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.     

International management of Atlantic salmon, Salmo salar L., by the North Atlantic Salmon Conservation Organization, 1984–1994

Abstract  The development of distant water fisheries at West Greenland and in the Northern Norwegian Sea during the 1960s and, more recently, in the Faroese zone, led to the establishment, in 1984, of the North Atlantic Salmon Conservation Organization (NASCO). This paper summarizes the progress made in the 10 years since this Organization was established towards achieving its goals of conservation, restoration, enhancement and rational management of salmon, Salmo salar L., stocks. Agreements on regulatory measures have reduced the proportion of the total catch taken by the distant water fisheries and, in the case of the West Greenland fishery, management is now firmly based on scientific advice. The stability created by these regulations has allowed NASCO to address the broader areas of conservation called for under the Convention and international action has been taken to address a number of threats to the resource, including the impacts of aquaculture, introductions and transfers, and fishing in international waters by non-contracting parties. These actions are described and the paper concludes that NASCO has a significant role to play in the future, both through continued regulation of the fisheries and by promoting international cooperation to address these broader threats to the resource.     

Marine temperatures experienced by postsmolts and the survival of Atlantic salmon, Salmo salar L., in the North Sea area

The survival of two Atlantic salmon stocks that inhabit rivers confluent with the North Sea was examined in respect to historical distributions of sea surface water temperatures. The rivers Figgjo and North Esk are relatively small salmon rivers in southern Norway and eastern Scotland, respectively. Wild salmon smolts have been tagged in these rivers since 1965. Tag returns were used to evaluate the survival of salmon in the North Sea. Survival rates of one-sea-winter (1SW) and 2SW fish were correlated within stocks, as well as between stocks. Survival rates were compared with the areal extent of thermal habitat in the north-eastern Atlantic Ocean. A positive correlation was found between the area of 8–10°C water in May and the survival of salmon. A reciprocal negative correlation was also found between survival and 5–7°C water in the same month. An analysis of sea surface temperature distributions for periods of good vs. poor salmon survival showed that when cool surface waters dominate the Norwegian coast and North Sea during May, salmon survival has been poor. Conversely, when the 8°C isotherm has extended northward along the Norwegian coast during May, survival has been good. The effect of water temperature distributions on the growth of postsmolts and other survival factors are discussed.     

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.     

Differences in sea migration between wild and reared Atlantic salmon (Salmo salar L.) in the Baltic Sea

The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 1991–1993 on wild and reared smolts of the Simojoki river salmon stock. The tag recovery data analysed by log-linear models revealed significant differences in both spatial and temporal sea migrations between the wild and reared salmon; the variation was attributed to the year of release and to the origin of the fish. Grilse accounted for the majority of reared returners (76%) but for a smaller proportion (46%) of the wild fish. The effect of smolt size could be studied only in the smolt groups tagged in 1991. Wild fish were more frequently (71%) caught in the Baltic Main Basin than were reared fish (51%) during their second sea year, and the size variation between wild and reared smolts did not explain the recovery site. No such differences in spatial distribution were found during the third sea year. The tagging place (hatchery/trap) of the reared fish did not affect their later sea migration. The differences in sea migration patterns suggest that the wild salmon are more vulnerable to the intensive salmon fishery in the Baltic Main Basin than are reared fish.     

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.     

Atlantic Salmon (Salmo salar L.) in Brittany and Lower-Normandy: Preliminary Observations on the General Characteristics of Adults

This paper presents some characteristics (length, weight and age) of adult Atlantic salmon in rivers of Brittany. The data were derived from 1,578 fish caught mainly by rod and line; the remainder being from net catches in estuaries and small samples taken outside the fishing season. The bulk of the fish spent 1 or 2 years in freshwater before smoltifying. Those fish becoming smolts at the end of 1 year were the most numerous (54%). The majority of the fish sampled were spring fish (82.4%). Very few summer fish were identified and those found were mainly grilse. The high proportion of spring fish in the sample is due to the high proportion of rod-caught fish. Probably a complete census over the year would show a larger proportion of grilse and summer fish as these are not subject to heavy rod-fishing the season being closed at the beginning of June. The relationship between sea age and river age and the influence of freshwater life upon the sea life were tested in four rivers. It seems probable that sea life depends chiefly on the river and the year of smoltification rather than on the freshwater stage. The biometric characteristics of Brittany salmon were compared with data from some foreign rivers. The angling exploitation of the spring Atlantic salmon in Brittany and the lack of data about summer migrations are discussed in regard to the future of this stock.     

Timing of migration and changes in age structure of Atlantic salmon, Salmo salar L., in the River Frome, a Dorset chalk stream, over a 24-year period

During the last 24 years, the number of Atlantic salmon, Salmo salar L., migrating up the River Frome has varied between < 1000 and > 4000 fish per annum. Most parr become smolts at age 1+ and there are three main sea ages of salmon returning to the river. Each sea age shows a bimodal pattern of migration in the river. Primary migration times for 3-sea-winter (3SW) fish are spring and autumn, with 2SW fish and grilse (1SW) peaking in summer and autumn. Some 28.4% of 3SW and 6.9% of 2SW fish are previous spawners. Multi-sea-winter fish historically passed over the counter in all months of the year, but recently, their numbers have declined, as have total numbers of fish of all ages. There has been a fundamental change in the age structure of the population, with grilse increasing in proportion over the study period. Photographs of fish, taken for validation of the counter, showed a significant decrease in mean length of grilse, but an increase in mean length for 2SW and 3SW fish. Rod catch data support these findings.     

Survival of reared Atlantic salmon and sea trout in relation to marine conditions of smolt year in the Baltic Sea

The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970–2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.     

Population decline in the endemic Atlantic salmon (Salmo salar) in Kapisillit River,Greenland

Atlantic salmon, Salmo salar L., is found throughout the North Atlantic, with thousands of rivers having spawning populations. In Greenland, spawning is limited to one river in West Greenland, the Kapisillit River (64?N), and the salmon are limited to the lower few kilometres of the system. Using mark–recapture, it was estimated the parr population was 5,953 individuals, and that the population size has declined by 52% since 1959. In spite of this decline, parr density remains high, being between 0.26 and 0.62 parr/m². Using a historical age‐length key, an estimated minimum of 635 smolt will have descended to the sea in 2017. These will be caught in a fishery currently subject to no regulatory measures and fishing remains the most likely driver of the population decline. The genetically distinct population is endemic to Greenland, and managers should implement measures to conserve this genetic integrity and local biodiversity.     

The Release of Reared Salmon Smolts (Salmo salar) into the Burrishoole River System (Western Ireland) and their Contribution to the Rod and Line Fishery

Reared salmon smolts have been released into the Burrishoole river system since 1957. From 1970 it has been possible to record all adult returns. Survival of reared smolts to adult, as measured by numbers of fish returning to fresh water, has averaged 2.1% cf. 8.4% for wild smolts. A number of factors contributing to the comparatively poor returns of reared smolts are outlined. The behaviour of reared adults on return to fresh water is described and it is concluded that once past their homing point, their contribution to the rod and line fishery diminishes. Thus, the exploitation rate of reared grilse below their homing point (L. Furnace) exceeds that above (L. Feeagh). In confirmation, whilst a positive correlation exists between total stock and total catch of reared grilse on L. Furnace, no significant correlation exists between stock and catch on L. Feeagh. In the Burrishoole system, the exploitation rate of wild grilse usually exceeds that of reared and it is only in L. Furnace that reared grilse make a significant contribution to the total rod catch. The mean exploitation rate obtained for Burrishoole reared grilse is low (10.8%) when compared with those recorded for salmon in other systems. The comparable exploitation rate of Burrishoole wild grilse is 14.3%.     

Strontium levels in scales and vertebrae of wild and hatchery-reared Atlantic salmon, Salmo solar L., smolts

Abstract. Analyses of the Sr-contenl in scales of Atlantic salmon, Salmo saiar L ., smolts from two hatcheries revealed significant ( P < 0·001) differences. These differences were reflected in the Sr-conients of their respective diets. Sr-values in scale samples taken from wild salmon smolts from 10 Norwegian rivers showed a considerable variation. Analysis of variance showed that sampling site was the major source of variation. In general the Sr-content in scales cannot be used to distinguish between hatchery-reared and wild salmon smolls, although the present results nevertheless suggest that such a distinction is possible in some localities.
The Sr-contenl of the vertebrae was somewhat lower than that of the scales in both hatchery-reared and wild smolls. The Sr-conient of the scales of sea trout, Salmo trutta L., smolts from one of the sampling sites was significantly ( P < 0·001) higher than that of scales of Atlantic salmon smolts from the same site. The Sr-content of the scales of all the salmon smolts sampled from a lake was higher than the values recorded for sea trout and salmon after their stay in the sea. Use of the Sr-contenl of the scales todistinguish between migratory and resident individuals should therefore be done with care.     

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.     

Angling Success for Atlantic Salmon (Salmo salar) in the River Wye in Relation to Effort and River Flows

Salmon catch and effort data for 1977 from ten private fisheries on the River Wye are analysed and discussed. The 555 salmon caught represent 10.7% of the total Wye rod catch and the mean angling success rate of 0.045 ± 0.022 salmon per angler-hour, equivalent to about 22 h per salmon, compares favourably with that for other rivers in Britain and Canada. Over 75% of all salmon caught were 2 sea-winter fish, the age distribution of salmon caught in the sample being similar to those for the whole river. Angling success in the upper reaches during July and August may have been reduced by limited availability affected by water temperatures too high for migration, Flows most suitable for successful angling in each fishery are compared in relation to distance from the estuary and long-term average daily flows. Salmon appeared to be caught nearer the crest of a spate in the upper compared with the lower reaches of the river and this difference is discussed.     

The development of fisheries at Bornholm,Denmark (Baltic Sea) during 1880s–1914

For centuries the abundance of fish in the Baltic Sea has had an important role in the economy of Bornholm, Denmark. This study examines the development of Bornholm fisheries between approximately 1880s and 1914 on the basis of the first officially recorded Danish fisheries data. Several species were caught, but prior to 1914 especially herring, salmon and cod were economically most important. In the late 19th and early 20th century, the catch composition changed fundamentally due to complex interactions between biological, technological and economical factors. During the late 19th century the catches of herring increased, while the catches of salmon and cod decreased. This development coincided with the introduction of new technologies and fishing practices, including motorization of fishing vessels and the exploitation of herring in previously unexploited areas (i.e., expansion of fishing areas). In addition, the salmon fishery diminished after the introduction of driftnets, which caught large numbers of small salmon. The high catches of small salmon, in combination with other factors, probably contributed to the decline in catches of larger salmon. Each of the fisheries for the three species required different types of vessels and gear, and when catches began to drop in the salmon fishery, Bornholm fishermen were faced with two options. They could either specialise in the growing herring fishery, thereby hoping to maintain their income level, or they could pursue a strategy of risk minimization, by acquiring an income outside the fishing industry, while continuing to work part-time in the salmon and cod fisheries. This dual tendency was intensified further as marine engines and new types of vessels were introduced, raising the costs of fishing.     

Safety and efficacy of emamectin benzoate administered in-feed to Atlantic salmon, Salmo salar L., smolts in freshwater, as a preventative treatment against infestations of sea lice, Lepeophtheirus salmonis (Krøyer)

The safety and efficacy of emamectin benzoate, administered in-feed to Atlantic salmon smolts, Salmo salar L., held in freshwater, was evaluated as a preventative treatment against sea lice, Lepeophtheirus salmonis, following transfer of fish to seawater.

In the safety study, salmon smolts held in freshwater were fed with diets containing emamectin benzoate at nominal doses of 0 (control), 50 (recommended dose) and 250 (5× recommended dose) μg kg⁻¹ fish day⁻¹ for 7 days (days 0–6). Actual dose rates, based on measured concentrations of emamectin benzoate in feed, differences in fish weight, and feed consumed, were 0, 54, and 272 μg kg⁻¹ day⁻¹, respectively. On day 9, fish were transferred to seawater and observed for 14 days. No differences in feeding response, coordination, behaviour, gross and histological appearance were observed between control fish and those that received 54 μg kg⁻¹ day⁻¹. Among smolts that received 272 μg kg⁻¹ day⁻¹, approximately 50% exhibited darker coloration, and one fish (1%) exhibited uncoordinated swimming behaviour. No pathognomonic signs of emamectin benzoate toxicity were identified.

In the efficacy study, smolts held in freshwater were fed an unmedicated ration (control group) or emamectin benzoate at 50 μg kg⁻¹ day⁻¹ (treated group) for 7 days (days 0–6). On day 9, fish were re-distributed to eight seawater tanks, each holding 30 control and 30 treated fish. On days 28, 56, 77 and 109, respectively, control and treated fish in two tanks were challenged with L. salmonis copepodites. When lice in each group reached chalimus stage IV, fish were sampled and the numbers of lice were recorded. Fish challenged at day 109 were sampled for the second time when lice were at the adult stage. Efficacy was calculated as the reduction in the mean number of lice on treated fish relative to the mean on control fish. Treatment with emamectin benzoate resulted in an efficacy of 85.0–99.8% in fish challenged at days 28–77, from the start of treatment, and lice counts were significantly lower (P<0.001) on treated fish than on controls. When fish challenged at day 109 were sampled at day 128, efficacy was 44.3%, but survival of chalimus to adult lice on treated fish was lower, and at day 159, efficacy had increased to 73%. These results demonstrate that treatment of salmon smolts with emamectin benzoate in freshwater was well tolerated and highly effective in preventing sea lice infestation following transfer of fish to seawater.     

Effects of Carlin tagging and fin clipping on survival of Atlantic salmon (Salmo salar L.) released as smolts

During the period 1976–1979, in total 2567 wild migrating smolts of Atlantic salmon were Carlin-tagged, 2528 were adipose fin-clipped and 2309 individuals were released unmarked into the River Imsa, S.W. Norway. The smolts were caught in a permanent Wolf trap. The adult return rate to the river was highest for unmarked and lowest for Carlin-tagged fish. The main reason for this is probably a mortality of smolts due to handling, anaesthesia and tagging.     

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.     

Remote straying of salmon (Salmo salar L.) from the Swedish west coast and possible effects on sea ranching operations

Some 6500 recaptures of salmon tagged as smolts on the Swedish west coast in 1966–1980 were reviewed for remote strayers. Only 27 (0.43%) were found, meaning no risk for the profitability of sea ranching activities. Along the Swedish west coast at least 450 kg salmon per 1000 released smolts can be expected to be caught by Swedish commercial fishermen, which makes public sea ranching highly profitable there.     

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.