Mitigation of acidified salmon rivers – effects of liming on young brown trout Salmo trutta

In southern Norway, 22 acidified rivers supporting anadromous salmonids were mitigated with lime to improve water quality and restore fish populations. In 13 of these rivers, effects on Salmo trutta and Salmo salar densities were monitored over 10–12 years, grouped into age 0 and age ≥ 1 year fish. These rivers had a mean annual discharge of between 4·9 and 85·5 m³ s^?1, and six of them were regulated for hydro‐power production. Salmo salar were lost in six of these rivers prior to liming, and highly reduced in the remaining seven rivers. Post‐liming, S. salar became re‐established in all six rivers with lost populations, and recovered in the seven other rivers. Salmo trutta occurred in all 13 study rivers prior to liming. Despite the improved water quality, both age 0 and age ≥ 1 year S. trutta densities decreased as S. salar density increased, with an average reduction of >50% after 10 years of liming. For age 0 year S. trutta this effect was less strong in rivers where S. salar were present prior to liming. In contrast, densities of S. trutta increased in unlimed streams above the anadromous stretches in two of the rivers following improved water quality due to natural recovery. Density increases of both age 0 and age ≥ 1 year S. salar showed a positive effect of river discharge. The results suggest that the decline in S. trutta density after liming is related to interspecific resource competition due to the recovery of S. salar. Thus, improved water quality through liming may not only sustain susceptible species, but can have a negative effect on species that are more tolerant prior to the treatment, such as S. trutta.     

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (F_ST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (F_ST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum F_ST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).     

Deposition of Atlantic salmon Salmo salar carcasses in a Scottish upland catchment

Radio telemetry was utilized to track 38 Atlantic salmon Salmo salar across space and time during and following their spawning run on the Conon system, Scotland. The data collected were used to assess the proportion of S. salar able to migrate successfully from an upland oligotrophic area of the catchment and the distribution of the carcasses of those fish that remained. Of these fish, 35% successfully migrated from the study area after the spawning period. The fish that remained were distributed approximately equally between riverine and lacustrine areas, but with a strong tendency to accumulate in regions of relatively slow water flows.     

The pre‐spawning migratory behaviour of Atlantic salmon Salmo salar in a large lacustrine catchment

The movements of adult Atlantic salmon Salmo salar were determined as they migrated to spawning habitats in a large lacustrine catchment, Lough Neagh, in Northern Ireland. The minimum average ground speed of S. salar through the lake was 2·1 km day^?1 and the mean residence time was 11 days. Tagged S. salar tended to actively migrate through the lake which represented a transitory habitat for adult S. salar. Migration time from the release site, through the lake, to a spawning tributary decreased during the migratory period. During the 4 year study period between 20·5 and 41·6% of tagged S. salar which entered the lake each year, explored at least one other channel before ascending the final spawning tributary. Exploratory behaviour was more likely in S. salar which spawned in the tributaries furthest from the sea. Exploratory behaviour was also more likely to occur during periods of reduced discharge in the natal stream. The fishery management implications of complex pre‐spawning behaviour in a mixed stock lacustrine system, are discussed.     

The influence of aquaculture unit proximity on the pattern of Lepeophtheirus salmonis infection of anadromous Salmo trutta populations on the isle of Skye,Scotland

A total of 230 anadromous Salmo trutta (brown trout) were sampled in five sheltered coastal fjords (or sea lochs) on the Isle of Skye, Scotland, U.K., in 2016 at varying distances from active Atlantic salmon Salmo salar farms. Statistical models were developed to investigate potential correlations between salmon lice Lepeophtheirus salmonis burdens on S. trutta hosts and their proximity to S. salar farm cages. Significant correlations were found between lice burdens and fish fork length and proximity to the nearest S. salar farm. The probability of the presence of L. salmonis on fish hosts increased with fish host size and with distance from the nearest S. salar farm, but total lice burdens were highest in fish sampled near S. salar farms and decreased with distance. The proportion of different life‐cycle stages of L. salmonis were also dependent on S. salar farm proximity, with higher juvenile lice numbers recorded at sites near S. salar farm cages. These results highlight the complexity of the relationship between S. trutta and L. salmonis infections on wild fish and emphasize the requirement of further research to quantify these effects to better inform conservation and management strategies, particularly in areas of active S. salar farm facilities.     

Characterization of an Atlantic salmon Salmo salar stream at the southern limit of its eastern Atlantic distribution

Ten reaches of an Atlantic stream located in north‐west Spain were sampled intensively during one summer to characterize the conditions where Atlantic salmon Salmo salar have been re‐introduced along the stream. Fish species richness and diversity showed a downstream increase, which was mainly attributed to the higher number of cyprinid species found in the lower reaches. Moreover, except for brown trout Salmo trutta that appeared to be the most ubiquitous species, the densities of the other species was higher in the lower than in the upper stream reaches. Redundancy analysis showed that the pattern of fish assemblages observed along the studied stream was mainly related to the expected gradient observed in the levels of dissolved oxygen, discharge and mean current speed. There was a significant differentiation between midstream and downstream reaches, both in terms of the composition of their fish assemblages and the freshwater habitat. This study emphasizes the importance of describing the variations in fish assemblages and habitat characteristics along a river to explore its relation to potential changes in the survival of fish populations. In particular, the development of habitat–fish relationships may be a useful tool for water managers to assess the effects of development and restoration projects on the habitat of S. salar.     

A review of the likely effects of climate change on anadromous Atlantic salmon Salmo salar and brown trout Salmo trutta,with particular reference to water temperature and flow

The present paper reviews the effects of water temperature and flow on migrations, embryonic development, hatching, emergence, growth and life‐history traits in light of the ongoing climate change with emphasis on anadromous Atlantic salmon Salmo salar and brown trout Salmo trutta. The expected climate change in the Atlantic is for milder and wetter winters, with more precipitation falling as rain and less as snow, decrease in ice‐covered periods and frequent periods with extreme weather. Overall, thermal limits for salmonids are species specific. Scope for activity and growth and optimal temperature for growth increase with temperature to an optimal point before constrain by the oxygen content of the water. The optimal temperature for growth decreases with increasing fish size and varies little among populations within species, whereas the growth efficiency may be locally adapted to the temperature conditions of the home stream during the growth season. Indirectly, temperature influences age and size at smolting through its effect on growth. Time of spawning, egg hatching and emergence of the larvae vary with temperature and selective effects on time of first feeding. Traits such as age at first maturity, longevity and fecundity decrease with increasing temperature whilst egg size increases with temperature. Water flow influences the accessibility of rivers for returning adults and speed of both upstream and downstream migration. Extremes in water flow and temperature can decrease recruitment and survival. There is reason to expect a northward movement of the thermal niche of anadromous salmonids with decreased production and population extinction in the southern part of the distribution areas, migrations earlier in the season, later spawning, younger age at smolting and sexual maturity and increased disease susceptibility and mortality. Future research challenges are summarized at the end of the paper.     

Brown trout (Salmo trutta,Linnaeus, 1758) spawning habitat improvement in the Ebro River Basin (eastern Spain)

Limestone streams in Spain are subject to calcium carbonate precipitation processes, which make the substrate very cohesive. Many populations of brown trout (Salmo trutta, Linnaeus, 1758) in those rivers show reduced production due to unsuitable spawning habitats. This study presents data on S. trutta spawning habitat improvement in the Guadalope River Basin during 2011. The main objectives of this study were to (i) modify the substrate in order to make it easier for S. trutta females to dig when spawning, (ii) test the effect of the gravel quality improvement on the habitat, (iii) analyse the effect of habitat improvement on S. trutta recruitment, and (iv) monitor habitat improvement effectiveness in the long term. Three manual measures were adopted to make the gravel suitable for S. trutta breeding: the separation of the embedded gravel using an iron bar by hitting it with a mace‐hammer (‘substrate breaking’), the spreading out of the gravel loosened by the previous action with a rake (‘raking up’), and thirdly the elimination of fine sediment depositions (‘substrate cleaning’). Results of the first year of monitoring (2012) in improved and control areas were compared. Methodology effectiveness assessment requires monitoring over a longer period, but the data from the first year after habitat improvement provide encouraging results, mainly dealing with changes in spawning habitat parameters and an increase in the S. trutta age‐0 class in the improved areas.     

Predictability of multispecies competitive interactions in three populations of Atlantic salmon Salmo salar

Juvenile Atlantic salmon Salmo salar from three allopatric populations (LaHave, Sebago and Saint‐Jean) were placed into artificial streams with combinations of four non‐native salmonids: brown trout Salmo trutta, rainbow trout Oncorhynchus mykiss, Chinook salmon Oncorhynchus tshawytscha and coho salmon Oncorhynchus kisutch. Non‐additive effects, as evidenced by lower performance than predicted from weighted summed two‐species competition trials, were detected for S. salar fork length (L_F) and mass, but not for survival, condition factor or riffle use. These data support emerging theory on niche overlap and species richness as factors that can lead to non‐additive competition effects.     

The effect of catch‐and‐release angling at high water temperatures on behaviour and survival of Atlantic salmon Salmo salar during spawning migration

In this study, behaviour and survival following catch‐and‐release (C&R) angling was investigated in wild Atlantic salmon Salmo salar (n = 75) angled on sport fishing gear in the River Otra in southern Norway at water temperatures of 16·3–21·1° C. Salmo salar were tagged externally with radio transmitters and immediately released back into the river to simulate a realistic C&R situation. The majority of S. salar (91%) survived C&R. Most S. salar that were present in the River Otra during the spawning period 3–4 months later were located at known spawning grounds. Downstream movements (median furthest position: 0·5 km, range: 0·1–11·0 km) during the first 4 days after release were recorded for 72% of S. salar, presumably stress‐induced fallback associated with C&R. Individuals that fell back spent a median of 15 days before commencing their first upstream movement after release, and 34 days before they returned to or were located above their release site. Mortality appeared to be somewhat elevated at the higher end of the temperature range (14% at 18–21° C), although sample sizes were low. In conclusion, C&R at water temperatures up to 18° C had small behavioural consequences and was associated with low mortality (7%). Nevertheless, low levels of mortality occur due to C&R angling and these losses should be accounted for by management authorities in rivers where C&R is practised. Refinement of best practices for C&R may help to reduce mortality, particularly at warmer temperatures.     

Vertebral deformities in interspecific diploid and triploid salmonid hybrids

Vertebral deformities in salmonid interspecific hybrids, some of which were triploidised, were assessed across three separate year classes during the freshwater life stage. Initially, eggs from a farmed Atlantic salmon Salmo salar were crossed with the sperm from a S. salar, arctic char Salvelinus alpinus or brown trout Salmo trutta. For S. salar × S. trutta, half the eggs were triploidised. In a second- and third-year class, the eggs from a farmed S. salar were crossed with the sperm from either a S. salar or a S. trutta, and half of each group was triploidised. In the two initial-year classes, all hybrids were larger than the S. salar controls, and triploid S. salar × S. trutta were larger than diploid counterparts. In the third-year class, the S. salar × S. trutta were smaller than the S. salar, in contrast to the initial 2 year classes, although the triploid hybrids were still larger than the diploids. In the third-year class, a high degree of spontaneous triploidy was also observed in the putative diploid groups (between 16 and 39%). Vertebral deformities were consistently higher in pressure-shocked triploids than diploids, irrespective of hybridisation, but there was no consistent effect of hybridisation among experiments. Although this study was not able to explain the contrasting results for vertebral deformities between year classes, triploid S. salar × S. trutta can demonstrate impressive freshwater growth that could be of interest for future farming programmes.     

Hybridization between Atlantic Salmon Salmo salar L. and Brown Trout S. trutta L. upon Artificial Propagation

Samples of Salmo salar and S. trutta were examined in 12 Russian fish hatcheries. With protein markers, hybrids of the two species were found in three hatcheries of the Baltic Sea basin. Some fishes had a phenotype intermediate between theS. salar and S. trutta phenotypes by morphological traits, but did not differ genetically from one of the parental species. Possible consequences of hybridization and ways to prevent it are discussed.     

Upstream migratory behaviour of wild and ranched Atlantic salmon Salmo salar at a natural obstacle in a coastal spate river

The upstream migratory behaviour of wild and ranched Atlantic salmon Salmo salar in a small Irish coastal spate river was investigated using acoustic telemetry. Prespawning migratory behaviour was investigated including movement patterns at a large natural waterfall in the lower reaches of the river. A strong diurnal pattern was observed for upstream migrants at the waterfall indicative of the need for daylight to ascend this complex natural obstacle to migration. Successful passage of the waterfall was also associated with distinct environmental conditions and no difference in migratory ability was detected between wild and ranched origin S. salar. Wild S. salar tended to exhibit a non‐erratic, stepwise upstream migration pattern after ascending the waterfall while ranched S. salar had an increased probability of displaying more erratic migratory behaviour. Wild S. salar penetrated further into the river catchment than ranched S. salar, although male ranched S. salar exhibited the greatest cumulative distance moved prior to the spawning period. The management implications of escaped or released ranched S. salar and movement at natural obstacles are discussed.     

Marine depth use of sea trout Salmo trutta in fjord areas of central Norway

The vertical behaviour of 44 veteran sea trout Salmo trutta (275–580 mm) in different marine fjord habitats (estuary, pelagic, near shore with and without steep cliffs) was documented during May–February by acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day v. night), season, seawater temperature and the body length at the time of tagging. Mean swimming depth during May–September was 1·7 m (individual means ranged from 0·4 to 6·4 m). Hence, S. trutta were generally surface oriented, but performed dives down to 24 m. Mean swimming depth in May–September was deeper in the near‐shore habitats with or without steep cliffs (2·0 m and 2·5 m, respectively) than in the pelagic areas (1·2 m). May–September mean swimming depth in all habitats was slightly deeper during day (1·9 m) than at night (1·2 m), confirming that S. trutta conducted small‐scale diel vertical movements. During summer, S. trutta residing in near‐shore habitat progressively moved deeper over the period May (mean 1·1 m) to August (mean 4·0 m) and then reoccupied shallower areas (mean 2·3 m) during September. In winter (November and February), individuals residing in the innermost part of the fjords were found at similar average depths as they occupied during the summer (mean 1·3 m). The swimming depths of S. trutta coincide with the previously known surface orientation of salmon lice Lepeophtheirus salmonis. Combined with previous studies on horizontal use of S. trutta, this study illustrates how S. trutta utilize marine water bodies commonly influenced by anthropogenic factors such as aquaculture, harbours and marine constructions, marine renewable energy production or other human activity. This suggests that the marine behaviour of S. trutta and its susceptibility to coastal anthropogenic factors should be considered in marine planning processes.     

Water discharge affects Atlantic salmon Salmo salar smolt production: a 27 year study in the River Orkla,Norway

A model that explains 48% of the annual variation in Atlantic salmon Salmo salar smolt production in the River Orkla, Norway, has been established. This variation could be explained by egg deposition, minimum daily discharge during the previous winter and minimum weekly discharge during the summer 3 years before smolt migration. All coefficients in the model were positive, which indicates that more eggs and higher minimum discharge levels during the winter before smolt migration and the summer after hatching benefit smolt production. Hence, when the spawning target of the river is reached, the minimum levels of river discharge, in both winter and summer, are the main bottlenecks for the parr survival, and hence for smolt production. The River Orkla was developed for hydropower production in the early 1980s by the construction of four reservoirs upstream of the river stretch accessible to S. salar. Although no water has been removed from the catchment, the dynamics of water flow has been altered, mainly by increasing discharges during winter and reducing spring floods. In spite of the higher than natural winter discharges, minimum winter discharge is still a determinant of smolt production. Hence, in regulated rivers, the maintenance of discharges to ensure that they are as high as possible during dry periods is an important means of securing high S. salar smolt production.     

Spawning-strategy-dependent diets in two North American populations of Atlantic salmon Salmo salar

The diet of repeat-spawner Atlantic salmon Salmo salar was investigated using carbon and nitrogen stable-isotope values from the outer growth band of scales, which reflect the fish's consumption and growth during their most recent marine phase. Isotope values for S. salar displaying different spawning strategies were compared between and within the Miramichi and Nashwaak Rivers, New Brunswick, Canada and a Bayesian mixing model was used to infer dietary contributions from potential prey items. Significant differences in the stable-isotope values were found among spawning strategies and between rivers, indicating differences in diet and feeding area, consistent with hypotheses. Bayesian mixing model results inferred the main prey items consumed during marine feeding by S. salar to consist of hyperiid amphipods and capelin Mallotus villosus for repeat alternate spawners from both rivers, sandlance Ammodytes sp. for repeat consecutive spawners from the Miramichi River and amphipods for repeat consecutive spawners from the Nashwaak River. These results demonstrate the diversity of feeding tactics among S. salar spawning strategies from the same river and between populations from different rivers. Accounting for differences in prey availability and the subsequent impact on S. salar diet and spawner return rates (i.e., marine survival) will facilitate the application of ecosystem-based management practices, such as ensuring that fisheries for forage species do not indirectly adversely affect S. salar return rates.     

Initial mortality of radio-tagged Atlantic salmon (Salmo salar L.) smolts following release downstream of a hydropower station

The fate of radio-tagged hatchery-reared salmon smolts (Salmo salar L.) was investigated in the tailrace of a hydropower station in the Danish River Gudenaa during 2 years. Seventeen and 27 smolts were tagged and released in late May 1996 and during April 1999, respectively. Out of the total of 44 smolts, only two were recorded to leave the river and enter the estuary. In both years of study, electrofishing was used to sample tagged smolts. In 1996, these attempts were unsuccessful, while in 1999, 19 (70% of total) transmitters were retrieved in the stomachs of pikeperch (Stizostedion lucioperca (L.)) and pike (Esox lucius L.), and in grey heron (Ardea cinerea L.) nests. The present study demonstrates that the physical condition of the watershed, with respect to spawning and rearing, may not be the only crucial determining factors of the success of reintroduction of Atlantic salmon. Hydropower plants not only block the free movement of migratory fish, hence preventing the utilisation of spawning and rearing areas, but also form the basis of areas with increased mortality.     

The gill maggot Salmincola salmoneus as an indicator of repeat spawning in Atlantic salmon Salmo salar

The potential of the gill maggot Salmincola salmoneus for use as an indicator of repeat spawning in Atlantic salmon Salmo salar was studied in adult fish captured during their return migration to the River Alta (n = 659) and River Namsen (n = 540) in Norway. Eighty‐eight and 49% of previous spawners identified by scale readings were infected with S. salmoneus in the two rivers, respectively. Salmincola salmoneus can be used as a reliable, rapid and objective field indicator of repeat spawning in S. salar as nearly all infected fish (99·4%) were identified as repeat spawners, although it is important to have appropriate background information on S. salmoneus prevalence on the postspawning individuals within the same population.     

Influence of the marine feeding area on the muscle and egg fatty‐acid composition of Atlantic salmon Salmo salar spawners estimated from the scale stable isotopes

Fatty acids in muscle tissue and eggs of female Atlantic salmon Salmo salar spawners were analysed to evaluate the dietary quality of their final feeding areas in the Baltic Sea. The final likely feeding area was identified by comparing stable carbon and nitrogen isotope composition of the outermost growth region (final annulus) of scales of returned S. salar with that of reference S. salar caught from different feeding areas. Some overlap of stable‐isotope reference values among the three areas, in addition to prespawning fasting, decreased the ability of muscle tri‐acylglycerols to discriminate the final likely feeding area and the area's dietary quality. Among three long‐chained polyunsaturated fatty acids, docosahexaenoic acid (DHA; 22:6n‐3), eicosapentaenoic acid (EPA; 20:5n‐3) and arachidonic acid (ARA; 20:4n‐6), the proportions of ARA in total lipids of spawning S. salar muscle and eggs showed a significant negative correlation with increasing probability of S. salar having returned from the Baltic Sea main basin (i.e. the Baltic Sea proper). The results suggest that ARA in muscle and eggs is the best dietary indicator for dietary characteristics of final marine feeding area dietary characteristics among S. salar in the Baltic Sea.