta yli 10 ºC:een ja joen virtaaman vähenemisestä. Vaelluskauden alkupuoliskolla smoltit olivat yleensä keskimäärin pitempiä ja aina keskimäärin vanhempia kuin myöhemmin vaeltavat. Monet vaelluskauden alkupuolella vaeltaneet smoltit ja lähes kaikki vaelluskauden loppupuolella vaeltaneet olivat alkaneet kasvunsa keväällä joessa jo ennen mereen tuloaan. Postsmolttien tarvitsema aika jokisuulta poistumiseen riippui pintaveden lämpötilasta meressä; se oli merkitsevästi lyhyempi keväinä, jolloin merivesi oli lämmintä kuin jos se oli kylmää. Jokisuulta lähdettyään postsmoltit vaelsivat etelään Perämeren itärannikkoa pitkin, jolloin merkkipalautusten maantieteellinen jakautuminen vastasi lämpimän veden vyöhykkeen keväistä esiintymistä rannikolla. Saavuttuaan loppukesällä Selkämerelle postsmoltit vaelsivat enimmäkseen länsirannikon läheisyydessä ja saapuivat Itämeren pääaltaalle myöhään syksyllä. 1990-luvun alkupuoliskolle saakka villien smolttien pituuden ja postsmolttien eloonjäännin välillä oli vain heikko positiivinen yhteys. Sen jälkeen kuitenkin smolttikoko pieneni, jolloin havaittiin merkitsevä positiivinen riippuvuus smolttikoon ja lohien merkkipalautusprosentin välillä; suurten smolttien eloonjäänti oli parempi kuin pienten. Merkkipalauksissa ei ollut merkitseviä eroja vuotuisen smolttivaelluksen alku- ja loppupuoliskolla merkittyjen smolttien välillä. Tutkituista ilmastollisista tekijöistä Pohjanlahden pintaveden lämpötilalla kesällä oli selvimmin vaikutusta villien postsmolttien eloonjääntiin. Postsmolttien eloonjäänti oli paras vuosina, jolloin pintaveden lämpötila vaihteli kesäkuussa Perämerellä välillä 9 – 12 ºC. Lisäksi villien postsmolttien eloonjäännillä oli merkitsevä positiivinen riippuvuus pintaveden lämpötilasta heinäkuussa Selkämerellä. Sen sijaan eloonjäänti ei riippunut merkitsevästi saaliskalojen (0+ silakka ja kilohaili) runsaudesta Selkämerellä ja Itämeren pääaltaalla. Tulokset osoittavat, että jos poikkeuksellisen kylmien ja lämpimien keväiden esiintyminen lisääntyy ilmastonmuutoksen seurauksena, se voi heikentää Itämeren villien lohikantojen säilymismahdollisuuksia. Viljeltyjen smolttien eloonjäänti voisi parantua, jos ne saataisiin ominaisuuksiltaan enemmän villien smolttien kaltaisiksi.
Article
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The activity patterns of animals, whether diurnal, crepuscular or nocturnal, are usually fixed endogenous rhythms, entrained by environmental Zeitgebers. Here we demonstrate that juvenile Atlantic salmon, Salmo salar, switch between diurnal and nocturnal foraging solely in response to environmental temperature, and independently of photoperiod and season. Above 10 degrees C juvenile Atlantic salmon fed predominantly during daylight, spending the night exposed in the water column but relatively quiescent. As temperature dropped below 10 degrees C they became increasingly nocturnal, hiding in refuges by day but emerging to feed at night. It has previously been shown that parallel physiological changes take place in the retinae of several species of salmonids: the quantity and composition of the visual pigments change so as to make the fish more dark adapted at low temperatures. As the fish were found to be far less aggressive by night than by day at all temperatures, the switch to nocturnal activity was also accompanied by a change in social structure. We suggest that this temperature-dependent strategy maximizes feeding efficiency in summer but reduces predation risk in winter.
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This study was designed to define the microhabitats selected in summer by juvenile Atlantic salmon (Salmo salar). Curves were developed describing the preference of 880 young salmon for water velocity at the fish's position (nose velocity), mean water column velocity, total water depth, and stream substrate size. Study sites were chosen in six morphologically diverse streams in Nova Scotia and New Brunswick during 1982–84. Of the four variables measured, only nose velocity chosen by both fry and parr was not significantly different among years or rivers. Atlantic salmon fry (< 65 mm) most frequently selected nose velocities between 5 and 15 cm∙s−1, small parr (65–100 mm) between 5 and 25 cm∙s−1 and large parr (> 100 mm) between 5 and 35 cm∙s−1. Apparently, juvenile salmon utilized water depths and stream substrates which varied within tolerable limits according to their availability in conjunction with preferred water velocities. Significant differences in the body shape and size of the pectoral fin of Atlantic salmon parr in different rivers did not influence the selection of nose velocities within the range of flow conditions sampled.
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1. The various commonly used methods of assessing, on the basis of gut contents, the food of those fishes which have a generalized diet are listed and discussed. Both during the present investigation and by re-examination of published data it is shown that, when a large number of fish are examined, and when the results are expressed comparably, i.e. each food item is shown as a percentage of the total food eaten, all methods give substantially the same results. Reasons are, however, given for rejection of the method based on the number of organisms eaten, and also of the practice of comparing data so obtained with counts of the organisms found in samples of small areas of the substratum. Such comparisons have in the past been used to give numerical expression to the availability of food organisms; but it is suggested that this would be better accomplished by using an arbitrary method of allocation of points on the basis of estimated volume of each food item present (a) in the fish guts, and (b) in general faunal collections from the habitat. The number of points, expressed as a percentage of the total, gained by any food item in the fish guts and in the general collections could then be compared. 2. The food of Gasterosteus aculeatus is studied, and it is shown that the diet, consisting chiefly of Crustacea and insects, changes slightly with season and with increase in size of fish. During the winter the fish eat less than at other times, and both sexes feed more sporadically than usual during the breeding season. It is suggested that the two biological races, A and B of Heuts, have different diets. 3. The food of Pygosteus pungitius is shown to be similar to that of Gasterosteus aculeatus, and to vary similarly. The data for the winter are, however, incomplete, and it appears that feeding in this species is more affected during the breeding season. 4. The food relations of three species of fish (G. aculeatus, Pygosteus pungitius and Rutilus rutilus) in a a small muddy Cheshire stream are discussed. It is shown that R. rutilus does not compete with the two other species, but that the diets of the two stickleback species are almost identical. It is suggested that differences in breeding habits enable these two species to live together as they nest in different areas, and that the number of each species is regulated by the fact that during the breeding season the males defend a definite territory round their nests. There is therefore only room for a certain number of nests of each species.
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Wild salmon (Salmo salar) parr and smolt were forced to swim against constant flow (50 cm.s−1) for 8 hours. Physiological properties describing the hormonal status, the energy metabolism and the ionic and osmotic balance of fish were measured from the fish prior to and at the end of the swimming test. Plasma cortisol levels were elevated in response to enforced swimming; the response of the smolt was clearly greater than that of the parr. Plasma thyroxine concentration increased in the parr but stayed at the initial level in the smolts. The parr consumed much of their coelomic fat, but the glycogen stores stayed nearly constant. The smolts had very low fat stores, and the glycogen stores were depleted in the test. The ionic and osmotic balance of the parr was stable in the test, but in smolts, the plasma Cl−1 and osmotic concentrations decreased and muscle moisture increased. The results indicate that downstream migration smolts have markedly lower physiological capacity for continuous swimming than parr.
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Brown trout and young Atlantic salmon in streams are selective in their habitat use, which is partially determined by hydro-physical conditions. Habitat selection may be quantified in models and combined with hydraulic models to evaluate instream habitat suitability. Fish occupancy of habitat depends on the fish species and size. Brown trout prefer deep stream areas with moderate to low water velocities and rocky substrates, whereas young Atlantic salmon chose more fast flowing and often shallower areas. Habitat selection has been quantified in static selection models which should be based on measures of habitat usage and availability (preferences) and combined with data on hydro-physical conditions to build predictive habitat hydraulic models. Such models assess habitat availability and capacity rather than discharge–biomass relationships. Limitations of static models in fish habitat studies are (1) the relevant hydro-physical variables are not included, (2) the interaction terms are difficult to quantify and not incorporated, (3) the hydraulic models may not operate on a spatial scale that is relevant to fish, (4) the models include spatial but only to a limited extent temporal heterogeneity in habitat conditions and (5) biotic factors are not included. Streams may be extremely heterogeneous ecosystems, both spatially and temporally, which may influence habitat selection and modelling. In response to varying habitat availabilities (stream size and structure, water flows) habitat selection in brown trout and young Atlantic salmon is dynamic and relatively flexible. Furthermore, changes in temperature may result in seasonal and daily niche shifts. Therefore unless the dynamic aspects of habitat selection are incorporated into the habitat models, long-term predictive power in habitat–hydraulic modelling is unlikely. However, habitat–hydraulic modelling is a useful tool in a ‘no net loss of habitat’ management strategy regardless of these shortcomings.
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It has been hypothesized that downstream migration of juvenile salmonids is initiated by physiological changes that occur during smoltification, which render the fish unable, or unwilling, to swim against currents that exceed 2 body lengths per second (BL ·s-1). This decline in ability, coupled with the increase in flow rate generally associated with the spring run-off, is thought to result in passive downstream displacement. To test this hypothesis, we measured holding ability of wild Atlantic salmon (Salmo salar) parr and swimming ability of wild, migrating Atlantic salmon smolts in the field under ambient environmental conditions. Atlantic salmon parr (fork length 4.8-13.1 cm) used their pectoral fins to anchor themselves for indefinite (i.e., >200 min) periods in water speeds up to 0.86 m ·s-1. Atlantic salmon smolts (fork length 12.4-21.1 cm) swam indefinitely against currents up to 1.26 m ·s-1, maintained velocities as high as 1.64 m ·s-1 for short periods (2-10 min), and made short bursts at speeds up to 1.95 m ·s-1. These findings indicate that absolute swimming performance is not impaired after smoltification and that wild Atlantic salmon smolts are capable of swimming at speeds much greater than 2 BL ·s-1, making it unlikely that they are involuntarily carried to the sea by river currents.
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Ultrasonic telemetry was used to follow the movements of two groups of smolts in the estuaries of two small Scottish rivers. Hatchery reared smolts released into a typical wedge flow, partially mixed estuary had movements which were dominated by the influence of tide on the direction of water flow. The net movements of wild native smolts in a two layer flow estuary, in which freshwater flow dominated, was downstream but were intermittent consisting of short steps and numerous long pauses. The hatchery reared smolts escaped from the estuary within a tidal cycle, moving out on an ebb tide. The wild smolts remained in the estuary for periods up to 108 h, none escaping within one tidal cycle.
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Behavioural and physiological mechanisms postulated for the control of downstream migration of Atlantic salmon smolts are reviewed briefly, and some new evidence is presented for their refusal to undergo sustained swimming. Although these mechanisms imply passive displacement as the primary means of emigration, it is likely that active components must also exist as the rates of travel of smolts through loch systems are only slightly slower than those recorded for river systems. The timing of these movements within 24 h periods is reviewed and it is shown that the predominantly nocturnal emigration pattern is evident on occasions in alevin, fry and parr stages also. Thus at migration the diel periodicity probably represents a seasonal locomotor rhythm which, under changed behavioural and physiological circumstances, results in downstream displacement.
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On 16 May 1983 1750 tagged 1+ and 1280 tagged 2+ Atlantic salmon smolts of the River Imsa strain were released into the River Imsa in the evening. In 1984 1409 tagged 2+ smolts were divided into three groups. The first was released at 09.00 h on 21 May, the second group at 21.00 h the same evening, and the third group was released at 21.00 h the next evening. The smolts were recaptured in a Wolf trap, 1000 m downstream. To test effects on smolt migration, the light at the trap (4 lux) was switched on and off successively every 15 min during the first night after the 1983 release. In 1984 the whole width of a fast flowing part of the river was constantly illuminated (88 lux) during the first night after the last release.Most smolts descended during the first 12 h after release, and the smolts released in the morning descended in considerable numbers during daytime. 2+ smolts descended faster than did 1+ smolts. Small 1+ smolts descended faster than larger 1+ smolts whereas large 2+ smolts descended faster than smaller ones. Smolts released in the evening descended faster than smolts released in the morning. Illumination of the river during the night reduced the speed of descent, the effect being most pronounced in the pool just above the trap. Males that were mature in autumn 1983 smoltified and descended when released in 1984, but they were smaller than immature males and females. It is suggested that wild and hatchery smolts staying in the river may be attracted to shoals of released hatchery smolts and join them when migrating downstream.
Temperature dependence of the switch between nocturnal and diurnal smolt migration in Atlantic salmon
  • J E Thorpe
  • N B Metcalfe
  • N H C Fraser
Thorpe, J. E., Metcalfe, N. B. & Fraser, N. H. C. (1994). Temperature dependence of the switch between nocturnal and diurnal smolt migration in Atlantic salmon. In High Performance Fish (Mackinlay, D. D., ed.), pp. 83-86. Vancouver: Fish Physiology Association.
The food of Atlantic salmon Salmo salar L. and brown trout Salmo trutta L. smolts during migration in the Orkla river
  • E Garnås
  • N A Hvidsten
Garnås, E. & Hvidsten, N. A. (1985). The food of Atlantic salmon Salmo salar L. and brown trout Salmo trutta L. smolts during migration in the Orkla river, Norway. Fauna Norvegica Series A 6, 24-28.
The feeding of Baltic salmon smolts in the river and in the sea
  • A R Mitans
Mitans, A. R. (1970). The feeding of Baltic salmon smolts in the river and in the sea. Journal of Ichthyology 10, 89-95.