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The importance of a constructed near-nature-like Danube
fish by-pass as a lifecycle fish habitat for spawning,
nurseries, growing and feeding: a long-term view with
remarks on management
Paul Meulenbroek
A
,
C
,Silke Drexler
A
,Christoffer Nagel
B
,Michael Geistler
A
and Herwig Waidbacher
A
A
University of Natural Resources and Life Sciences, Vienna, Institute of Hydrobiology and Aquatic
Ecosystem Management, Gregor-Mendel-Strasse 33, A-1180 Vienna, Austria.
B
Technical University of Munich, Department for Ecology and Ecosystem Management, Chair of
Aquatic Systems Biology, Mu
¨hlenweg 22, D-85354 Freising, Germany.
C
Corresponding author. Email: paul.meulenbroek@boku.ac.at
Abstract. Major sections of today’s rivers are man made and do not provide the essential requirements for riverine fish.
A nature-like fish by-pass system in Vienna-Freudenau was assessed for its function as a fish habitat. The study was
conducted continuously over 3 years; 15 years after construction of the by-pass. The chosen nature-like construction of the
by-pass system functions like natural tributaries. More than 17 000 fish and 43 species, including several protected and
endangered species, in all life stages, including eggs, larvae, juveniles and adults, were captured. Furthermore, the
indicator species of the free-flowing Danube, nase (Chondrostoma nasus) and barbel (Barbus barbus), migrated into the
fish by-pass and successfully spawned before returning. Therefore, our results suggest that by-pass systems can function as
an important habitat for the conservation of native fish fauna. The heterogenic habitat configuration provides conditions
for all ecological guilds and, consequently, increases biodiversity. Finally, approved management tools are discussed. We
suggest that fish by-pass channels may be suitable at other sites in the Danube catchment.
Additional keywords: Barbus barbus, by-pass management, Chondrostoma nasus, cyprinids, large river.
Received 26 March 2018, accepted 2 August 2018, published online 17 October 2018
Introduction
Hydromorphological alterations for navigation, flood protec-
tion, hydroelectric power generation, as well as the disconnec-
tion of tributaries, have resulted in riverine habitat degradation
and fragmentation, especially in large rivers such as the Danube
(Schiemer 2000;Morley and Karr 2002;Dudgeon et al. 2006).
These habitat modifications affect the integrity and diversity of
freshwater biota (Karr et al. 1985;Allan and Flecker 1993;
Richter et al. 1996). Lack of functional spawning grounds,
nursery habitats and reduced connectivity are now considered to
be limiting factors for riverine fish populations (Keckeis and
Schiemer 2002;Jungwirth et al. 2003;Pander and Geist 2010;
Jungwirth et al. 2014).
According to the key objective of the European Water
Framework Directive (WFD), all waterbodies in the EU
need to achieve good ecological status. This is defined in
Annex V of the Water Framework Proposal as a slight departure
from the biological community (fish, benthic invertebrates
and aquatic flora) that would be expected in conditions
of minimal anthropogenic impact (European Parliament
and the Council of the European Union 2000). For WFD
implementation, among others, the Austrian legal framework
(NGP, Gewa¨sserbewirtschaftungsplan 2009) focuses on the
provision of a longitudinal migration for aquatic organisms by
installing fish by-passes. Investigations showed that a free
passage alone does not improve the ecological status of a river
satisfactorily in many cases (Schmutz 2012;Reyjol et al. 2014;
Harreiter et al. 2015). To improve connectivity, near-natural
fish by-passes provide passage for a wider range of species, age
classes and sizes and are, therefore, preferred over hard techni-
cal fish by-passes (Jungwirth et al. 1998;Calles and Greenberg
2007;Tummers et al. 2016).
There are many river restoration projects (e.g. the creation of
gravel banks, riparian bays and channel systems or lateral
connections of waterbodies) that attempt to create and restore
important key habitats for the different life stages of endangered
species such as spawning grounds, and larval and juvenile
habitats to strengthen fish populations (Schiemer and Waidba-
cher 1992;Barlaup et al. 2008;Pulg et al. 2013;Geist and
Hawkins 2016;Pander and Geist 2016;Zauner et al. 2016;
Meulenbroek et al. 2018;Waidbacher et al. 2018). Near-natural
by-pass solutions can provide both, namely, possibility of
CSIRO PUBLISHING
Marine and Freshwater Research, 2018, 69, 1857–1869
https://doi.org/10.1071/MF18121
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SPECIAL ISSUE
migration as well as the provision of the abovementioned key
habitats. There are limited studies, mostly focusing on salmo-
nids at by-passes in smaller rivers, showing this multifunctional
role for different species (Eberstaller et al. 1998;Parasiewicz
et al. 1998;Calles and Greenberg 2007;Gustafsson et al. 2013;
Pander et al. 2013;Tamario et al. 2018). However, to the best
of our knowledge, the present study is the first on a large river
such as the Danube (mean annual discharge 1910 m
3
s
1
,
Niedero¨sterreich 2018) that considers all life stages of fish
deriving from a broad range of species. Besides improved
connectivity, shown by Eberstaller et al. (2001) in 2000, there
have been first indications of possible additional benefits, such
as spawning activities during this time.
The objective of the present study is to assess the near-natural
by-pass system in Freudenau-Vienna as a habitat. We hypothe-
sised that the fish by-pass would provide habitat for spawning,
nurseries, growth and feeding. Furthermore, following the
principles of ecological guilds (Balon 1990;Schiemer and
Waidbacher 1992), the heterogenic configuration should pro-
vide conditions for different species compositions. Therefore,
we sampled fish larvae, juveniles, and adult fish and analysed
species occurrences and spatial and temporal differences of
assemblage structure.
Materials and methods
Study sites
The Hydropower plant (HPP) Vienna-Freudenau is the newest
HPP in the Danube (mean discharge 1910 m
3
s
1
) and was built
in 1998. A fish migration by-pass system was incorporated with
two major components, namely, a near-natural by-pass channel
and a near-natural pool pass (Fig. 1). The fish by-pass starts
500 m downstream of the HPP, with a delta system in the tail-
water that has calm, shallow waters over some 200 m with two
permanent wetted channels. The subsequent semi-natural
by-pass channel has an average slope of 0.7% and is situated in
a 7-m-wide riverbed with and an average current speed of
,0.6 m s
1
. The first 160-m length is straight (hereafter called
the straightened section), followed by a 300-m-long meandering
section (hereafter meandering section) and a 140-m-long bran-
ched section.
One of the branches is blocked by a beaver dam and has calm
to stagnant water for ,50 m (hereafter stagnant sidearm). The
remaining section of 170 m up to the weir is straight again. The
total length of this free-flowing section is ,1000 m. The channel
bottom was constructed with a 1-m-thick layer of gravel and
sand; subjacent is a 0.4-m-thick silt layer that seals the fish by-
pass. Some rifle-pool sequences are developed and very dense
riparian vegetation has been well established, consisting mainly
of willows and alders.
The uppermost part of the system is a pool pass of 19 pools
(20–40 m in length and 3–16 m in width), with a minimum of
70 m
2
per pool, a water-level difference of 11 cm from pool to
pool, and a total length of 420 m (hereafter pool pass). It is
characterised by a pool depth of 1.5 m, different flow conditions
(from reverse flow to velocities up to 1 m s
1
), high abundances
of reeds and macrophytes. There are big boulders (30–50 cm) at
the ramps between the pools and different substrate patterns,
ranging from gravel to very fine sediments with high quantities of
xylal. Sediments ranged from megalithal to pelal (O
¨norm-6232
1995;Fig. 2). The straightened and meandering sections were
rather similar, showing a high percentage (,80%) of lithal
(mega, macro, meso, microlithal) fractions. In contrast, the pool
pass and the stagnant sidearm exhibited higher percentages
(,65%) of finer fractions (Akal, Psammal and Pelal). At the
fishing points, water depth, river width and flow velocities were
measured (Fig. 3). Mean flow velocity was calculated following
Kreps (1975). For a detailed description of the by-pass, see
Eberstaller et al. (1998). The discharge of the by-pass is not
constant but changes depending on the discharge of the Danube
and the season, ranging from 1.5 m
3
s
1
in winter to a maximum
of 3.6 m
3
s
1
during higher discharges in the main river (Table 1).
Pool pass Stagnant sidearm Meandering section Straightened section River mouth
Tailwater
Flow direction
Impoundment
420 m
0.54%
50 m
0.26%
300 m
0.91%
160 m
0.93%
200 m
0.99%
Fig. 1. Fish by-pass system Freudenau. Triangles indicate fish larvae sampling points, circles indicate electrofishing points; length of
the sections (m) and slope (%) are given beneath each section name (adapted after Eberstaller et al. 2001).
1858 Marine and Freshwater Research P. Meulenbroek et al.
Field sampling, identification and data analyses
The main sampling campaign comprised point abundance
sampling by electrofishing (EF) of juvenile and adult fish (Copp
and Penˇa´z 1988) from January 2014 to December 2015. For EF,
the backpack-generator ELT60-IIH (Hans Grassl GmbH,
Scho¨nau am Ko¨nigssee, Germany) was used, according to the
code of practice and national standard in Austria (Haunschmid
et al. 2010). The generator operates with direct current at 1.3 kW
and 500 V. The same two points (,20 m
2
) in each section
(straightened section, meandering section, pool pass and stag-
nant sidearm) were fished approximately every 2 weeks, with a
total of 225 sampling events. A constant fishing effort was
applied for each sampling event and the catch per unit effort
(CPUE) was used for further analyses on the basis of these data.
Additional fishing was undertaken at selected habitats in 2013
and 2016 (22 sampling events). Fish larvae were sampled from
April to September 2015 with the same drift nets as described by
Meulenbroek et al. (2018), in 27 sampling events at the fol-
lowing three different locations: one at the beginning of the pool
pass, one at the end of the pool pass and one at the end of the
straightened section (triangles in Fig. 1). Fish larvae samples
were taken approximately once per week, always during the
night with an exposure time of 12 h (CPUE). All juvenile and
adult specimens were identified to species level by using mor-
phological characters (Wiesner and Zauner 1999), counted, and
their total length was measured. Early life stages of fish were
identified to family level and further processed as described in
Meulenbroek et al. (2018). A subsample of 560 individuals was
analysed with mt-DNA barcoding to species level (Hebert et al.
2003). The selection criteria for the individuals to be barcoded
were the abundance within each family found in a sampling site
for each calendar week and the potential number of species. The
latter represents the proportions of potential species for each
family (Cyprinidae: 32 species; Gobiidae and Cottidae: 5 spe-
cies; Percidae: 9 species; compare with Meulenbroek et al.
2018). The calculated number of individuals for barcoding was
then randomly selected by the randomise tool in Excel (2016).
The primers FishCo1-F and FishCo1-R (Baldwin et al. 2009)
were used, and for some individuals we also used the cyto-
chrome bprimers KAI_F and KAI_R (Kotlı´k et al. 2008).
Straightened
0
20
40
Cumulative percentage
60
80
100
Meandering Stagnant Pool pass
Megalithal Macrolithal Mesolithal Microlithal Akal Psammal Pelal
Fig. 2. Choriotop distribution (according to O
¨norm-6232) at four different
sections of the by-pass where fish sampling was performed.
⫺0.8
0
Depth (m) v mean (m s⫺1)
12345678910 012345678910
012345678910
⫺0.6
⫺0.4
⫺0.2
0.2
0.4
0.6
0.8
1.0
0
⫺0.8
Depth (m) v mean (m s⫺1)
⫺0.6
⫺0.4
⫺0.2
0.2
0.4
0.6
0.8
1.0
0
⫺0.8
⫺0.6
⫺0.4
⫺0.2
0.2
0.4
0.6
0.8
1.0
0
⫺0.8
⫺0.6
⫺0.4
⫺0.2
0.2
0.4
0.6
0.8
1.0
0
Straightened Pool pass
012345678910
Meandering Stagnant
River width (m)
Mean velocity (m s⫺1)
Water depth (m)
Fig. 3. Water depth, river width (m) and mean flow velocity (v mean, m s
1
; after Kreps 1975) for each of the sampled sections
(straightened section, meandering section, stagnant sidearm and pool pass).
Nature like fish by-passes as lifecycle habitat Marine and Freshwater Research 1859
The affiliation to guilds followed Schiemer and Waidbacher
(1992), as well as Zauner and Eberstaller (1999), and was
expanded for Neogobius melanostumus,Ponticola kessleri,
Babka gymnotrachelus and Lepomis gibbosus (compare with
Table 2;Kottelat and Freyhof 2007). Kruskal–Wallis test was
performed using SPSS (IBM SPSS Statistics for Windows, ver.
24.0, IBM Corp., Armonk, NY, USA), followed by a Dunn–
Bonferroni post hoc test. Significance was accepted at P,0.05.
Frequency-of-use graphs (FUG) were calculated as normalised
probability density functions ranging from 0 to 1 (Raleigh et al.
1984;Melcher and Schmutz 2010):
FUGi¼Ri
CR½maxð1Þ
where R
i
is the class frequency and R
[max]
is the maximum class
frequency.
To visualise spatial differences, 95% confidence intervals
(Sachs 2004), histograms, line plots and a Venn diagram were
compiled. Non-metric multidimensional scaling (NMS; Kruskal
1964) was applied to the ecological guild data (densities were
ln(xþ1) transformed, for juveniles and adults separately).
Indicator species analysis (ISA; Dufreˆne and Legendre 1997)
identified fish species and life stages that serve as indicators for
different habitats and seasons. Only species with an indicator
value (IV) of .25 and P,0.05 (Monte Carlo permutation test)
were considered. NMS and ISA were performed with PC-ORD
(ver. 5.33, MjM Software, Gleneden Beach, OR, USA). Length–
frequency diagrams were used to illustrate population structure,
showing the relationships between length classes and relative
frequency.
Results
During the entire study period, 17 200 fish were caught, com-
prising 6800 adults, 3900 juveniles and 6500 larvae. In total, 43
species from 12 families were detected with spatial and seasonal
variations.
Habitat function
Most of the Danube fish species used the system at all life stages
every year (compare with Table 2). There is an apparent increase
in numbers of species from downstream (17) to upstream (29;
Fig. 4). The straightened section is characterised by a relatively
low number of species (17) and has the highest proportion of
rheophilic species (58% of juveniles, 69% of adults), such as the
nase (Chondrostoma nasus) and the barbel (Barbus barbus). The
subsequent meandering section provides habitats for at least 27
species, with an increased proportion of eurytopic specimens
(49% of juveniles, 54% of adults). In the meandering section, we
caught all species that occurred in the straightened section in
similar quantities, plus additionally 10 further species. Stagno-
philic species such as European bitterling (Rhodeus amarus),
three-spined stickleback (Gasterosteus aculeatus), rudd (Scar-
dinius erythrophthalmus) or tench (Tinca tinca) were almost
exclusively caught in the pool pass (5% of juveniles, 8% of
adults) and in the stagnant sidearm (22% of juveniles, 35% of
adults). The latter exhibits the highest proportion of this guild.
Furthermore, hundreds of juvenile cyprinid fish used the dam
cavities for overwintering.
The pool pass was dominated by eurytopic species (86% of
juveniles, 77% of adults). There were highly significant differ-
ences in the distribution of adult and juvenile fish for the
different flow guilds among sections in the by-pass (Kruskal–
Wallis; P,0.05). The post hoc test showed the following
significant differences (Dunn–Bonferroni test, P,0.05): fish
communities differed between the straightened section and
stagnant sidearm for all guilds; the straightened section and the
pool pass differed for all guilds except for rheophilic B; the
meandering section and the stagnant sidearm differed for all
guilds except for rheophilic A, whereas only the stagnophilic
showed significant differences between the meandering sec-
tion and the pool pass; when comparing the stagnant sidearm
and the pool-pass, only rheophilic B and eurytopic guilds
differed.
The results of the NMS analysis (Fig. 5), presented as joint
plot (cut-off value r
2
¼0.3), showed a relationship between
riverbed slope and ecological traits of juvenile and adult fish
species. On the basis of the NMS scatterplot, samples from
sections with a higher slope were noticeably separated from the
remaining samples. Samples from the straightened and
meandering section were close to each other on the NMS
scatterplot, exhibiting a rather similar faunal composition. In
contrast, samples from the pool pass and stagnant sidearm were
predominantly on the right side of the dashed line. Fifteen
species were caught in all sections, whereas four species
occurred only in the stagnant sidearm, one only in the straight-
ened section, three only in the Meandering section, and three
only in the Pool pass (Fig. 6). Overlaps indicated the number of
species caught in more than one section (e.g. 22 species were
caught in both the stagnant sidearm and the pool pass).
The indicator species analyses (Table 3) showed some fish
species that serve as indicators for the different sections of the
Table 1. Work regulation of the minimum values for the variable discharge in the fish by-pass system depending on the discharge of the Danube and
the season (adapted after Renner 2012)
Season Discharge of the Danube (m
3
s
1
) Weir flow (L s
1
) Pool pass (L s
1
)PBy-pass stream (L s
1
)
Winter ,3000 600 900 1500
(Dec.–Feb.) .3000 3100 500 3600
Spawning season ,2000 900 900 1800
(Mar.–May) .2000 3100 500 3600
Summer ,3000 900 900 1800
(Jun.–Nov.) .3000 3100 500 3600
1860 Marine and Freshwater Research P. Meulenbroek et al.
by-pass. For the pool pass, those were Neogobius melanostomus,
Alburnus alburnus, adult individuals of Squalius cephalus and
Gasterosteus aculeatus, and juvenile Chondrostoma nasus. The
best indication was given by Neogobius melanostomus (IV:
juvenile ¼55.2; adult ¼50.2). For the straightened section, only
adult Cottus gobio was listed. For the meandering section, no
species met the chosen indicator species criteria (IV .25,
Monte Carlo permutation test: P,0.05). For the stagnant
sidearm, the best indicator species were Rhodeus amarus (IV:
juvenile ¼55.7; adults ¼63.9), followed by juvenile Squalius
cephalus and adult Proterorhinus marmoratus. Eleven species
were detected throughout the whole year in the system (chub
(Squalius cephalus), trout (Salmo trutta), bullhead (Cottus
gobio), bleak (Alburnus alburnus), roach (Rutilus rutilus),
Table 2. Presence or absence of fish species at adult, juvenile, larval and egg stage for the entire study period (2013–2016)
EN, endangered; VU, vulnerable; NT, near threatened, according to Wolfram and Mikschi (2007). The affiliation to ecological guilds (habitat and
reproduction) follows Schiemer and Waidbacher (1992), as well as Zauner and Eberstaller (1999) and was slightly expanded for Neogobius melanostumus,
Ponticola kessleri,Babka gymnotrachelus and Lepomis gibbosus (compare with Kottelat and Freyhof 2007). These categories are denoted by: A, Schiemer and
Waidbacher (1992);B,Zauner and Eberstaller (1999). eury, euryotopic; limn, limnophilic; rheo, rheophlic; pel, pelagophil; phyt, phytopil; lith, lithophil; psam,
psammophil; ostrac, ostracophil; speleo, speleophil
Family Species Guild Life stage
Habitat Reproduction Adult Juvenile Larvae Eggs
Anguillidae Anguilla anguilla eury. pel. x
Centrarchidae Lepomis gibbosus limn. phyt. x
Cobitidae Cobitis taenia
A
, VU rheo. B phyt. x
Cottidae Cottus gobio
A
, NT rheo. A lith. x x
Cyprinidae Abramis brama rheo. B phyt./lith. x
Alburnoides bipunctatus rheo. A lith. x x
Alburnus alburnus eury. phyt./lith. x x x
Aspius aspius
A
, EN rheo. B lith. x x x
Ballerus sapa EN rheo. B lith. x x x
Barbus barbus NT rheo. A lith. x x x x
Blicca bjoerkna eury. phyt./lith. x x
Carassius gibelio eury. phyt. x
Chondrostoma nasus NT rheo. A lith. x x x x
Cyprinus carpio EN eury. phyt. x
Gobio gobio rheo. A psam. x
Leuciscus idus EN rheo. B lith. x x x x
Leuciscus leuciscus NT rheo. A phyt./lith. x x x
Pelecus cultratus
A
, NT eury. pel. x
Rhodeus amarus
A
, VU limn. ostrac. x x x
Romanogobio vladykovi
A
rheo. A lith. x x
Rutilus pigus
A
, EN rheo. A lith. x x
Rutilus rutilus eury. phyt./lith. x x x
Scardinius erythrophthalmus limn. phyt. x
Squalius cephalus eury. lith. x x x x
Tinca tinca VU limn. phyt. x
Vimba vimba VU rheo. B lith. x
Gasterosteidae Gasterosteus aculeatus limn. phyt. x x
Gobiidae Babka gymnotrachelus limn. speleo. x x
Neogobius melanostomus eury. speleo. x x x x
Ponticola kessleri eury. speleo. x x x x
Proterorhinus marmoratus EN eury. speleo. x x x x
Lotidae Lota lota VU eury. lith./pel. x
Nemacheilidae Barbatula barbatula rheo. A psam. x
Percidae Perca fluviatilis eury. phyt./lith. x x x
Sander lucioperca NT eury. phyt. x x x
Sander volgensis EN rheo. B phyt./lith. x
Zingel zingel
A
, VU rheo. A lith. x x
Gymnocephalus cernuus eury. phyt./lith. x
Salmonidae Hucho hucho
A
, EN rheo. A lith. x
Oncorhynchus mykiss rheo. A lith. x
Salmo trutta NT rheo. A lith. x x
Thymallus thymallus VU rheo. A lith. x
Siluridae Silurus glanis VU eury. phyt. x x x
A
Listed in Annex II of the Flora–Fauna–Habitat Directive (Richtlinie-92/43/EWG 1992).
Nature like fish by-passes as lifecycle habitat Marine and Freshwater Research 1861
spirlin (Alburnoides bipunctatus), round goby (Neogobius mel-
anostomus), barbel, bitterling, three-spined stickleback, and
nase). Some species, such as grayling (Thymallus thymallus),
gudgeon (Gobio gobio), Danube salmon (Hucho hucho), carp
(Cyprinus carpio), stone loache (Barbatula barbatula) and
sichel (Pelecus cultratus), were detected only once or very
rarely in catches. However, there were also seasonal differences
in the occurrence of species. In general, the lowest abundances
Rheophilic B
100%
90%
80% 697
711
2
5
8
70%
60%
50%
40%
30%
20%
10%
Juvenile
Stangnant sidearm (27)
Adult Juvenile
Pool pass (29)
Adult Juvenile
Meandering section (27)
Adult Juvenile
Straightened section (17)
Adult
0%
Relative abundance
6
Limnophilic
1
9
8
3
5
5
8
2
142
9
8
4
2
7
8
3
Eurytopic Rheophilic A
Fig. 4. Relative abundances of flow guilds and number of species for the given sections. Bars indicate
relative abundance of flow guilds for juvenile and adult of all caught fish; numbers within each category
show the number of species per guild; numbers in parenthesesindicate total numberof species per section.
Axis 1
Axis 2
Habitat
Straightened
Meandering
Stagnant
Pool pass
Slope (%)
Rheophilic A (A)
Rheophilic A (J)
Rheophilic B (J)
Rheophilic B (A)
Limnophilic (A)
Limnophilic (J)
Eurytopic (J)
Eurytopic (A)
Fig. 5. Joint plot of the non-metric multidimensional scaling analysis for fish samples of the by-pass
system Freudenau (n¼193 fishing events); vector (joint plot cut-off value r
2
¼0.3) represents the slope
(in %), stress: 10.87 for the three-dimensional solution (number of iterations ¼500). Stars indicate scores
for each ecological guild (juveniles and adults are separated).
1862 Marine and Freshwater Research P. Meulenbroek et al.
were found for most species during winter (November–
February), except for bleak, Danube whitefin gudgeon
(Romanogobio vladykovi) and dace (Leuciscus leuciscus),
which inhabited the system numerously in this period. This
seasonal pattern is clearly illustrated in Fig. 7, which illustrates
the frequency of species occurring in the by-pass for adult
individuals of Barbus barbus (most abundant from May to
September), Alburnus alburnus (most abundant from September
to January), Chondrostoma nasus (maximum in April) and
Romanogobio vladykovi (most abundant from October to
December). On the basis of these data, the only significant (P
#0.05) indicator species identified as colonising the by-pass
system in spring was the adult stages of Chondrostoma nasus.In
summer, the adult stages of Barbus barbus and Squalius
cephalus were identified as indicator species. Adult Neogobius
melanostomus and juvenile Chondrostoma nasus were indicator
species in autumn, and adult Cottus gobio and Alburnus albur-
nus were indicator species in winter (Table 3).
There were noticeable differences for all caught nase for the
four investigated sections (Fig. 8), specifically the following:
(1) length classes from 200 to 350 mm were highly
underrepresented;
(2) juveniles were found in all four sections, with higher
abundance in the stagnant sidearm and especially the pool
pass; and
(3) larger individuals (.350 mm) were almost exclusively
found in the meandering and straightened sections.
Spawning and fish larval drift
Nase showed a distinct seasonal pattern. In both years, the adult
individuals migrated in high numbers into the fish by-pass at the
beginning of April and remained there for ,4 weeks. The nase,
together with chub, which were most frequent in May, and
barbel in July, were the most frequently found species.
Spawning activities were observed multiple times, especially in
riffle sections, for these species (Fig. 3). A single pool and one
riffle section were fished carefully quantitatively during a single
spawning event, showing massive spawning runs of nase. In the
pool, which had a surface area of 30 m
2
, 44 adult individuals
were caught with a mean weight of 1.5 kg. A further 10 indi-
viduals were caught in the adjacent 20-m
2
riffle section. Esti-
mates of fish biomass calculated from these data equated to 22 t
of fish per hectare in the pool and 7 t of fish per hectare in the
riffle section. We collected a total of 6557 fish larvae,
Table 3. Monte Carlo permutation test of significance of observed
maximum indicator value (IV) for each species (IV .25 and P,0.05)
for the different habitats within the by-pass system and seasons, based
on 1254 randomisations and 4999 permutations (compare Dufre
ˆne and
Legendre 1997)
Species Life stage Habitat IV Mean s.d. P-value
Cottus gobio Adult Straightened 30.6 14.8 3.13 0.0006
Rhodeus amarus Adult Stagnant 63.9 8.0 2.93 0.0002
Rhodeus amarus Juvenile Stagnant 55.7 6.4 2.90 0.0002
Squalius cephalus Juvenile Stagnant 40.4 14.4 3.42 0.0002
Proterorhinus
marmoratus
Adult Stagnant 27.8 5.8 2.50 0.0002
Neogobius
melanostomus
Juvenile Pool pass 55.2 9.6 3.21 0.0002
Neogobius
melanostomus
Adult Pool pass 50.2 17.6 3.88 0.0002
Chondrostoma nasus Juvenile Pool pass 35.1 15.5 4.13 0.0014
Squalius cephalus Adult Pool pass 33.4 22.9 3.10 0.0058
Alburnus alburnus Adult Pool pass 33.3 18.9 5.68 0.0200
Alburnus alburnus Juvenile Pool pass 32.4 10.5 4.11 0.0004
Gasterosteus aculeatus Adult Pool pass 31.6 8.0 3.12 0.0002
Chondrostoma nasus Adult Spring 46.2 16.5 3.44 0.0002
Barbus barbus Adult Summer 30.0 16.2 3.10 0.0012
Squalius cephalus Adult Summer 32.7 22.7 2.97 0.0064
Neogobius
melanostomus
Adult Autumn 25.1 17.4 3.67 0.0392
Chondrostoma nasus Juvenile Autumn 32.0 15.2 3.89 0.0024
Cottus gobio Adult Winter 32.6 14.6 2.97 0.0006
Alburnus alburnus Adult Winter 37.9 18.7 5.26 0.0026
Jan
0
0.2
0.4
0.6
FUG
0.8
1.0
B. barbus A. alburnus C. nasus R. vladykovi
Feb Mar Apr Ma
y
Jun Jul Au
g
Sept Oct Nov Dec
Fig. 7. Seasonal frequency-of-use graphs for 2014 and 2015 (frequency-
of-use-graph) of the by-pass for adult individuals of Barbus barbus (mean
standard deviation: s¼0.07), Chondrostoma nasus (s¼0.02), Alburnus
alburnus (s¼0.09) and Romanogobio vladykovi (s¼0.12).
Straightened section (17)
Stagnant sidearm (27)
Meandering section (27)
1
4
1
5
15
2
1
3
3
3
Pool pass (29)
Fig. 6. Venn diagram summarising the relations between the different
sections and the number of fish species (early life stages of fish were
excluded). Each circle represents one section; overlaps indicate number of
species caught in more than one section.
Nature like fish by-passes as lifecycle habitat Marine and Freshwater Research 1863
representing 22 species, from three sampling points. The fol-
lowing results presented a clear picture of the spatial distribution
and spawning-guild composition:
(1) in the first pool, a mixed set of fish larvae drifted into the
system (n¼2465);
(2) the sampling point downstream at the end of the pool pass
(n¼2734) was dominated by speleophilic (75–85%) and equal
shares of lithophilic and phytophilic (1–16%) species; and
(3) in contrast, in the stream section (n¼1358), most of the
larvae caught consisted of lithophilic species (55–66%),
followed by speleophilic species (26–39%; Fig. 9).
These pronounced differences were also reflected in family
and species compositions at the three sampling sites (Table 4). In
the uppermost pool of the pool pass, the majority of the caught
fish larvae were from European catfish (Silurus glanis: 37%),
followed by invasive Gobiidae, namely the round goby
(Neogobius melanostomus: 20%) and bighead goby (Ponticola
0
50–100 100–150 150–200 200–250 250–300
Length classes (mm)
Individuals (n)
300–350 350–400 400–450 450–500 ⬎500
50
100
150
200
250
Stangnant sidearm Pool pass Straightened section Meandering section
Fig. 8. Length–frequency of nase (Chondrostoma nasus) for four different sections of the fish by-pass
system.
0
First pool
Relative abundance (%)
Last pool Stream
10
20
30
40
50
60
70
80
90
Lithophil
Phyto/lithophil
Phytophil
Speleophil
Fig. 9. Confidence intervals (95%; Sachs 2004) on relative abundance of
all caught fish larvae appointed to their spawning guild for each of the drift
net sampling points (compare with Fig. 1); ostracophilics were excluded.
Table 4. Relative distribution (%) of all caught fish larvae species and
families separated for all sampling sites
n, 560 barcoded larvae of 6571 caught larvae
First pool Last pool Stream
Cyprinidae 24.5 9.3 68.1
Alburnus alburnus 1.2 0.4 1.2
Aspius aspius
A
1.3 0.7 5.6
Ballerus sapa 0.1
Barbus barbus
A
14.3 3.2 25
Chondrostoma nasus
A
3.3 2.1 16.9
Leuciscus idus
A
0.4 0.2 0.6
Leuciscus leuciscus 0.2 0.2
Rhodeus amarus 1.4 0.4
Romanogobio vladykovi 0.3
Rutilus rutilus 2.4 0.5 1.3
Rutilus virgo 0.1
Squalius cephalus
A
1.3 0.1 17
Gobiidae 34.1 83.5 24.7
Babka gymnotrachelus 2.4
Ponticola kessleri
A
10.9 21 8.5
Neogobius melanostomus
A
20.8 62.5 15
Proterorhinus marmoratus
A
1.2
Percidae 4.1 3.4 7.1
Gymnocephalus cernuus 2.4 0.1
Perca fluviatilis 0.1 1.5 0.6
Sander lucioperca 1.3 1.6 2
Zingel zingel 0.3 0.2 4.6
Siluridae 37.4 3.8
Silurus glanis
A
37.4 3.8
A
Additional, drifting eggs were genetically confirmed.
1864 Marine and Freshwater Research P. Meulenbroek et al.
kessleri: 11%). In addition, barbel formed a relatively high
percentage (14%) of the fish larvae caught. The remaining
12 species were found in lower frequencies. The species infor-
mation from the last pool and the stream presented a contrasting
picture: whereas the stream was dominated by cyprinids (68%),
the pool pass clearly showed high proportions of Gobiidae
(84%). Of the 22 species of fish larvae collected, nine were also
detected by mt-DNA analysis of drifting eggs (Table 4).
Discussion
Our observations and the occurrence of the majority of the
Danube fish species from different life stages validates our
hypothesis that the fish by-pass provides habitat for spawning,
nurseries, growing and feeding for a wide range of species. In
total, 43 species deriving from 12 families were detected. These
included eight species classified as endangered, eight species
classified as vulnerable and a further seven species classified as
near threatened for Austria (Wolfram and Mikschi 2007). On a
European scale, nine species are listed in Annex II of the Flora–
Fauna–Habitat Directive (Richtlinie-92/43/EWG 1992) and,
consequently, locations where these species occur must be
managed in accordance with the ecological needs of the species.
Furthermore, the occurrence of various life stages of protected
species such as Aspius aspius,Barbus barbus,Chondrostoma
nasus,Leuciscus idus,Proterorhinus marmoratus,Rhodeus
amarus,Ballerus sapa or Rutilus pigus highlighted the impor-
tance of such fish by-passes for their conservation. In total, the
43 verified species represent three-quarters of all species that
were sampled in all habitat assemblages of the Viennese Danube
waterbodies in 2013–2016 (Waidbacher et al. 2016). Most of the
missing species are rare in the area, such as Acipenser ruthenus,
Misgurnus fossilis,Romanogobio kesslerii or Ballerus ballerus.
It remains unclear why other species, such as Gymnocephalus
schraetser,Esox lucius or Zingel streber do not inhabit the fish
pass, and why the abundance of top predators seems to be low.
Whether the sampled juvenile and adult fish originate from
downstream or also from upstream sections remains uncertain
because no traps were used. In a monitoring study conducted by
Eberstaller et al. (2001) in 2000, the downstream migration was
evaluated as negligible, mainly consisting of a few juvenile
individuals of Alburnus alburnus and Blicca bjoerkna. These
authors also found that mainly ‘indifferent’ fish species, espe-
cially the bleak, white bream, European roach, vimba and zobel,
traverse the entire by-pass system into the impoundment. Only a
few individuals of stagnophilic species were detected; however,
they are also rare in the tailwater of the power plant. During the
spawning season in spring, nase and barbel migrate into the
bypass channel in high abundance. Whereas barbel frequently
ascends into the impoundment via the pool pass, comparatively
few nase traverse the entire system. These authors concluded
that the Freudenau bypass channel can be classified as broadly
functional (Eberstaller et al. 2001).
Habitat function for fish species, young-of-the-year classes
and adults
The taxonomic composition and distribution of the fish fauna
varied among the different sections and seasons, and it is likely
that this was related to the high variability of the habitat con-
ditions (such as, for example, water depth, flow velocities and
substrate). This is in line with one of the key elements of ecol-
ogy, namely, that habitat heterogeneity increases biodiversity
(Ricklefs and Schluter 1993). Additionally, large organic debris
is often added from the well-stocked riparian zone, which also
has a positive effect on the richness of biota (Crook and
Robertson 1999;Dossi et al. 2018). Noteworthy is the stagnant
sidearm, which is clogged by a beaver dam, with its calm-water
conditions that rarely exist in by-passes. This section supple-
ments the range of available habitats and this is reflected in the
proven fish community, which shows a high proportion of
stagnophilic species. The increase in species number from the
straightened section to the meandering section can be explained
by the complex hydrodynamics of convex and concave riv-
erbanks in short succession that produce the sequence of shal-
low, calm-flowing habitats and deep fast-running sections. This
fast-changing sequence produces a variety of essential habitat
types in immediately adjacent spots. (Gorman and Karr 1978;
Garcia et al. 2012).
The pool pass differs substantially in its habitat specifications,
by providing deeper areas with low flow velocity, large boulders
at the ramps between the pools, and a well-established riparian
vegetation with high proportions of reed and different substrate
patterns, ranging from gravel to very fine sediments with a high
component of xylal. It shows an abundance of the invasive round
goby (Neogobius melanostomus), which prefers the above-
mentioned large boulders or riprap structures (Ahnelt et al.
1998;Borcherding et al. 2013;Brandner et al. 2015;Meulenbroek
et al. 2018). The reed belt provides shelters andhabitats for small
species and masses of young-of-the-year fish from all guilds.
It remains unclear whether the apparently under-represented
length classes (200–350 mm) of Chondrostoma nasus were
caused by either
(1) the low attractiveness of the by-pass system for these length
classes, or
(2) the limited abundance of these length classes in the tailwater
of the main river channel.
Evidence in support of the second point comes from the
monitoring of the by-pass entrance conducted by Eberstaller
et al. (2001) who reported that nase in the 200–350-mm length
classes were migrating into the system in the Year 2000, and
from our survey of the Danube from 2013 to 2015, in which
these length classes were under-represented in the main channel
(Waidbacher et al. 2016).
Little is known about a self-sustaining Salmo trutta popula-
tion within the river Danube, but this species is considered rare
(Schiemer and Spindler 1989). It is worth mentioning that some
of the caught individuals are most likely to derive from stocking
activities, indicated by body pigmentation and deformations of
gills and fins (Arndt et al. 2001;Aparicio et al. 2005), and some
from autochthonous populations. In total, we caught 48 Salmo
trutta individuals throughout the years and seasons, ranging
from 100 mm to 350 mm in total length. Most of the adults were
caught in late autumn, which corresponds to their
spawning season, whereas the presence of some smaller indivi-
duals in summer indicated that reproduction had occurred in the
system.
The peaks of Chondrostoma nasus and Barbus barbus
are linked to their spawning seasons, whereas those of
Nature like fish by-passes as lifecycle habitat Marine and Freshwater Research 1865
Romanogobio vladykovi and Alburnus alburnus indicate their
use of the system as a winter habitat. The latter was also
confirmed in the indicator-species analyses. The provision and
accessibility of winter habitats are essential for fish communi-
ties, especially in highly degraded river systems (e.g. Schlosser
1995;Cunjak 1996).
Spawning function and fish larvae drift
The observed migration of the indicator species of the free-
flowing Danube, nase and barbel, and their multiple spawning
acts within the fish by-pass are comparable to those described in
natural streams and tributaries of the Danube (Keckeis 2001;
Ovidio and Philippart 2008;Melcher and Schmutz 2010) and
highlight the quality of the fish by-pass system as a functional
spawning habitat.
In total, the 22 genetically verified species of fish larvae
represent nearly half of all species sampled in the by-pass. This
does not necessarily mean that the others do not reproduce there,
because they might either spawn at other sites or avoid drifting
(Reichard and Jurajda 2007). Artificially built systems often
provide functional spawning grounds (Pander and Geist 2016;
Meulenbroek et al. 2018), which can be assessed by the occur-
rence of early life stages of fish (Pavlov 1994). The differences
in composition and abundance of larval species among the three
sampling points are particularly pronounced and indicate a
locally separated reproduction of different fish species.
Most of the catfish larvae (Silurus glanis) in the first pool
were caught on a single day together with catfish eggs. This
indicates that spawning took place in the area upstream of the
first net. Other species found in the first net drifted in a balanced
distribution and were derived from somewhere in the Danube
upstream. Not all drifting larvae were collected in the first net;
some of the species by-passed the first net and were found in the
second net in the last pool of the pool pass. The large differences
between the two sampling points demonstrated the contribution
of the stretch between as a reproduction area. As mentioned
above, the high proportion of speleophilic species (in particular
Gobiidae) at the second sampling point originates from the rock
habitats found in ramps and ripraps within the pool pass. The
repeated capture of Rhodeus amarus larvae indicated the occur-
rence of mussels, which are a prerequisite for the reproduction of
this ostracophilic species (Mills and Reynolds 2003).
The third and most downstream larval sampling point in the
stream section was clearly dominated by lithophilic cyprinids,
primarily bynase, barbel and chub. This showed that the observed
spawning acts resulted in successful reproduction. In comparison
to a larval-drift investigation undertaken several kilometres
upstream (Meulenbroeket al. 2018), additional species of drifting
larvae (Silurus glanis,Proterorhinus marmoratus,Ballerus sapa
and Romanogobio vladykovi) were detected only in the described
fish by-pass. Further investigations are needed to provide a clear
explanation for this. However, the distribution of the caught
larvae in the stream section of the by-pass is comparable to that
of gravel bars in the remaining free-flowing Danube and its
tributaries. The distribution of larvae caught in the pool pass is
more similar to that of riprap sections in the main channel
(Lechner et al. 2010,2014;Melcher and Schmutz 2010;Ramler
et al. 2016;Meulenbroek et al. 2018).
Management aspects
Even though close-to-nature types of fish passes are easier to
maintain (Food and Agriculture Organization of the United
Nations and the Deutscher Verband fu
¨r Wasserwirtschaft und
Kulturbau e.V. 2002), these artificial systems need continuous
management to function sustainably. Besides maintenance of all
technical facilities, ecological maintenance needs to be imple-
mented. Currently, the plant operator needs to ensure a free
passage of fish by an official notification. This comprises mainly
the yearly removal of beaver dams and log or driftwood jams
(Renner 2012).
In general, higher discharge provides better passage, but it
needs to be in accordance with the morphology of the fish pass
(Food and Agriculture Organization of the United Nations and
the Deutscher Verband fu
¨r Wasserwirtschaft und Kulturbau e.V.
2002). The critical swimming capacity (Plaut 2001) of different
species and life stages needs to be taken into account to facilitate
a complete passage through the by-pass. Furthermore, the
stability of the spawning habitats must be guaranteed for nearly
4 weeks for successful spawning to occur (Hauer et al. 2007). In
2016, a high discharge occurred for a long time shortly after
migrating nase arrived, resulting in zero catches and the loss of a
whole generation of young-of-the-year fish (P. Meulenbroek
and H. Waidbacher, unpubl. data). Hydrological disturbances or
dynamic floods are still recommended as they can ‘clean up’ the
interstitial of the sediments, which enhances the habitat not only
for fish and egg development but also for other organisms such
as macroinvertebrates (Dudgeon et al. 2006;Dole-Olivier 2011)
or biofilms (Boulton 2007). Implementing this ‘cleaning up’ of
the sediments before the migration season, which is not linked to
a particular date but more to water temperature, discharge and
other parameters (Northcote 1984), is recommended.
Another fact that should be considered is the deepening of the
by-pass riverbed. After 17 years of operation, the whole system
deepened by an average of 24 cm, resulting in a loss of more than
3000 m
3
of gravel (Hagel and Westermayr 2016). Compensation
is required to ensure system stability and to fill up the developed
washouts, which can impede some species from swimming
upstream and we recommend implementing this at 10-yearly
intervals. Gravel addition could also create or improve suitable
spawning grounds (Pulg et al. 2013). This demonstrates the
absolute need for continuous management actions to secure the
positive ecological values for fish and other riverine faunal
elements.
Conclusions
Most species from the Austrian Danube were observed using a
man-made by-pass and some have accepted the surroundings as
habitats for different life stages. The diversity of species and
sizes of the colonised fish, as well as the evident reproduction of
some, correspond to a situation in a natural sidearm or tributary
(Haunschmid et al. 2006). Therefore, the fish by-pass may serve
as a key habitat for the conservation of a variety of endangered
species. Furthermore, we have shown that the heterogenic
configuration provides conditions for different ecological guilds
and, consequently, increases biodiversity. The spatial extent of
by-passes is limited in comparison to the degradation and dis-
connection of former habitats, and the habitat quality of these
1866 Marine and Freshwater Research P. Meulenbroek et al.
artificial systems may be lower than that of natural habitats.
Nevertheless, near-nature fish by-passes have high potential as a
remediating or mitigating measure (Quigley and Harper 2006;
Tamario et al. 2018) and this was clearly visible in the present
study. Future studies should focus on the influence ofnear-nature
fish by-passes on the population size, population dynamics, and
the production of offspring of the protected and endangered fish
species in the context of the lost habitats and fish populations in
the river system. However, such artificial systems need to be
managed continuously to function sustainably.
Almost 60% of all Austrian waterbodies are affected by
interruption of river continuity, whereby, in larger rivers
(.100 km
2
catchment), 26% derives from hydropower plants.
More than 70% of these facilities are not passable at present
(Gewa¨sserbewirtschaftungsplan 2015). Until now, the focus for
the implementation of fish by-passes has mostly been to enable
migration corridors. Accepting the Danube as an originally
braided river with highly diverse habitats (Hohensinner et al.
2013), a systematic approach to the creation and connection of
habitats is necessary to improve the ecological situation of such
a system under pressure and to achieve the requirements
formulated in the EU–WFD. Especially in highly modified
waterbodies, the provision of functioning spawning and juvenile
habitats are two of the most essential tasks to strengthen the
remaining fish stocks (Keckeis and Schiemer 2002;Pander and
Geist 2010;Jungwirth et al. 2014;Waidbacher et al. 2018). In
planning river modifications, interruptions, or by-passes, the
ecological functioning of these key habitats must be
incorporated.
Conflicts of interest
The authors declare that they have no conflicts of interest.
Acknowledgements
The authors thank C. Dorninger for technical assistance and P. Rauch,
D. Stadler, W. Westermayr and T. Hagl who assisted with field work.
H. Meulenbroek improved the English. S. Krumbo¨ ck and V. Waidbacherare
thanked for laboratory assistance. The study was partly financed and enabled
by Verbund Hydro Power GmbH; special thanks go to Roswitha Renner and
Mr Ringel.
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