Introduction and dispersion of the bullhead, Cottus gobio L., in a sub-Arctic salmon river in northern Finland

The bullhead, Cottus gobio L., was observed in the River Utsjoki in the far north of Finland for the first time in 1979. This river is a tributary of the sub-Arctic River Teno, which flows into the Arctic Ocean and is one of the most important salmon rivers in northern Europe. The bullhead does not belong to the natural fish fauna of the Finnish river systems flowing into the Arctic. It was probably introduced by anglers as bait or accidentally during the water exchange activities as salmon fry were transferred from Finnish hatcheries to northern Norway. A viable bullhead population has dispersed through the River Utsjoki catchment. In 1995 its range extended from the lowest part of the river to a point 43 km upstream. It is highly probable that it has also entered the mainstem of the River Teno.     

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.     

An experiment in the planting of Atlantic salmon, Salmo salar L., fry

Abstract. During April 1989, approximately 45000 swim-up Atlantic salmon, Salmo salar L., fry were planted in the upper tributaries of the Lui Water, a tributary of the River Dee, north-east Scotland, at densities of between 2-6 and 3-3/m². Adult salmon are normally excluded from this stream by a waterfall. In mid-August 1989. a 5-km length of stream was surveyed for juvenile salmon and it was estimated that about 230000+ salmon were present. It was estimated that 518% of the original planted fry survived from April to August 1989. A further survey in August 1990 showed that about 3400 1 + salmon remained. Of the 0 + salmon 14·5% survived from August 1989 to August 1990.     

Feeding of Atlantic salmon, Salmo salar L., parr in the subarctic River Teno and three tributaries in northernmost Finland

Abstract— Available food and feeding of Atlantic salmon parr were studied in different parts of a subarctic river in northernmost Finland, the River Teno: in the large main stem, in a major tributary influenced by lakes, and in two minor tributaries. The highest density of benthic organisms was found in an outlet of a lake, but drifting organisms were the most abundant in the small tributaries. Migrant parr that move to the small tributaries from the main stem have better food resources in their new habitat. Plecoptera nymphs were the most important prey for salmon parr early in the spring. Later in the summer dipterans were usually the dominant prey group. In September the importance of Plecoptera was emphasized in the small tributaries and the lake outlet, and that of Trichoptera in other sites. Drifting prey was used more by large than by small parr. The utilization of drifting food appeared to be related to the water temperature rather than to the availability of the drift.     

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 age structure and distribution of Atlantic salmon parr, Salmo salar L., in small tributaries and main stems of the subarctic River Teno, northern Finland

Parr of the Atlantic salmon in subarctic River Teno, northern Finland (70°N, 28°E), are found to migrate to small tributaries that are not spawning areas for the adult salmon. The age distribution of the salmon differs significantly between these brooks and the natal rivers, the parr in the brooks being typically 2–4 years old, whereas those in the main rivers were mostly of age 0–2 years. The older fish were found in the uppermost regions of the brooks. The ratio of salmon to brown trout decreases towards the upper reaches of the brooks. At least some of the young salmon overwinter in the brooks. However, the number of fish and the area inhabited diminished towards the autumn in some occasions. It is suggested that this migratory behavior may be an adaptational phenomenon that could cause variation in survival among young salmon in the River Teno.     

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.     

Population genetics of salmon, Salmo salar L., in northern Russia

Abstract. The results of population genetics studies associated with the problem of the restoration of Atlantic salmon, Salmo salar L., in Russia are presented. The peculiarities of using donor material for artificial population formation when the native gene pool has been lost are discussed. Genetic polymorphism in 19 salmon populations from different areas (within Russia) were investigated and analysed. Intrapopulation genetic differences of the Pechora River Atlantic salmon are shown. The role of monitoring in salmon culture is discussed and the results of an ecologo-genetic evaluation of natural and artificial salmon populations are presented.     

Temporal changes in ascendance and in‐season exploitation of Atlantic salmon,Salmo salar,inferred by a video camera array

Abstract Weekly exploitation rates of Atlantic salmon, Salmo salar L., in the River Utsjoki, Finland, were estimated from catch reports during the 2003, 2004, 2006 and 2007 seasons, and recordings of all Atlantic salmon ascending the river using a submerged video camera array. In all years, mean weekly fishing mortality rates were significantly higher in June than July to August, with a falling trend throughout the fishing season. Owing to overlap in size between one‐sea‐winter (1SW) and multi‐sea‐winter (MSW) salmon, the estimated fishing mortality rates were based on all sea‐age categories combined. By the second week in June, 18.3–34.7% of large salmon (only MSW) had ascended, compared with 2.2–6.4% of small salmon (1SW and some MSW). Indirectly, the earlier start of ascent of large salmon to River Utsjoki indicated that in‐river fishing mortality of MSW salmon is higher than for 1SW salmon. A later opening of the fishing season may be used to reduce the in‐river fishing mortality, especially for the MSW component.     

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.     

Road culvert restoration expands the habitat connectivity and production area of juvenile Atlantic salmon in a large subarctic river system

The effects of restoration of impassable road culverts on the distribution of juvenile Atlantic salmon, Salmo salar L., were evaluated in seven small tributaries of the subarctic River Teno system, northernmost Finland/Norway. Restoration enabled the passage of juvenile salmon through the culverts and increased the distribution area of salmon parr in the seven streams by tens or hundreds of metres, depending on the natural slope of the tributary with a total of ≈1 km new area for ascending juveniles. Areas upstream of the culverts were colonised after varying number of years, mostly 2–3, following restoration. Age‐1 and age‐2 parr were the first salmon age groups entering the new territory after removal of the migration barrier. Although the restoration measures were conducted at the downstream outlet area of the culverts only, the connectivity was improved and increased the production area accessible to juvenile salmon. Such removal of migration barriers and securing habitat connectivity by passable culverts should be taken into account in environmental management strategies of river systems safeguarding the essential habitats of salmonid fish.     

Evidence of straying from wild Atlantic salmon,Salmo salar L., smolt transportation experiments in northern Scotland

Over a period of 4 years a total of 10482 Atlantic salmon, Salmo salar L., smolts were transported from the River Bran, one of the tributaries of the Conon River system in Ross-shire, Scotland and released at a point below all the dams on this river system. The percentages of smolts recaptured as adult fish from the 4 years of the experiments were 2.8, 2.2, 2.2 and 4.0. Ninety per cent of the fish were recaptured within the Conon River system by anglers and at trapping sites or in neighbouring coastal nets, while the remaining 10% were taken in Greenland waters. The recapture of tagged fish in other tributaries of the river system indicated that there was a certain amount of straying, with a straying rate of 18.9%, although it was only appreciable in the tributary nearest to the Bran. The results are compared with those obtained from smolt transportation experiments in northern England where no adult trapping facilities were available. Discussion covers the probability of straying as a result of hydroelectric and water abstraction schemes involving inter-river transfer.     

Larval fish movement in the River Lohajang,Tangail, Bangladesh

Larval fish drift and distribution patterns were studied in the River Lohajang, a tributary of the River Jamuna, Bangladesh, during the monsoon seasons (June - October) of 1992, 1993 and 1994. Larval fish drift peaks with the water levels of the River Jamuna. In all three years, the Indian carp, Catla catla, Labeo rohita L. and Cirrhinus mrigala, were associated with the first pulses, but were absent from drift samples after September. Hilsa species and other non-identified fish species were found throughout the monsoon season. The highest larval fish densities were found in the upper strata of the river, near the shore. The drift densities of these fish became evenly distributed through time. Water turbidity probably influenced the vertical and horizontal distribution, and the absence of diel patterns. The most important species in the larval drift were Hilsa spp., followed by Labeo rohita L., Glossogobius giuris L. and Rasbora daniconius. Among the major carp, Labeo rohita was the dominant species, contributing 80-90%. The impact of flood control programmes on the reproduction of riverine and larval fish drift is discussed.     

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.     

Historical changes in age structure of Atlantic salmon, Salmo salar L., in the River Eo, northern Spain

Abstract Variations in the age structure and number of Atlantic salmon, Salmo salar L., running into the River Eo in northern Spain were assessed from catch records of sport fisheries for the period 1949 to 1991. The analysis focused on two periods (1951–1960 and 1981–1991) for which more complete data on fish number, size and age were available. Over the whole period, no evidence for a decline in the total salmon run was found, but the last few years revealed significant changes in both the number and age structure of salmon caught. The catches were mainly composed of multi-sea-winter salmon, and this did not change from the 1950s to the 1980s as the proportion of grilse remained unchanged. However, since 1980, there was a marked decline in both the proportion of 3-sea-winter salmon (versus 2-sea-winter fish) and the frequency of previous spawners among the multi-sea-winter salmon. These changes were probably linked because most of the previous spawners re-entered the river as 3-sea-winter or older salmon. River age of salmon caught in the 1950s was low and constant (about 90% of the catch had smolted at one year) but the mean age of smolting increased in the 1980s.     

Microfauna associated with amoebic gill disease in sea-farmed Atlantic salmon, Salmo salar L., smolts

A study of microfauna, associated with pathological changes in the gills of Atlantic salmon, Salmo salar L., was conducted over 2001-2002. Monthly samples of 1(+) salmon smolts were taken, protozoan populations were quantified and gill health was assessed histologically. Protozoan densities were correlated with pathological changes, in order to determine their possible role in lesions in the gills. The most severe gill tissue changes were observed in summer/autumn and the least in spring. A diverse polyphyletic protozoan community was observed colonizing the gills, including Neoparamoeba sp., other amoebae, scuticociliates, Ichthyobodo-like flagellates, trichodinid ciliates and prostomatean ciliates. The earlier gill tissue changes in the gill were not always associated with the presence of these microorganisms, whereas amoebae (other than Neoparamoeba sp.), Ichthyobodo-like flagellates and trichodinid ciliates correlated with augmenting gill lesions. Neoparamoeba sp. was present, but its abundance did not correlate with the disease. This study suggests that a diversity of protozoans including Ichthyobodo-like flagellates, trichodinid ciliates and amoebae other than Neoparamoeba sp. are involved in the aetiology of amoebic gill disease in the Irish situation.     

Genetic differentiation and gene flow in Atlantic salmon,Salmo salar L.: a case study of the River Shannon system in Ireland

An analysis of the effect of an extensive restocking programme on genetic differentiation and gene flow of the Atlantic salmon, Salmo solar L., in the River Shannon system in Ireland is presented, using allozyme electrophoresis to estimate population differentiation. The observed differentiation in the River Shannon is attributed to random genetic drift, caused by small effective population sizes in some of the tributaries, coupled with the confounding effect of extensive stocking of indigenous fish which were largely derived from a single tributary. The methods of analysis are assessed by applying numerical resampling techniques to determine confidence levels for each of the estimates.     

Riverine and early marine survival of stocked salmon smolts, Salmo salar L., descending the Testebo River, Sweden

Abstract  A combination of radio and acoustic telemetry was used to monitor the out-migration of hatchery-reared Atlantic salmon, Salmo salar L., in the River Testebo, its estuary and coastal system. As with many other Baltic rivers, the hydropower regulated River Testebo once had a self-sustaining salmon population that is now extinct. Substantial losses of smolts in the river (48–69%) and inner part of the estuary (43–47%) were found, but after leaving the estuary, the success of post-smolts moving out of the Bay was sufficiently high (83–89%) to conclude that habitat within the bay is not a factor limiting initial marine survival. The results suggest that hatchery-based recovery of a wild salmon population in the river will not be successful unless other actions, such as habitat improvement, are included.     

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.     

Relationships between juvenile salmon, Salmo salar L., and invertebrate densities in the River Tana, Norway

Abstract –  Juvenile salmon density was related to invertebrate density in 13 streams within the River Tana, northern Norway. There were only small, nonsignificant, differences in benthic density between streams with and without juvenile salmon. All streams with a high density of juvenile salmon had low benthic densities at the stream mouth. Juvenile salmon were not found, or were in very low densities, in streams where the benthic density at the stream mouth was as high or higher than that in the stream. A multiple regression model showed that parr density was related negatively to benthic density at the stream mouth, water velocity and pH, and positively to benthic density within the stream and the proportion of the substratum covered by moss. The amount of overhanging cover in the different streams explained 93% of the variation in the drift density of terrestrial invertebrates in August. The highest densities of juvenile salmon were found in streams with riparian vegetation, and were thus associated with an abundant supply of drift food, especially terrestrial invertebrates.