river bed construction: impact and habitat restoration

Aquaculture and Fisheries Management 1992, 23, 489-498

River bed construction: impact and habitat restorationfor juvenile Atlantic salmon, Salmo salar L., and browntrout, Salmo trutta L.

N. A. HVIDSTEN & B.O. JOHNSEN Norwegian Institute for Nature Research,Trondheim, Norway

Abstract. TTie River S0ya, Norway, was canalized for agricultural purposes. In order tocompensate for damage to the Atlantic salmon, 5a/moja/ar L. and brown trout, Salmo trutta L.populations, different weirs were built. The aims of this study were firstly to analyse the effectsof weirs covering the river bank and entire river bottom with blasted stones and secondly, toanalyse the effects of sediments transported by freshets on the downstream salmon and troutpopulations after canalization. Restoration of the river bottom with blasted stones providedsalmon with more substrate spaces. Densities of trout increased after the river bank wascovered with stones. Sediments transported downstream from the canalized river stretchdecreased the densities of juvenile salmon and trout.

Introduction

The River S0ya, Norway, was drained for agricultural purposes in the period 1985-88. Part ofthe river is now a canal, which has shortened the total river length by 2-5 km, on a river stretchpreviously measuring about 7-5 km. Weirs were built to compensate for the impact ofcanalization on fish populations. The objectives of this investigation were: (1) to measurehabitat preference among juvenile salmon, Salmo salarL., and brown trout, Salmo trutta L.,in natural and different artificial weirs and (2) to analyse the effects of transported sedimentin the drained area on the densities and interspecific competition of downstream salmon andtrout populations.

Study area

The River S0ya is situated in north-western Norway. The drainage area is dominated bymountains with some lakes. The river is influenced by rapid changes in water flow and lowwater temperatures.

A 10-km-long stretch (Fig. 1) of the River S0ya produces salmon and trout. The river wasshortened between Haugafossen and Kvendb0brua (Fig. 2). On the canalized area, watervelocities are generally highest upstream and gradually diminish downstream to Hauga-fossen. Upstream near Kvendbebrua, the bottom substrate consists of stones measuring5-lOcm in diameter. The stony substrate particles gradually decrease in size to fine sand inthe downstream sections of the canalized area.

Correspondence: Dr N.A. Hvidsten, Norsk Institutt for Naturforskning, Tungasletta 2, N-7004 Trondheim,Norway.

489

490 N. A. Hvidsten (fe 5. O. Johnsen

Figure 1. The River S0ya before canalization, showing the stations for electrofishing.

The river banks along the canal were filled with stones ^ l m in diameter to preventerosion. Nine weirs were built on the canalized stretch (Fig. 2). Seven of the weirs wereconstructed as 'Syvde' weirs (Beheim, Jensen, Mellquist & Vasshaug 1977), while two of theweirs were constructed using stones in 60-m-Iong stretches covering the entire width of theriver (20 m). Stones of sizes 0-20 (stations 4c and 5b) and 0-40 (stations 4b and 5a) cm wereplaced in two separate areas each 30 m long.

Prior to drainage works, seven stations were chosen for analyses. Three stations weresituated downstream of the canalized area (stations 6,7 and 8). Stations 1 and 2 were situatedupstream from the drained area and station 4 and station 5 were located on the subsequentlydrained area. Stations 1, 2, 6, 7 and 8 were reference stations outside the canalized areas.These stations have a natural substratum. The bottom substratum at stations 1 and 2 isdominated by stones with diameters of 10-15 cm. The bottom substratum on stations 6,7 and8 consists of stones with diameters of 5-30cm. After regulation two new stations were chosenupstream of the drained area (station la and station 3). Station la is just downstream ofstation 1, and has the same bottom substratum as station 1. Stones with diameters of 5-15 cmdominated the bottom substratum at station 3. At station 4, the bottom consisted of smallstones of 2-5cm diameter prior to regulation. Station 5 was partly covered with stones of5-lOcm in diameter and sand.

Figure 2. The new river stretch between Haugafossen and Kvendb0brua after canalization. The nine weir sites arcshown.

Effect of river bed construction 491

The drained areas at station 4a-d consist of four stations, and station 5a-c consists ofthree stations. Stations 4a, 4d and 5c are reference stations, where only the river banks werecovered with stones. At stations 4b and 5a the entire river bed was covered by 0-40cmdiameter stones. At stations 4c and 5b, the river bed was covered by stones of 0-20cmdiameter. Only blasted stones were used.

Huge amounts of fine sediments were transported downstream from the canalized areaduring and after the digging operations when building the canal.

Analyses of adult fish scales indicated mean (X) smolt age of 3-7 and 3-1 for salmon andtrout respectively.

Methods

Juvenile salmonids were sampled using electric fishing gear. Each area was fished three timesand densities were estimated according to Zippin's method (Zippin 1958; Bohlin 1984).Salmon and trout were treated separately using the year classes >0+ in the density estimates.Large and frequent changes in water discharges made estimates of salmonid densities difficultin the River S0ya. Differences in the estimates are caused partly by variation in catchingefficiency during the sampling period due to changes in water discharge (Jensen & Johnson1988). There is no tendency for either particularly low or high density values from any seasonof sampling in the period 1984-1990 in the River S0ya.

Results

The upstream area

The numbers of juvenile salmon and trout at stations 1 and 2 varied between 1 and 28 perand 5 and 38 per lOOm^ respectively. In this section of the river, the unstable river bed

covered with round stones may negatively influence the densities of juvenile salmonids.

The canalized river stretch

Salmon. Densities of juvenile salmon of age >0+, have increased at stations 4b and 4c (areascovered with stones), compared to the densities estimated prior to drainage activities (Fig. 3).Estimates were about seven fish per lOOm^ prior to draining. After restoration densities ofjuvenile salmon varied from 25 to 125 individuals per 100 m .̂ In the reference stations,densities varied from 7 to 64 juveniles per lOOm .̂ On most occasions, the densities ofjuveniles were higher on these stations compared to undisturbed conditions prior to thedraining operation.

The densities of juvenile salmon at station 5 may have increased above the densities priorto the draining activity in 1990 at stations 5a and 5b. Salmon juveniles were foundoccasionally in reference station 5c.

The age distribution of the salmon gave higher numbers of older parr of age 3, 4 and 5after the regulation in stations 4a-d and 5a-c (Fig. 4).

Trout. The densities of juvenile trout at stations 4c and 4d have increased after canalizationcompared to station 4 (Fig. 5). The number of juvenile trout per lOOm^ has probably

492 N. A. Hvidsten & B. O. Johnsen

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Figure 3. Densities of salmon >0+ in the River Soya before, during (1985-1988) and after (1989-1990) thecanalization.


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m a 1987^ 1 9 9 5Bra 1984

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Figure 4. Age distribution of juvenile salmon in the period 1984-1989 at the drained area before and after thecanalization.

increased at stations 4a and 4b compared to station 4 prior to regulation, as the densities inmost cases were higher at the reference stations.

The densities of trout at stations 5b and 5c have increased in 1990 compared to previousfigures from the canalized area. In station 5a, the density may have increased after regulation,but the difference is insignificant (P > 0-05).

Analysis of the age distribution of trout after regulation shows higher numbers of olderfish than prior to the canalization and the covering of the river bank with stones (Fig. 6).

Tke downstream area

Densities of juvenile salmon and trout downstream from the canalized area have decreased inthe period 1984-1988. Changes were most conspicuous at station 6. Estimates of troutdensities downstream from the canalized area (stations 6-8) have increased from the lowestvalues in 1988 and 1989, but are still lower than before the upstream regulation, with theexception of station 8- The recorded numbers of juvenile saimon were lowest in 1990, afterrelatively high density values in 1989.

The number of juvenile salmon increased compared to total numbers of juvenilesalmonids downstream from the regulated area in the period 1984-1989. Significantcorrelation occurred at stations 7 and 8(P < 0-05) (Table 1). A comparison ofthe frequencyof salmon in the total estimates upstream of the canalized area with the frequencydownstream yielded higher figures in the downstream area in the period 1984-1989 (Fig. 7).

Discussion

Densities of juvenile salmonids increased when blasted stones were used to cover the riverbed and the river bank. Territories for salmonids increased when the river bottom wascovered with stones. This accords with Rimmer, Paim & Saunders (1983), Bachman (1984)


brown trout

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Figure 5. Densities of trout >0+ in the River Soya before, during (1985-1988) and after the canalization 1989-1990.


Table I. Frequency of juveniles (>0+) of salmon at the different stations comparedto the numbers of salmon and trout (%) in the period 1984-1989 in the River Soya

Station

1la23 = 2b44a4b4c4d55a5b5c678

1984

70

29

67

73

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1985

57

18

54

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1986

31

30

677124

1987

69

15

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848630

1988

385210

19383217

90370637618

1989

504033^

68706621

997120768342

and Elliott (1984), who reported that the number of hiding places is related to the availabilityof substrate spaces for juvenile salmonids.

Densities of juvenile salmon were higher where the entire river width was covered withstones, compared to the reference areas, where stones only covered the river bank. Thedensities of trout were the same as on the reference area in stations covered with stonesthroughout the entire river bed. Water velocity increases from the river bank to the middle ofthe river. This accords with other reports on preference for higher water velocity by juvenilesalmon than juvenile trout (Heggberget 1974; Karlstrom 1977; Heggenes & Saltveit 1990).

79

oi iuveniltfs 'K

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Figure 6. Age distribution of juvenile trout in the period 1984-1989 at the drained area before and after thecanalization.


of iuvenile salnon '/.

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1984 1985 1986 1987 19B8 1989

Figure 7. Frequency of juvenile salmon upstream and downstream of the canalized area.

The number of juvenile trout and salmon increased considerably after habitat restoration inmost cases. Elevated water levels after weir construction on stations 4c and 4d may partlyexplain higher trout densities, as reported by Saunders & Smith (1962) and Mellquist (1986),reviewing other authors. The populations of juvenile salmon and trout were recruitedthrough immigration at least in 1987 and 1988. The age distribution of salmon and trout afterrestoration yielded higher numbers of older fish. Prior to canalization, the substratum wasdominated by stones of 2-5cm, and 2—4 year old juveniles were seldom caught at stations 4and 5. Probably few smolts were produced on this river stretch due to lack of hiding places forlarger parr. Effects of building weirs in the river bed have been partially reduced due todrifting substratum clogging hiding places for salmonids on the canalized area. The riverbanks in the canalized area are now gradually stabilizing as small trees and grasses appear inaddition to blasted stones, covering large parts of the river banks.

At stations 4b and 4c, stones of size 2-5 cm diameter have gradually filled up the spacesbetween the stones placed in the river as a consequence of canalization. In the winter of1987-88, the weirs at stations 4b and 4c were restored by lifting up the blasted stones. Thismight explain lower densities of juvenile salmon in 1990 than 1989 at stations 4b and 4c.However, densities of juveniles were generally low in 1990. Extremely high freshets prior tothe electrofishing might have caused large sediment transport. The low effect of placingstones in the river bed at stations 5a and 5b may have been greater transportation of very finesediments covering the river bed just upstream of station 5. Low numbers of recruits to thisarea might also explain low densities in the years 1987 and 1988. Densities and production ofsalmonids have been reported to be reduced due to sediment transportation (Bjornn,Brusven, Molnau, Milligan, Klamt, Chacho & Schaye 1977; Lloyd, Koenings & LaPerriere1987). Berg & Northcote (1985) reported that dominant hierarchies disappeared in cohosalmon, Oncorhynchus kisutch (Walbaum), when disturbed by sediment transport.

The juvenile salmon densities were generally low at reference stations 1 and 2, during theperiod 1984-1990, being particularly low in 1990. Reduced recruitment of the salmon due toless spawning success might explain the low densities of salmon parr. The densities of salmonat the weirs (stations 4b, 4c, 5a and 5b) were still highest in 1990 of all stations examined.


The reference sites (stations 1 and 2) have had varying densities of brown trout. The densitiesat station 1 seem unchanged in the period, but the numbers of trout in station 2 haveincreased. Probably the trout preferred the new niches in the downstream canalized area.

Huge amounts of fine sediments have been transported from the canalized area to theareas downstream during and after the digging operations when building the canal. Largeamounts of sand are found in the river estuary. Densities of juvenile salmonids have beenreduced as a consequence of the sediment transportation. Frequencies of trout compared tosalmon have decreased relative to the total densities of juveniles. Brown trout prefer the riverbank areas with low water velocities (Heggenes & Saltveit 1990). As the stones were coveredwith fine sediments and sand, especially in 1988, few hiding places were left for trout.Sediment transport may reduce insect fauna as reported in a review by Cederholm & Salo(1979), reducing the food availability for salmon and trout.

In 1989, densities of salmon and trout seem to have increased compared to the lowestvalues recorded following regulation. The values for salmon are still low in 1990. Sedimentsare not stabilized after the canalization and large freshets result in huge transport ofsediments downstream from the canalized river stretch and the rest of the downstream area.

Weir building may obviously represent great potential for producing juvenile salmonidsin restored rivers. In the River S0ya, canalization prior to the construction of the weirsreduced the numbers of substrate spaces due to clogging from drifting sediments. Thepositive effect to salmonid production may eventually be lost unless weirs are alsomaintained.

Until now, canalization has had a major effect on salmonids living downstream from thecanalized areas. Large freshets still transport considerable amounts of fine sedimentsdownstream in the river, but not so much as during the canalization works. Increaseddensities of trout in 1990 may be indicative of more stabilized conditions after thecanalization, which was carried out over the period 1985-1988.

Acknowledgments

We want to thank Amy Lightfoot for improving the English. The project was supported bythe Norwegian Water Resources and Energy Administration, the Norwegian Concession FeeFund.

References

Bachman R.A. (1984) Foraging behaviour of free-ranging wild and hatchery brown trout In a stream. Transactions ofthe American Fisheries Society 113, 1-32.

Berg L. & Northcote T.G. (1985) Changes in territorial, gill-flaring, and feeding behavior in juvenile coho salmon{Oncorhynchus kisutch) following short-term pulses of suspended sediment. Canadian Journat of Fisheries andAquatic Sciences 42, 1410-1417.

Beheim E., Jensen K.W., Meilquist P. & Vasshaug 0. (1977) Biotopforbedring i regulerte vassdrag. Rapport fra'iakseterskelutvalget'. VN-Rapport 3. NVE-Vassdragsdirektoratet, 29 pp. [In Norwegian.]

Bjornn T C , Brusven M. A., Molnau M.P., Milligan J.H., Klamt R. A.. Chacho E.& Schaye C. (1977) rranjpor/o/granitic sediment in streams and iu effects on insects and fish. Forest, Wildlife and Range Experiment Station.University of Idaho, 43 pp.

Bohlin T. (1984) Kvantitativt elfiske efter lax och oring — synspunkter och rckommandationer. English summary:Ouantative electrofishing for salmon and trout — views and recommendations. Report of the Institute ofFreshwater Research of the Swedish National Board of Fisheries, Drottningholm 4, 1-33.


CederholmC.D. & Salo E.O. (1979) The effect of logging road landslide siltation on the salmon and trout spawninggravels of Stequaleho Creek and the Clearwater River basin. Jefferson County, Washington, 1972-1978. FinalReport, part III. Fisheries Research Institute, College of Fisheries, University of Washington.

Elliott J.M. (1984) Numerical changes and population regulation in young migratory trout, Salmo trutta, in a LakeDistrict stream. Journal of Animal Ecology S3, 327-350.

Heggberget T. (1974) Habitatsvaig hos yngel av laks, Salmo salar L. og flrret Salmo trutta L. Kongelige NorskeVidenskapers Setskap Museet Rapport Zoolog'isk Serie 12, 1-76. [In Norwegian.)

Heggenes J, & Saltveit S.J. (1990) Seasonal and spatial microhabitat selection and segregation in young Atlanticsalmon, Salmo salar, and brown trout, S. trutta, in a Norwegian river. Journal of Fish Biology 36, 707-720.

Jensen A. & Johnsen B.O. (1988) The effect of river flow on the results of electrofishing in a large Norwegian salmonriver. Verhandlungen und Internationale Vereinigung Limnologie 23, 1724-1729.

Karlstrom 0 . (1977) Biotopva! och besattningstethet hos lax och oringungar i svenska vattendrag. Informationfr&n Sotvattenslaboratoriet, Drouningholm 6, 1-72.

Lloyd D.S., Koenings J.P. & LaPerriere J.D. (1987) Effects of turbidity in fresh waters of Alaska. North AmericanJournal of Fisheries Management 7, 18-33,

Mellquist P. (1986) Life in Regulated Streams. The Weir Project. Norwegian Water Resources and EnergyAdministration, Oslo.

Rimmer D.M., Paim U. & Saunders R. (1983) Autumnal habitat shift ofjuvenile Atlantic salmon (5a/moja/flr) in asmaller river. Canadian Journal of Fisheries and Aquatic Sciences 40, 671-680.

Saunders J.W. & Smith M.W. (1962) Physical alteration of stream habitat to improve brook trout production.Transactions of the American Fisheries Society 91, 185-188.

Zippin C. (1958) The removal method of population estimation. Journai of Wildlife Management 22, 82-90.

river bed construction: impact and habitat restoration

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