Atlantic salmon, Salmo salar L., and sea trout, Salmo trutta L., passage in a regulated northern river – fishway efficiency, fish entrance and environmental factors

The number and the size of Atlantic salmon, Salmo salar L., using the Isohaara fishway was elevated by increasing the fishway discharge and by changing the type of the pass entrance. The fishway is intended to help fish bypass a hydroelectric station located close to the mouth of the large, regulated River Kemijoki, in northern Finland. Multi-sea-winter (MSW) salmon returned to the river mouth during peak flows in early June but did not use the fishway until 1 month later. Their number in the fishway was positively correlated with the tailwater level. One-sea winter (1-SW) salmon, which arrived approximately one month later, started to enter the fishway without corresponding delays. In autumn, a high tailwater level and a small drop at the fish entrance seemed to be needed for the successful passage of these small-sized salmon and sea trout, Salmo trutta L.     

The interpretation of anglers'records (trout and sea trout, Salmo trutta L., and salmon, Salmo salar L.)

Abstract. This paper collates the results of five surveys which were made to determine the average catch of anglers not making a return at game fisheries. This pilot study shows that the average catch per licence/permit ( ) may be estimated from the average and the proportion of unprompted returns, (m_u) and (p_u) respectively, by the empirical equation , for trout and sea trout, Salmo trutta L. Salmon, Salmo salar L., catcrl'rates may be best estimated from . Typical values for United Kingdom waters are tabulated for recent seasons. Rod catches of stillwater and migratory fish closely follow the Negative Binomial distribution. Their means and variances are shown to be related empirically by Taylor's power law: for daily records of stillwater trout and migratory fish , and for annual records of salmon , and of sea trout . A method of estimating the expected frequency distribution of catches at any water is shown. The distribution is truncated where the nil catches are not recorded and is censored where the bag is limited; here a microcomputer was programmed to compute the estimates of the mean and variance of the catches and to print the full distribution.     

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.     

Assessment of a semi-quantitative electric fishing sampling technique for juvenile Atlantic salmon, Salmo salar L., and trout, Salmo trutta L., in small streams

Abstract. A semi-quantitative technique of assessing trout, Salmo trutta L., and juvenile Atlantic salmon, Salmo salar L., stocks in small rivers is described and evaluated against a commonly used quantitative method. The results indicate that for 0+ and >0+ juvenile salmon and trout a log₁₀ n+1 transformed linear regression accounted for between 68.6% and 90.2% of the total variance in the relationship between the semi-quantitative and the quantitative result. The use of a categorization system is recommended and the applicability of the technique to an extensive juvenile salmonid monitoring programme is described.     

Saltwater performance of triploid Atlantic salmon Salmo salar L. brown trout Salmo trutta L. hybrids

Survival and growth in a saltwater net-pen of sexually immature triploid Atlantic salmon Salmo salar L. brown trout Salmo trutta L. hybrids was comparable to that of immature diploid Atlantic salmon. Following 17 months of freshwater rearing, the experimental fish were individually tagged and transferred to a saltwater net-pen where they were raised communally for 376 days. Initial and final average weights were 158 and 760 g per fish for the diploid Atlantic salmon and 209 and 1010 g per fish for the triploid hybrids; weights for the hybrids were significantly larger in both cases (α= 0.05). Survival from transfer to harvest was 43% for the Atlantic salmon and 48% for the hybrids. Average specific growth rate of fish which survived to harvest was 0.42% day^-1 for Atlantic salmon and 0.41% day^-1 for hybrids; these values were not significantly different. No significant differences were observed in average condition factor and dress-out percentage between crosses. Average gonadal weights and gonadosomatic indices were not significantly different for male diploid Atlantic salmon, female triploid hybrids and male triploid hybrids, but were significantly greater for female Atlantic salmon.     

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.     

Morphometric and meristic characters in salmon, Salmo salar L., trout, Salmo trutta L., and their hybrids

Abstract. Atlantic salmon, Salmo salar L., brown trout, S. trutta L. and their hybrids are normally identified in the field by empirical differences in maxilla length, thickness of the caudal peduncle, degree of forking of the tail and overall body conformation. This study quantifies these characters and analyses their variations in electrophoretically identified hatchery-reared individuals. Means and variances of the various measures are presented for salmon, trout and hybrids separately. The morphometric characters do not satisfactorily distinguish hybrids from the pure species, and even within samples of the pure species, some individuals will be misclassified as hybrids, or as members of the opposite species, if single characters are used on their own. Hybrids often resemble one or other parent species in one or more characters and are less often intermediate in phenotype. Triploidized hybrids are more like salmon than diploid hybrids are, and triploidized salmon are not different from diploid salmon. The results confirm that frequencies of hybrids of these species cannot be reliably assessed by morphological characters alone, and even for individuals of the pure parental species, independent confirmation of species status is advisable. Early reports of hybrid frequencies in wild stocks should be treated with caution, and apparently higher levels of hybridization in more recent studies compared with older, traditional surveys may simply reflect the greater precision of electrophoretic identification.     

Proliferative kidney disease in brown trout, Salmo trutta L., and Atlantic salmon, Salmo salar L., parr: histopathological and epidemiological observations

Abstract Proliferative kidney disease (PKD) has been described in cultivated rainbow trout but major outbreaks have not been recognized in other fish species. This paper describes light and electron microscope studies and presents epidemiological data of a disease condition in first summer Salmo salar parr and S. trutta , which is associated with an organism apparently identical to the aetiological agent of PKD. The pathology was found to be similar to that described for PKD in rainbow trout. The epidemiology in the brown trout suggests that exposure after May does not result in infection. The data from the outbreaks in two salmon hatcheries appear complex and suggest different susceptibilities to PKD among salmon originating from different sources.     

River bed construction: impact and habitat restoration for juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

The River Soya, Norway, was canalized for agricultural purposes. In order to compensate for damage to the Atlantic salmon, salmo salar L. and brown trout, Salmo trutta L. populations, different weirs were built. The aims of this study were firstly to analyse the effects of weirs covering the river bank and entire river bottom with blasted stones and secondly, to analyse the effects of sediments transported by freshets on the downstream salmon and trout populations after canalization. Restoration of the river bottom with blasted stones provided salmon with more substrate spaces. Densities of trout increased after the river bank was covered with stones. Sediments transported downstream from the canalized river stretch decreased the densities of juvenile salmon and trout.     

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.     

Studies on the pathology of ulcerative dermal necrosis (UDN) in Swedish salmon, Salmo salar L., and sea trout, Salmo trutta L., populations

Abstract. Continuous visual observations of the signs of development of UDN in fish kept in aquaria were correlated with light microscopical, transmission and scanning electron microscopical results. The initial signs of the disease are circles of pathologically-changed epidermis. The tight junctions of the squamous cells within these mucus-free areas disintegrate resulting in the loss of the protective function of the zonula occludens. Subsequently, the intercellular spaces dilate and communicate with the exterior. Necrosis of the epidermal cells due to hydromineral disturbances occur simultaneously with fungal infections and marked responses of the melanophores. It is suggested that the fungal infections are triggered by metabolites of the necrotic epidermal cells. The epidermal cells are shed and the fungus determines the further course of the disease which terminates in large ulcers covered with fungal hyphae. No substantial evidence for the presence of a virus could be recorded at any stage of the disease. It is concluded that the disease is a squames and the fungus appears as an opportunist which causes the eventual death of the fish.     

River temperature and adult anadromous Atlantic salmon,Salmo salar,and brown trout,Salmo trutta

Abstract In terms of the spawning migration of adult salmon, Salmo salar L., water flow is often considered the primary factor controlling river entry and fluctuations in flow controlling when the fish subsequently migrate upstream. However, water temperature has also been suggested to modify the spawning migration of salmon, particularly their movements within estuaries and the timing of freshwater entry. Freshwater temperature is more likely to impact salmonid biology than flow, particularly in relation to temperature dependant metabolic costs, time of spawning and fecundity. Therefore, temperature may be more of a factor regulating salmonid populations in fresh water than flow itself. This study focuses on two aspects of the impact of temperature on salmonids in fresh water: first, how salmon may modify their behaviour to adapt to changes in temperature and second the potential relationship between temperature, environmental conditions (e.g. water quality) and physiology (e.g. maturation and olfaction) in regulating adult migration.     

Phylogenetic analysis of intestinal bacteria of freshwater salmon Salmo salar and sea trout Salmo trutta trutta and diet

The microbial diversity of culturable intestinal microflora of wild freshwater salmonid fishes salmon Salmo salar and sea trout Salmo trutta trutta juveniles (0+ years old) from the same environmental conditions were investigated by means of molecular identification techniques and analysis of diet. Significant differences in the intestinal microbial diversity were observed in different fish species. The predominant group in the intestinal tract of the salmon comprised representatives of the Enterobacteriaceae family (23%), Plesiomonas (19.2%) and Carnobacterium (15.3%). Predominant microbiota in sea trout intestinal tract were Enterobacteriaceae (52%), Aeromonas (22%) and Pseudomonas (14%). The results show that Enterobacteriaceae were predominant in the intestinal tract of the salmon and the sea trout juveniles raised on diets of different compositions. However, molecular identification of the intestinal microbiota at the species or genus level revealed differences in these fish species. Bacteria in the hindgut of salmon included Pragia and Serratia. However, bacteria in the gut contents of sea trout from the Enterobacteriaceae family were Buttiauxella, Enterobacter, Moellerella, Pantoea, Rahnella and Tiedjeia arctica. A novel phylotype of T. arctica is harbored in the intestinal tract of wild salmon, and may correspond to a previously undescribed species.     

Diel migration pattern of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) smolts: an assessment of environmental cues

The timing of smolt migration is a key phenological trait with profound implications for individual survival during both river descent and the subsequent sea sojourn of anadromous fish. We studied relationships between the time of smolt migration, water temperature and light intensity for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta). During 2006–2012, migrating smolts descending the southern Norway River Storelva were caught in a rotary screw trap located at the river mouth. The date of 50% cumulative smolt descent correlated significantly with the date when the river temperature exceeded 8°C for both Atlantic salmon and sea trout smolts. In 2010, smolts of both species were passive integrated transponder (PIT)‐tagged, and the diel timing of their migration was precisely documented. The degree of night migration decreased in both species as the river temperature rose, and at temperatures above 12–13°C, more smolts migrated during day than during night. A multinomial model was fitted for estimating temperature and species effects on probabilities of migration during night, daytime, dusk and dawn. Atlantic salmon smolts preferred migrating under lower light intensities than sea trout smolts during early, but not late spring when both species migrated during bright daylight. In accordance with the early‐season tendency to migrate at night, Atlantic salmon smolts migrated more during darker hours of the day than sea trout. In both species, smaller smolts migrated under dark conditions than during light conditions. Most of the findings on thermal, light and temporal effects on the observed smolt migration pattern can be explained as adaptations to predation avoidance.     

Morphological variability of Atlantic salmon Salmo salar and brown trout Salmo trutta in different river environments

Abstract – The morphologies of sympatric juvenile Atlantic salmon Salmo salar parr and brown trout Salmo trutta parr were compared between Irish rivers with contrasting hydraulic environments – a high‐gradient and a low‐gradient tributary from the River Barrow catchment, south‐east Ireland and a high‐gradient river from the Burrishoole catchment, west of Ireland. The two catchments differ markedly in mean annual precipitation (849.0 mm year^?1 and 1370.3 mm year^?1 for the Barrow and Burrishoole catchments, respectively). Parr of both species demonstrated morphological variation between and within catchments. Changes in metrics such as pectoral fin length, body depth and body length between and within catchments suggest that hydraulic forces were a major determinant of morphological variation. Both species from the Burrishoole catchment had relatively larger pectoral fins, longer heads, larger eyes and shallower bodies than conspecifics from the Barrow catchment. In rivers subject to frequent rainfall‐driven high discharges, such as those in the Burrishoole catchment, a more fusiform body and head shape coupled with larger pectoral fins may reduce the energetic cost of maintaining position in the water column, as well as increase stability and manoeuvrability. The larger eyes in both parr species in the Burrishoole catchment could further be a response to the lower visibility of the more turbid and peat‐stained waters or to the reduced prey availability. The results of this study demonstrate that local adaptation to flow conditions can yield morphologically distinct populations and that multiple species can exhibit parallel phenotypic responses to changing environmental conditions.     

A comparison of the effects of overhead cover on the growth,survival and haematology of juvenile Atlantic salmon,Salmo salar L., brown trout,Salmo trutta L., and rainbow trout,Salmo gairdneri Richardson

The provision of floating annular covers to conventional tangential-flow rearing tanks significantly increased the growth rate of underyearling Atlantic salmon during the period July–November. This increased the proportion of potential 1-year-old smolts from 22% to 38% of the population (P < 0.001). Salmon without access to overhead cover grew slowly and showed haematological signs of a chronic stress response (thrombocytopenia and lymphocytopenia). Mortality rates and the incidence of disease in the young salmon were not influenced by the provision of cover. Overhead cover had no significant effects on any of these parameters in underyearling brown trout and rainbow trout. These findings are discussed in relation to feeding opportunity and behavioural differences between speices.     

Reared Atlantic salmon, Salmo salar L., in the catches of a salmon fishery on the western coast of Scotland

Abstract. Atlantic salmon, Salmo salar L., of reared origin were distinguished from putative wild fish In the catch of a single salmon fishery in western Scotland. Fish in seven day-catches made in 1990 totalling 403 individuals were assessed formally for the presence of residual fin damage resulting from culture. Fifty-eight (22%) were classed as being of reared origin, on the basis of previously established criteria. Additional examination of 54 of these fish showed the pigment canthaxanthin to be detectable in the tissues of 35 (65%), which were therefore classed as having escaped from sea cages. Among the fish in which canthaxanthin could not be detected, a further 17 individuals (31% of the total) were classed as having escaped from sea cages on the basis of scale growth patterns. Scale growth patterns in the remaining two fish (4% of the total) were consistent with their having grown in a hatchery before escape or release.