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HomeMy WebLinkAboutAPA1750Introduction POPULATIONS OF PRESMOLT ATLANTIC SALMON (SALMO SALAR L.)AND BROWN TROUT (SALMO TRUTTA L.)BEFORE AND AFTER HYDROELECTRIC DEVELOPMENT AND BUILDING OF WEIRS IN THE RIVER SKJOMA,NORTH NORWAY. TOR G.HEGGBERGET The river Skjoma was a cold and grey river before the regulation in 1977.The river was regulated in such a way that the elevated parts of the water shed were eliminated,which caused increased water temperatures and reduced silt transportation in summer. After the reduction of the waterflow,the density o~presmolt salmonides inc 2eased from about 8 fish/100 m to about 30 fish/100 m •The increase of the proportion of salmon from 40 to 60 in the period from 1976 through 1980 is likely to be a result of the met- hods used.The.productive area of the river was reduced by about 2/3 in the same period.In the weir magazines, the trout became the dominating species,while the salmon was the dominating one at the river stations. The growth of presmolt salmon and trout increased by 1,5-2,0 cm for each year-class in the period from 1978 through 1980.The reasons for the increasing growth is mainly an effect of the increased water temperatures, which were caused both by the hot summers in 1978 and 1979 and the elimination of the elevated,cold tributa- ries.There were no significant differences in growth of the fish ~aught at the river stations and in the weir magazines.Though the production of salmon and trout did not seem to be reduced significantly,the sport fishery for salmon and sea trout have been redu- ced to a minimum during the research period,mainly because of the obstructipn~"~M~~~t~emA~~~&~ors caused by the low water Fl~Y'Ii;.If.B:!f Tt~,w~Mtrac-' tion.ARCTIC ENVIRONj\~;:"'L INFORMATION AND D,\-iTTER 707 A STREET ANCHORAGE,AK 99501 The biological effects of hydroelectric power development in streams in Norway are relatively poorly documented.No comple~e studies discribing the biological situation before and after hydroelectric developmen~have been carried out (Gunner¢d &Mellquist 1979). However,several studies describing the biolog~cal sit~ation after different kinds of hydroelectric development Qr~carried out during the last years (Jensen 1979,Li11ehammer 1979,Borgstr¢m 1981, ~kken et al.1981). The present study started in 1976,one year before hydroelec- tric development and building of weirs in the Skjoma (Heggberget 1977).This study continued in the years 1977 through 1980.The aim of the study was to analyse the density and growth of presmolt salmonids during the first years after hydroelectric development and establishing of the weirs. The study area The river Skjoma (68 0 15'N,17 0 30'E)is situated near Narvik, Northern Norway (Fig.1).In its unregulated condition,the catch- ment area of the Skjoma was 850 km 2 •Due to the hydroelectric development the catchment area was reduced by about 70%. NOROELV / o 2 3km ...---....1---11 S0RELV c;:Irz--~.,f----65 " Fig.1.Map of the investigated localities and position of the weirs in the Skjoma. In its natural condition the Skjoma was a glaciated river with cold and turbid water in summer.After the water abstraction, the glacier parts of the drainage area was deleted,and the stream ..-~,.#,--..of changed into warmer and less turbid condition in the summer.The mean flow was reduced from 31 m3 /sec to 8,7 m3 /sec,and the mean water temperatures increased by 1,8 oC in June,3,3 0 C in August and 2,7 oC in September.(Norwegian Water Resources Electricity Board -hydrology departement). The dominating fish species are Atlantic salmon (Salmo salar L.)and brown trout (Salmo trutta L.).There are small populations of arctic charr (Salvelinus alpinus L.),flounder (Platichtys flesus L.),threespined stickleback (Gasterosteus aculeatus L.)and eel (Anguilla anguilla L.). The Skjoma is accessible for upstream migrating fish for about 13 kID,but the reduced waterflow after the regulation has caused difficulties for upstreams migrating salmon.The river sport fishery for salmon has been reduced to a minimum after 1976. In the years 1976-1979 there were established three weirs in the river.The length of the weir basins varies from 1 to 3 kID. Artifical fishways are established through the weir dams. Methods and material The fish were sampled by electrofishing at 15 different localities, which were fished 3 periods each year in 1976,1977,1978 and 1980. The density of fish were computed by successive removal method (Zippin 1958)after three fishing runs.The habitat selection of the fish was described by recording distance from the river bank, water depth,water velocity and size distribution of the substratum for each fish caught. Electrofishingwas impossible in the deeper parts of the weir basins,and therefore some of the fish from these areas were sampled by using small nets. The material consisted of a total of 13740 fish (1976:.1359, 1977:6064,1978:2448 and 1980:3869).Most of the fish were re- leased after a super fica 1 examination,while some of the fish were conserved for further analyses (age,growth and stomach contents). Differences in habitat selection in salmon and trout were tested by using a chi-square test (Gutman and Wilks 1967). Results The density of salmon and trout increased from a mean of 8 fish/IOO m2 before,to 28 fish/IOO m2 after hydroelectric development (Fig.2).The main reduction in waterflow was accomplished during the first half of August 1977.Therefore,the results from 1977 are divided into two groups,before and after Au9~~·t 10:1977.The density of fish increased with the reduction of waterflow,indicat- ing that the fish were forced together in a narrow watercovered area,without any signs of mortality.Both before and after the .reduction of the drainage area,the density of fish was highest in the upper part of the stream.The mean density at the upper part (st.4-12)in 1977-80 was 36 fish/100 m2 while the corresponding numbers for the lower part (st'-.12-23)was 19 fish/100 m2 in the same period. 7 .-1976t~Zl45 2-Before 10/8-77J-,~W.er 1018-77~:\98~ Fig.2.Mean density of salmon and trout in the different parts of the river .in the research period.Sampling was carried out from July 10.to September 10.each year. The composition of the species varied both between and within the research localities (Table 1).The mean catch of salmon in- creased from about 38%in 1976 to about 62%in 1980.Within each station,there was great variation,but for most of the localities there was a clear tendency towards an increasing proportion of salmon.For st.11 and 21,which were transformed from river to weir basins in 1977 and 1979 respectively,there was a decreasing proportion of salmon following the building of the weirs.In 1976, the mean proportion of salmon was 38%,both with and without st. 11 and st.21.For 1980,however t the mean proportion of salmon was 62%for all the research localities,while the salmon consti- tuted 69%if sampling localities in the weir basins (st.11 &21) are deleted,thus indicating a tendency of reduction of salmon in the weir basins. Table 1.Percentage occurrence of s~lmon at different stations in the river Skjoma,1976-1980. Station " " II II " " II II II II " II Mean 4 5 6 7 9 11 12 13 15 17 18 19 20 21 23 1976 70 60 70 40 40 45 50 40 30 5 20 15 35 20 30 38 1977 80 60 20 10 60 15 90 80 45 10 10 45 15 15 45 42 1978 85 60 30 35 60 20 90 80 40 10 70 45 40 65 30 50 1980 70 80 90 75 60 15 90 70 65 40 . 70 70 60 15 60 62 Downstreams side of the weir. Weir basin finished 1977 • Downstream side of the weir. weir basin finished 1979. In addition to the main sampling stations (4-23),a number of locations in several parts of the weir basins were fished.In all the parts of the weirs basins,except a narrow zone along the weir dam,the trout was the dominating species (Table 2). A significant increase in growth was observed both for salmon and trout during the research period (Fig.3 and 4).There were minor differences between fish caught in 1976 and 1977,while 2+ and 3+salmon caught in 1980 were 1,8-2,3 em larger than fish of corresponding age caught in 1976-77.Similar growth differences were also observed for trout. Table 2.Percentage catch of salmon and trout and age composition from different biotope types with regard to the weirs. Salmon (%)Trout (%) Total 0+1+>1+Total 0+1+>1+ Sampling stations apart from the weirs 71,6 20,2 28,2 51,6 28,4 17,4 29,1 53,5 Sampling stations in the weir basins 16,5 1,4 58,1 40,5 83,5 3,1 17,1 79,8 Sampling stations along the weir dams 69,9 3,6 41,4 55,1 30,1 0,7 29,7 69,7 Sampling stations down streams the weirs 68,9 31,1 2 I 1976 1977 1978 1980 95 'I.confidence intervol 1+2+3+ Iv,]e Fig.3.The growth of trout in the years 1976,1977,1978 and 1980. Analyses of scales and otholits indicate that the growth in- crease started in the period 1978-1980.The oldest fish (3+,4+) caught in 1980 had a longer distance between the two last winter zones than between the previous ones. The growth increase indicates reduced smoltification age for salmon and trout in the river Skjoma.The frequency of 4+and older salmon caught in 1976 was 17,7%,while the frequency of 4+and '1 1976 -----1977 --1978 -'-'~1980I95".~onfiden~e inte'vo\ 0+1+3+·:..;. Fig.4.The growth of salmon in the years 1976, 1 9 7 7 , 1978 and 1980. older salmon in the 1980-catches was 7,4%.The corresponding numbers .for trout were 3,6%in 1976 and 0,9%in 1980. There were no systematic differences in growth between the different parts of the stream,neither in the weir basins nor in the parts of the river between the weirs when electrofished samples were compared.The fish caught by nets in the weir basins showed a better growth than fish sampled by electrofishing in the weir basins. This is due to nonrandom sampling of the stock,and indicates that growth of fish caught by nets and electric fishing gear should not be compared. Both the results from 1976 and 1980 showed significantly diffe- rent habitat selection in salmon and trout.The salmon was caught further from the river banks,at deeper water and at higher water velocities than the trout.The described segregation of these spe- cies when ocurring sympatrically,is a result of competition (Kar1- str¢m 1977,Heggberget 1977,1982).At the sampling localities between the weirs,there was a tendency towards more distinct segre- gation in 1980 than in 1976.This might be the result of the in- creased density of fish and hence increased interspecific competi- tion.In the weir basins (not including the area near the weir dam),however,significantly different segregation to the same degree between the two species was not observed.There was a ten- dency that most of the salmon was observed further from the banks than the trout,while the majority of trout was caught in deeper Table 3.Distribution of ..lmon and trout compared to dhtance from the banks,..ater depth,"ater velocity and substratum tested by chi-square test. plO.01~**piO.OS="S =Salmon T =Trout Station Distribution compared to 2 p Numbers of fish x Distance from the banks 1 m 46.24 ** 2 m 18.89 ** 4 10 em 23.32 **5 =169 Water depth 20 em 41.53 **T =91 30 em 13.83 ** 0.1 mls 60.53 ** Water vel ocl ty 0.2 mls 71.75 ** 0.3 mls 20.22 ** Substratum 30 em 2.38 Distance from the banks 1 m 62.24 ** 2 m 39.33 ** 5 10 em 34.00 **S =174 Water depth 20 em 36.03 **T =73 30 em 26.92 ** 0.1 mls 30.33 ** W~ter velocity 0.2 mls 66.91 .** 0.3 mls 15.76 ** Substrotum 30 em 2.12 Di stance from the banks 1 m 0.11 2 m 0.36 7 10 em 0.79 5 =31 Water depth 20 em 0.28 T =11 30 em 0.62 0.1 mls 0 Water velocity 0.2 mls 0 0.3 mls 0 Substratum 30 em 7.45 Distance from the banks 1 m 65.86 ** 2 m 24.27 ** 9 10 em 28.10 **S =190 Water depth 20 em 42.48 **.T =128 30 em 46.50 ** 0.1 mls 86.92 ** Water velocity 0.2 mls 42.48 ** 0.3 mls 14.38 ** Substratum 30 em 7.44 ** Distance from the banks 1 m 3.87 * 2 m 13.06 **Weir-10 em 0 S =20 basin Water depth 20 em 1006 T =39 C -A 30 em 0.23 0.1 mls 0 Water vel ocl ty 0.2 mls 0 0.3 mls 0 Substratum 30 em 0.72 Table 3,continued. Station Distribution compa'red to p Number$of fish Of stance from the banks 1 m 0.95 2 m 0.13 Weir 10 em 0.13 S •71 C -B Water depth 20 em 0.32 T ..12 30 em 0.17 0.1 mI$0.42 Water velocity 0.2 mi.0.52 0.3 mI$0.09 Substratum 30 em 5.99 * Distance from the banks 1 m 2.65 2 m 0 Welr-10 cm 0.40 S ..5 basin Water depth 20 em 0.93 T ..40 11 -A 30 em 0.21 0.1 mI$3.20 Water vel ocl ty 0.2 m/$0 0.3 mI$0 Substratum 30 em 2.18 Di stance from the bank.1 m 13.24 ** 2 m 3.22 Weir 10 em 21.77 **S"155 11 - B Water depth 20 em 16.50 **T ..56 30 em 1.31 0.1 mI$28.83 ** Water velocity 0.2 mI$35.74 ** 0.3 mI$21.79 ** Sub.tratum 30 em 1.90 Distance from the banks 1 m 34.09 ** 2 m 25.93 ** 12 10 em 11.65 **S •418 Water depth 20 cm 5.85 *T •44 30 em 2.74 0.1 mI$17.10 ** Water velocity 0.2 mi.30.30 ** 0.3 mI$18.44 ** Substratum 30 em 10.12 ** Olstance from the banks 1 m 35.74 ** 2 m 19.29 ** 13 10 em 12.60 **S ..165 Water depth 20 em 9.71 **T ..73 30 em 8.66 ** 0.1 mi.37.36 ** Water velocity 0.2 mI$14.04 **0.3 mI$0.44 Substratum 30 em 2.27 Distance from the banks 1 m 96.02 ** 2 m 16.36 ** 15 10 em 3.74 S ..136 Water depth 20 em 0.21 T ..107 30 em 3.75 0.1 mI$24.52 ** Water velocity 0.2 mI$32.31 ** 0.3 mls 19.84 ** Substratum 30 em 0.01 Table 3.conti nue. Stltlon Dlstrl butl on compared to 2 p Numbers of fish Distance f rom the bank!1 m 29.54 ** 2 m 32.90 ** 17 10 em 0.76 S =49 Water depth 20 em 0.01 T •78 30 em 0.35 0.1 mls 55.46 ** Water velocity 0.2 mls 68.93 ** 0.3 mls 48.75 ** Substratum 30 em 0.76 Distance from the banks 1 m 9.20 ** 2 m 11.68 ** Welr-10 em 6.51 *S·15 basin Water depth 20 em 0.73 T·28 11 - C 30 em 3.10 0.1 mls 0.07 Water vel ocl ty 0.2 mls 1.91 0.3 mls 0 Substratum 30 em 0.21 Distance from the banks 1 m 0.02 2 m 0.01 Welr-10 em 0.46 S =26 basin Water depth 20 em 0.07 T •27 21 -A 30 em 0.03 0.1 mls 0.12 Water velocity 0.2 mI,0.002 0.3 mls 0.98 Substratum 30 em 1.05 Distance from the bank,1 m 1.35 2 m 4.47 *Weir 10 em 6.06 *S·142 21 - B Water depth 20 em 0.46 T •129 30 em 0.12 0.1 mls 1.82 Water velocity 0.2 mls 0.59 0.3 mls 0.22 Substratum 30 em 0.78 OJ stance f rom the bank.1 m 0.07 2 m 0.07 Wei r-10 em 0.03 S •8 basin 'Water depth 20 em 1.09 T •227 21 - C 30 em 1.54 0.1 mls 0 Water velocity 0.2 mls 0 0.3 mi.0 Substratum 30 em 0.09 Olstanee from the bank.1 m 122.10 ** 2 m 107.45 ** 18 10 em 94.35 **S •211 Water depth 20 em 14.44 **T =99 30 em 0 0.1 mi.44.63 ** Water velocity 0.2 mi.115.60 ** 0.3 m/s 81.30 ** Substratum 30 em 0.09 TobIe 3.continued. Station Distribution compared to p Number.of fish Distance from the banks 1 ..5.57 * 2 m 8.66 ** 19 10 em 8.00 **S =131 Water depth 20 em 10.98 **T =80 30 em 8.93 ** 0.1 mi.11.66 ** Water velocity 0.2 mi.23.78 ** 0.3 mi.28.22 ** Substratum 30 em 0.02 Distance from the bank.1 ..3.12 2 m 12.95 ** 20 10 em 12.96 **S =93 Water depth 20 em 23.08 **T •63 30 em 18.25 ** 0.1 mi.30.23 ** Water velocity 0.2 mI.22.83 ** 0.3 mi.14.47 ** Sub.tratum 30 em 0 Distance from the bank.1 m 28.00 ** 2 m 39.67 ** 23 10 em 31.33 **S =227 Water depth 20 em 50.80 **T=162 30 em 17.17 ** 0.1 mI.122.27 ** Water velocity 0.2 mI.9S.77 ** 0.3 mi.74.53 ** Sub.tratum 30 em 0.94 water than the salmon (Table 3).Also along the weir dams-there were minor differences in habitat selection for salmon and trout, though there was a tendency towards more frequent observations of salmon at higher water velocities'than for trout (Table 3). Discussion The reduced water flow and reduced turbidity of the Skjoma have increased the catch effeciency in general and especially the catch- ability for salmon.As a result of the increased visibility in the water,the smaller fish (0+,1+)were easier to observe after than before the reduction of the river catchment area.Due to the reduced water level the midstream dwelling fish became more catchable by electrofishing after the hydroelectric development.Because of the susceptibility for salmon to be located further from the river banks than trout (Karlstr¢m 1977,Heggberget 1977),an increased electrofishing effeciency for salmon would be expected.The results are to some extent affected by the methods used,but is mainly a consequence ofa real increase of fish density. The density of presmolt sa1monides after hydroelectric develop- ment has increased by about twice to three times in the Skj~ma.At the same time,the permanent watercovered area of the stream has been reduced to about one third. The total production of presmolt salmonides is therefore not supposed to have been negatively affected by the flow reduction in the river.A positive effect is the growth increase in salmon and trout. The increase of water temperatures in ·the summers after the hydroelectric development in the Skjoma is due both to the reduction of the glacier fed areas and the hot summers in Northern Norway 1978-80 compared to 1975-76.The mean air temperatures in June, July and August 1977-1980 were 1,9 -3,3 0 C higher than the mean temperatures for the same period in 1975-1976 (Norwegian Meteoro- logical Institute).Growth analyses of presmolt salmonids in an- other glacier fed North Norwegian stream which was not regulated, showed a growth increase of 0,5-1 cm in the period 1978-1980.com- pared to that of the period 1975-1977 (Jensen &Johnsen 1982). The simultaneous increase of the fishes'growth and density is in contrast to the results from several other studies.Horton (1961),Le Cren (1965),(1973)and Mortensen (1975)showed that growth rates in presmolt salmon and trout were negatively corre- lated to fish density.Those studies were made in rivers which were not changed during the research period.Welch (1952)claims that high turbidity modifies productivity in general and Hynes (1970)states that high turbidity has a negativ photosyntetic ef- fect.This indicates that there has been an increase in the gene~ ral productivity in the Skjoma dur.ing the research period.The stomach contents of the fish were not changed qualitatively during the research period (Heggberget i982).Quantitative bottom or drift fauna studies have not been made in the Skjoma,but other studies in North Norwegian streams indicate increasing densities of bottom invertebrates with decreasing water flow (Koksvik 1977). A proportionally greater part of the drainage area consists of woodland (birch)after the hydroelectric development.Allochto- nous material from leaf woods affects the fish production in streams positively (Egg1ishaw 1964,1967,Lil1ehammer 1975).An'effect of the establishment of the weir basins might be an increased deposi- tion and a decreased outwashing of organic material and food orga- nisms (B~kken et ale 1981).It was observed an increased produc- tion of algae during the research period.Huntsman (1948)observed that increased growth og algae lead to increased density of pre- smolt streamdvelling salmon. The changes in the species composition between salmon and trout in the weir basins,are likely to be due to the different competetiveness under varying physical conditions.Several studies (Lindroth 1955,Kalleberg 1958,Karlstr~m 1977,Heggberget 1981) have shown that the trout is the more competetive at slow flow, while salmon is dominating at localities with higher water velo- cities.Also in the Skjoma there was observed a dominance of salmon midstream and in the fast-flowing parts of the river (Heggberget 1977).When establishing weir basins,the most striking physical change is the reduction of water velocity.The decline of salmon in the weir basins is therefore explained by the fitness for slow- flowing water in trout. Other studies (Lindroth 1965,Heggberget 1974,1981),indicate that in areas where the one of these species are absent,or occurs only in.small numbers,the dominating species tends to occupy and utilize microhabitats otherwise occupied by the other one.On this background it is possible that the trout will become even more dominating in the weir basins in the future,especially if station- ary populations of brown trout is developing in the weir basins after some time. Though the smolt production in the Skjoma i~general did not seem to be negatively affected by the hydroelectric development, the reduced water flow has reduced the ascent of spawners of salmon and trout.The upstream migration of spawners seem to be sufficient to maintain the reproduction,but it is not sufficient to maintain the formerly excellent sport fishery for atlantic salmon and sea trout in the Skjoma.The weir basins seem however to be important. resting places for the few large salmon and trout still entering the river. Acknowledgements Drs.Tor B.Gunner~d and Arnfinn Langeland kindly reviewed a pre- liminary version of the manuscript.This study was supported by the Directorate for Wildlife and Freshwater Fish,Norwegian Water Resources Electricity Board and the "Weir Project". References Borgstr¢m,R.1981:0rretbestanden i M¢rked¢la etter bygging av terskler.Terskelprosjektet,nr.17.NVE-Vassdragsdirektoratet 1981. 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