Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

Determination of body shape variation in Irish hatchery‐reared and wild Atlantic salmon

Discriminant function analysis was used to distinguish morphologically between samples of parr, smolts and adult Atlantic salmon Salmo salar from several hatchery and river systems in Ireland. The effect of habitat shift was investigated in Atlantic salmon parr. Parr grown from the eyed‐egg stage with a non‐sibling group in a hatchery environment, came to resemble the mean body shape of their host hatchery Atlantic salmon stock more closely than that of a full sibling group grown at their natal hatchery. Wild Atlantic salmon smolts differed in shape from hatchery‐reared smolts. This difference was less pronounced, but still statistically significant when wild adults were compared with hatchery‐reared adults captured in the coastal drift‐net fishery after a year spent at sea. Rearing conditions had a significant impact on the production and growth of fish body shape. This in turn may have affected adaptability and survivorship of ranched Atlantic salmon in the marine environment.     

Migration of hatchery‐reared Atlantic salmon and wild anadromous brown trout post‐smolts in a Norwegian fjord system

Hatchery‐reared Atlantic salmon Salmo salar ( n  = 25) and wild anadromous brown trout (sea trout) Salmo trutta ( n  = 15) smolts were tagged with coded acoustic transmitters and released at the mouth of the River Eira on the west coast of Norway. Data logging receivers recorded the fish during their outward migration at 9, 32, 48 and 77 km from the release site. Seventeen Atlantic salmon (68%) and eight sea trout (53%) were recorded after release. Mean migratory speeds between different receiver sites ranged from 0·49 to 1·82 body lengths (total length) per second (bl s⁻¹) for Atlantic salmon and 0·11–2·60 bl s⁻¹ for sea trout. Atlantic salmon were recorded 9, 48 and 77 km from the river mouth on average 28, 65 and 83 h after release, respectively. Sea trout were recorded 9 km from the release site 438 h after release. Only four (23%) sea trout were detected in the outer part of the fjord system, while the rest of the fish seemed to stay in the inner fjord system. The Atlantic salmon stayed for a longer time in the inner part than in the outer parts of the fjord system, but distinct from sea trout, migrated through the whole fjord system into the ocean.     

The occurrence of Eubothrium crassum (Cestoda: Pseudophyllidea) in salmon Salmo salar and trout S. trutta of the River Exe

The occurrence, size and maturity changes of Eubothrium crassum (Bloch) have been studied in smolts and adult Salmo salar L. and S. trutta L. of the River Exe and other Devon rivers. Adult parasites are found only in salmon and sea trout migrating up river to spawn, and range in size from small and immature to large and mature, though rarely gravid, worms. It is concluded that they are acquired and only become fully mature in the sea and gravid worms are lost on or before arrival in the rivers. During the upstream migration of salmon there is a selective loss of larger and more mature parasites and the survivors do not increase in length or mature, due to the intestine becoming a progressively more unfavourable habitat as the period of fasting continues. As few salmon return to the sea, most parasites in fresh-water die before breeding. In sea trout, which feed in fresh-water, parasites continue to grow and probably mature on return to the sea. It is suggested that parasites found in adult Atlantic and Pacific salmon in rivers are rarely, if ever, the same worms that are carried to sea by smolts but are of marine origin and are residues of a marine life cycle, and that two biological races of E. crassum exist, a marine and a fresh-water one.     

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).     

Female and male contribution to egg size in salmonids

Egg size contributes to other life history traits of an individual. It is traditionally considered as a maternally determined characteristic to which the male does not have any direct contribution. However, a recent finding in insects suggests that males can affect egg size also directly. In fish, the male effect could take place only during egg swelling, as the final egg size is reached after that. We studied egg size in four freshwater salmonid species (the land-locked Atlantic salmon, the brown trout, the Arctic charr and the lake trout) right after fertilisation (initial egg size) and after the swelling phase (final egg size). The results showed that the final egg size is affected not only by the initial egg size but also by both the female and the male through the process of egg swelling. This study suggests that paternal contribution may form a previously largely ignored source of variation in early life history traits in salmonid fish.     

Polymorphism of smolts of pink salmon <Emphasis Type="Italic">Oncorhynchs gorbuscha</Emphasis> in the Indera River (Kola Peninsula)

Pink salmon introduced into the White Sea started to exploit as spawning grounds middle and upper reaches of the river 20 years after its appearance in the Indera River. As a result of this, the migration pathway of smolts and late smolts appeared in addition to early smolts. The intraspecies polymorphism of smolts is confirmed by differences of early and late smolts by body length and weight, migration dates, food spectrum, and indices of stomach fullness. The food spectra of late juveniles of pink salmon coincide with those of parr of Atlantic salmon Salmo salar and of brown trout S. trutta. Greater abundance of late migrants of pink salmon may cause competition of these species for food.     

Evaluation of biochemical methods for the non‐destructive identification of sex in upstream migrating salmon and sea trout

Female‐specific markers of reproductive activity [plasma 17β‐oestradiol (E2), vitellogenin (VTG) and alkali‐labile phosphoprotein phosphorous (ALP)] were measured over 12 months in a captive population of brown trout Salmo trutta . During the early months of the reproductive season (February to May) and using the concentration of plasma E2 or plasma ALP as a marker for females the proportion of fish in which sex was misidentified was high (15–50%). The misidentification rate was considerably lower (1–8%) using plasma VTG. Preliminary evaluation of a commercial immunochromatographic VTG test system as a screen for the presence or absence of VTG in plasma from brown trout provided results that were consistent with those obtained from direct measurement of plasma VTG levels by enzyme‐linked immunosorbent assay (ELISA). These preliminary conclusions were verified by sampling upstream‐migrating anadromous brown trout, sea trout, and Atlantic salmon Salmo salar trapped over a 6 month period. Plasma E2 levels did not satisfactorily discriminate between male and female sea trout and Atlantic salmon. Plasma VTG levels in both species, however, were bimodally distributed and it was assumed that this divergence corresponded to male (plasma VTG levels <10 μg ml⁻¹) and female (plasma VTG levels >800 μg ml⁻¹) fishes. Plasma ALP provided a more accurate indication of sex in the wild Atlantic salmon and sea trout than was suggested by the pilot study on captive brown trout. The commercial immunochromatographic VTG test system provided results that were wholly consistent with the data obtained from the trapped fishes by direct measurement of plasma VTG.     

Parental influence on growth rate, smolting rate and survival in hatchery reared juvenile Atlantic salmon, Salmo salar

The proportion of potential 1-year smolts, their mean length, the mean length of potential 2-year smolts, and the mortality rate in four half-sib families of Atlantic salmon, reared under four contrasted conditions of overhead cover, is shown to be inffuenced primarily by genetic factors (89.9%, 86.1 %, 82.7% and 80.2% of total variance respectively). Variation between families in smolting rate and mortality rate is influenced by both parents, but more by the male than the female. Variation in mean length is influenced almost entirely by the female parent. These results are discussed in relation to previous findings on bimodality of size distribution and inheritance of growth and mortality characteristics in Atlantic salmon.     

Size-independent growth in fishes: patterns, models and metrics

A combination of a dynamic energy budget (DEB) model, field data on Atlantic salmon Salmo salar and brown trout Salmo trutta and laboratory data on Atlantic salmon was used to assess the underlying assumptions of three different metrics of growth including specific growth rate (G), standardized mass‐specific growth rate (G_S) and absolute growth rate in length (G_L) in salmonids. Close agreement was found between predictions of the DEB model and the assumptions of linear growth in length and parabolic growth in mass. Field data comparing spring growth rates of age 1+ year and 2+ year Atlantic salmon demonstrated that in all years the larger age 2+ year fish exhibited a significantly lower G, but differences in growth in terms of G_S and G_L depended on the year examined. For brown trout, larger age 2+ year fish also consistently exhibited slower growth rates in terms of G but grew at similar rates as age 1+ year fish in terms of G_S and G_L. Laboratory results revealed that during the age 0+ year (autumn) the divergence in growth between future Atlantic salmon smolts and non‐smolts was similar in terms of all three metrics with smolts displaying higher growth than non‐smolts, however, both G_S and G_L indicated that smolts maintain relatively fast growth into the late autumn where G suggested that both smolts and non‐smolts exhibit a sharp decrease in growth from October to November. During the spring, patterns of growth in length were significantly decoupled from patterns in growth in mass. Smolts maintained relatively fast growth though April in length but not in mass. These results suggest G_S can be a useful alternative to G as a size‐independent measure of growth rate in immature salmonids. In addition, during certain growth stanzas, G_S may be highly correlated with G_L. The decoupling of growth in mass from growth in length over ontogeny, however, may necessitate a combination of metrics to adequately describe variation in growth depending on ontogenetic stage particularly if life histories differ.     

EROD activity in gill filaments of anadromous and marine fish as a biomarker of dioxin-like pollutants

The applicability of a gill filament-based ethoxyresorufin O-deethylase (EROD) assay, originally developed in rainbow trout, was examined in Atlantic salmon (Salmo salar), Arctic charr (Salvelinus alpinus), Atlantic cod (Gadus morhua), saithe (Pollachius virens) and spotted wolffish (Anarhichas minor). All species but spotted wolffish showed strong EROD induction in tip pieces of gill filaments following 48 h of exposure to waterborne beta-naphthoflavone. Atlantic salmon parr, smolts held in freshwater and smolts transferred to seawater showed EROD induction of similar magnitude. Arctic charr, differing 11-fold in body weight, showed similar EROD activities as expressed per gill filament tip. Laboratory exposure of saithe to water and sediments collected at polluted sites, resulted in strong EROD induction. In conclusion, the gill filament assay seems useful for monitoring exposure to aryl hydrocarbon receptor agonists in various species. Furthermore, smoltification status, water salinity and body size proved to have minor influence on gill filament EROD activity. However, the results in spotted wolffish show that some species may be less suitable for monitoring using the gill assay. Assessment of gill filament EROD activity in fish exposed to polluted water and sediments in the laboratory proved to be an easy and cost-effective way to survey pollution with dioxin-like chemicals.     

Migration of farmed adult Atlantic salmon with and without olfactory sense, released on the Norwegian coast

The dispersal and migration of farmed Atlantic salmon, Salmo salar , allowed to escape during the summer was studied. Three groups of 4–year–old fish of the River Imsa stock were released in coastal waters off south-western Norway: one group, with functional olfactory organs, was released at a fish farm 4 km away from the R. Imsa; two other groups, one with transected olfactory nerves and the other with functional olfactory organs, were released in the sea 90 km from the R. Imsa. To compare them with the migration pattern of reared, large smolts of the Imsa stock, a group of 3 + smolts was released in the R. Imsa.
Adults of salmon released as 3–year–old smolts homed with high precision to the R. Imsa. Four– year–olds released in the sea were recaptured in the fjord and in the coastal current, the majority north of the places of release. Immatures migrated to feeding areas in the North Atlantic. Matures seemed to enter rivers at random when ready to spawn. There was no difference in migration pattern between anosmics and controls. The olfactory sense was not mandatory for entering fresh water. The results indicate that the homing behaviour of Atlantic salmon is not a direct consequence of a single imprinting of the smolts, and that there is not a direct genetic link for return to a particular river. The present results support the sequential imprinting hypothesis proposed by Harden Jones (1968).     

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.     

Predation on Atlantic salmon and sea trout during their first days as postsmolts

Radio-tagged smolts of Atlantic salmon Salmo salar and sea trout Salmo trutta were predated heavily by sea birds after crossing the saline limit in the estuary of the River Skjern, Denmark. Most predation took place within the first 9 h after estuarine entry. The field data do not contradict the hypothesis of maladaptive anti-predatory behaviour.     

Migrations of sea trout, Salmo trutta L., from the Vardnes river in northern Norway

During the periods 1956–1963 and 1967–1970 traps were operated to catch upstream- and downstream-migrating sea trout, Salmo trutta L. A total of 15 788 sea trout were tagged, using Carlin tags. The number of recaptures made in the traps was 4481, of which 1796 were recaptured more than once.
The distribution of the 2122 recaptures in the sea provides a picture of the sea-migration pattern. Of the sea recaptures, 52.8% were reported as within a distance of 3 km from the river mouth, compared to 0.7% more than 80 km away. All the different size-groups of sea trout were represented among both the long-distance and the short-distance migrants. The results of this study of sea trout migrations are discussed in relation to the published results for Atlantic salmon, Salmo salar L., and sea charr, Salvelinus alpinus (L.), from the same river.
The four highest values recorded for mean distance of daily travel away from the river were 20, 8, 8 and 6km day⁻¹ by smolts and 6, 6, 5 and 5km day⁻¹ by larger-sized sea trout.
Recaptures of tagged sea trout in rivers other than the Vardnes totalled 506, of which 306 had been tagged as smolts. The calculated minimum percentage of stray is 15.5%. The proportion of sea trout from the Vardnes river that actually spawn in other rivers is not known. No significant difference in length distribution was found between the sea trout caught in the Vardnes river and those caught in other rivers. An hypothesis concerning the selective advantages of straying by anadromous salmonids living in small rivers is discussed.     

Effects of species, culture history, size and residency on relative competitive ability of salmonids

The relative competitive ability of juvenile farm and wild salmonids was investigated to provide insight into the potential effects of introduction of cultured salmon on wild Pacific salmonid ( Oncorhynchus ) species. Aquarium experiments involving equal contests ( i.e. size matched, simultaneously introduced individuals) indicated that two wild coho salmon Oncorhynchus kisutch populations were competitively equal to a farm coho salmon population. In equal contests between farm Atlantic salmon Salmo salar (Mowi strain) and these wild coho salmon populations or coastal cutthroat trout Oncorhynchus clarki clarki , Atlantic salmon were subordinate in all cases. When Atlantic salmon were given a residence advantage, however, they were competitively equal to both wild coho salmon populations, but remained subordinate to coastal cutthroat trout. When Atlantic salmon were given a 10–30% length advantage, they were competitively equal to one wild coho salmon population but remained subordinate to the other. In equal contests in semi-natural stream channels, both wild coho and farm Atlantic salmon grew significantly more in the presence of the other species than when alone. It appears that coho salmon obtain additional food ration by out competing Atlantic salmon, whereas Atlantic salmon were stimulated to feed more in the presence of coho salmon competitors. These results suggest that wild coho salmon and cutthroat trout should out compete farm Atlantic salmon of a similar size in nature. As the relative competitive ability of Atlantic salmon improves when they have a size and residence advantage, should feral populations become established, they may exist on a more equal competitive footing owing to the long freshwater residence of Atlantic salmon.     

Evidence for non-random spatial positioning of migrating smolts (Salmonidae) in a small lowland stream

1. The ontogenetic development of anadromous salmonids includes downstream emigration of immature individuals from freshwater towards the marine environment. Although this migration of juvenile salmonids (smolts) may be associated with severe mortalities, only limited attention has been paid to the spatial positioning of smolts in small streams. 2. Using a novel approach, this study examined the vertical and horizontal positioning of brown trout and Atlantic salmon smolts while performing downstream migration in a small lowland stream. 3. Pre‐smolts of indigenous and hatchery‐reared (F1) brown trout (Salmo trutta), and two different populations of Atlantic salmon (S. salar), were tagged with passive integrated transponder (PIT) tags and subsequently released upstream of an antenna array consisting of five circular swim‐through PIT antennas. Antennas were positioned in order to determine whether the migrating smolts were bottom or surface oriented, and if they were oriented towards the mid‐channel or the stream bank. 4. During the smolt emigration period, data describing both the detection of the migrating fish and the amount of water passing through the antennas were collected. This was accomplished in order to determine if the fish were performing active positioning behaviour independently of the vertical and horizontal discharge distributions in the stream. 5. The results showed that the smolts migrated in a non‐random spatial pattern independently of the stream discharge distributions. Vertically, the indigenous brown trout and the Atlantic salmon demonstrated a preference for the bottom orientated positions. In contrast, the distribution of the F1 brown trout was not different from the discharge distribution. The latter observation suggests random vertical positioning, which may be indicative of inferior migratory performance. Horizontally, all tested smolt populations strongly preferred the mid‐channel positions. 6. The discharge‐corrected preferences for certain spatial positions suggest that smolt emigration is not entirely a matter of passive displacement in lowland streams. 7. Anthropogenically altered channels may inhibit or delay downstream emigration of smolts resulting in increased mortalities. Given that the smolts in this study actively selected spatial positions in the mid‐channel and near the bottom, it is suggested that deep, mid‐channel furrows may be used to help guide migrating smolts past adverse habitats in lowland streams.     

Riverine, estuarine and marine migratory behaviour and physiology of wild and hatchery-reared coho salmon Oncorhynchus kisutch (Walbaum) smolts descending the Campbell River, BC, Canada

Eighty coho salmon Oncorhynchus kisutch smolts (40 wild and 40 hatchery-reared) were surgically implanted with acoustic transmitters and released into the Quinsam River over 2 days. Differences in physiology, travel time and migratory behaviour were examined between wild and hatchery-reared fish. In addition, tagged and control fish of both wild and hatchery-reared stock were raised for 3 months following surgery to compare survival and tag retention. Detection ranges of the acoustic receivers were tested in the river, estuary and ocean in a variety of flow conditions and tide levels. Receivers were placed in the river, estuary and up to 50 km north and south from the river mouth in the marine environment. Wild smolts were significantly smaller by mass, fork length and condition factor than hatchery-reared smolts and exhibited significantly higher levels of sodium, potassium and chloride in their blood plasma than hatchery-reared smolts. The gill Na⁺K⁺-ATPase activity was also significantly higher in the wild coho smolts at the time of release. Ninety-eight per cent of wild and 80% of hatchery-reared fish survived to the estuary, 8 km downstream of the release site. No difference was found in migration speed, timing or survival between smolts released during daylight and those released after dark. Wild smolts, however, spent less time in the river and estuary, and as a result entered the ocean earlier than hatchery-reared smolts. Average marine swimming speeds for wild smolts were double those of their hatchery-reared counterparts. While hatchery smolts dispersed in both a northward and southward direction upon entering the marine environment, the majority of wild smolts travelled north from the Campbell River estuary. The wild coho salmon smolts were more physiologically fit and ready to enter sea water than the hatchery-reared smolts, and as a result had higher early survival rates and swimming speeds.     

Smolt size in relation to age at first maturity of Atlantic salmon (Salmo salar): the role of lacustrine habitat

Back-calculated growth and size of Atlantic salmon smolts were compared in two groups of river systems in Newfoundland. One group consisted of rivers dominated by lacustrine habitats while the other had rivers characterized by fluvial habitats. Back-calculated length at each age and smolt size was significantly higher for the rivers dominated by lacustrine habitats. Associated with this was a lower proportion of maiden large salmon in adult returns. These findings are discussed in relation to density in fresh water, environmental conditions at sea, and life-history strategy.     

Relationships between smolt size, postsmolt growth and sea age at maturity in Atlantic salmon ranched in the Baltic Sea

Tagging data were used to examine the relationships between smolt size, post-smolt growth and sea age at first maturity for the short-migrating Neva strain of Atlantic salmon ( Salmo salar L.) ranched in the Bothnian Sea and the Gulf of Finland. The results provided evidence that post-smolt growth was influenced by both relative and absolute smolt size. For both sea-areas, 2-year smolts of small relative size within a release group grew more rapidly in the sea than did smolts of higher relative, but equivalent absolute size. The negative influence of increasing relative smolt size on marine growth was, however, outweighed by the stronger positive influence of increasing absolute smolt size. A 160-mm increase in smolt size (140–300 mm) resulted in an overall growth advantage of about 1 year. In the Bothnian Sea, the predicted mean length after 1 year in the sea was 288 ± 25 mm for 140-mm smolts and 560 ± 16 mm for 300-mm smolts. Under the more favourable conditions of the Gulf of Finland, the respective mean lengths were 369 ± 15 mm and 613 ± 12 mm. The sea age at first maturity was inversely related to both freshwater and marine growth rates. For both sea areas, large smolts yielded proportionately more grilse than did small ones. Smolt years with good post-smolt growth rates yielded more grilse than did years with poor growth rates. The overall level of grilsing was higher in the Gulf of Finland than in the Bothnian Sea. These results suggest that the relationships between smolt size, post-smolt growth and age at first maturity in the sea are influenced by the environmental conditions of the respective sea area. A framework explaining the links between smolt size, marine growth, survival and sea age at maturity in Neva salmon is presented for the Gulf of Finland and the Bothnian Sea.