ECOLOGIA BALKANICA2019, Special Edition 2 pp. 107-115

Ichthyofauna of the Iskar River Section Affected by theHydropower Cascade „Middle Iskar”

Luchezar Z. Pehlivanov*, Stefan A. Kazakov

Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences,2 Major Yurii Gagarin Str., Sofia, BULGARIA

*Corresponding author: luchezarpehlivanov@gmail.com

Abstract. The study aimed to assess the impact of a cascade of 5 small hydropower plants on thecomposition and structure of the recent fish communities in the affected area of Iskar River. Thefield surveys cover the river sections up- and downstream the 5 dams, the dam lakes and the fishpasses of the dams. Fifteen fish species were recorded in the dam lakes. Limnophylous andeurytopic species predominate the communities. The high species richness and the presence oflarge-size fishes can be considered as an indirect indication of relatively stable conditions in thedam lakes. Eleven fish species were found in the studied river stretches. The ecological status ofthese river sections assessed by standardized fish indices were determined as Good and High. Boththe abundance and size-age compostion of some type-specific species correspond to “Favorable”conservation status. Totally 8 fish species were recorded to migrate up- and downstream throughthe fish passess. The most intensive upstream migration was found at the end of May when 2 to 5species migrated through different dams.

Key words: HPPs, Fish communities, Iskar River, Dams, Fish passes, Ecological state.

Introduction The effects of construction and operation of

hydropower facilities over the fish communitiesin rivers are widely discussed because of theirwidespread and increasing numbers asrenewables. The main reported impacts arerelated to the hydro-morphological pressure:barrier effect (river fragmentation and breakingthe migration corridors), change of hydrologicaland morphological features (depths, currentvelocities, sediment’s granulometry etc.), waterabstraction (LUCAS & MARMULLA, 2000; LUCAS

& BARAS, 2001; STEINMETZ & SUNDQVIST, 2014,VASSILEV et al., 2016; UZUNOVA et al., 2017, etc.).Although the general impacts of the smallhydropower plants (HPPs) on the river fishcommunities are well known the specific effectsare quite variable depending on the river

typology, fish zone, types of the HPP (run-of-the-river hydroelectrics or diverted),constructive features of the facilities and somefurther pressures.

The aim of this study was to establishthe current state of fish community in a semi-mountain river zone after construction of acascade of 5 small run-of-the-river HPPs andto check the use of the fish passes at the damsof HPPs by fishes for upstream migrations.So far no data on these issues have beenpublished for this hydroelectric facility, andfor Bulgaria they are too scarce as a whole.

Materal and MethodsStudy area. Iskar River is the longest

right Danube tributary on Bulgarian territoryand the only one which crosses the Balkan

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Ichthyofauna of the Iskar River Section Affected by the Hydropower Cascade „Middle Iskar”

Mountain through a gorge 84 km long. Alongthe gorge, its middle course is a high-flow,fast-flowing river. The “Middle Iskar” HPPcascade is built in the middle part of theIskar’s gorge. The cascade covers about 30km along the river including 5 HPPs(downstream): “Prokopanik”, “Tserovo”,“Lakatnik”, “Svrajen” and “Opletnya” (Fig.1). The distance between the HPPs“Prokopanik” and “Tserovo” is about 6 km,those between “Tserovo” and “Lakatnik” –about 16 km while the 3 HPPs “Lakatnik”,“Svrajen” and “Opletnya” are immediatelyadjacent consequtively. Two tributariesinflow in the Iskar River close to the cascadearea. The Batuliyska River inflows upstreamthe Prokopanik dam lake and theGabrovnitsa River inflows downstream theOpletnya HPP.

Fig. 1. Location of the hydropower cascadeand the surveyed HPPs.

Study was carried out in 2017 and 2018comprising spring, summer and autumnseasons. The surveys cover three types ofhabitats within the studied area: 1) dam lakesof the HPPs; 2) free sections of the Iskar Riverbetween the HPPs and these situatedimmediately upstream and downstream thecascade and 3) fish passes at the dams alongwith surveys on the passage of fish upstreamthrough the fish passes. Fish were sampledusing different methodology and toolsrelevant to both the concrete task and thehabitat features as follows:

In the dam lakes fish were sampledby multimesh gillnets and fish traps whichwere put during the night time. Theprocessing of fish samples in situ included:species identification of the caught fish,measuring their individual total length (TL)and determination of the number and thetotal weight of each species;

In the targeted free river sectionsmultihabitat fish sampling with electricitywas performed by wading according CENEN 14011 in accessible zones using backpackelectrofishing devices. The accessible riversection between the HPPs „Lakatnik“ and„Svrajen“ is quite short and does not allow tofind two separate sampling sites (one –downstream „Lakatnik“ and other –upstream „Svrajen“) and that‘s why there isonly one site. No sampling sites were alsofound between the HPPs „Svrajen and„Opletnya“ since there is no free river sectionat all (the dam lake of HPP “Opletnya”begins immediately downstream the fishpass of HPP “Svrajen”). The processing offish samples in situ included: speciesidentification of the caught fish, measuringtheir individual total length (TL) anddetermination of the number and the totalweight of each species;

In the fish passes fish were sampledusing electrofishing. Fish caught in thedifferent sections of each fish pass (lower,middle and upper) after species identificationand determination of the individual lengthwere counted and released in the river or inthe dam lake;

For checking upstream fishmigrations through fish passes speciallyconstructed fish trap (Fig. 2A) was putagainst the upper exit of the fish pass (Fig.2B). After several trial catches the fish trapswere then put only during the night time.The processing of fish samples in situincluded: species identification of the caughtfish, measuring their individual length (TL)and determination of the number of eachspecies.

The ecological state of the surveyed riversections was assessed by Biological Quality

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Element (BQE) Fish using the intercalibratedType-specific Bulgarian Fish Based Index(TsBFI) (Adapted method for BQE Fish inrivers of national types R2, R4, R7, R8 Typespecific Bulgarian Fish Based Index (TsBFI),2016) in 5 grades as defined by the OrdinanceH-4/2012. Furthermore the status of typespecific and indicator species was assessedaccording the approach used in the NationalSystem for Monitoring of Biological Diversity(National System For Monitoring ofBiological Diversity, 2014).

Fig. 2. Fish trap for studying the upstreamfish migrations through the fish passes.Upper – general view; Lower – working

position. Results and Discussion Fifteen fish species of 5 familes were

found in the dam lakes of the cascade, most ofthem native for the studied area accordingthe historical data (DRENSKI, 1921; PASPALEV

& PESHEV, 1958). The species composition indifferent dam lakes was almost the same(Table 1). Roach R. rutilus characterizes withhigher abundance in the fish communities ofdam lakes sharing about 60% of fish caught.It is remarkable however the presence ofVimba bream which is considered as arheophylous migrating species.

Four species are considered alien for thestudied area: L. gibbosus and P. parva are alienspecies in Bulgarian ichthyofauna, reportedfirst for the Danube and adjacent wetlands;currently widespread all over the country,manly in standing waters and slow-flowingrivers; C. gibelio is considered native in theLower Danube region. It was considered aninvader upstream along the tributariesalthough in the middle stream of the Iskar itsstatus (native/alien) is unclear; A. brama is anative species for Bulgaria but it is not typicalfor the middle stream of the Iskar River. Mostlikely it was translocated in the Middle Iskardam lakes. A special case is with the Carp C.carpio, considered a native species in theDanube basin but currently in the MiddleIskar it is probably presented by stockedculture form instead a wild form presented inthe past.

A total of 11 fish species of 3 Familieswere found in the surveyed free river sections(Table 2). The fish communities there werepredominated by few rheophylous speciesalthough some eurytopic and evenlimnophylous species are presented.

The same three alien fish species werefound in the river sections but presented withvery low abundance (Table 2).

Ecological state of the surveyed riversections has been determined according theindicative ichthyological parameters as High orGood (Table 3) but the assessment of the sectiondownstream the HPP “Lakatnik” should beconsidered only as approximate because of thebig depth and the fast current the zone wherestandard electrofishing by wadding could beapplied is very restricted. Standard samplingwith electricity was also inapplicabledownstream the HPP “Svrajen“. The highestvalues of the index TsBFI were calculated for the

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Ichthyofauna of the Iskar River Section Affected by the Hydropower Cascade „Middle Iskar”

longest free river section between the HPPs “Prokopanik” and “Tserovo” (Table 3).

Table 1. Species composition of the fish communities in the dam lakes of surveyedHPPs. Legend: * - alien species for the region.

FamilySpecies

Dam lakesProkopanik Tserovo Lakatnik Svrajen Opletnya

EsocidaeEsox lucius Linnaeus, 1758 + + + + +CentrarchidaeLepomis gibbosus (Linnaeus, 1758)* + + + + +CyprinidaeAbramis brama (Linnaeus, 1758)* + + + +Alburnus alburnus (Linnaeus, 1758) + + + + +Barbus petenyi Heckel, 1852 + + + + +Carassius gibelio (Bloch, 1782)* + + + + +Cyprinus carpio Linnaeus, 1758 + + + + +Gobio gobio (Linnaeus, 1758) + + + + +Pseudorasbora parva (Temminck & Schlegel, 1846)*

+ + + + +

Rhodeus amarus (Bloch, 1782) + + + + +Rutilus rutilus (Linnaeus, 1758) + + + + +Squalius cephalus (Linnaeus, 1758) + + + + +Vimba vimba (Linnaeus, 1758) + + +PercidaePerca fluviatilis Linnaeus, 1758 + + + + +SiluridaeSilurus glanis Linnaeus, 1758 + + +

Table 2. Composition of the fish communities in the river sections along the studiedarea. Legend: Pr – Prokopanik, Ts – Tserovo, La – Lakatnik, Op – Opletnya, DL – dam lake,HPP – Hydropower Plant, (Up) – upstream, (D) – downstream; Ab – abundance: N –ind./ha, B – kg/ha. Legend: * - alien species for the region.

FamilySpecies

Ab Pr DL(Up)

Pr HPP(D)

Ts DL(Up)

Ts HPP(D)

La DL(Up)

La HPP(D)

Op HPP(D)

Fam. CyprinidaeAlburnoides bipunctatus(Bloch, 1782)

N 860 680 833 3045 2160 1067 3400B 3.900 6.360 6.042 12.727 11.840 4.667 15.000

Alburnus alburnus N 80 750 400 75B 0.960 3.545 0.733 0.225

Barbus petenyi N 720 1720 1708 1454 7820 133 2375B 9.420 44.400 57.250 19.954 11.160 1.333 24.975

Carassius gibelio* N 120 125 23B 5.880 1.083 0.045

Gobio gobio N 200 320 167 636 160 600 375B 2.200 5.600 2.208 3.590 0.560 2.867 3.700

Pseudorasbora parva* N 42 67B 0.375 0.467

Rutilus rutilus N 640 750 68B 1.600 13.167 0.068

Rhodeus amarus N 40 267B 0.120 0.200

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FamilySpecies

Ab Pr DL(Up)

Pr HPP(D)

Ts DL(Up)

Ts HPP(D)

La DL(Up)

La HPP(D)

Op HPP(D)

Squalius cephalus N 520 750 909 880 1200 300B 7.900 44.083 10.727 11.040 14.000 9.975

Fam. Centrarchidae Lepomis gibbosus* N 80

B 2.320Fam. CobitidaeCobitis elongatoides Bacescu & Mayer, 1969

N 50B 0.250

Table 3. Ecological state of the surveyed Iskar River sections according TsBFI.

Site – river section Value of TsBFI EQR Ecological stateUpstream DL “Prokopanik” 50 0.71 GoodDownstream HPP “Prokopanik” 74 1 HighUpstream DL “Tserovo” 68 1 HighDownstream HPP “Tserovo” 57 0.83 HighUpstream DL “Lakatnik” 50 0.71 GoodDownstream HPP “Lakatnik” 64 0.97 HighDownstream HPP “Opletnya” 63 0.96 High

In most of the surveyed river sectionsthe numbers, the biomass and the sizestructure of some common type-specific andindicator fish species, such as: S. cephalus, B.petenyi, A. bipunctatus and G. gobio,correspond to “Favorable” ecological state.High abundance and biomass, as well as thegood age structure of A. bipunctatus and B.petenyi are remarkable as far as they are anindicator species for this river zone andfurthermore B. petenyi is a species enlisted inthe Annex 2 of the Habitat Directive.

The conservation status of B. petenyi inthe river section downstream the HPPs“Lakatnik” was assessed as “Unfavorable”according the parameters Abundance,Biomass and Age structure. Although asmentioned above the results for this riversection are only approximate this is anexpected result taking in account themodified habitat of this species there.

The most intensive upstream fishmigrations through the fish passes wererecorded in May-June when as it is wellknown the most intensive spawning occurs(KARAPETKOVA & ZHIVKOV, 1995) butupstream movements occurred until the endof August even outside the spawning period

(Table 4). During the period 27.05-01.06upstream fish migrations were recordedthrough the fish passes of all 5 dams but ofdifferent intensity (presented as fishspecimens per night). As one can see (Table 4),the highest number of migrating speciesoccurred at the both “Lakatnik” and“Opletnya” dams while only one species B.petenyi was recorded to migrate through thefish pass at the HPP “Tserovo”. The highestintensity of migration through the fish passwas recorded at the HPP “Opletnya” and thelowest one – at the HPP “Svrajen. Later theintensity of the upstream migration of fishsharply decreased and only single specimensof 2-3 species were found to move upstreamthrough the fish passes at the three lowerdams (Table 4).

The biggest proportion amongst the fishesmigrating upstream through all the fish passesshared B. petenyi (39 – 100%), A. bipunctatus (1,7– 24%) and S. cephalus (1,5 -32%).

Fish were found in the fish passesthroughout the period from May to Augustand as the water temperature drops in theautumn all they leaved this habitat. Thequantitative features of fish into the fishpasses were surveyed during the period of

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most intensive upstream fish migration(27.05-01.06.2018 г.). In three out of fivesurveyed fish passes fish were sampledtwice – before and after checking of theupstream migration. In the other 2 of themsampling by different reasons wasperformed only after finishing the checkingand removal of the fish trap. During thesampling some solid wast (plastic and

wooden debris) blocking the fish pass wereremoved.

The highest species richness was recordedin the fish pass of HPP “Lakatnik” (Table 5)including typical rheophlyous species (i.e., A.bipunctatus, B. petenyi) but also somelimnophylous and eurytopic species, such as: A.alburnus, C. gibelio, G. gobio, L. gibbosus, R.amarus, R. rutilus, S. cephalus.

Table 4. Intensity of the upstream movements of fishes through the fish passes as fishspecimens per night during the studied period.

Dam Prokopanik Tserovo Lakatnik Svrajen Opletnya

Number of specimens per night

Date

Species 27.0

5-01

.06

27.0

5-01

.06

27.0

5-01

.06

10-1

1.06

29-3

1.08

27.0

5-01

.06

29-3

1.08

27.0

5-01

.06

10-1

1.06

A. bipunctatus 4 1 3 91A. alburnus 5 4 1B. petenyi 47 35 23 1 6 1 5 285 2B. barbus 1G. gobio 8L. gibbosus 1S. cephalus 1 19 1 5 1 6 1V. vimba 3 2Total number 53 35 59 2 15 2 8 386 3

Table 5. Number of fishes found in the fish passes. legend: 1 – at the evening (beforeputting the fish trap); 2 – on the next morning (after the removal of the fish trap).

HPP Prokopanik Tserovo Lakatnik Svrajen OpletnyaSampling 1 2 1 2 1 2 2 2Species Fish numberA. bipunctatus 7 52 98 25 10 199 35A. alburnus 11 1 10 454 346 69B. petenyi 33 4 212 76 42 18 293 26C. gibelio 30C. nasus 1G. gobio 16 10L. gibbosus 5 12 1P. fluviatilis 1R. amarus 8R. rutilus 4 57 9S. cephalus 2 6 77 51 19 1V. vimba 2 10 8 10 1Total number 51 13 296 688 563 166 522 62

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After the sampling and removing of fishfrom the fish pass before the fish trapputting, the next morning in the fish passwere recorded 6 species the most abundantbeing rheophylic and highly mobile species,such as: A. alburnus (64%), A. bipnctatus(16%) and B. petenyi (9%).

Only in the fish pass of HPP “Opletnya”was found the typical migrating speciesChondrostoma nasus even if presented onlywith single specimens against with the highproportions of both the B. petenyi (65%) andA. bipunctatus (28%).

As the water temperature drops in theautumn all the fishes leave the fish passes.

As can be seen by the obtained results,the species typical for standing and/or slowflowing waters predomined in the speciescomposition of fish communities in the damlakes what is an expected result taking inaccount the hydrological features. Most ofthe recorded species are native inhabitants ofthis part of the Iskar River but thehydromorphological modifications createmore favorable conditions for thelimnophylous and eurytopic species. Thepopulations of some of these species whichare atypical for the Iskar gorge area probablydeveloped after their introduction here forrecreational fishing or as a result ofunintentional translocation when stockingwith game species.

Currently the alien species for the areashare about 18% of the total fish communityabundance, with Prussian carp sharing thehighest propotion (12,7%) and the Stonemorocco – the lowest (< 1%) what suggeststhat the expectations for the rapiddevelopment of invasive alien species in thedam lakes of the cascade are not justified sofar.

The diversity of the species compositionand and the presence of big-size fishes (suchas: Carp, Prussian carp, Perch, Pike and Welscatfish) could be cosidered as an indirectsign for stable environmental conditions inthe dam lakes.

The fish communities in the surveyedfree river sections are predominated by

native species typical for this section of theIskar River (DRENSKI, 1921; PASPALEV &PESHEV, 1958; KARAPETKOVA, 1994). Thecomparison with the available historical data(DRENSKI, 1921; PASPALEV & PESHEV, 1958)shows reducing of the species richness ofnative fish community in the area of theIskar Gorge along the years together withappearance of some new species. Howeverthe absence of recent published data doesnot allow evaluate the specific impact of the“Middle Iskar” hydropower cascade amongthe complex of anthropogenic pressures andimpacts in this area. Eventually despite thepresences of some species indicative for daminfluence (such as, R. rutilus) and of 3 alienspecies in the river fish communities theyobviously play negligible role in thecommunity structure because of their verylow abundance. This result refutes somepredictions for significant increase of theimportance of these species in the nativeriver communities within the area of thecascade.

The Good and High ecological state ofthe surveyed river sections determined byBQE Fish and the Favorable conservationstatus of the type-specific species suggestrelatively little impact of the HPP facilitieson the fish communities in the river sectionsbetween the dam lakes if these river sectionsare more than 5 km long as well as on theseupstream and downstream the cascadewhen there are less affected tributaries likethe rivers Batuliyska and Gabrovnitsa.

Twelve species occurred in the fishpasses in the spring-summer periodrepresent a wide range of ecological guilds –from typical rheophylous to more or lesseurytopic. Although the obtained resultsgive a reason to conclude that during thenight fish from the river actively colonize thefish pass moving upstream only part of themmainly typical rheophylous species realizeeffective upstream migration passingsuccessfully from the lower to the upperwater level of the dams. The rest of speciesmost probably use the fish passes astemporary habitat and stepping stone bio-

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Ichthyofauna of the Iskar River Section Affected by the Hydropower Cascade „Middle Iskar”

corridor until the end of summer. Theseasonal dynamics of the migrationobviously is related to the reproductivebehavior of river fishes. Although therheophylous and mobile speciespredominate in the composition of migratingfishes in all fish passes of the cascade boththe species composition and the intensity ofmigration are quite different in the differentfish passes. The available data do not allowus yet to speculate concerning the reasonsfor the different features of the upstreammovements of fish through the fish passes ofthe “Middle Iskar” HPP cascade. Thereforefurther investigations on these issues arevery recommendable.

ConclusionsDifferent fish communities composed

mainly of native species are formedcorresponding to the two main types ofhabitats (dam lakes and river zones) in theIskar river section affected by thehydropower cascade “Middle Iskar”. Thetwo communities are not strongly isolatedbut clearly distinguish by the dominatingspecies complexes. The influence of the damlakes’ fish communities over the river one ismanifested through spreading of speciesonly in restricted river sections both up-anddownstream.

The fish passes are a transient habitatused temporary by river fish species duringthe spring-summer period when upstreammovements of fish occur. Rheophylous andsome eurytopic species use successfully thefish passes for upstream migrations whichare most intensive during the spawningperiod but occur also later in summer.Significantly wider range of species (incl.more eurytopic species) uses the fish passesas temporary habitats and stepping stonebio-corridor moving upstream. Impairedfunction of the fish passes occurs whenfloating plastic waste and/or wooden debrisblock the upstream exit or the submergedorifices of the fish pass.

Currently no indications of worseningneither of the ecological state of the “free”

river sections longer than 5 km between theHPPs and both up- and downstream the“Middle Iskar” hydropower cascadedetermined through standard fish basedindices nor of the status of type-specificspecies inhabiting there. A certainimportance for maintaining good ecologicalstate of the Iskar River sections up- anddownstream the hydropower cascadeprobably have the good condition of thetributaries. Negative effect of damconstruction over the rheophylous speciesoccurs only in highly modified zonesimmediately downstream the dams in caseof cascade construction where no enoughlong river sections exist.

Acknowledgements. This work waspartially supported by the Bulgarian Ministryof Education and Science under the NationalResearch Programme “Young scientists andpostdoctoral students” approved by DCM #577 / 17.08.2018 and partially - by the Instituteof Biodiversity and Ecosystem Research in theBulgarian Academy of Sciences under theproject "Changes in the composition, structureand functioning of freshwater and marineecosystems affected to a different degree byhuman activity - Phase 2".

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Received: 30.09.2019Accepted: 13.12.2019

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