Return migration of one‐sea‐winter Atlantic salmon in the River Tana

Fifty‐three one‐sea‐winter Atlantic salmon Salmo salar (45–63 cm L _T) were radio‐tagged in the Tana fjord, Barents Sea, in 1995. Thirty‐seven fish (70%) entered the freshwater zone of the River Tana in an average of 3 days after release in the fjord. The migration speeds in the lowest river section below the first riffle area were significantly higher than in the subsequent river section below the second riffle area. Similarly, the observed time spent in the first riffle area was significantly lower than in the next riffle area. The majority of Atlantic salmon entered the river during the hours of high tide and the subsequent ebb tide. In addition, most river entries were recorded around midnight. No effects of river flow on the river entry or migration speed were detected, but the migration speed of Atlantic salmon in both river sections examined was greater at lower temperatures. Twenty‐eight fish (72%) were recaptured in the river, 71% of them with weirs and gillnets, and 29% by rod and line. Over half of the Atlantic salmon (54%) were recaptured within 3 weeks following river entry, and within the first 100 km of the river (56%). The results are discussed in relation to earlier studies on multi‐sea‐winter Atlantic salmon in the River Tana.     

Return migration of Atlantic Salmon in the River Tana: Phases of migratory behaviour

From a total of 174 multi-sea-winter Atlantic salmon radio tagged in the Tanafjord (northern Norway, 70°N) during 1992 and 1993, 48 Atlantic salmon were followed from entering the River Tana until spawning. Three phases were identified: (1) migratory, direct or stepwise migration to, or close to the position held at spawning; (2) search, movements both up and down river at or close to the position held at spawning; (3) holding, a period without movements prior to spawning. During the migratory phase, Atlantic salmon migrated directly to near the spawning area, or stopped between one and nine shorter periods during the upstream migration. Number of stops increased with increasing migratory distance in 1993, but no such correlation was found in 1992. The highest migratory speeds were recorded in the lower parts of the river. A distinct change in migratory pattern was found in 67% of the Atlantic salmon near or at the area held at spawning. Most common was a search phase of erratic movements with more than one down river movement. After the movement terminated, 96% of the Atlantic salmon had a period when no or little movement was recorded until spawning (on average 55 days in 1992 and 51 days in 1993). There was no preference for staying at, up or down river from the spawning area during this holding period. Early ascending Atlantic salmon migrated to spawning areas further from the mouth than the later arriving Atlantic salmon in 1993, but not in 1992. The proportion of time spent on the migratory phase increased, while the proportion of time spent on the holding phase decreased with increasing distance to the spawning area.     

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.     

Early-season river entry of adult Atlantic salmon: its dependency on environmental factors

River entry of adult Atlantic salmon Salmo salar into the River Tornionjoki, monitored during three migration seasons (1997–1999) by horizontal split-beam hydroacoustics, started early in June when water temperature was c . 9° C and when the discharge varied between 1700 and 2000 m³ s⁻¹. In 1997 and 1999, migration peaked during the latter half of June, 17 days after the peak flood, at water temperatures ranging from 11· 5 to 18·2° C. Few statistically significant correlations were observed between river entry and six measured environmental factors and those that were significant were not persistent over the years. The strongest correlation (  r  = −0·60) was between the number of upstream migrants and seawater level, with a time lag of 1 day in 1998. In 1998 and 1999, no clear diurnal migration pattern was observed, although in 1997 the intensity of midday migration was higher than that of the midnight migration. It is concluded that environmental factors have little effect on river entry of Atlantic salmon in a large pristine river located at high latitude.     

Synchrony in the downstream migration of smolts and upstream migration of adult Atlantic salmon in the subarctic River Utsjoki

Seasonal and diel migration timing of wild Atlantic salmon Salmo salar smolts and adults were investigated annually (2001–2004) in the subarctic River Utsjoki, a tributary of the large River Teno (70° N), using underwater video monitoring. Submerged video cameras provided information on the exact timing and intensity of both migrations in a natural river channel, without disturbing the fish. In contrast to the mainly nocturnal migration pattern described from temperate rivers, the River Utsjoki smolts and adults migrated throughout the day. Smolts migrated most intensively during hours of rising (0300–0900 hours) and high sun (0900–1500 hours), while adults favoured the period of low sun (2100–0300 hours). Smolt migrations started in June and lasted on average 42 days. Adults usually ascended the site 2–3 weeks before the first descending smolts were observed and the adult migrations extended to the end of August. Seasonal synchrony was observed between smolt and adult migrations in years of slowly warming water, whereas in a year of exceptionally warm early summer (2002), smolts migrated earlier than adults. Thus, water temperature seemed to be an important environmental factor triggering the smolt migration, while the migration of adults was probably more fixed to a certain season. Weak positive correlations between fish counts and water temperature were observed, indicating that increasing water temperature may have promoted both smolt and adult migrations. The influence of discharge was negligible, although increasing discharge late in the season may have activated the remaining individuals in both groups.     

Are there Atlantic salmon in the River Tambre?

Genetic variability and assignment analyses demonstrate that the 60 returning adult salmon to the Tambre River in 2005 were not the product of an incipient wild population, but the result of straying from a neighbouring river.     

The environmental requirements of Atlantic salmon, Salmo salar L., during their passage through the Thames Estuary, 1982–1989

An examination of data on both water quality in the Thames Estuary and on the count of salmon. Salmo salar L . trapped in fresh water above the head of the tide in 1982–1989, was carried out to establish statistical correlations. The annual and monthly return of salmon as 1-sea-winter fish (grilse) in June to September was negatively correlated with water temperature, a nil catch being associated with a maximum value of 24.2° C, coupled with lower values maintained over substantial lengths of the estuary (e.g. 21.5° C over a distance of no more than 50 km). The annual return was negatively related to the extent to which the estuary was predicted to be lethal from the combination of low concentration of dissolved oxygen (DO) and high temperature, a reduction in DO of 1 mg 1 ¹ being equivalent to an increase in temperature of 4° C. The annual return for the whole year was directly related to the return in July to September. Depending upon the year, the monthly returns were related to both DO and temperature; they were reduced to a tenth at a 95 percentile DO of 2.7 mg 1 ¹, whilst the weekly catches were reduced to zero at 2.4 mg 1⁻¹. Weekly catches increased with river flow and daily catches increased with both river flow and tidal height. The few mortalities observed in the estuary in July are generally related to the quality of the water as predicted from the combination of high temperature and low DO.     

Atlantic salmon straying from the River Imsa

Mean estimated straying rate for Atlantic salmon Salmo salar L. leaving the River Imsa as smolts during 1976–1999 was 15% for hatchery fish and 6% for wild conspecifics. Hatchery Atlantic salmon selected for production traits during four or more generations strayed >50%. The straying rate was higher for Atlantic salmon staying 2 rather than 1 year at sea before attaining maturity. For spawning, 96% of the strays entered streams within 420 km from the River Imsa, and c . 80% entered streams within 60 km of the mouth of the River Imsa, whether the fish were wild or hatchery released. Within the 60 km zone, the number of strays caught in a river increased with the Atlantic salmon catch in that river, but there was no significant relationship between straying rate and water discharge or distance from the river to the River Imsa. The observed straying rate of hatchery Atlantic salmon decreased with increasing number of fish entering the River Imsa. Sexual maturation as parr did not influence the tendency to stray. The results suggest that the establishment of temporary zones, free of fish farms, outside important Atlantic salmon rivers by the fisheries authorities in Norway should be large, whole fjords, to be effective.     

Halastatic demineralization in the vertebrae of Atlantic salmon, during their spawning migration

There was a significant decrease ( c . 10%) in the mineralization of the vertebrae, determined by analysis of quantitative microradiographs, of both sexes of Atlantic salmon Salmo salar during their ascending spawning migration. The decrease was hypothesized to be the result of halastatic demineralization, i.e. the removal of mineral substance with no degradation of the organic matrix of the bone.     

Counterintuitive migration patterns by Atlantic salmon Salmo salar smolts in a large lake

What little is known about the seaward migration of Salmo salar smolt migration through standing waters indicates that it is both slow and results in high mortality rates, compared with riverine migration. This may be partly because smolts in lakes need to swim more actively and require more complex directional cues than they do in rivers. In this telemetry study of smolt migration through Loch Lomond, S. salar smolts made repeated movements in directions away from the outflowing river, which considerably increased migration time.     

Daily growth increments in the otoliths of Atlantic salmon parr, Salmo salar L., and the influence of environmental factors on their periodicity

Daily increments were demonstrated in the sagitta otoliths of fast- and slow- growing Atlantic salmon parr, Salmo salar L., when held under natural photoperiod and temperature. Otolith increments continued to be deposited at a daily rate when fish were held under constant light and/or temperature and on single or multiple feeding regimes. However abnormally short photoperiods of 6L: 6D induced two increments per day. The results suggest that an endogenous rhythm, synchronized lo light/dark transitions within a 24 h period, controls otolith increment deposition.     

Thyroid hormone levels in Atlantic salmon (Salmo salar) during the return migration from the ocean to spawn

Serum thyroid hormone levels were determined in adult Atlantic salmon ( Salmo salar ) of both sexes caught in the ocean and at a sequence of locations on their return migration to spawn. Tri-iodothyronine (T₃) levels were greatest in fish caught in coastal or estuarine waters or in a river near head-of-tide. T₃ levels were lower in fish caught in rivers throughout the angling season and lowest in those captured entering a tributary near spawning. Thyroxine (T₄) levels were lowest in immature fish captured in the ocean in winter but raised in fish captured in spring; many of the latter group showed endocrine evidence of their becoming sexually mature. T₄ levels were greatest in fish captured in coastal waters and progressively lower in fish captured in an estuary, near head-of-tide and in rivers. T₄ levels in fish captured at tributary entry near spawning exceeded those in fish caught in rivers earlier in the year. In general, these data support the hypothesis that motor activity level in migrant fish is a determinant of thyroid status.     

Genetic variation among Atlantic salmon in six Spanish rivers

Genetic variation at 24 protein loci was studied in six wild Spanish populations of Salmo salar . Temporal allelic variation and gene flow existing between rivers point out the similarity of these Spanish populations as well as the effect of stocking in the actual pattern of allelic distribution.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Tidal and diel timing of river entry by adult Atlantic salmon returning to the Aberdeenshire Dee, Scotland

The timing of salmon migration from the estuary of the Aberdeenshire Dee into the river in relation to tidal phase and time of day was studied by combined acoustic and radio-tracking of individual fish and by analysing records of untagged fish from a resistivity fish counter 0.8 km upstream from the tidal limit. Up-estuary movements that led to river entry were predominantly nocturnal and tended to occur during the ebb tide. Penetration into the non-tidal reaches of the river also tended to occur at night, but the timing of salmon movements was no longer significantly associated with tidal phase. The tracking data suggested that the reduction in the strength of the association between salmon migration and tidal phase resulted from variability in rates of progress from the estuary to the river. This variability may have been random, or related to changes in migratory behaviour during entry to fresh water. A simulation of upstream progress by groups of salmon illustrated the decay of the relationship between salmon movements and tidal phase and the advance of the average tidal phase of observed salmon movements with increasing distance of the observation point from where movements were initiated. The magnitude of these effects depended on the average rate of upstream progress and variability in the rate of progress, being greatest when upstream progress was slow and variable. These results highlight a limitation of point observations of migration with regard to identifying environmental stimuli for migration and quantifying their effects.     

Relationship between freshwater angling catch of Atlantic salmon and stock size in the River Bush, Northern Ireland

Over 25 years rod catches of Atlantic salmon Salmo salar increased proportionately as stock size increased ( r ²=0·581, P <0·001), with no overall trend between exploitation rate and stock size ( r ²=0·016, P >0·5). On a 15 year sub-set of these data annual effort ( P =0·804) and flow ( P =0·339) had little significance relative to stock size ( P <0·01) on variation in rod catches. Stock size, time, effort and flows had no influence on inter-annual variation of rod exploitation rate ( r ²=0·094, P =0·880). Pairwise correlation between variables confirmed these results. In 1998, weekly effort contributed significantly to overall catch variation ( P <0·001), while weekly flow did not ( P =0·438). These results are discussed in relation to the utility of rod catch data for deriving estimates of stock for spawning target compliance purposes.     

Temporal and environmental influences on the variation in Atlantic salmon smolt migration in the Burrishoole system 1970–2000

The relationships between a number of environmental variables and the number of Atlantic salmon Salmo salar smolts migrating in the Burrishoole system, western Ireland, were examined over a 30 year period from 1970 to 2000. The number of Atlantic salmon smolts recorded migrating downstream decreased significantly from an annual mean of 11 579 in the 1970s to a mean of 6272 in the 1990s. The primary factor in the decline in Atlantic salmon numbers was a consistent decline in the number of returning adults from the 1970s until the mid 1990s. Timing of the smolt migrations (runs) was consistent throughout the three decades. The mean durations of the smolt runs were 102·1, 92·6 and 103·2 days for the 1970s, 1980s and 1990s respectively. Mean surface water temperatures at key points in the smolt run for the three decades were also similar. Mean water temperature values were 5·4, 5·1 and 5·3° C at the start of the smolt runs and 15·2, 14·9 and 15·3° C at the end of the smolt runs in the 1970s, 1980s and 1990s respectively. Multivariate analysis identified two groups of environmental variables which had a significant influence on the daily smolt catch. One group of variables dominated by photoperiod and temperature operated prior to the smolt run and was considered to regulate the development of smoltification. The second group of variables dominated by total light and water level operated within the smolt run and was considered to control daily smolt migration.     

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.     

Muscle development in Atlantic salmon (Salmo salar) embryos and the effect of temperature on muscle cellularity

Salmon eggs were incubated at 5, 8 or 11° C from fertilization to hatching. At Gorodilov stages 25, 27, 29, 31 and 33 transverse sections of whole embryos (at somite level 10–15) were prepared for histochemistry and electron microscopy. At every stage up to hatching, cross–sectional areas of the embryos were not different between temperatures, and from stage 27 onwards there was also no difference in the ratio of white to red muscle. However, there were more muscle fibres but of smaller average diameter in both the red and white muscle for the colder temperature embryos. At hatching there were also more nuclei (per cross–section) in the colder embryos but more nuclei per muscle fibre in the warmer embryos. In all cases the 8° C embryos were intermediate between 5 and 11° C embryos in their muscle parameters. Fast and slow muscle fibres could only be distinguished in the embryos by alkali–stable ATPase reactions. Succinic dehydrogenase activity was low in embryonic fish. No differences between the temperature groups were detected in the histochemical reactions for either ATPase or succinic dehydrogenase activities.