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.     

Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 1970s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon × trout hybrids have been recorded in rivers situated near intensive salmon farming in Norway and Scotland, which may be indicative of a breakdown in reproductive isolation between salmon, Salmo salar L., and brown trout, Salmo trutta L. In the present study, salmonid fry and 0+ parr were collected from rivers in western Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2%). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic salmon female × brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 catchments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the first extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.     

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.     

Revealing trends in densities of juvenile Atlantic salmon, Salmo salar L., in the subarctic River Teno using cluster analysis on long-term sampling data

The density of juvenile Atlantic salmon, Salmo salar L., was monitored at 57 sites representing different habitats in the River Teno and two of its major tributaries from 1979 to 1995. Cluster analyses were used to combine sites with similar densities and to study trends in densities within clusters. It was found that management measures have played an important role in maintaining salmon stocks and there was some evidence of increasing juvenile salmon densities. Parr densities decreased significantly in one cluster containing 45% of the sites studied in the River Utsjoki, whereas densities increased significantly in one cluster in the River Teno and in one cluster in the River Inarijoki containing 38% of the sites in these rivers. Fry densities increased significantly in two clusters containing 16% of all the sites studied in the three rivers. In general, the mean densities in successive years in the clusters were independent. The results demonstrate the value of long-term monitoring in ecological investigations.     

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.     

Influence of climate and abundance on migration timing of adult Atlantic salmon (Salmo salar) among rivers in Newfoundland and Labrador

Evidence mounts for the influence of climate variability on temporal trends in the phenology of many organisms including various species of fish. Accordingly, we examined variation in adult Atlantic salmon Salmo salar run timing in thirteen Newfoundland and Labrador rivers where returns were monitored at fishways or fish‐counting fences. Run timing varied significantly among rivers with the median date of return differing by up to 5 weeks. Duration of runs was generally short with most adults returning over a period of three to 5 weeks. A mixed model analysis incorporating a first‐order autoregressive error structure was used to generalise changes in run timing among all monitored rivers. Results indicated that the median date of return has advanced by almost 12 days over a 35‐year interval from 1978 to 2012, while several individual rivers have advanced by almost 21 days. The influence of climate on median timing was evident when the simultaneous effects of both climate and salmon abundance were controlled. We found earlier runs associated with overall warmer climate conditions on the Newfoundland and Labrador Shelf. Results contrast with those from the north‐east Atlantic where Atlantic salmon are returning later in some rivers coincident with warming climate conditions.     

Density of brown trout, Salmo trutta L., and Atlantic salmon, Salmo salar L., parr after point and scatter stocking of fry

Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., fry were point and scatter stocked in the early part of June at densities of 63–263 fry 100 m⁻² per species in the River Viantienjoki, a small river in northern Finland, and their population densities were assessed in late summer. Both species were always stocked together in similar quantities. Point stocking was used in the first 2 years and scatter stocking in the following 2 years. In point stocking, there was no correlation between the distance from the stocking sites (maximum = 250 m) and parr density in census sites ( r = −0.013 and 0.019 for brown trout and Atlantic salmon, respectively). The stocking density of fry did not influence parr density in August by either method or by species. Stocking density explained only from 11% to 23% of the parr survival depending on the species or stocking method. The mean densities of Atlantic salmon and brown trout parr did not differ significantly from each other at any fishing site ( P > 0.05). Both point and scatter stocking appear to be suitable methods for use in small rivers. The parr densities depend more on the other factors (e.g. habitat quality) than the stocking method, and the choice between methods could be based on the time and labour available.     

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.     

Accuracy of snorkelling counts in assessing spawning stock of Atlantic salmon,Salmo salar,verified by radio‐tagging and underwater video monitoring

Abstract The accuracy of underwater snorkel counts of adult Atlantic salmon, Salmo salar L., was analysed in a small tributary of the River Teno before and during the spawning period using two independent verification methods (video counts and radio‐tagged fish) with known numbers of fish. The observation efficiency of snorkelling crews was generally high, but that of an inexperienced crew (65–72%) was significantly lower than an experienced crew (81–82%). Habitat type affected the accuracy of snorkelers, especially those of the inexperienced crew. The efficiency of snorkelers was high in pool‐type habitats (75–100%) but decreased considerably in faster flowing turbulent habitats (43–82%), where the inexperienced crew provided the lowest estimates. Thus, snorkel counting is a valid method for estimating Atlantic salmon spawning population densities in small rivers with adequate underwater visibility and with a sufficiently experienced snorkelling crew.     

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.     

A genetic tool for evaluating male‐mediated stock introgression in Atlantic salmon

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Laboratory and field investigations of salmon lice [Lepeophtheirus salmonis (Krøyer)] infestation on Atlantic salmon (Salmo salar L.) post-smolts

Hatchery‐reared 1‐year‐old Atlantic salmon post‐smolts (Salmo salar L.), artificially infected with salmon lice [Lepeophtheirus salmonis (Krøyer)] copepodids, were found to suffer from primary alterations (increased cortisol levels) at early lice stages. Secondary alterations, such as osmotic stress (increased chloride levels), first occurred after the preadult stages of the lice appeared. Fish with the highest salmon lice infections died throughout the experiment. Seven years of field investigation of Trondheimsfjorden showed that Atlantic salmon post‐smolts descending coastal waters can become heavily infected with salmon lice. The migrating post‐smolts were only infected with the chalimus stages, showing that the fish had only recently left the rivers. The infection level, however, varied considerably between the years, and, in 1998, the infection was higher than previous years. The experimental results have been combined with the field data to appraise the consequences of the infection.     

Effects of diet supplementation of soya‐saponins,isoflavones and phytosterols on Atlantic salmon (Salmo salar,L) fry fed from start‐feeding

A 14‐week trial was conducted to investigate the effects of antinutritional factors (ANFs) commonly present in soybean ingredients, singly and in combination, on Atlantic salmon (Salmo salar L.) fed from start‐feeding. The experimental diets consisted of a negative control fish meal diet (FM), and a positive control diet with 167 g kg^?1 soybean meal inclusion (SBM) and four diets based on the FM diet supplemented with 2 g kg^?1 soya‐saponins (SAP), 1.5 g kg^?1 isoflavones (IFL), 0.3 g kg^?1 phytosterols (PHS) or a mixture of these (MIX). Fish fed the SAP diet showed significantly higher growth performance than those fed FM, while the IFL treatment significantly decreased growth performance of salmon fry. Fish fed the IFL diet had significantly lower maltase activity and higher trypsin activity in proximal intestine than fish fed the FM diet. Histological differences were observed in the liver of fish fed the IFL diet, characterized by reduced size of the hepatocytes. Fish fed the PHS and IFL diets showed the highest frequencies of skeletal deformities among the six treatments. In conclusion, the results indicate that purified isoflavones may negatively affect growth performance, intestinal function, liver metabolism and bone formation of salmon fry.     

Occurrence of canthaxanthin in Atlantic salmon, Salmo salar L., fry in Irish rivers as an indicator of escaped farmed salmon

The rapid growth of Atlantic salmon, Salmo salar L., culture in north-western Europe has given rise to concerns regarding the biological consequences of fish farm escapes on wild salmonid populations. Canthaxanthin, a carotenoid pigment additive to farmed salmon feed which is passed from females to their progeny, may be used as an indicator of the numbers of escaped farmed salmon which spawn in the wild. In the present study, thin-layer chromatography and high-performance liquid chromatography (HPLC) were used to screen emergent Atlantic salmon fry sampled from seven river catchments in Ireland for canthaxanthin. The incidence of fry containing canthaxanthin at greater than trace levels (<5% of total carotenoid pigment) was 0–4%, with an average of 1.7%, among the seven rivers sampled, indicating that the progeny of farmed salmon were present at similarly low frequencies. Canthaxanthin was detected at trace levels in an unexpectedly high proportion (35%) of salmon fry. Canthaxanthin was present at levels exceeding trace amounts in 24% of 21 non-anadromous brown trout, Salmo trutta L., sampled from six Irish rivers and present at trace levels in a further 57% of the fish, indicating that dietary canthaxanthin is freely available to salmonids in Irish rivers. The widespread presence of trace levels in salmon fry may be attributable, at least in part, to the increased sensitivity of the HPLC methods and to rapid dietary uptake during early post-emergence feeding.     

An examination of the use of angling data to estimate total returns of Atlantic salmon, Salmo salar, to two rivers in Newfoundland, Canada

Conservation spawning requirements (limits) for Atlantic salmon, Salmo salar L., have been developed for a number of rivers in Newfoundland. Status of stocks is evaluated against these limits and scientific advice and recommendations on levels of recreational catch provided to resource managers. A critical factor in this process is the determination of total returns. This has been accomplished for 30–40% of the rivers in question over the years through the use of fish counting fences and traps installed in fishways. In Newfoundland, it is not logistically or financially possible to operate fish counting facilities in all of the approximately 200 Atlantic salmon rivers. Thus angling catch data have been used as an alternative means of estimating total returns to rivers without counting facilities. Estimates of total returns were three to four times higher than the actual values for one of the two rivers studied and deviated from the actual by as much as 60% for the other. Management implications of the approach are discussed.     

Variability in performance in wild Atlantic salmon, Salmo salar L., fry from a single redd

Dispersal and growth were studied in Atlantic salmon, Salmo salar L., fry from a natural isolated redd. The distribution of fry leaving the redd was strongly peaked, 80% being caught within a 2-week period. Early in the dispersal period, all fry leaving the redd had remnants of yolk sac and had not fed; by half way through the dispersal period, no fry had any visible yolk, but 35% still had empty stomachs. Fry leaving the redd during the first half of the dispersal period tended to settle in different first feeding sites than those dispersing later. Predation on fry by larger salmonids was frequent, especially during dispersal. Coefficients of variation for length, weight and condition factor increased significantly over the study period and for individually recognised fry, growth rates varied markedly. Thus, individual salmon fry differ in physical status on emergence from the redd and these differences are amplified during the first few weeks after emergence.     

Spawning of escaped farmed Atlantic salmon, Salmo salar L., in western and northern Scottish rivers: egg deposition by females

Abstract. In 1991, the progeny of female Atlantic salmon, Salmo salar L., were sampled at emergence from sites in 16 rivers in western and northern Scotland. The progeny of farmed females that had escaped from sea-cages were identified by detecting the presence of maternal canthaxanthin in the juveniles' pigment load. Canthaxanthin was detected among fish sampled from 14 of the 16 rivers examined. Overall, 109 of the 2373 fry sampled carried canthaxanthin with an average frequency over all the rivers examined of 5·1%. This value will underestimate the real frequency of occurrence of the progeny of escaped farmed salmon: some escapees do not contain canthaxanthin and male fish do not contribute to the pigment load of their progeny.     

Effect of sudden increase in discharge in a large river on newly emerged Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) fry

Abstract– Downstrcam displacement of salmonid fry due to flow increase, from 12–15 m³/_s to >100 m³_s, was documented in the river Suldalslåen, Western Norway. In May only fry of brown trout ( Sulmo trutta ) occurred in the drift, while from the beginning of June only newly emerged Atlantic salmon ( Sulmo sular ) were found. The maximum number of Atlantic salmon fry drifting during a single day was estimated to be 17 000 individual. Their density in the drift was higher during the night than during the day, and their appearance in the drift coincided with the predicted period of emergence. Total brown trout numbers in the drift were estimated to vary between 4000 and 16 000 per day. Fry displaced downstream from the lowermost part of the river were lost from the population. The total losses were estimated to be between 75 000 and 100 000 Atlantic salmon fry which represents between 5.6 and 11.1%) of juvenile mortality during the first year of life.     

Modelling the impacts of removing seal predation from Atlantic salmon, Salmo salar, rivers in Scotland: a tool for targeting conflict resolution

Abstract  Bioenergetics were used to model the potential impacts on adult Atlantic salmon, Salmo salar L., stocks and rod fisheries of removing harbour seals, Phoca vitulina L., from three rivers of different scales in the Moray Firth, Scotland, viz: the Spey (large), Conon (medium) and Moriston (small). Overall, seals had the greatest impact on the Moriston, where removal of a single animal could increase cumulative catch by 17% during the fishing season. On the Conon and Spey the impacts were negligible, and resulted in increased catches of <1% annually. On all rivers eliminating seal predation had the greatest impact during the spring due to the smaller size of spring salmon sub-stocks. A generalised model of seal removal illustrated that stocks and catches increased by >33% in rivers with monthly rod catches ≤10 fish, but declined to <10% for rivers with catches >34 fish. The outputs of the models are qualitative, but provide a management tool for targeting action to resolve seal-salmon fishery conflict. Smaller salmon population units, and spring salmon sub-stocks and fisheries in particular, are most vulnerable to predation. The merit of this approach is discussed regarding the management of Special Areas of Conservation for salmon and seals.     

Deep habitats are important for juvenile Atlantic salmon Salmo salar L. in large rivers

Linnansaari T, Keskinen A, Romakkaniemi A, Erkinaro J, Orell P. Deep habitats are important for juvenile Atlantic salmon Salmo salar L. in large rivers. Ecology of Freshwater Fish 2010: 19: 618–626. © 2010 John Wiley & Sons A/S Abstract – Juvenile Atlantic salmon were studied by underwater video surveillance and self contained underwater breathing apparatus‐diving in deep (i.e. >1.0 m), fast flowing areas of two large river systems (River Teno, River Tornionjoki) in northern Finland. Both video and diving data indicated that young‐of‐the‐year (YOY) salmon (0+) and salmon parr (>0+) readily utilised habitats deeper than 1 m. Young‐of‐the‐year salmon and parr were observed through a range of 0.5–1.9 m and 0.4–2.2 m, respectively. A negative linear relationship between the mean abundance of YOY salmon and mean depth was noted from the diving transects. Salmon parr were similarly abundant throughout the range of depths studied. Video data showed that deep habitats were used throughout the summer (June–August). It was concluded that deep, fast‐flowing areas in large rivers may constitute a significant habitat resource for juvenile salmon that has not been traditionally accounted for when estimating salmon production.