Effects of the Faroese long-line fishery, other oceanic fisheries and oceanic variations on age at maturity of Icelandic north-coast stocks of Atlantic salmon (Salmo salar)

Investigations were conducted on the effects of oceanic variations (as indicated by sea temperatures) and catches of the Faroese, Norwegian Sea and West Greenland salmon fisheries on the sea-age composition of Atlantic salmon (Salmo salar L.) from 22 Icelandic north-coast rivers. Catches of grilse in rivers were strongly correlated among the 22 rivers, as were the ratios of grilse caught to two-sea winter (2SW) salmon caught the next year. Four of the 22 rivers showed increasing ratios over time and three of these rivers, all in the northeast, had significantly higher mean ratios after the expansion of the Faroese fishery than before (P < 0.05). No evidence was found from ratios for the other 18 rivers to suggest that Faroese fishing was significantly depleting those stocks, even though 13 micro-tagged north-coast salmon had been recovered in the fishery in the 1988–1989 season. No evidence was found that Norwegian Sea or West Greenland fisheries affected stock composition. The mean April–May sea temperature prior to when the smolts enter the sea was significantly and positively related to ratios in eight of the 22 rivers. This result, along with frequent significant correlations in ratios among rivers, indicated that more rapid growth of smolts in their first summer may have increased grilse to 2SW salmon ratios on several rivers.     

The smolt run and postsmolt survival of Atlantic salmon, Salmo salar L., in relation to early summer water temperatures in the northern Baltic Sea

Abstract –  The timing of the smolt migration of Atlantic salmon, Salmo salar L., was investigated during 1972–2002 in the Simojoki, a river flowing into the northern Baltic Sea. The onset of the smolt run was positively correlated with the river water temperature; a rise in water temperature above 10 °C being the main proximate environmental triggering factor. There was also a weaker correlation between the decreasing river discharge in the spring and the onset of the smolt migration. The duration of the main run was shorter in the years when the onset of the smolt run was delayed. No differences were found in the onset timing or in the duration of the smolt run between wild smolts and semi-wild smolts released into the river as parr. A polynomial equation fitted to the annual data on the survival of Carlin-tagged wild smolts and the sea surface temperature (SST) in June off the river mouth appeared to follow a dome-shaped pattern. Survival was lower in cold early summers (SST <9 °C) than in those with an average SST (9–11.9 °C), and lower again, although not significantly, in warm early summers (SST ≥12 °C). Too low and probably also too a high water temperature in early summer could thus be one of the underlying reasons for the fluctuations observed in postsmolt survival in the Baltic Sea.     

Atlantic salmon smolts in the Irish Sea: First evidence of a northerly migration trajectory

Results from an acoustic telemetry study revealed for the first time a northerly migration route for Atlantic salmon (Salmo salar L.) smolts leaving the east coast of Ireland. Atlantic salmon smolts were tagged in spring 2019 in the Castletown and Boyne rivers. Three tagged smolts registered on disparate deep‐water offshore marine receivers as they travelled northwards out of the Irish Sea through the North Channel. One fish had migrated an estimated 250 km in a period of 32 days. The remaining two individuals were detected on receivers located off the Northern Ireland coast, further corroborating the northward migration of salmon smolts through the Irish Sea.     

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.     

Diurnal variation in thermal environment experienced by salmonids in the North Pacific as indicated by data storage tags

Eight temperature-recording data storage tags were recovered from three salmonids in Alaska (pink and coho salmon and steelhead trout) and five chum salmon in Japan after 21–117 days, containing the first long-term records of ambient temperature from Pacific salmonids migrating at sea. Temperature data imply diel patterns of descents to deeper, cooler water and ascents to the surface. Fish were found at higher average temperatures at night, with narrower temperature ranges and fewer descents than during the day. Fish tagged in the Gulf of Alaska were at higher temperatures on average (10–12°C) than chum salmon tagged in the Bering Sea (8–10°C). Chum salmon were also found at a wider range of temperatures (−1–22°C vs 5–15°C). This is probably related both to the different oceanographic regions through which the fish migrated, as well as species differences in thermal range and vertical movements. Proportions of time that individual fish spent at different temperatures seemed to vary among oceanographic regions. Steelhead trout may descend to moderate depths (50 m) and not be limited to the top few metres, as had been believed. Japanese chum salmon may seek deep, cold waters as they encounter warm surface temperatures on their homeward migrations. Temperature data from all fish showed an initial period (4–21 days) of day and night temperatures near those of sea surface temperatures, suggesting a period of recuperation from tagging trauma. A period of tagging recuperation suggests that vertical movement data from short-term ultrasonic telemetry studies may not represent normal behaviour of fish. The considerable diurnal and shorter-term variation in ambient temperatures suggests that offshore ocean distribution may be linked more to prey distribution and foraging than to sea surface temperatures.     

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.     

Evaluation of a new management scheme for Norwegian Atlantic salmon Salmo salar

Atlantic salmon, Salmo salar L., stocks are decreasing worldwide, and major efforts are underway to conserve populations that are threatened. In a recent effort to conserve the Norwegian salmon, the Norwegian government implemented in 2007 a scheme called National Atlantic Salmon Watercourses and Fjords (NASW), where 52 rivers were given special protection. Here, the scheme is described and it is evaluated if and to what extent the scheme has led to changes in management practice. After the implementation of the NASW scheme, fewer plans for development (e.g. hydropower plants, water abstraction, flood and erosion control) at the within‐river level were accepted for NASW‐rivers than for rivers not in the scheme. However, at the same time, there was a tendency for fewer plans overall being sanctioned in all types of rivers probably due to overall changes in management practices not related to the NASW scheme. In total, these changes could lead to increased protection of Norwegian Atlantic salmon populations.     

Potential impact of climate warming on recreational fishing opportunities for Atlantic salmon, Salmo salar L., in Newfoundland, Canada

Potential impacts of climate warming on recreational fishing opportunities were addressed by assessing the frequency and extent that Atlantic salmon, Salmo salar L., rivers in Newfoundland have been closed over a 25-year period (1975 to 1999) because of warm water temperatures and low water levels. On average, approximately 28% of all salmon rivers were temporarily closed annually, with over 70% affected in some years. This has resulted in a loss of 35–65% of the potential fishing days available in some salmon fishing areas with the collective 1995 to 1999 period being impacted the most over the past two decades. Geographically, west and south-west coastal rivers were affected less from environmental closures than east and some south coast fishing areas. A trend for increased closures of rivers related to environmental reasons could reduce the economic importance of the recreational salmon fishery and also make it more difficult to assess the status of Atlantic salmon stocks, which is a requirement for conservation of the resource.     

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.     

Comparison of growth rate in Atlantic salmon,pink salmon,Arctic char,sea trout and rainbow trout under Norwegian farming conditions

Growth rates of Atlantic salmon, pink salmon, Arctic char, sea trout and rainbow trout were compared under Norwegian farming conditions. During the juvenile, freshwater period, growth was fastest in pink salmon, followed by rainbow trout and Arctic char. Freshwater growth of sea trout and, especially, Atlantic salmon, was slow. After transfer of smolts or fingerling to sea water, Arctic char failed to survive the autumn. Sea water growth of sea trout was slow, but the three species, rainbow trout, Atlantic salmon and pink salmon, all grew rapidly through all seasons. When in sea water, rainbow trout and pink salmon were regularly attacked by vibriosis, while Atlantic salmon were rarely attacked, and sea trout never. It is concluded that, for commercial farming in Norway, rainbow trout are of value for production of fish of any size up to 3–4 kg, and pink salmon for production of small fish of 0.5–1.5 kg. Atlantic salmon is the only species suitable for production of a very large salmonid, i.e., more than 4–5 kg.     

Prospects for the future of pink salmon in three oceans: From the native Pacific to the novel Arctic and Atlantic

While populations of other migratory salmonids suffer in the Anthropocene, pink salmon (Oncorhynchus gorbusca Salmonidae) are thriving, and their distribution is expanding both within their natural range and in the Atlantic and Arctic following introduction of the species to the White Sea in the 1950s. Pink salmon are now rapidly spreading in Europe and even across the ocean to North America. Large numbers of pink salmon breed in Norwegian rivers and small numbers of individuals have been captured throughout the North Atlantic since 2017. Although little is known about the biology and ecology of the pink salmon in its novel distribution, the impacts of the species' introduction are potentially highly significant for native species and watershed productivity. Contrasts between pink salmon in the native and extended ranges will be key to navigating management strategies for Atlantic nations where the pink salmon is entrenching itself among the fish fauna, posing potential threats to native fish communities. One key conclusion of this paper is that the species' heritable traits are rapidly selected and drive local adaptation and evolution. Within the Atlantic region, this may facilitate further establishment and spread. The invasion of pink salmon in the Atlantic basin is ultimately a massive ecological experiment and one of the first examples of a major faunal change in the North Atlantic Ocean that is already undergoing rapid changes due to other anthropogenic stressors. New research is urgently needed to understand the role and potential future impacts of pink salmon in Atlantic ecosystems.     

Survival of Atlantic salmon captured in and released from a commercial trap-net: Potential for selective harvesting of stocked salmon

The major wild Atlantic salmon stocks in the Baltic Sea began to recover in the late 1990s. This recovery has been partly due to strict regulations in the Gulf of Bothnia that effectively prevent salmon fisheries during the peak migration. About half of the migrating salmon, however, are reared fish that could be harvested. We simulated a limited trap-net fishery that selectively harvested reared salmon and released wild fish, and studied the survival and migration of the released salmon. We tagged and released 1970 salmon caught in the trap-nets along the coast in 2001 and 2002. The mean maximum capture and release induced mortality of salmon was 11%, ranging between 4% and 21% in different release groups by year, sea age and number of releases. The cumulative mortality for the total salmon population on their spawning migration in the Gulf of Bothnia was below 5%, and it would not increase considerably after the first capture and release events, provided fishing effort is not excessive and fish are handled properly. Survival of trap-net captured and released Baltic salmon appears high and their migration behavior is not altered due to this handling. Several preconditions, however, should be considered before selective fishing is introduced in the Gulf of Bothnia salmon fishery.     

Relationships between marine growth and marine survival of one sea winter Atlantic salmon, Salmo salar L., from the River Bush, Northern Ireland

An examination of marine growth/marine survival relationships in Atlantic salmon, Salmo salar L., was carried out, based on scale growth measurements in relation to two indices of marine survival in wild fish from the River Bush, Northern Ireland. The survival of cohorts to the Irish/Northern Irish coast (prefishery) and to fresh water was statistically unrelated to variation in growth from smolt migration to the end of the first winter at sea ( P  > 0.1; – P  > 0.7). Marine growth of 1+ smolts decreased significantly during the period of the study ( P  < 0.05), but growth of 2+ smolts did not change ( P  > 0.05). The variability in marine growth was much less than the variation in natural survival at sea, suggesting that factors instead of or in addition to, growth influence natural survival in the marine environment. Survival to fresh water was not related to survival to the coast ( P  > 0.4), although it was inversely correlated with exploitation rate ( P  < 0.01). These results are discussed in relation to the use of freshwater returns to assess marine survival and the potential for the variation in natural marine mortality to influence total life-cycle variation.     

High numbers of farmed Atlantic salmon. Salmo salar L., observed in oceanic waters north of the Faroe Islands

Abstract. To estimate the proportion of escaped fanned Atlantic salmon. Salmo salar L., at the feeding grounds in the north-east Atlantic Ocean, samples of salmon caught with long-lines north of the Faroe Islands were examined. Identification of reared fish was carried out using scale analysis. The proportion of fanned fish was estimated to range from 25 to 48% in the different samples, suggesting that high numbers of escaped farmed salmon occur in the Norwegian Sea. The farmed fish were significantly smaller in size than the wild salmon. Although it is suggested that most of the farmed fish are of Norwegian origin, farmed fish of Scottish, Faroese and Irish origin are also believed to be present. If not accounted for, high numbers of reared salmon in fisheries and stocks will seriously affect the assessments of fisheries and stocks of wild salmon.     

Spatial correlation patterns in coastal environmental variables and survival rates of salmon in the north-east Pacific Ocean

We examined spatial correlations for three coastal variables [upwelling index, sea surface temperature (SST), and sea surface salinity (SSS)] that might affect juvenile salmon ( Oncorhynchus spp.) during their early marine life. Observed correlation patterns in environmental variables were compared with those in survival rates of pink ( O. gorbuscha ), chum ( O. keta ), and sockeye ( O. nerka ) salmon stocks to help identify appropriate variables to include in models of salmon productivity. Both the upwelling index and coastal SST were characterized by strong positive correlations at short distances, which declined slowly with distance in the winter months, but much more rapidly in the summer. The SSS had much weaker and more variable correlations at all distances throughout the year. The distance at which stations were no longer correlated (spatial decorrelation scale) was largest for the upwelling index (> 1000 km), intermediate for SST (400–800 km in summer), and shortest for SSS (< 400 km). Survival rate indices of salmon showed moderate positive correlations among adjacent stocks that decreased to zero at larger distances. Spatial decorrelation scales ranged from approximately 500 km for sockeye salmon to approximately 1000 km for chum salmon. We conclude that variability in the coastal marine environment during summer, as well as variability in salmon survival rates, are dominated by regional scale variability of several hundred to 1000 km. The correlation scale for SST in the summer most closely matched the observed correlation scales for survival rates of salmon, suggesting that regional-scale variations in coastal SST can help explain the observed regional-scale covariation in survival rates among salmon stocks.     

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.     

A survey of the distribution of the PKD‐parasite Tetracapsuloides bryosalmonae (Cnidaria: Myxozoa: Malacosporea) in salmonids in Norwegian rivers – additional information gleaned from formerly collected fish

The malacosporean Tetracapsuloides bryosalmonae was detected in kidneys from Atlantic salmon parr in 64 of 91 sampled Norwegian rivers. Using real‐time PCR, this parasite was found to be present in Atlantic salmon parr in rivers along the whole coast, from the northernmost and southernmost areas of the country. In addition, T. bryosalmonae was found in kidneys from brown trout parr in 17 of 19 sampled rivers in south‐east Norway, and in Arctic charr sampled in the River Risfjordelva, located at the northernmost edge of the European mainland. In conclusion, T. bryosalmonae has a widespread distribution in salmonids in Norwegian watercourses. Proliferative kidney disease (PKD) caused by T. bryosalmonae and PKD‐induced mortality has been observed in salmonids in several Norwegian rivers and it can be speculated that more PKD outbreaks will occur as a result of climate change.     

Evidence for spatial coherence in time trends of marine life history traits of Atlantic salmon in the North Atlantic

A hierarchical Bayesian life cycle model is presented that considers spatial covariation of marine life history traits of Atlantic salmon (Salmo salar) populations in the North Atlantic. The model is based on a collective analysis of the dynamics of 13 stock units (SUs) from two continental stock groups (CSGs) in North America and Southern Europe in a single hierarchical model over the period 1971–2014. The model sets up a new assessment framework for Atlantic salmon stocks. It also provides a framework to investigate the drivers of changes in Atlantic salmon population dynamics including disentangling the effects of fisheries from those of environmental factors in a hierarchy of spatial scales. It is used to test the hypothesis of a strong spatial synchrony in marine life history dynamics of Atlantic salmon populations. The trends in two key parameters associated with the early marine phase of the life cycle are estimated: (i) the marine survival during the first summer–autumn spent at sea and (ii) the proportion of fish maturing after the first winter at sea. The results provide evidence of a decline in the marine survival together with an increase in the proportion of fish that mature after the first winter at sea, common to all SUs. Our results show an increased coherence in the covariations of trends in these two marine life history traits related to geographic proximity of SUs which support the hypothesis of a coherent response of geographically proximate Atlantic salmon populations that likely share similar migration routes.     

Effects of juvenile size at release and early marine growth on adult return rates for Hokkaido chum salmon (Oncorhynchus keta) in relation to sea surface temperature

Using path analyses, we investigated relationships between size at release from hatcheries, the early marine growth of juveniles, and adult return rates for chum salmon from five river stocks of Hokkaido, Japan, in relation to sea surface temperature during ocean residence. Marine growth was estimated using scales collected from 11 760 adults of age 0.3 (1980–2004). The growth and survival of each stock appeared to have a different suite of regulatory processes. Interannual variability in return rates was mainly regulated by size at release in two stocks from the Sea of Okhotsk. A similar relationship was found in one stock from the Sea of Japan, but growth during coastal residency also affected their return rates. In two stocks from the Pacific coast of Hokkaido, variability in return rates was not related to size at release or to the coastal growth of juveniles, but with offshore growth in the Sea of Okhotsk, the nursery area for juveniles after leaving Japanese coastal waters. Whereas coastal growth tended to be negatively correlated with size at release in some stocks, offshore growth was positively associated with the August–November sea surface temperature in all stocks. This study confirmed that mortality of juvenile salmon occurred in two phases, during the coastal residency and the late period of the growing season, but the relative importance of both phases varied by stock and region, which probably regulated year‐class strength of Hokkaido chum salmon.     

Lessons from the past: investigating historical data from bluefin tuna fisheries

In 1963, the leading fisheries targeting Atlantic bluefin tuna ( Thunnus thynnus ) in the Norwegian Sea and North Sea suddenly collapsed without any warning. Little is known about this collapse and several hypotheses have been put forward, such as changes in migratory routes, recruitment failure or eradication of a sub-population: all of these hypotheses could result from natural causes and/or from overfishing. To help explain this mysterious event, an original data set of the main bluefin tuna fisheries of the 20th century, including total catch and size composition of the catch, has been compiled and analysed. The results reveal a strong and unambiguous link between the Nordic purse seine and Spanish trap fisheries during the 1950s and 1960s. However, this link vanished during the 1970s. In addition, the North-west Atlantic and Mediterranean trap fisheries appeared also to be partially connected to the Nordic fisheries. During the 1950s and 1960s, the main migration routes of bluefin tuna were probably from the Mediterranean spawning grounds and from the West Atlantic coasts to the Norwegian coast and North Sea, which were probably a key feeding ground at that time. The analyses also lead to the conclusion that interactions between environmental, trophic and fishing processes have probably affected bluefin tuna migration patterns which would have finally caused the Nordic fisheries collapse. This retrospective analysis finally leads to an original – albeit more speculative – hypothesis concerning Atlantic bluefin tuna population structure, therein conjectured as an assemblage of at least three sub-populations.