Straying of Atlantic salmon, Salmo salar, from delayed and coastal releases in the Baltic Sea, with special focus on the Swedish west coast

Abstract  Atlantic salmon, Salmo salar L., reared from two Baltic strains were released around the islands Bornholm and Møn in the Baltic Sea between 1995 and 1999. A total 600 000 reared salmon were released from net pens using the delayed release technique, keeping the salmon in net pens for approximately 3 months after smolting, and 208 000 were released directly from the hatchery. Of these, 15 958 were tagged with Carlin tags. Additionally, 65 300 coded wire tagged salmon were released as delayed release salmon close to Bornholm in 2000. Recaptures from the five years of Carlin tagged releases varied between 2.8% and 21.2% (average 13.1%). Most recaptures were from within the Baltic Sea (average 98%), but some were recaptured outside the Baltic Sea, either in the sea (1%) or in fresh water (1%). Recaptures outside the Baltic Sea and in fresh water were higher for releases at Møn in the western part of the Baltic, than releases at Bornholm. Straying rates from the releases into six rivers on the Swedish west coast were estimated using information from capture in traps and sport and broodstock fisheries. The proportion of straying salmon in rivers on the Swedish west coast was about 3.8% of the salmon run, but with large variations between rivers. Releases were discontinued because of possible deleterious effect on the local wild salmon populations.     

Comparison of the distribution and homing of hatchery-reared and wild Atlantic salmon,Salmo salar L., from north-east England

Hatchery-reared Atlantic salmon, Salmo salar L., parr and wild smolts which were microtagged and released into rivers in north-east England were recaptured by the fisheries at West Greenland, Faroes and on the Irish west coast, by the homewater net fisheries and by rod fisheries in rivers in north-east England. No significant differences were observed in the patterns of exploitation of hatchery-reared and wild fish in the distant water fisheries. The distribution of tag recoveries in coastal waters was strongly influenced by the pattern of fishing effort which was concentrated in the middle of the fishery area, but recaptures tended to be biased towards the river of release. There were significant differences in the distribution of recaptures of fish released in different rivers and between hatchery-reared and wild fish from the River Wear. There were also differences in the timing of recaptures of hatchery and wild fish from this river in the coastal net fishery. Very few fish were recaptured in rivers other than the one in which they were released, and there was no significant difference between straying rates for hatchery and wild fish. The relative numbers of recaptures in the coastal fishery and in the home river was the same for translocated hatchery fish and wild fish, thus refuting the suggestion that hatchery-reared fish have a reduced ability to home. The distribution of recaptures within fresh water provides clear evidence of tributary-specific homing of hatchery-reared fish.     

Dispersion of stocked brown trout, Salmo trutta L., and landlocked salmon, Salmo salar L., from stocking sites in Lake Oulujärvi, Finland

Movement and recaptures of two hatchery-reared brown trout, Salmo trutta L., stocks and landlocked salmon, Salmo salar L., released at different sites in regulated Lake Oulujärvi, were studied in relation to release site. Five groups of fish from each stock were released in approximately equal numbers. Most of the fish released in June and July were recaptured within 3 months, whereas the majority of the fish released in early winter (October and November) were caught the following spring, about 7–9 months after stocking. The release site had a significant effect on recapture rate. The results showed that fishing restrictions targeted mainly at gill net fishing are needed to preserve the stocked fish from overfishing. Significantly fewer recaptures were observed from the landlocked salmon stocking compared with brown trout. The recaptures from the landlocked salmon stocking indicated more active movement and less clumping compared with the two brown trout stocks.     

The straying of Icelandic ranched Atlantic salmon, Salmo salar L.: release and recapture techniques

The straying rate of ranched Atlantic salmon, Salmo salar L., into rivers in Iceland was estimated on the basis of coded wire tag recoveries. Out of a total of 15 158 recaptured tagged salmon from the releases between 1987 and 1992, 189 fish (1.3%) strayed into 25 out of the 79 salmon rivers observed. Most of the strayers were found in neighbouring rivers to the ranching stations. There were no significant differences in straying rate between different age classes of salmon returning from the same smolt year class. Generally, there was a delay of 26-27 days in running time between strayers in rivers and salmon returning to the ranching station. Higher straying rates were observed for ranching stations using riverine traps than for stations using estuary traps. By close inspection of seven key rivers in Iceland and assuming a 50% exploitation rate (fishing effort) in the rivers, on average, 2.1% of the returning salmon in ranching were estimated to stray to native salmon rivers over the years. A limitation of using these numbers to estimate geneflow between ranching stocks and wild populations is discussed.     

Recovery and re-establishment of Atlantic salmon, Salmo salar, in limed Norwegian rivers

A total of 21 acidified Norwegian rivers are now being limed to re‐establish or restore Atlantic salmon, Salmo salar L., stocks. Natural reproduction of Atlantic salmon was evident 1 year after the first year of liming in all rivers that had lost their native stocks (n = 9) except for one river. The density of fry (age 0+) developed significantly more rapidly in rivers that supported remnant stocks than in rivers that had lost their stocks, based on data 5 years after treatment. Nine of the study rivers were supplied with hatchery‐reared salmon, mainly unfed fry. Of the rivers with lost stocks, those which were supplied with fish had significantly higher densities than those that were not enhanced. On the other hand, rivers with remnant stocks that were supplied with fish had significantly lower densities of salmon fry than those that did not undergo such mitigation measures. In 2001, all limed rivers yielded 41.9 t of salmon.     

Above parr: Lowland river habitat characteristics associated with higher juvenile Atlantic salmon (Salmo salar) and brown trout (S. trutta) densities

Understanding juvenile salmonid habitat requirements is critical for their effective management, but little is known about these requirements in lowland rivers, which include important but unique salmonid habitats. We compared the relative influence of in-stream Ranunculus cover, water depth, prey abundance, distance upstream and two previously unexplored factors (water velocity heterogeneity and site colonisation potential) on summer densities of juvenile Atlantic salmon and brown trout. We applied electrofishing, habitat surveys and macroinvertebrate kick sampling, and calculated the site colonisation potential from salmon redd surveys across 18–22 sites in a lowland river in 2015–2017. Due to a recruitment crash in 2016, models including and excluding this unusual year were explored. Excluding 2016 data, juvenile salmon densities showed a positive association with Ranunculus cover and numbers of nearby upstream redds, and a negative association with distance upstream from the tidal limit. Trout densities were positively associated with velocity heterogeneity, indicating a potential indirect influence of Ranunculus mediated by water velocity. When including 2016, year had the largest effect on densities of both species, highlighting the impact of the recruitment failure. These findings uncover interspecific differences in the habitat requirements of juvenile salmonids in lowland rivers. Velocity heterogeneity and site colonisation potential had high explanatory power, highlighting that they should be considered in future studies of habitat use. These findings demonstrate that temporal replication and recruitment dynamics are important considerations when exploring species–habitat associations. We discuss potential management implications and argue that Ranunculus cover could be an important management tool in conservation of lowland salmonids.     

The role of river flow in the migration of adult Atlantic salmon,Salmo salar,through estuaries and rivers

An assumption of Atlantic salmon, Salmo salar L., management is that river flow influences their upstream migration; but the quantitative relationships between flow and movements are variable and inconsistent. A consensus has emerged that standard rules on salmon flow needs for water management across all rivers are unattainable. However, this does not preclude an underlying conceptual framework that should guide a consistent scientific basis to flow management for protecting salmon. This article examines the potential for a conceptual framework from a British Isles perspective, assuming that upstream migration is an emergent property from selection to optimise lifetime fitness. The template for this adaptation is the diversity of river types and hydro‐morphology in British rivers coupled with life‐history and migration adaptations that may be influenced by flow‐related factors. Models of adult salmon migration through estuaries and rivers are outlined that may provide a basis for developing generic guidance.     

Differences in longitudinal distribution patterns along a Honshu stream of brown trout <Emphasis Type="Italic">Salmo trutta</Emphasis>, white-spotted charr <Emphasis Type="Italic">Salvelinus leucomaenis</Emphasis> and masu salmon <Emphasis Type="Italic">Oncorhynchus masou</Emphasis>

Brown trout Salmo trutta were first introduced into Japan in 1892, and they currently naturally reproduce in several rivers in Honshu and Hokkaido, Japan. Although negative impacts of brown trout introductions on native salmonid fishes have been documented in some Hokkaido rivers, studies of ecological interactions between brown trout and native salmonid fishes on Honshu are limited. In this study, we describe the longitudinal distribution patterns of introduced brown trout, white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou in a 4 km stretch of a stream in central Honshu. Underwater observations were conducted in all pools within upstream, middle and downstream sections (190–400 m in length) of this stretch in order to estimate the densities of these species. Only white-spotted charr was observed in the upstream section, while brown trout and masu salmon were observed in the middle and downstream sections. Masu salmon densities, however, were much lower than brown trout densities. In the downstream section, white-spotted charr was absent. These results are consistent with results from previous studies of Hokkaido rivers, where it was found that white-spotted charr in low-gradient areas tend to be displaced by brown trout.     

Dispersal of large farmed Atlantic salmon, Salmo salar, from simulated escapes at fish farms in Norway and Scotland

Abstract  Individually tagged farmed Atlantic salmon, Salmo salar L., were released from fish farms in simulated escapes in Scotland ( n  =   678) and Norway ( n  =   597) to compare migratory behaviour and dispersal. Large fish (510–870 mm fork length) were released to simulate the escape of aquaculture growers. Fish were released in spring and all recaptures of tagged fish were reported during summer and autumn of the year of release. Recapture rates were respectively 7.0 and 0.6% of the salmon released in Norway and Scotland, indicating a higher mortality or a lower exploitation or reporting rate for Scottish fish. Recaptures of Norwegian fish were all from Norway and mostly within 150 km of the release site; 64% were taken by anglers in fresh water. By contrast, the three salmon recaptured from the release in Scotland were reported from Norway (Hardangerfjord and Lofoten) and western Sweden (River Göta); two detached tags were found on beaches in Scotland north of the release site. These findings establish the capacity for long distance dispersal among escapees from aquaculture facilities and suggest a net easterly bias in long distance dispersal of salmon escaping from Scottish fish farms.     

Effect of smolt age on migratory behaviour of Baltic salmon, Salmo salar L., transplanted to the east Atlantic

Abstract. This paper describes tests which were made to determine if smolt age at release influences the migratory pattern of three different stocks of salmon, Salmo salar L. The fish were hatchery-reared and released in two different rivers, the River Imsa and the River Akerselv. Based on the tag returns we found that Baltic salmon from the River Neva, USSR differed in migratory pattern from two Norwegian stocks from the River Lone and the River Imsa. A large proportion of the 2+ River Neva smolts stayed in the fjord during the summer and autumn after release. On the other hand, 2+ smolts of the Norwegian stocks left the fjord and migrated to the feeding areas in the Norwegian Sea within a short time after release. The 1+ smolts of all stocks showed the same migratory pattern as the 2+ smolts of Norwegian origin. We propose that the observed differences in migratory pattern are influenced by the developmental rate of the smolts. The effect of developmental rate on the migration may differ among stocks. Our results show that it is possible to develop a fjord fishery by releases of 2+ smolts of Neva salmon. However, such releases must be carried out with the utmost caution, preferably in fjords with no salmon rivers, so that the possibility of gene flow between populations is minimized.     

Ecology and movement of juvenile salmonids in beaver-influenced and beaver-free tributaries in the Trøndelag province of Norway

There is concern that expanding beaver (Castor fiber) populations will negatively impact the important economic, recreational and ecological resources of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) populations in Europe. We studied how beaver dams influenced habitat, food resources, growth and movement of juvenile Atlantic salmon and trout on three paired beaver-dammed and beaver-free (control) tributaries of important salmon rivers in central Norway. Lotic reaches of beaver-dammed and control sites were similar in habitat and benthic prey abundance, and ponds were small (<3,000 m²). Though few juvenile salmonids were detected in ponds, trout and salmon were present in habitats below and above ponds (comprising 9%–31% and 0%–57% of the fish collected respectively). Trout dominated control sites (79%–99%), but the greatest proportion of Atlantic salmon were upstream of beaver ponds (0%–57%). Growth rates were highly variable, with no differences in growth between lotic reaches of beaver-dammed and control sites. The condition and densities of juvenile salmon and trout were similar in lotic reaches of beaver-dammed and control sites, though one beaver-dammed site with fine sediment had very few juvenile salmonids. Beaver dams did not block the movement of juvenile salmonids or their ability to use upstream habitats. However, the degree of repeated movements and the overall proportion of fish moving varied between beaver-dammed and control sites. The small scale of habitat alteration and the fact that fish were able to move past dams makes it unlikely that beaver dams negatively impact the juvenile stage of salmon or trout populations.     

Migratory behaviour of adult wild and escaped farmed Atlantic salmon, Salmo salar L., before, during and after spawning in a Norwegian river

The migratory behaviour of adult wild and escaped farmed Atlantic salmon, Salmo salar L., before, during after spawning in the River Namsen, Norway, was analysed using radio telemetry. The fish were caught, radio tagged and released into the fjord between 7 and 25 km from the river mouth. A significantly higher proportion of wild (74%) than farmed (43%) salmon was subsequently recorded in the river. Wild salmon (33%) were more frequently captured in the sea and in rivers than farmed salmon (14%). The migration speed from release to passing a data logger 11 km upstream from the river mouth was not significantly different between wild (20.6 km day^?1) and farmed (19.8 km day^?1) salmon. Wild salmon tagged when water flow in the river was increasing had a significantly higher migration speed than wild salmon tagged when water flow was decreasing. This was not true for farmed salmon. Farmed salmon were distributed significantly higher up the river than wild salmon during spawning, although both types of fish were found together in spawning areas. Thus, there was no geographical isolation to prevent spawning between wild and escaped farmed salmon. Farmed salmon had significantly more and longer up- and downstream movements than wild salmon during the spawning period. Unlike farmed salmon, the number of riverine movements by wild salmon increased significantly when variation in water flow increased. A smaller proportion of wild (9%) than farmed (77%) salmon survived through the winter after spawning.     

Effects of smolt size and timing of migration on recovery rate of wild masu salmon <Emphasis Type="Italic">Oncorhynchus masou</Emphasis>

Little is known about the survival rate of wild masu salmon Oncorhynchus masou. To examine the effects of smolt length and migration timing on the recovery rate of wild masu salmon, we reanalyzed past tagging and recovery data (1993–1994). The tagging study was conducted in the Shokanbetsu River, northern Japan; 863 wild masu salmon smolts were captured, tagged, and released in a downstream site, and a total of 19 fish were recovered in coastal fisheries and in the natal river the following year. The data were analyzed by a logistic regression analysis with recapture as a response variable and tagging date and smolt length as explanatory variables; the tagging date had a significant effect on the recapture rate, whereas the effect of smolt size was not significant. Despite the small number of recaptures, this study indicates that migration timing is a factor affecting the marine survival of wild masu salmon smolts, although this conclusion has been repeatedly documented for other species of Pacific salmon Oncorhynchus spp.     

Influence of gear switching on recapture of Atlantic salmon (Salmo salar) in catch‐and‐release fisheries

Anglers that release Atlantic salmon (Salmo salar) in recreational fisheries do so with the intention that the fish will survive and contribute to succeeding generations. In some instances, salmon that are released may be recaptured, but mechanisms associated with recapture are unclear. To test whether gear avoidance influences recapture rates, we analysed data from tagging programmes in major Norwegian Atlantic salmon fishing rivers to determine how frequently salmon were recaptured by different gear than that by which they were initially captured (i.e. gear switch). Among 339 salmon captured, externally tagged and released in 2012 and 2013, 46 (14%) were recaptured; 70% of these recaptured salmon exhibited gear switch. To test whether this gear switch percentage could be expected in the absence of gear avoidance, a simulation was conducted, which accounted for variation in catch probability among rivers and across time with different gear types based on comprehensive catch data. Each simulation step provided a simulated rate of gear switch under the null hypothesis of no gear avoidance. A distribution was generated, which described the probability that we would observe 70% gear switch. The simulated results indicated that this rate of gear switch was highly unlikely (P = 0.003) if recapture gear is assumed to be independent of initial capture gear, suggesting that salmon avoided familiar gear types. Changes to behaviour after release, including learned hook avoidance, may explain our observation of gear avoidance by recaptured salmon.     

Immunoreactive somatolactin cells in the pituitary of young,migrating, spawning and spent chinook salmon,Oncorhynchus tshawytscha

Immunocytochemical techniques using an antiserum to cod somatolactin (SL) demonstrated the presence of SL cells in the intermediate lobe of the pituitary in Oncorhynchus tshawytscha. The cells were small in yearling fish. Two groups of maturing fish were studied. In the spring run salmon collected in April and May during the upstream migration, the SL cells appeared stimulated. In September, during spawning, SL cell stimulation was maximal with indices of hypertrophy and degranulation often more marked in females than in males. In the other group, salmon of the fall run collected in the Pacific Ocean in August had well developed gonads, large gonadotropes and abundant SL cells. In spawning salmon (September) the SL cells were stimulated, mainly in females. However, the final stimulation was less intense than in spring run spawning fish. The SL cells were smaller, without evident granule release, but still abundant in spent salmon of the fall run caught at the end of November. Various factors (time spent in rivers before spawning, starvation, decalcification, stress, hypothalamic influences) were considered which might explain differences between spring and fall run salmon. These observations suggest that SL may play a role in the control of gonadal maturation in chinook salmon as it may also do in sockeye and chum salmon previously studied, and that SL cells may be sensitive to the ambient salinity.     

Upstream migration of Atlantic salmon at a power station on the River Nidelva, Southern Norway

The upstream migration of 17 radio-tagged adult Atlantic salmon, Salmo salar L., past hydroelectric developments on the River Nidelva, Southern Norway, was examined. Salmon migrated quickly from the site of release in the lower part of the river up to the tunnel outlet of Rygene power station, but were substantially delayed at the outlet. The salmon stayed in the outlet area for 0–71 days (median = 20), and mainly took up a position inside the dark power station tunnel. Water discharge in the tunnel was 57–176 m³ s⁻¹, while residual flow in the river between the outlet and the dam 2.5 km further upstream was 3 m³ s⁻¹. Ten salmon passed the outlet and entered the residual flow stretch, but none passed the dam. Six of the 10 salmon returned to the tunnel outlet. No major migration barriers were identified in the residual flow stretch, suggesting lack of motivation among the salmon to migrate due to either low water discharge compared with the main river, or several minor migration barriers along the river stretch.     

The risk of individual fish being captured multiple times in a catch and release fishery

The proportion of angled Atlantic salmon Salmo salar L. being caught and released has increased. If individuals are repeatedly captured, this may have fish welfare consequences. Of 995 Atlantic salmon tagged during catch and release in eight Norwegian rivers, 10% were captured twice, while 3% were captured three times within the same fishing season. The probability that released salmon were captured again decreased with decreasing time left of the fishing season, decreased for larger‐sized fish and varied among rivers/years. Increased exploitation rates within the river, indicating an increased fishing pressure, strongly increased the probability that fish would be recaptured. However, the proportion of salmon caught a second time was much lower than the total exploitation rates in the same rivers (which was on average 46%). For fish tagged in the sea, the likelihood of being angled decreased with time since entering the river, which may explain why the recapture rates of caught and released fish were lower than the total exploitation rates.     

Piscine reovirus (PRV) in wild Atlantic salmon,Salmo salar L., and sea‐trout,Salmo trutta L., in Norway

This is the first comprehensive study on the occurrence and distribution of piscine reovirus (PRV) in Atlantic salmon, Salmo salar L., caught in Norwegian rivers. PRV is a newly discovered reovirus associated with heart and skeletal muscle inflammation (HSMI), a serious and commercially important disease affecting farmed Atlantic salmon in Norway. A cross‐sectional survey based on real‐time RT‐PCR screening of head kidney samples from wild, cultivated and escaped farmed Atlantic salmon caught from 2007 to 2009 in Norwegian rivers has been conducted. In addition, anadromous trout (sea‐trout), Salmo trutta L., caught from 2007 to 2010, and anadromous Arctic char, Salvelinus alpinus (L.), caught from 2007 to 2009, were tested. PRV was detected in Atlantic salmon from all counties included in the study and in 31 of 36 examined rivers. PRV was also detected in sea‐trout but not in anadromous Arctic char. In this study, the mean proportion of PRV positives was 13.4% in wild Atlantic salmon, 24.0% in salmon released for stock enhancement purposes and 55.2% in escaped farmed salmon. Histopathological examination of hearts from 21 PRV‐positive wild and one cultivated salmon (Ct values ranging from 17.0 to 39.8) revealed no HSMI‐related lesions. Thus, it seems that PRV is widespread in Atlantic salmon returning to Norwegian rivers, and that the virus can be present in high titres without causing lesions traditionally associated with HSMI.     

Retrospective analysis of Atlantic salmon (Salmo salar) marine growth and condition in the northwest Atlantic based on tag‐recovery data

The life history of North American Atlantic salmon (Salmo salar) is characterized by extensive round‐trip migrations between freshwater rearing habitats and marine feeding grounds off the coasts of Canada and Greenland. Growth is rapid during the marine migration, and growth rate and condition factor may be indicators of salmon health during this period. Growth data were evaluated from a tag‐recovery program conducted from 1969 to 1991 using hatchery‐reared Atlantic salmon smolts released in the Penobscot River, Maine, U.S.A. Information from recaptures of 3167 salmon that were at large in the marine environment for 1 month to 3 yr was analyzed. Length–weight measurements coupled with time‐at‐large data were used to estimate von Bertalanffy and allometric growth parameters specific to the marine phase. Variations in growth and condition factor in relation to smolt age, release date, and temperature conditions in the northwest Atlantic were also examined. The von Bertalanffy k parameter declined with ordinal release date, indicating faster growth rates during the first year of smolts released earlier in the spring. The 2‐yr‐old smolts had a larger k than 1‐yr‐old smolts, although 1‐yr‐old smolts grew to a larger asymptotic size. Sea surface temperature had variable effects on growth parameters and condition factor, with temperature at the beginning of the migration and in overwintering habitat during the first year at sea having the greatest influence on length–weight relationships. Determining the mechanisms that influence growth of individuals during the marine phase will help elucidate the factors responsible for historic growth trends, establishing a baseline for current research.     

Increased abundance of European eel (Anguilla anguilla) in acidified Norwegian rivers after liming

The prevalence and abundance (density 100 m^?2) of European eel (Anguilla anguilla) were studied by means of electrofishing in 13 acidified rivers in Norway that had been limed to restore acceptable water quality for Atlantic salmon (Salmo salar). Status of Atlantic salmon in these rivers varied prior to liming, from being entirely lost in six highly acidified rivers and in different stages of decline in seven less acidified rivers. Four of the rivers are heavily affected by hydropower development. The abundance and prevalence of European eel increased significantly during the study period. The best model for predicting eel abundance was that with four explanatory variables: time after liming, time after liming squared, status of salmon stocks and hydropower regulation. The eel density was expected to increase by a factor of almost 5 after 10 years of liming. The model also predicts that a river with a formerly reduced Atlantic salmon stock has a 2.8 times higher density of eel than rivers with formerly lost salmon stocks. Before liming, European eel were on average recorded at 15 and 41% of the sampling stations in rivers with formerly lost and reduced Atlantic salmon populations, respectively, increasing to 49 and 68% in individual rivers, respectively, after 10 years of liming. The recovery of European eel in these formerly acidified rivers by means of liming took place during the same period as their abundance declined in other parts of their distribution area in Norway and elsewhere in Europe.