Carlin tag recoveries as an indicator of predation on salmon smolts by goosanders and red-breasted mergansers

Between 1984 and 1990 a total 221 Carlin tags used to mark salmon Salmo salar smolts in the River North Esk, NE Scotland, were recovered from the stomachs of goosanders Mergus merganser and red-breasted mergansers M. serrator . Both Carlin-tagging and adipose-clipping affected the predation of salmon smolts by sawbill ducks. The mean (± S.D.) sizes of tagged smolts taken by both species were similar (117 ± 3 mm) and significantly smaller than the mean sizes of smolts in the river, possibly due to a reduction in the swimming performance of small smolts bearing tags. Large adipose-clipped smolts (±mean smolt size) were predated significantly more than unclipped smolts, but no such difference was observed for small smolts (相似文献   

Divergence in smolt production from the stocking of 1-summer-old and 1-year-old Atlantic salmon parr in a northern Baltic river

One‐year‐old and 1‐summer‐old salmon parr have been stocked in the Simojoki, a northern Baltic river, to protect the natural salmon stock from extinction. This enhancement practice was studied during a 5‐year stocking period, when an average of 14.3 1‐summer‐old parr and 5.4 1‐year‐old parr was needed to produce one sea running smolt. The mean yield from stocking was 70 smolts per 1000 stocked 1‐summer‐old parr and 186 smolts per 1000 stocked 1‐year‐old parr. The mean cost of smolts produced by stocking 1‐year‐old and 1‐summer‐old parr was 1.2 and 2.6 times the cost of producing one 50 g smolt in the hatchery, respectively. There was a negative relationship between density of wild parr and length of smolts stocked as 1‐summer‐old parr in the river, but no relationship between wild parr density and length of smolts stocked as 1‐year‐old parr. This indicated that 1‐summer‐old parr were more susceptible than the older parr to competition with wild parr. Considering production costs and smolt yield, stocking of 1‐year‐old parr appeared to be more profitable than that of 1‐summer‐old parr in the enhancement of the endangered salmon stock.     

Long-term study of the ecology of wild Atlantic salmon smolts in a small Norwegian river

Backcalculated lengths at the end of the first growth season in wild Atlantic salmon Salmo salar differed significantly between parr smolting at age 1, 2 and 3 years over a period of 11 years (i.e. 1983–1993). Mean body lengths of the respective age groups at the end of the first growth period were 11·1, 6·2 and 4·7 cm, respectively. The mean percentage distribution of fish smolting at age 1, 2 and 3 was 14, 78 and 7%, and the mean smolt age was 1·95 years. Mean lengths at smolting of age groups 1, 2 and 3 were 13·6, 15·8 and 17·5 cm, respectively. Females outnumbered males among the downstream migrating smolts with a mean sex ratio (females/ males) estimated at 1·61, with a significant female surplus in 7 of the 11 years sampled. Of the smolts sampled, 14% exhibited enlarged gonads indicative of parr maturation, and all were males (37% of the parr males sampled). Mean annual smolt density from 1975 to 1996 was 13·4 individuals 100 m⁻² ranging between 0·3–31 smolts 100 m⁻². Mean densities (100 m⁻²) of the smolts aged 1, 2 and 3 years were 1·5, 9·3 and 0·9 fish, respectively. Mean annual biomass for the 22-year period (1975–1996) was estimated at 437 g 100 m⁻², with a range of variation from 136 to 683 g 100 m⁻². Smolt age 2 made up 81% of the mean annual biomass (355 g 100 m⁻²) and smolt age 1 and 3, 8% (35 g 100 m⁻²) and 11% (47 g 100 m⁻²), respectively.     

The survival of semi‐wild, wild and hatchery‐reared Atlantic salmon smolts of the Simojoki River in the Baltic Sea

The recapture rate and survival of hatchery‐reared Atlantic salmon Salmo salar stocked as 1 year‐old parr (semi‐wild) with that of hatchery‐reared Atlantic salmon stocked as 2 year‐old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986–1988 and 1992. The recapture rate of the semi‐wild groups varied from 1·0 to 13·1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1·7–17·0%). The recapture rate of the semi‐wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery‐reared groups stocked as smolts (1·3–6·3%). The survival of semi‐wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery‐reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery‐reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.     

Myosin heavy chain expression in the red, white, and ventricular muscle of juvenile stages of rainbow trout.

Juvenile stages of rainbow trout, smaller parr and older juveniles, termed smolts, show differences in red muscle contractile properties: parr red muscle has faster kinetics and a faster maximum shortening velocity than smolt red muscle. A developmental reduction in the number of MHC isoforms as detected by SDS-PAGE between parr and smolt has also been observed. To investigate whether this shift in contractile kinetics results from differential gene expression, three different MHC cDNA fragments, one each from red, white, and ventricular muscle, were identified. The red muscle and ventricular forms are novel MHCs, and the white muscle form is identical to a published MHC from adult trout white muscle. Tissue and developmental stage-specific expression patterns of these MHC isoforms were examined using isoform-specific RT-PCR. Ventricular muscle typically showed only the ventricular form; 60% parr and 80% smolts expressed the ventricular form only. Approximately half of the white muscle samples of either parr or smolts, 58% and 50%, respectively, expressed only white muscle MHC. Red muscle samples were the most heterogeneous, with red muscle MHC found in combination with either the white or ventricular form or both. Combining samples from the anterior and posterior, 8% of parr red muscle samples expressed solely the red muscle MHC form, and 30% of smolt red muscle samples expressed the red muscle form alone. Variations in the relative contribution of each MHC to the red muscle of parr and smolt may explain observed differences in protein composition and contractile properties. J. Exp. Zool. 290:751-758, 2001.     

Consumption of Atlantic salmon smolts and parr by goosanders: estimates from doubly-labelled water measurements of captive birds released on two Scottish rivers

The field metabolic rates (FMRs) of nine captive goosanders, Mergus merganser , released on two Scottish rivers were estimated using the doubly-labelled water (DLW) technique. Mean (± S.E.) FMR was 2.322±0.239ml CO₂ g⁻¹ h⁻¹ and daily energy expenditure (DEE) 1939 ± 184 kJ per day. This was significantly greater (x 1.5) than previous estimates which assumed DEE to be three times the basal metabolic rate (BMR) based on regression equations predicting BMR from body mass. FMR of captives and dietary data from previous studies were used to estimate daily consumption of salmon, Salmo salar , smolts and parr by natural populations of these ducks on the river North Esk, north-east Scotland. Goosanders are likely to consume 480-522 g fish per day of which two-thirds are juvenile salmon; equivalent to a daily intake of 10-11 smolts and 48-52 parr. Annual predation of smolts by goosanders was estimated to be between 8000 and 15 000 or 3 and 16% of annual production.     

Feeding of wild and hatchery reared Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.) smolts during downstream migration

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.     

The relationships between anadromy, sex ratio and parr growth of Arctic charr in a lake in North Norway

Relationships between the degree of anadromy, sex ratio and parr growth of Arctic charr Salvelinus alpinus were investigated in Lake Storvatn in Hammerfest town, by estimating the densities of resident and anadromous charr in the lake, and by comparing the smolt characteristics of fish captured in the outlet river with those of fish of the same age in the lake. About two-thirds of the charr older than 5 years were anadromous. The sex ratio of smolts was approximately 1 : 1 and the frequency of mature male parr in the lake was low. As females tended to dominate the mature portion of the anadromous population, there was probably a relatively higher mortality among sea-run male smolts. The youngest smolts (3+ and 4+) were probably recruited from fast-growing parr in the littoral zone, while older smolts (>4+) may have been recruited from all habitats in the lake.     

Behavioural changes during the parr–smolt transformation in coho salmon Oncorhynchus kisutch: is it better to be cool?

Behavioural changes that occur during the parr–smolt transformation were investigated in juvenile coho salmon Oncorhynchus kisutch. Fish from two populations were examined from the Fraser River catchment in British Columbia, Canada; a short and a long-distance migrating population. Fish showed a significant decrease in condition factor and significant increase in gill Na⁺K⁺-ATPase activity during the spring indicating that they became competent smolts, but no difference between populations. Temperature preference trials were conducted using a shuttlebox system throughout the spring. Mean temperature preference did not differ between the two populations, but preferred temperature decreased with development from 16.5 ± 0.3°C for parr to 15.5 ± 0.4°C for smolts. Mean swimming velocity was also greater in smolts than parr, but there was no difference between the two populations. The preference for warmer water temperature observed for parr in early spring may be advantageous for stimulating smolt development. Preference for slightly cooler temperatures observed for smolts would sustain elevated seawater tolerance during the smolt window by a short time and may ensure successful transition to the marine environment.     

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

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

The diet of red-breasted mergansers (Mergus serrator) during the smolt run in N.E. Scotland: the importance of salmon (Salmo salar) smolts and parr

Salmon ( Salmo salar ) formed the largest proportion by mass offish found in the stomachs of red-breasted mergansers ( Mergus serrator ) shot during the smolt run in north-east Scotland. Salmon parr represented approximately two-thirds by mass of juvenile salmon eaten, whilst smolts were present in a smaller proportion than previously predicted. The median lengths of parr and smolts eaten were 70 and 115 mm, respectively; the latter being significantly smaller than the smolt population sampled during annual production estimates on the river.     

Differential transferrin gene expression in Atlantic salmon (Salmo salar L.) freshwater parr and seawater smolts

The Atlantic salmon has a complex life-cycle in which it encounters a salinity barrier initially upon migration to the sea as a young smolt and later as an adult salmon returning to its natal river. Concurrent with seawater migration is a process termed smoltification which is a series of metabolic changes which transform the freshwater parr into smolts adapted for life in the marine environment. To gain an understanding of events occurring at the molecular level in the salmon liver during this developmental process, a cDNA library prepared from post-smolt salmon liver mRNA was screened with total liver cDNA probes synthesised from parr and smolts. Clones which hybridised more strongly to the smolt probe than the parr probe were chosen as candidates, for an analysis of liver gene expression implicated in seawater adaptation. Many of these cDNA clones encoded the iron binding protein transferrin. Transferrin mRNA levels were determined to be significantly higher in seawater smolt salmon than in freshwater smolts implying that transferrin may play a role in seawater adaptation.     

Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

Smolt traits (length, age) and timing of smolt migration of wild Atlantic salmon, Salmo salar L., were investigated in the Simojoki River, northern Baltic Sea. The aim was to determine whether they responded to changes in parr length, parr density and temperature from 2000 to 2014. Annual electrofishing surveys and smolt numbers determined parr densities by springtime trapping in the river mouth. During the smolt trapping period captured parr and smolts were aged from scales. Water temperature was measured daily. Mean length decreased from 137 mm (TL) to 129 mm among 2‐year‐old smolts, and from 150 mm to 139 mm among 3‐year‐olds. Median date of the smolt migration was 10 days earlier, from early June to late May during the study period, linked to the rise in air temperature in May at the nearby Kemi‐Tornio airport. However, the median day temperature and the mean daily water temperatures during the second (Q₂) and third (Q₃) migration quartiles did not change. This implied that migration began when a suitable water temperature was reached, independent of the date.     

Daily endocrine profiles in parr and smolt Atlantic salmon

To elucidate possible mechanisms behind the endocrine control of parr–smolt transformation, the daily plasma profiles in thyroid hormones (TH; free thyroxine (FT₄), total thyroxine (TT₄), and total 3,5,3′-triiodothyronine (TT₃)), growth hormone (GH) and cortisol were studied in Atlantic salmon parr and smolts under simulated-natural winter (8 L:16D) and spring (16.5 L:7.5D) photoperiods, respectively. Overall, TT₄, TT₃ and GH levels were higher in smolts than in parr, whereas FT₄ levels fluctuated within the same range in parr and smolts. Significant diurnal changes in plasma TH were present in parr. Both FT₄ and TT₄ levels increased during the photophase and decreased during the scotophase, while TT₃ levels followed an inverse pattern. Growth hormone showed no significant changes in parr. Changes in FT₄, TT₄, GH, and cortisol, but not TT₃, levels, were observed in smolts with peak levels during both the photophase and scotophase for FT₄, TT₄ and GH. Plasma cortisol was not assayed in parr but in smolts the peaks were associated with dusk and dawn. In addition to the general increases in TH, GH and cortisol, the distinct endocrine differences in nighttime levels between parr in the winter and smolts in the spring suggest different interactions between TH, GH, cortisol and melatonin at these different time points. These spring scotophase endocrine profiles may represent synergistic hormone interactions that promote smolt development, similar to the synergistic endocrine interactions shown to accelerate anuran metamorphosis. The variations in these diurnal rhythms between parr and smolts may represent part of the endocrine mechanism for the translation of seasonal information during salmon smoltification.     

Physiological stress responses to confinement in diploid and triploid Atlantic salmon

Confinement of juvenile Atlantic salmon Salmo salar from four different populations (all–female diploids, all–female triploids, mixed sex diploids and mixed sex triploids) either before (FW parr) or after (SW smolts) transfer to sea elevated plasma cortisol and plasma lactate from control levels irrespective of ploidy status. Both before and after confinement, plasma cortisol levels in SW smolts (6–174 ng ml^–1) were higher than those in FW parr (4–58 ng ml^–1), which possibly reflected the physiological challenge of acclimation to SW. Mixed sex populations of SW smolts had higher cortisol levels than all–female populations. The duration of confinement (1, 3 or 6 h) affected the magnitude of the plasma cortisol and lactate responses in SW smolts. Plasma cortisol levels in diploid and triploid SW smolts subjected to 2 h of confinement decreased to pre–stress levels within 6 h post–confinement. Plasma lactate levels were not significantly different from pre–stress levels 48 h after confinement. As no difference exists in primary and secondary stress responses of Atlantic salmon of differing ploidy status, it is unlikely that differences in mortality rates between diploid and triploid populations under commercial conditions can be attributed to differences in their physiological responses to periods of stress lasting up to 6 h.     

Choice of feeding station in Atlantic salmon, Salmo salar, parr: effects of predation risk, season and life history strategy

The current speed at which underyearling salmon parr held feeding stations was examined from late summer to early winter in laboratory flume tanks that offered a choice between (a) areas with high water flow, high food availability but high predation risk and (b) areas with low flow, little food but shelter from predators. In August, those fish that would become smolts aged 1 + (and which by late winter formed the upper modal group, UMG, of the bimodal size distribution) adopted positions in faster currents than did the fish which would take a further year to reach the smolt stage (the lower modal group, LMG). However, the chosen current speed of UMG fish decreased through the period of study, so that by December all fish were found in areas of low flow, and hence little food. Both date and water temperature had independent effects on the chosen current speed of UMG fish.
The effect of predation risk was investigated using a model trout. A brief sight of this predator caused 47% of fish that had been in the main, exposed currents to move to slacker, sheltered areas; they took 1 h, on average, to return to their previous position. The fish that remained in position upon seeing the predator reduced their rate of tail beating, presumably increasing crypticity. Eventual UMG parr were less likely than were LMG fish to move away upon seeing a predator. The fish moved to faster currents than normal 2–3 h after seeing the predator, possibly compensating for the earlier reduction in feeding rate.     

Growth and development of a bimodal length-frequency distribution during smoking in a wild population of white-spotted charr in northern Japan

Smolt age and size, and individual growth during smoking were examined in a wild population of white-spotted charr, Salvelinus leucomaenis , in a small stream in northern Japan. Smolt size (110–210 mm F.L.) differed neither between sexes nor among age cohorts of 2–7 years. Back-calculation of riverine growth of smolts using otoliths showed a gradually increasing discrepancy in fork lengths of these age cohorts in both males and females, the faster-growing individuals smolting at an earlier age. A mark—recapture study on individually-identified fish revealed that the initial (spring-summer) length-frequency distribution of parr was unimodal, but became slightly skewed to the larger side in autumn, resulting in a bimodal distribution being evident by the following spring, with the upper modal group primarily composed of smolts. However, closer examination revealed that both male and female potential (subsequent) smolts were larger than the potential parr in the previous spring, the minimum lengths of the former being around 90–100 mm f.l. Specific growth rates of the potential smolts remained relatively high after summer through to the following spring, whereas those of the potential parr decreased after summer, despite having been similar between the two groups during the previous spring and summer. These observations revealed that the developmental triggering of smelting possibly occurs in or following the summer season.     

The effect of environmental conditions on Atlantic salmon smolts’ (<Emphasis Type="Italic">Salmo salar</Emphasis>) bioenergetic requirements and migration through an inland sea

The timing of the juvenile Atlantic salmon ocean-entry is considered a critical stage in the species’ life-history. Entry into the ocean at suboptimal times can have negative survival impacts on entire smolt cohorts. Previous studies have identified smolt residency time in the Bras d’Or Lakes as highly variable and correlated with body condition. This study combines energetic modelling using Dynamic Energy Budget (DEB) theory with acoustic telemetry to mechanistically link smolt bioenergetics to their migration strategy within the Bras d’Or. This study examines two main questions: 1) what is the relationship between smolts’ bioenergetics and smolts’ migration strategy, and 2) what effect would warmer water temperature have on smolts’ energetic requirements? Simulation results indicate that smolts requiring more food are more likely to exit the Bras d’Or during the observation period. The results also suggest higher lake temperature would result in faster depletion of smolt energy reserves, which is predicted to favour smolts migrating to the ocean sooner.     

Body composition and energy allocation in life-history stages of brown trout

Energy contents of immature parr and smolts, and mature resident and anadromous brown trout Salmo trutta sampled from a small stream in southern Norway were estimated from lipid, protein and carbohydrate concentrations. In immatures the lipid concentrations were highest in parr in the autumn. Mean lipid concentrations increased significantly with age in parr sampled in autumn (1·3% in age 0+ to 3·4% in age 3+), whereas they did not in smolts. The lipid concentrations of parr in spring were not significantly different from those of similarly aged smolts. By contrast, the relative water content (%) decreased with age in parr in the autumn and increased with age in smolts, mean values being slightly higher in smolts (78%) than in parr (77%). Protein and carbohydrate concentrations did not vary with age in the immature fish, mean protein concentrations being 18·0, 17·5 and 16·8% in parr in the autumn and spring, and in smolts, respectively. In residents, the concentrations of lipids were lower and of water higher, in age group 1 than in older fish, whereas there was no significant variation with age amongst anadromous trout. The energy concentration of 2+ smolts (349 kJ 100 g^-1) was similar to that of 0+ parr in the autumn. Mean somatic energy density in autumn was 1·1 times higher in freshwater residents than in parr at age 1+ (407 and 387 kJ 100 g^-1) and marginally different at age 2+ (462 and 426 kJ 100 g^-1, respectively). The energy contents per unit mass of residents were 1·3–1·6 times that of similar aged smolts. Mean somatic energy density of anadromous trout (504 kJ 100 g^-1) was higher than that of residents (455 kJ 100 g^-1). Somatic energy, lipid and protein concentrations were correlated highly with water contents of all life stages, age and sex groups.     

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.