Periodicity in Atlantic salmon Salmo salar L. smolt migration

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.     

Infection of Atlantic salmon, Salmo salar L., by Gyrodactylus salaris, Malmberg 1957, in the River Lakselva, Misvær in northern Norway

Gyrodacrylus salaris was most probably introduced to the River Lakselva in 1975 through stocking of Atlantic salmon from an infected hatchery. The parasite population grew rapidly, and the parasite spread throughout the entire watercourse during the summer of 1976. This epidemic situation led to mortality among the young Atlantic salmon, and the density of salmon parr was heavily reduced from 1976 to 1977. The density of salmon parr has remained close to zero since then, while there are no apparent trends toward decrease or increase in the density of brown trout. In spite of the reduced density of young salmon, a new epidemic has developed each year among the few young 0+ and 1+ Atlantic salmon present in the river. Results from successive sampling during the summer of 1987, 1988 and 1989 indicate that most of the presmolt salmon are attacked during their first summer or autumn of life. The infection develops into an epidemic during the first autumn, winter or the next summer. The build-up of the parasite burden on the fish leads in turn to mortality. Norwegian Atlantic salmon probably have no resistance against G. salaris, since the parasite has recently been introduced to Norwegian rivers.     

Cytogenetic studies of Atlantic salmon, Salmo salar L., in Scotland

Chromosome numbers for Atlantic salmon, Salmo salar L., from ten populations in Scotland were ascertained. The majority of fish had 2n = 58, NF = 74 karyotypes, and no polymorphisms between populations were found. The findings suggest that Atlantic salmon in Scotland are cytogenetically homogeneous.     

Predation on hatchery-reared and wild smolts of Atlantic salmon, Salmo salar L., in the estuary of River Orkla, Norway

Predation on wild and hatchery-reared Atlantic salmon smolts was studied in the estuary of River Orkla. Cod and saithe congregating in the estuary were the most serious predators on smolts. There was no difference between the mortality rates of wild and hatchery-reared smolts. Predation by cod was estimated at 20%. No evidence was found to indicate selective predation on the smallest wild and hatchery-reared smolts.     

Cranial nodules in farmed Atlantic salmon, Salmo salar L., fry

Cranial nodules are described from Atlantic salmon, Salmo salar fry hatched from certain egg batches. Conspicuous, smooth, mostly unilateral nodules were recorded on the cranium. Histologically, the cerebellum appeared normal, but was displaced dorsally. Karyorhectic malpighian cells were absent from the epidermis and the meninges appeared normal. There was no apparent contact of the cerebellum with the water, or evidence of fungal, bacterial or parasitic infection. Mortality of up to 15% as recorded. The aetiology of this condition is currently unknown.     

Predation on salmon smolts, Salmo salar L., in the estuary of the River Surna, Norway

The post-smolt stage of salmon, Salmo salar L., is subject to heavy mortality in Norway. Each year 35 000 hatchery-reared smolts are stocked in the River Surna. Smolt stocking at sea and in the fjord resulted in recovery rates of adult salmon which were up to three times better than those resulting from river stockings. This was supposed to be due to by-passing some predators. In 1984 and 1985 predators and their predation on smolts were studied in the R. Surna estuary. Cod, Gadus morhua , were found to prey heavily on smolts, and a total mortality up to 24.8% was found in a small restricted area. Cod are thought to assemble in the Surna estuary in the spring, foraging on the smolt run.     

Mitochondrial DNA variation in Atlantic salmon, Salmo, salar L., populations

Mitochondrial DNA (mtDNA) variation was examined in 209 Atlantic salmon from two river systems, the R. Itchen in Hampshire (Southern England) and the R. Conwy (North Wales). Within each system, five spawning sites were sampled. Four enzymes (AVA II, HAE III, HINF I and MBO I) revealed restriction fragment polymorphisms that were informative. Ten clonal lines were observed. These clones were differentially distributed between the two river systems and single clonal types were found to predominate at several spawning sites. MBO I variants were found in salmon from the R. Itchen but not the R. Conwy. A significant heterogeneity in frequency distribution of clonal lines between parr and smolt assemblages of the same year class within the R. Itchen was detected. This heterogeneity suggests that differences in survival or migratory behaviour may be identified by changes in mtDNA clonal frequencies.     

Mortality of pre-smolt Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., caused by fluctuating water levels in the regulated River Nidelva, central Norway

The river Nidelva, situated in central Norway, is regulated for production of electricity. Water discharge may vary from 150 to 30 m3 s -1 over a period of 10 min at the outlet of the power stations. The water level then sinks 50 cm during the next 30 min. The Nidelva produces both salmon and trout. Water fluctuations were found responsible for large losses of O+ salmon and trout. The recruitment of salmon was concluded as satisfactory, while recruitment of trout was reduced as a result of stranding.     

Post-smolt migration of Atlantic salmon, Salmo salar L., from the Simojoki river to the Baltic Sea

The migration pattern of Atlantic salmon ( Salmo salar L.) post-smolts in the Baltic Sea was investigated based on tag recoveries of Carlin-tagged wild and hatchery-reared smolts released in the Simojoki river in 1972–2005. Exact date of sea entry was known only for the wild smolts. Tag recoveries of wild salmon in the estuary within 10 km from the river mouth were received on average 3.5 days (±2.0 SD) after release. Time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river ( R ² = 0.625, P = 0.004), being shorter in years with warmer than colder sea temperatures. Outside the estuary, the wild and hatchery-reared post-smolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the tag recoveries coinciding with the warm thermal zone in the SST occurring along the coastal area. After arriving in the southern Gulf of Bothnia in late summer the post-smolts mostly migrated near the western coast, reaching the Baltic main basin in late autumn. The relationships between the marine conditions and migration patterns are discussed.     

Fundamental research problems in the management of Atlantic salmon, Salmo salar L., in Atlantic Canada

There are two fundamental types of research problems facing the management of Atlantic salmon in Atlantic Canada. First, there are problems associated with assessment of individual and mixed stocks: these can be divided into estimating optimal spawning requirements, estimating current spawning escapements, and forecasting available harvests at least one year in advance. Second, there are problems related to the large number of Atlantic salmon stocks, at least 350, and the small number of biological staff. One solution to many of these problems is to focus research efforts on small index rivers where upstream and downstream fish migrations are counted and randomly sampled. These rivers can be used to estimate optimal yield and to forecast harvest in distinct geographical areas. They also provide base-line data to examine phenomena such as fluctuations in ocean survival, density-dependent growth and mortality, or even large-scale environmental perturbations.     

The deposition of otolith rings in Atlantic salmon, Salmo salar L., embryos

Salmon embryos, age 48–81 days post-fertilization (dpf), were incubated under three different temperature and photoperiod regimes to test the effects of incubation conditions on otolith ring deposition. Otolith rings were first observed at 50 dpf and certain developmental events were marked by distinct, heavy rings. The rate of ring deposition increased with increasing temperature and exposure to light, ranging from 0·50 to 2·33 rings per day. The possible relation to physiological activity as measured by oxygen consumption is discussed.     

Migratory and reproductive behaviour of female adult Atlantic salmon, Salmo salar L., in a spawning stream

Migration and spawning behaviour of eight Atlantic salmon adult females were analysed by radio-tracking in relation to the degree of sexual maturity in a spawning tributary of the R. Sélune. Six of them were grilse and four of them were ripe. All the fish migrated upstream until reaching their spawning site at a distance of 4–12 km from the trap. The daily migration rate up to this site was inversely correlated with the length of the female. Spawning occurred in all fish at the same time when the water temperature increased dramatically. Spawning lasted 1–10 days according to the fish. After spawning, females quickly moved downstream only small distances and then stayed in approximately the same location until death. This study concluded that ripeness did not influence behaviour on the spawning migration and describes certain characteristics of the reproductive phase in a spawning tributary.     

Tracking Atlantic salmon smolts, Salmo salar L., through Loch Voil, Scotland

Twenty-two salmon smolts, Salmo salar L., carrying miniature sonic tags were tracked individually for periods of up to 175 h in Loch Voil, Scotland, during May 1979 and 1980. Activity was predominantly nocturnal, 80% occurring between 21.00 and 06.00 hours, and was apparently undirected. Average velocities during this active interval were 0.6 body lengths per second (bl s⁻¹), with 98 and 93% of the time spent moving at less than 2 and less than 1 bl s⁻¹, respectively. The rates of downstream displacement were 0.04 bl s⁻¹ in 1979 and 0.01 bl s⁻¹ in 1980. The direction of displacement of smolts and of movement of water at a depth of 1 m was positively correlated ( P <0.001) and smolt displacement was biassed slightly ahead of water movement. Mean step lengths were 141 and 200 m in 1979 and 1980, respectively. Rates of downstream passage of 327 ICES plate-tagged smolts released 16.8 km upstream of the fish trap at Clunie dam, Loch Tummel, during the spring migrations of 1975 and 1976 averaged 0.13 bl s⁻¹ in each year: net surface water movement was about 3.7 times this rate during the same intervals. These data are consistent with the model of passive smolt migration postulated by Tytler et al. (1978) and suggest that the active component required to ensure passage through a loch (Thorpe & Morgan, 1978) is very small.     

The food of Atlantic salmon, Salmo salar L., caught by long–line in northern Norwegian waters

A total of 1145 stomachs from Atlantic salmon caught over the shelf off Helgeland/Trøndelag, and in the oceanic waters off Andenes, northern Norway during late winter–spring, 1969–1972 were examined. Food was found in 52.9% of the stomachs examined. The most important food items found in fish caught in the Helgeland/Trøndelag area were euphausids and hyperid amphipods while the myctophid Benthosema glaciale , the squid Gonatus fabricil and euphausids were found most frequently in the salmon caught off Andenes. Most salmon had preyed upon only one species, and few stomachs contained three or more prey species. The type of food did not appear to be related to the length of the fish. It is suggested that some mesopelagic feeding occurred.     

Mating of anadromous Atlantic salmon, Salmo salar L., with mature male parr

Sexually mature male parr successfully mated with female anadromous Atlantic salmon, Salmo salar , in the absence of anadromous males. There was no significant difference between the proportion of eggs fertilized by mature male parr and anadromous males. One of the demographic consequences of our observations is that overfishing could eventually eliminate anadromy in male members of a population.     

Movements of Atlantic salmon, Salmo salar L., in an estuary in south-west England

In the years 1985–1987, salmon were tracked in the estuary of the River Fowey, Cornwall using the 'sonar buoy' system developed at the M.A.F.F. Fisheries laboratory and described by Solomon & Potter (1988). Sixty-one salmon were caught and tagged in the estuary, 25 with radio and 36 with combined radio and acoustic transmitters. About half these fish were not recorded entering the freshwater reaches of the R. Fowey, and from tag recoveries it seems likely that about 27% were returning to other rivers. For fish returning to the R. Fowey the time between tagging and entry into fresh water varied between 9 h and 130 days. In the estuary the fish largely moved in the same direction as the tidal currents, although movements over the ground against the current were more common in the bottom 2 km and the top 2 km. During periods of low river-flow in summer all salmon tracked in the estuary dropped back out to sea where they stayed for varying periods: some of these made several sorties past the estuary mouth and on occasions well up the estuary before finally entering fresh water. During periods of higher river discharge, in both the summer and the autumn, some fish remained in the estuary for longer periods, sometimes holding position in deep, sheltered water. Most fish entered fresh water at night during periods of increased freshwater discharge, although at very high flows fish also entered in daylight.     

Movement rhythms in juvenile Atlantic salmon, Salmo salar L.

Nocturnal downstrean migration of juvenile Atlantic salmon is usually interpreted as increased locomotor activity. The frequency of downstream passages of 0–1 + salmon in an endless stream channel was greater by night than by day in both smoking and non-smolting fish in autumn and spring. Movement increased at dusk, and decreased after dawn. Mature male 1 + fish moved slightly less than immatures in October, but significantly more in November. Total movement frequency was lower at full moon than at other moon phases, and movement was reduced when the moon was up. Under turbid conditions by day, the threshold water velocity inducing nett downstream movement was 8.2 cm s⁻¹, and the relative velocity of fish swimming downstream was never more than one third that of fish holding station at the normal maximal flow of 25–30 cm s⁻¹.
At the end of their first growing season in October, fish which had been offered food continuously through 24 h did not differ in size from those fed by day only, but the latter were significantly larger than those offered food only at night.
We conclude that: (1) the fish fed actively by day, and not by night; (2) station-holding represented activity, and downstream nocturnal movement represented relative inactivity (displacement) which occurred on loss of visual orientation, hence migration resulted from reduced activity; (3) lack of displacement in early autumn has adaptive value for maturing fish, but not for non-spawners.     

Predation on adult Atlantic salmon, Salmo salar L., by otters, Lutra lutra (L.), within the River Dee system, Aberdeenshire, Scotland

Predation on adult salmon, Salmo salur L., by otters, Lurra lutra (L.), varied seasonally on the R. Dee, Aberdeenshire, Scotland, being highest during the spawning season in winter. Predation is described for some tributaries of the river. Male fish were caught by otters more often than females, and it is suggested that they were most vulnerable during their extensive excursions up and down stream, particularly as they negotiated shallow riffles. Otters appeared to prey upon healthy fish rather than those infested with Saprolegnia sp. but there was no difference in the size, freshwater- or sea-age offish killed by otterscompared with 'kelts' which had died non-violently. At least some of the otters obtained most of their daily food requirements by catching a single salmon per night.
Considerable numbers of adult salmon may be killed by otters during the spawning period but it is suggested that, because most are males, this is unlikely to affect the breeding success of the salmon population. Most predation occurs outside the fishing season and so is unlikely to reduce the numbers of salmon caught by anglers.     

Biochemical genetics of the Atlantic salmon Salmo salar L.

In recent years, blood group and protein polymorphisms in Atlantic salmon have been investigated extensively with a view, primarily, to their use in identifying individuals of different spawning populations present in high seas fisheries. Erythrocyte antigens, haemoglobins, serum proteins and various tissue enzymes—mainly esterases and de-hydrogenases—have been studied by electrophoretic and immunological techniques. These studies are reviewed here for the first time.
Many of the protein systems exhibit multiple components and this fact, together with cytological evidence, indicates the occurrence of tetraploidy in the course of Salmonid evolution. The significance of a tetraploid origin in the evolution and ecological adaptation of Salmonids is discussed briefly.
Some protein systems studied exhibit phylogenetic variation, and analyses of phenotype ratios and allele frequencies indicate that the populations of different river systems are genetically distinct. Allele frequencies have not yet been shown to be stable from generation to generation however, and some of the factors likely to affect allele frequencies are discussed.
Different spawning populations can not be identified in high seas fisheries using these protein characters, although it may be possible to identify the continent of origin (N. America or Europe) of some individuals. Indeed, it has been proposed independently by two groups that North American and European populations of Atlantic salmon be assigned to different sub-species viz. S. s. americanus and S. s. europaeus respectively. The contradictory evidence on which these taxa are proposed is discussed, together with the evidence for other population groups proposed in the European part of the salmon's range. The possible role and future direction of studies on the biochemical genetics of salmon are outlined.