Geographical differences in marine feeding of Atlantic salmon post‐smolts in Norwegian fjords

Stomach content analyses were conducted on Atlantic salmon Salmo salar post‐smolt (average size, 119–154 mm fork length, L _F) caught in eight large Norwegian fjord systems along a north–south geographical axis during 1998–2001. In general, post‐smolts from southern Norway showed low feeding intensity in the fjords, whereas extensive feeding was observed in fjords in the northern and middle parts of Norway. The marine diet mainly included different crustaceans and in particular marine pelagic fish larvae (sand-eels Ammodytes spp., herring Clupea harengus and gadoids), but with a substantial spatial and annual variation in prey diversity and feeding intensity. Insects were most frequently taken in the estuary, although fishes often made a large contribution in mass. In contrast, fishes, and to some extent various crustaceans (particularly Hyperiidae, Gammaridae, Euphausiacea and Copepoda) dominated the diet in the middle and outer parts of the fjords, where post‐smolts also fed more extensively than in the inner part. The results indicate that extensive feeding immediately after sea entrance may be more common for post‐smolts in the northern and middle parts of Norway, than in the southern fjords. The observed differences in post‐smolt feeding may be due to spatial and temporal differences in prey availability within and between the different types of fjord systems, and this might influence post‐smolt growth and survival.     

Survival and behaviour of post‐smolt Atlantic salmon in coastal habitat with extreme tides

The early marine migration of 55 Atlantic salmon post‐smolts tagged with acoustic transmitters was automatically monitored using 13 to 25 km long arrays of receivers deployed inside the Bay of Fundy, a coastal system on the east coast of Canada. The survival of post‐smolts from groups with short‐ and long‐term transmitters in coastal habitat up to 10 km from the river was 92 to 100%, indicating a successful transition to salt water and departure. Migration for 68 to 77% of post‐smolts followed a direct route and it was rapid (transit time usually <12 h). Post‐smolts initially migrated in a south to south‐west direction ( i.e . orientation towards the mouth of the bay) and they were aggregated near the coast. Post‐smolts with long‐term transmitters were monitored 20 km from the river where they continued to be aggregated, moving near the coast through a 'common corridor', and their survival to that point was at least 84%. Post‐smolts from both groups travelled heading out of the coastal system during ebb tides. Flood tides interrupted migration, and they caused changes in travel direction and delays in departure for post‐smolts not leaving by a direct route. Monitoring of coastal habitat inside the Bay of Fundy intercepted 62% of migrating post‐smolts with long‐term transmitters returning after an initial absence of 2 to 22 days. Returning post‐smolts displayed a resident behaviour, using the habitat monitored inside the Bay of Fundy during July and August.     

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.     

Does increased abundance of sea lice influence survival of wild Atlantic salmon post‐smolt?

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years.     

Diel activity patterns of juvenile Atlantic salmon in rivers with summer water temperature near the temperature‐dependent suppression of diurnal activity

Direct day and night underwater observations of juvenile Atlantic salmon Salmo salar during summer and autumn showed a duality in response to temperature between 7 and 11° C for young‐of‐the‐year (YOY) Atlantic salmon. They were predominantly diurnal in early summer and nocturnal in late summer although water temperatures were similar. Post‐YOY Atlantic salmon did not show a strong response to temperature as they were mostly nocturnal during the study period. It is suggested that the difference in activity patterns between YOY and post‐YOY Atlantic salmon can be explained by size‐dependent trade‐off between growth and predation risk.     

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.     

Physiological characteristics of wild Atlantic salmon post-smolts during estuarine and coastal migration

Changes were measured in some of the major physiological variables associated with seawater adaptability, growth and energetics in wild Atlantic salmon Salmo salar smolts and post-smolts migrating from the river and through the estuary, fjord and coastal areas in the River Orkla and the Trondheimsfjord, Norway during late May to early June. Gill Na⁺,K⁺-ATPase activity increased to levels of 12–16 µmol ADP mg protein⁻¹ h⁻¹ in post-smolts caught in higher salinity zones, probably representing long-term levels of Atlantic salmon post-smolts in oceanic conditions. Muscle moisture was regulated within narrow limits (77·7–78·7%) in fish from all zones during both years, suggesting that post-smolts adapt to marine conditions without any long-term disturbance of hydro-mineral balance. Lipid and glycogen content showed a general trend towards depletion from the river, through the fjord and into the ocean. There was, however, no significant change in protein content. The present results confirm that smolts are naturally 'energy deficient' during downstream migration, and suggest that post-smolts also mobilize energy reserves during their early marine phase, while protein is allocated for somatic growth. Plasma growth hormone (GH) levels increased transiently during passage through the estuary and fjord, with lower levels observed in post-smolts caught off-shore, i.e. in fish which were feeding on marine prey and had adapted to the marine environment. These physiological changes may confer substantial selective advantages during the critical early marine phase of anadromous salmonids, and hence are adaptive for long-term survival in sea water.     

Comparison of metazoan parasites of Atlantic cod, Gadus morhua, from three geographical areas of coastal Newfoundland

This study compared metazoan parasites in the digestive tract of Atlantic cod, Gadus morhua , captured from 3 geographical areas off the northeastern (NE), southeastern (SE), and southwestern (SW) coasts of Newfoundland. Samples were obtained by line trawl and frozen after capture, and the digestive tract was subsequently examined for metazoan parasites following conventional parasitological methods. Mean abundance of nematodes, but not the trematodes, and an acanthocephalan was significantly greater in cod taken from the NE than from the SE or SW areas, which were generally similar. Four species of trematodes, 3 nematodes, and an acanthocephalan, Echinorhynchus gadi , were the dominant parasites observed. Differences in mean abundance among samples from different areas might be related to the types of prey consumed rather than to environmental factors. The dominant parasites were probably acquired by cod on the NE coast after feeding on capelin, Mallotus villosus , which serve as paratenic hosts for several helminth species. Previous studies on tagging, feeding, and prevalence of 3 parasites have revealed differences in the populations of cod from the NE than from the SW or SE areas. Results from the present study, indicating geographical differences in parasite mean abundance and prevalence from the digestive tract of Atlantic cod between SW and NE areas, support the view of distinct coastal cod stocks off Newfoundland.     

Changes in salmon smolt ages in the Welsh River Dee over a 66 year period

Analysis of Atlantic salmon Salmo salar scales from the River Dee in Wales between 1937 and 2002 revealed a marked increase in the proportion of 1 + year smolts from c . 5% in the period to 1949, to almost 40% in the decade to 2002. Due to regulation flows post‐1964 over the period of change the Dee became colder during the period April to October, indicating that factors other than climate change are likely to be involved.     

Food partitioning between coexisting Atlantic salmon and brook trout in the Sainte‐Marguerite River ecosystem, Quebec

Food resource partitioning between similar‐sized, sympatric Atlantic salmon Salmo salar and brook trout Salvelinus fontinalis was examined as a possible mechanism enabling their coexistence in a stream (Allaire) of the Sainte‐Marguerite River ecosystem, Quebec, Canada. Fish stomach contents and invertebrate drift were collected concurrently during three diel cycles in August to September 1996. The food and feeding habits of an allopatric brook trout population in a nearby stream (Epinette) were studied for comparison. The diel feeding rhythms of the two coexisting fish species were similar. The composition of their diet, however, showed significant differences. Atlantic salmon predominantly (60–90%) fed on aquatic insects, mainly Ephemeroptera (35–60% of the diet). The brook trout mostly (50–80%) fed upon the allochthonous terrestrial insects (mainly adults of Coleoptera, Hymenoptera and Diptera) which comprised 5–40% of the stream drift. The allopatric brook trout fed opportunistically on the more abundant aquatic insects and terrestrial insects rarely formed 25% of its diet. The allopatric trout fed nearly twice as much as the sympatric brook trout during a day. The results suggest that the differences in feeding by brook trout in the two streams (with and without Atlantic salmon) are the result of inter‐specific interaction with Atlantic salmon and are not related to the differences in food availability between the two streams. Food resource partitioning between Atlantic salmon and brook trout may be viewed as an adaptive response resulting in a greater exploitation of available resources and coexistence.     

Migration patterns and navigation cues of Atlantic salmon post-smolts migrating from 12 rivers through the coastal zones around the Irish Sea

The freshwater phase of the first seaward migration of juvenile Atlantic salmon (Salmo salar) is relatively well understood when compared with our understanding of the marine phase of their migration. In 2021, 1008 wild and 60 ranched Atlantic salmon smolts were tagged with acoustic transmitters in 12 rivers in England, Scotland, Northern Ireland and Ireland. Large marine receiver arrays were deployed in the Irish Sea at two locations: at the transition of the Irish Sea into the North Atlantic between Ireland and Scotland, and between southern Scotland and Northern Ireland, to examine the early phase of the marine migration of Atlantic salmon smolts. After leaving their natal rivers' post-smolt migration through the Irish Sea was rapid with minimum speeds ranging from 14.03 to 38.56 km.day⁻¹ for Atlantic salmon smolts that entered the Irish Sea directly from their natal river, to 9.69–39.94 km.day⁻¹ for Atlantic salmon smolts that entered the Irish Sea directly from their natal estuary. Population minimum migration success through the study area was strongly correlated with the distance of travel, populations further away from the point of entry to the open North Atlantic exhibited lower migration success. Post-smolts from different populations experienced different water temperatures on entering the North Atlantic. This was largely driven by the timing of their migration and may have significant consequences for feeding and ultimately survivorship. The influence of water currents on post-smolt movement was investigated using data from previously constructed numerical hydrodynamic models. Modeled water current data in the northern Irish Sea showed that post-smolts had a strong preference for migrating when the current direction was at around 283° (west-north-west) but did not migrate when exposed to strong currents in other directions. This is the most favorable direction for onward passage from the Irish Sea to the continental shelf edge current, a known accumulation point for migrating post-smolts. These results strongly indicate that post-smolts migrating through the coastal marine environment are: (1) not simply migrating by current following (2) engage in active directional swimming (3) have an intrinsic sense of their migration direction and (4) can use cues other than water current direction to orientate during this part of their migration.     

Temporal variation in abundance, return rate and life histories of previously spawned Atlantic salmon in a large subarctic river

The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.     

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.     

Experimental and natural host specificity of Loma salmonae (Microsporidia)

The microsporidian Loma salmonae (Putz, Hoffman & Dunbar, 1965) Morrison & Sprague, 1981 has caused significant gill disease in Pacific salmon Oncorhynchus spp. Host specificity of the parasite was examined experimentally by per os challenge of selected salmonids and non-salmonids with infective chinook salmon O. tshawytscha gill material. Pink Oncorhynchus gorbuscha and chum salmon O. keta, brown Salmo trutta and brook trout Salvelinus fontinalis, and chinook salmon (controls) were positive, whereas Atlantic salmon Salmo salar and Arctic char Salvelinus alpinus were negative. In addition, no non-salmonids were susceptible to experimental exposure. Wild Pacific salmon species in British Columbia, Canada, were examined for L. salmonae during their freshwater life history stages (smolts, prespawning, spawning). All stages were infected, although infections in smolts were only detectable using a L. salmonae-specific PCR test. Many previous Loma spp. described from Oncorhychus spp. are likely L. salmonae based on host, parasite morphology, and site of infection.     

Size and age of maturity of Atlantic salmon correlate with the North Atlantic Oscillation Index (NAOI)

Sea‐age at maturity of Atlantic salmon Salmo salar decreased with increasing values of the seasonal NAOI from February to April. Body mass increment from smolts to adults of one‐sea‐winter Atlantic salmon increased with increasing NAOI in May at the time when the juveniles moved to sea.     

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.     

Seaward migrating subyearling chinook salmon avoid overhead cover

Approximately three‐quarters of migrating autumn (fall) chinook salmon Oncorhynchus tshawytscha smolts avoided a covered channel and selected an uncovered channel when presented with a choice in an experimental flume. Rejection of overhead cover occurred prior to, but was rare post‐channel entrance. Smolts may selectively reject riparian cover as an adaptive behavioural response to minimize predation risk and enhance rates of migration. The findings have implications for fish bypass design and riparian habitat and culvert restoration.     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Differential lifetime success and performance of native and non‐native Atlantic salmon examined under communal natural conditions

The lifetime success and performance characteristics of communally reared offspring of wild native Burrishoole (native), ranched native (ranched) and non‐native (non‐native) Atlantic salmon Salmo salar from the adjacent Owenmore River were compared. Non‐native 0+ year parr showed a substantial downstream migration, which was not shown by native and ranched parr. This appears to have been an active migration rather than competitive displacement and may reflect an adaptation to environmental or physiographic conditions within the Owenmore River catchment where the main nursery habitat is downstream of the spawning area. There were no differences between native and ranched in smolt output or adult return. Both of these measures, however, were significantly lower for the non‐native group. A greater proportion of the non‐native Atlantic salmon was taken in the coastal drift nets compared to the return to the Burrishoole system, probably as a result of the greater size of the non‐native fish. The overall lifetime success of the non‐native group, from fertilized egg to returning adult, was some 35% of native and ranched. The ranched group showed a significantly greater male parr maturity, a greater proportion of 1+ year smolts, and differences in sex ratio and timing of freshwater entry of returning adults compared to native, which may have fitness implications under specific conditions.     

Estimating multiple years,tributary-specific,and overall Atlantic salmon smolt abundance in a large Canadian catchment using capture–mark–recapture experiments

Population monitoring of Atlantic salmon (Salmo salar L.) abundance is an essential element to understand annual stock variability and inform fisheries management processes. Smolts are the life stage marking the transition from the freshwater to the marine phase of anadromous Atlantic salmon. Estimating smolt abundance allows for subsequent inferences on freshwater and marine survival rates. Annual abundances of out-migrating Atlantic salmon smolts were estimated using Bayesian models and an 18-year capture–mark–recapture time series from two to five trapping locations within the Restigouche River (Canada) catchment. Some of the trapping locations were at the outlet of large upstream tributaries, and these sampled a portion of the total out-migrating population of smolts for the watershed, whereas others were located just above the head of tide of the Restigouche River and sampled the entire run of salmon smolts. Due to logistic and environmental conditions, not all trapping locations were operational each year. Additionally, recapture rates were relatively low (<5%), and the absolute number of recaptures was relatively few (most often a few dozen), leading to incoherent and highly uncertain estimates of tributary-specific and whole catchment abundance estimates when the data were modeled independently among trapping locations and years. Several models of increasing complexity were tested using simulated data, and the best-performing model in terms of bias and precision incorporated a hierarchical structure among years on the catchability parameters and included an explicit spatial structure to account for the annual variations in the number of sampled locations within the watershed. When the best model was applied to the Restigouche River catchment dataset, the annual smolt abundance estimates varied from 250,000 to 1 million smolts, and the subbasin estimates of abundance were consistent with the spatial structure of the monitoring programme. Ultimately, increasing the probabilities of capture and the absolute number of recaptures at the different traps will be required to improve the precision and reduce the bias of the estimates of smolt abundance for the entire basin and within subbasins of the watershed. The model and approach provide a significant improvement in the models used to date based on independent estimates of abundance by trapping location and year. Total abundance and relative production in discrete spawning, nesting, or rearing areas provide critical information to appropriately understand and manage the threats to species that can occur at subpopulation spatial scales.