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

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1 Aquaculture and Fisheries Management 1992, 23, River bed construction: impact and habitat restoration for juvenile Atlantic salmon, Salmo salar L., and brown trout, 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 to compensate 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 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. Introduction The River S0ya, Norway, was drained for agricultural purposes in the period Part of the river is now a canal, which has shortened the total river length by 2-5 km, on a river stretch previously measuring about 7-5 km. Weirs were built to compensate for the impact of canalization on fish populations. The objectives of this investigation were: (1) to measure habitat 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 sediment in the drained area on the densities and interspecific competition of downstream salmon and trout populations. Study area The River S0ya is situated in north-western Norway. The drainage area is dominated by mountains with some lakes. The river is influenced by rapid changes in water flow and low water temperatures. A -km-long stretch (Fig. 1) of the River S0ya produces salmon and trout. The river was shortened between Haugafossen and Kvendb0brua (Fig. 2). On the canalized area, water velocities are generally highest upstream and gradually diminish downstream to Haugafossen. Upstream near Kvendbebrua, the bottom substrate consists of stones measuring 5-lOcm in diameter. The stony substrate particles gradually decrease in size to fine sand in the downstream sections of the canalized area. Correspondence: Dr N.A. Hvidsten, Norsk Institutt for Naturforskning, Tungasletta 2, N-04 Trondheim, Norway. 489

2 490 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 ^lm in diameter to prevent erosion. Nine weirs were built on the canalized stretch (Fig. 2). Seven of the weirs were constructed as 'Syvde' weirs (Beheim, Jensen, Mellquist & Vasshaug 1977), while two of the weirs were constructed using stones in 60-m-Iong stretches covering the entire width of the river ( m). Stones of sizes 0- (stations 4c and 5b) and 0- (stations 4b and 5a) cm were placed in two separate areas each m long. Prior to drainage works, seven stations were chosen for analyses. Three stations were situated downstream of the canalized area (stations 6,7 and 8). Stations 1 and 2 were situated upstream from the drained area and station 4 and station 5 were located on the subsequently drained 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 is dominated by stones with diameters of -15 cm. The bottom substratum on stations 6,7 and 8 consists of stones with diameters of 5-cm. After regulation two new stations were chosen upstream of the drained area (station la and station 3). Station la is just downstream of station 1, and has the same bottom substratum as station 1. Stones with diameters of 5-15 cm dominated the bottom substratum at station 3. At station 4, the bottom consisted of small stones of 2-5cm diameter prior to regulation. Station 5 was partly covered with stones of 5-lOcm in diameter and sand. Figure 2. The new river stretch between Haugafossen and Kvendb0brua after canalization. The nine weir sites arc shown.

3 Effect of river bed construction 491 The drained areas at station 4a-d consist of four stations, and station 5a-c consists of three stations. Stations 4a, 4d and 5c are reference stations, where only the river banks were covered with stones. At stations 4b and 5a the entire river bed was covered by 0-cm diameter stones. At stations 4c and 5b, the river bed was covered by stones of 0-cm diameter. Only blasted stones were used. Huge amounts of fine sediments were transported downstream from the canalized area during 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 and trout respectively. Methods Juvenile salmonids were sampled using electricfishinggear. Each area wasfished three times and 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 difficult in the River S0ya. Differences in the estimates are caused partly by variation in catching efficiency during the sampling period due to changes in water discharge (Jensen & Johnson 1988). There is no tendency for either particularly low or high density values from any season of sampling in the period 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 per and 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 (areas covered 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 of juvenile salmon varied from 25 to 125 individuals per 0 m^. In the reference stations, densities varied from 7 to 64 juveniles per loom^. On most occasions, the densities of juveniles were higher on these stations compared to undisturbed conditions prior to the draining operation. The densities of juvenile salmon at station 5 may have increased above the densities prior to the draining activity in 1990 at stations 5a and 5b. Salmon juveniles were found occasionally in reference station 5c. The age distribution of the salmon gave higher numbers of older parr of age 3, 4 and 5 after 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 canalization compared to station 4 (Fig. 5). The number of juvenile trout per loom^ has probably

4 492 N. A. Hvidsten & B. O. Johnsen 60 salmon Stl n n St2/St3 Jl rh O SI5 SI O St7 - St8 JS o!^ o il DATE (Tear, month, day) Figure 3. Densities of salmon >0+ in the River Soya before, during ( ) and after ( ) the canalization.

5 Effect of river bed construction 493. Nunbers oi juveniles V, ma 1987 ^1995 Bra Year classes Figure 4. Age distribution of juvenile salmon in the period at the drained area before and after the canalization. 5+ increased at stations 4a and 4b compared to station 4 prior to regulation, as the densities in most cases were higher at the reference stations. The densities of trout at stations 5b and 5c have increased in 1990 compared to previous figures 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 older fish 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 in the period Changes were most conspicuous at station 6. Estimates of trout densities downstream from the canalized area (stations 6-8) have increased from the lowest values in 1988 and 1989, but are still lower than before the upstream regulation, with the exception of station 8- The recorded numbers of juvenile saimon were lowest in 1990, after relatively high density values in The number of juvenile salmon increased compared to total numbers of juvenile salmonids downstream from the regulated area in the period Significant correlation occurred at stations 7 and 8(P < 0-05) (Table 1). A comparison ofthe frequency of salmon in the total estimates upstream of the canalized area with the frequency downstream yielded higher figures in the downstream area in the period (Fig. 7). Discussion Densities of juvenile salmonids increased when blasted stones were used to cover the river bed and the river bank. Territories for salmonids increased when the river bottom was covered with stones. This accords with Rimmer, Paim & Saunders (1983), Bachman (1984)

6 494 N. A. Hvidsten & B. O. Johnsen so 6O SO 1GO brown trout Sti rh St2/Sl3 SI4 n J] logo SO lo " St6 St7 rii St8 n n 3 g S ^ o o V «in CO CO CD,- O n o «t o ^- a> a q o o DATS (Toar, month, day) Figure 5. Densities of trout >0+ in the River Soya before, during ( ) and after the canalization

7 Effect of river bed construction 495 Table I. Frequency of juveniles (>0+) of salmon at the different stations compared to the numbers of salmon and trout (%) in the period in the River Soya Year Station la 2 3 = 2b 4 4a 4b 4c 4d 5 5a 5b 5c $« ^ and Elliott (1984), who reported that the number of hiding places is related to the availability of substrate spaces for juvenile salmonids. Densities of juvenile salmon were higher where the entire river width was covered with stones, compared to the reference areas, where stones only covered the river bank. The densities of trout were the same as on the reference area in stations covered with stones throughout the entire river bed. Water velocity increases from the river bank to the middle of the river. This accords with other reports on preference for higher water velocity by juvenile salmon than juvenile trout (Heggberget 1974; Karlstrom 1977; Heggenes & Saltveit 1990). oi iuveniltfs 'K 79 SO t 3+ Yffor classes Figure 6. Age distribution of juvenile trout in the period at the drained area before and after the canalization.

8 496 N. A. Hvidsten & B. O. Johnsen of iuvenile salnon '/. 2S B 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 in most cases. Elevated water levels after weir construction on stations 4c and 4d may partly explain higher trout densities, as reported by Saunders & Smith (1962) and Mellquist (1986), reviewing other authors. The populations of juvenile salmon and trout were recruited through immigration at least in 1987 and The age distribution of salmon and trout after restoration yielded higher numbers of older fish. Prior to canalization, the substratum was dominated by stones of 2-5cm, and 2 4 year old juveniles were seldom caught at stations 4 and 5. Probably few smolts were produced on this river stretch due to lack of hiding places for larger parr. Effects of building weirs in the river bed have been partially reduced due to drifting substratum clogging hiding places for salmonids on the canalized area. The river banks in the canalized area are now gradually stabilizing as small trees and grasses appear in addition 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 spaces between the stones placed in the river as a consequence of canalization. In the winter of , the weirs at stations 4b and 4c were restored by lifting up the blasted stones. This might explain lower densities of juvenile salmon in 1990 than 1989 at stations 4b and 4c. However, densities of juveniles were generally low in Extremely high freshets prior to the electrofishing might have caused large sediment transport. The low effect of placing stones in the river bed at stations 5a and 5b may have been greater transportation of very fine sediments covering the river bed just upstream of station 5. Low numbers of recruits to this area might also explain low densities in the years 1987 and Densities and production of salmonids have been reported to be reduced due to sediment transportation (Bjornn, Brusven, Molnau, Milligan, Klamt, Chacho & Schaye 1977; Lloyd, Koenings & LaPerriere 1987). Berg & Northcote (1985) reported that dominant hierarchies disappeared in coho salmon, Oncorhynchus kisutch (Walbaum), when disturbed by sediment transport. The juvenile salmon densities were generally low at reference stations 1 and 2, during the period , being particularly low in Reduced recruitment of the salmon due to less spawning success might explain the low densities of salmon parr. The densities of salmon at the weirs (stations 4b, 4c, 5a and 5b) were still highest in 1990 of all stations examined.

9 Effect of river bed construction 497 The reference sites (stations 1 and 2) have had varying densities of brown trout. The densities at station 1 seem unchanged in the period, but the numbers of trout in station 2 have increased. 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 the areas downstream during and after the digging operations when building the canal. Large amounts of sand are found in the river estuary. Densities of juvenile salmonids have been reduced as a consequence of the sediment transportation. Frequencies of trout compared to salmon have decreased relative to the total densities of juveniles. Brown trout prefer the river bank areas with low water velocities (Heggenes & Saltveit 1990). As the stones were covered with 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 lowest values recorded following regulation. The values for salmon are still low in Sediments are not stabilized after the canalization and large freshets result in huge transport of sediments downstream from the canalized river stretch and the rest of the downstream area. Weir building may obviously represent great potential for producing juvenile salmonids in restored rivers. In the River S0ya, canalization prior to the construction of the weirs reduced the numbers of substrate spaces due to clogging from drifting sediments. The positive effect to salmonid production may eventually be lost unless weirs are also maintained. Until now, canalization has had a major effect on salmonids living downstream from the canalized areas. Large freshets still transport considerable amounts of fine sediments downstream in the river, but not so much as during the canalization works. Increased densities of trout in 1990 may be indicative of more stabilized conditions after the canalization, which was carried out over the period Acknowledgments We want to thank Amy Lightfoot for improving the English. The project was supported by the Norwegian Water Resources and Energy Administration, the Norwegian Concession Fee Fund. References Bachman R.A. (1984) Foraging behaviour of free-ranging wild and hatchery brown trout In a stream. Transactions of the American Fisheries Society 113, 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 and Aquatic Sciences 42, 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 TC, 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 of Freshwater Research of the Swedish National Board of Fisheries, Drottningholm 4, 1-33.

10 498 N. A. Hvidsten & B. O. Johnsen CederholmC.D. & Salo E.O. (1979) The effect of logging road landslide siltation on the salmon and trout spawning gravels of Stequaleho Creek and the Clearwater River basin. Jefferson County, Washington, Final Report, 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 Lake District stream. Journal of Animal Ecology S3, Heggberget T. (1974) Habitatsvaig hos yngel av laks, Salmo salar L. og flrret Salmo trutta L. Kongelige Norske Videnskapers Setskap Museet Rapport Zoolog'isk Serie 12, [In Norwegian.) Heggenes J, & Saltveit S.J. (1990) Seasonal and spatial microhabitat selection and segregation in young Atlantic salmon, Salmo salar, and brown trout, S. trutta, in a Norwegian river. Journal of Fish Biology 36, 7-7. Jensen A. & Johnsen B.O. (1988) The effect of riverflowon the results of electrofishing in a large Norwegian salmon river. Verhandlungen und Internationale Vereinigung Limnologie 23, Karlstrom 0. (1977) Biotopva! och besattningstethet hos lax och oringungar i svenska vattendrag. Information fr&n Sotvattenslaboratoriet, Drouningholm 6, Lloyd D.S., Koenings J.P. & LaPerriere J.D. (1987) Effects of turbidity in fresh waters of Alaska. North American Journal of Fisheries Management 7, 18-33, Mellquist P. (1986) Life in Regulated Streams. The Weir Project. Norwegian Water Resources and Energy Administration, Oslo. Rimmer D.M., Paim U. & Saunders R. (1983) Autumnal habitat shift ofjuvenile Atlantic salmon (5a/moja/flr) in a smaller river. Canadian Journal of Fisheries and Aquatic Sciences, Saunders J.W. & Smith M.W. (1962) Physical alteration of stream habitat to improve brook trout production. Transactions of the American Fisheries Society 91, Zippin C. (1958) The removal method of population estimation. Journai of Wildlife Management 22,

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