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.     

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.     

The onset of downstream movement of juvenile Atlantic salmon, Salmo salar L., in a chalk stream

The downstream movements of wild Atlantic salmon, Salmo salar L., from their established feeding territories in the River Itchen, Hampshire, UK, were logged continuously over an 11-month period using a passive integrated transponder (PIT) antenna system. The time of these movements was then related to a number of monitored and calculated environmental parameters. Initial downstream movement of smolts in April was correlated with the onset of darkness, at which time salmon moved from their established feeding territories alone. No relationship was found between the number of smolts migrating and river flow or maximum daily water temperature. The timing of downstream movement of parr between October and March was random with regard to sunset and time of maximum daily water temperature, suggesting the environmental cue that initiates movement may be different outside the spring smolt period.     

A comparative study of juvenile salmon density in 20 streams throughout a very large river system in northern Norway

Abstract – The objective was to compare juvenile salmon density in 20 streams throughout the very large River Tana, northern Norway, and to relate variation in density to a suite of environmental factors. Four sampling sites were electrofished in each stream (one at the mouth of the stream and three within the stream) in August and October 2000, 2001, 2002. 0+ salmon parr were absent from seven streams, present at the mouth of 11 streams, and present within only two streams, both of which were probably spawning streams. Older parr migrated upstream into most streams and their highest densities were usually found in streams flowing directly into the spawning habitat in the three largest tributaries of the Tana or the river itself. Juvenile salmon were sparse or absent in streams flowing into smaller tributaries. Most streams with high parr densities were those of dense riparian vegetation that provided terrestrial invertebrates as drift food for the salmon parr, cover for fish, cooler stream temperatures in summer, and food for benthic stream invertebrates that were also a source of food for the parr.     

Quantifying the effectiveness of regional habitat quality index models for predicting densities of juvenile Atlantic salmon (Salmo salar L.)

Abstract –  The suitability of using regional Habitat Quality Index (HQI) models for predicting distributions of juvenile salmon ( Salmo salar L.) was examined using data acquired from selected rivers within Québec, Canada. Regional HQI models were derived by identifying preferences of salmon fry and parr for the habitat properties of flow velocity, water column depth and granulometric index (an index of substrate size) for groups of rivers. These were compared with local HQI models, derived from preferences for habitat properties within individual rivers. Relationships between HQI values and densities of juvenile salmon were established through the use of nonlinear regression. In all cases, the regional HQI models were less effective for explaining distributions of juvenile salmon than local HQI models based on individual rivers. Regional HQI models were relatively ineffective when the habitat characteristics of the river to which they were applied differed greatly from the characteristics of the other rivers within the region. It is inferred that the relationship between the effectiveness of the regional HQI model and the difference in characteristics between individual rivers and those of the region may be used in an a priori determination of whether a regional HQI may be applied effectively to any given river.     

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.     

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.     

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.     

Lake-use by juvenile Atlantic salmon (Salmo salar L.) and other salmonids in northern Norway

Abstract– The utilization of lakes, and inlet and outlet streams by juvenile Atlantic salmon ( Salmo salar L.), brown trout ( Salmo trutta L.) and Arctic charr ( Salvelinus alpinus (L.), were investigated in 16 watercourses northern Norway, all known to inhabit salmon stocks. In lakes, fish were caught by small mesh size gill nets, while in rivers fish were caught electrofishing. In the shallow littoral (0-3 m depth) there were juvenile salmon in 15 of 19 investigated lakes, juvenile trout in 17 and juvenile charr in seven. Trout dominated significantly in numbers in the shallow littoral of seven lakes, while salmon and charr dominated in three lakes each. When trout and salmon were frequent in the shallow littoral, charr was usually not present in this habitat, but were found in the profundal zone in most of the lakes. Atlantic salmon parr utilized both shallow and deep lakes, and used both stones and macrophytic vegetation as shelter. The utilization of lakes by salmon parr seemed to be closely related to utilization of small inlet streams for spawning. In most inlet and outlet streams salmon dominated over trout in numbers, while charr were absent. This is the first documentation of lake-use by naturally occurring salmon parr in Scandinavia.     

Changes in the density of introduced bullhead, Cottus gobio L., and its impact on juvenile Atlantic salmon, Salmo salar L., densities in a sub-Arctic salmon river in northern Finland

A newly introduced species, the bullhead, Cottus gobio L., was observed for the first time in the River Utsjoki, a tributary of the River Teno, in 1979. The River Teno is one of the most important Atlantic salmon, Salmo salar L., rivers in Northern Europe and its tributary the River Utsjoki is an important salmon spawning and nursery area. The densities of bullhead were lowest in the areas to which the species has spread most recently and highest in the areas downstream of its introduction point. Densities were markedly lower upstream. Statistical analysis showed that the presence of the bullhead had not affected juvenile salmon densities in the River Utsjoki.     

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.     

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.     

Relationships between marine growth and marine survival of one sea winter Atlantic salmon, Salmo salar L., from the River Bush, Northern Ireland

An examination of marine growth/marine survival relationships in Atlantic salmon, Salmo salar L., was carried out, based on scale growth measurements in relation to two indices of marine survival in wild fish from the River Bush, Northern Ireland. The survival of cohorts to the Irish/Northern Irish coast (prefishery) and to fresh water was statistically unrelated to variation in growth from smolt migration to the end of the first winter at sea ( P  > 0.1; – P  > 0.7). Marine growth of 1+ smolts decreased significantly during the period of the study ( P  < 0.05), but growth of 2+ smolts did not change ( P  > 0.05). The variability in marine growth was much less than the variation in natural survival at sea, suggesting that factors instead of or in addition to, growth influence natural survival in the marine environment. Survival to fresh water was not related to survival to the coast ( P  > 0.4), although it was inversely correlated with exploitation rate ( P  < 0.01). These results are discussed in relation to the use of freshwater returns to assess marine survival and the potential for the variation in natural marine mortality to influence total life-cycle variation.     

Homing and straying of hatchery-reared Atlantic salmon,Salmo salar L., released in three rivers in Norway

Releases of hatchery-reared smolts of Atlantic salmon, Salmo salar L., in the Rivers Orkla, Gaula and Alta, Norway yielded large percentages of strays (194, 13.7 and 13.8% respectively), based on recaptures from anglers. Homing precision to the rivers of release was independent of the release site on the river. Distribution of recaptured adult salmon within the rivers was dependent on the release site; upstream releases yielded more upstream recaptures than did downstream releases.     

Estimates of the annual loss of Atlantic salmon, Salmo salar L., in Norway due to acidification

Abstract. The pre-acidification level of Atlantic salmon, Salmo salar L., production in Norway was estimated from accessible rearing habitat, potential smolt production, smolt to adult survival, and mean adult weight. Atlantic salmon have been virtually lost in 25 rivers due to acidification. In total, 340.6km of river is affected, and this corresponds to a rearing area of 2044.8 ha. The total annual number of adult salmon lost was estimated to be between 92016 and 306720 individuals, weighing some 345-1150 tonnes.     

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.     

Screamer disease in Atlantic salmon, Salmo salar L., in Chile

Investigation of cranial and spinal deformities in Atlantic salmon smolts from Chile immediately after sea transfer, over two successive crops, demonstrated ankylosis of the mandibular articulation, spinal foreshortening, fracture of vertebrae and rarefaction of osseous and cartilaginous tissues including the operculum. As a result the mouth was permanently agape and opercula could be folded back on themselves. All affected fish had been transferred to sea water at 20 °C plus and fed on particular commercial diets. Fish in cooler areas, or on diets high in vitamin C and phosphorus in the high temperature zone, were clinically normal. The condition has not recurred following dietary adjustment.     

Dietary phosphorus requirement of juvenile Atlantic salmon, Salmo salar L.

The objective of this study was to determine the dietary phosphorus (P) requirement of juvenile Atlantic salmon, Salmon salar L. Triplicate groups of fish (mean initial weight 1.4 g) were fed semipurified, casein-gelatine-based diets containing one of five levels of P (4, 8, 10, 15 and 25 g kg⁻¹) from Ca(H₂PO₄)₂·H₂O, or a commercial feed (17 g kg⁻¹ P) for 9 weeks. Weight gains did not differ significantly among treatment groups fed the experimental diets but were slightly less than gains in fish fed the commercial feed. Feed efficiency (wet weight gain/dry feed consumed) was similar in all groups, averaging 1.45. Availability of dietary P, estimated from apparent retention and apparent digestibility, was 86%. Whole-body P concentrations declined in fish fed diets containing less than 10 g kg⁻¹ P. Fitting a logistic curve to dietary P vs. whole-body P concentrations indicated that a minimum of 11 g kg⁻¹ dietary P (9 g kg⁻¹ digestible P) was required by juvenile Atlantic salmon to maintain whole-body P concentrations at initial levels. Calculation of a dietary requirement using a simple factorial model which incorporated measurements of P availability, feed efficiency and normal whole-body P concentration indicated that the dietary requirement was approximately 10 g kg⁻¹. The dietary requirement established in this study (10–11 g kg⁻¹) is higher than previously reported for Atlantic salmon or other fishes. Possible reasons for the wide range of reported dietary P requirements in fishes are discussed.