Effects of a Small Hydropower Station Upon Brown Trout Salmo Trutta L. in the River Hoz Seca (Tagus Basin, Spain) One Year after Regulation

Abstract

A small hydroelectric power station was built in 1993 on the River Hoz Seca (Tagus basin, central Spain). Pre- and post-regulation studies provided the opportunity to test the early effects of this disturbance on the brown trout Salmo trutta L. population. Before and after comparisons of population density and biomass, age composition, growth and production were made upstream and downstream of the diversion dam. The effects of disturbance on benthic macroinvertebrates were also analysed but no changes in abundance were detected. The downstream estimated population densities and biomass of trout showed a decrease of about 50 and 43%, respectively, following regulation. Examination of length-for-age tables revealed no obvious change in growth but a significant difference in age structure. The main consequence of the imposed fluctuating flow regime was a serious reduction in trout production caused by a loss of suitable habitat and a loss of juveniles. Copyright © 1999 John Wiley & Sons, Ltd.

Full text

REGULATED RIVERS: RESEARCH & MANAGEMENT
Regul. Rivers: Res.Mgmt.
15:
477—484 (1999)
Short Communication
EFFECTS OF A SMALL HYDROPOWER STATION UPON BROWN
TROUT SALMO TRUTTA
L. IN THE RIVER HOZ SECA (TAGUS
BASIN, SPAIN) ONE YEAR AFTER REGULATION
ANA ALMODÓVAR* AND GRACIELA G. NICOLA
Department of Ecology, Agricultura) Research Institute of Madrid (IMIA), El Encín, PO Box 127, E-28800 Alcalá de Henares,
Madrid, Spain
ABSTRACT
A small hydroelectric power station was built in 1993 on the River Hoz Seca (Tagus basin, central Spain). Pre- and
post-regulation studies provided the opportunity to test the early effects of this disturbance on the brown trout
Salmo
trutta
L. population. Before and after comparisons of population density and biomass, age composition, growth and
production were made upstream and downstream of the diversion dam. The effects of disturbance on benthic
macroinvertebrates were also analysed but no changes in abundance were detected. The downstream estimated
population densities and biomass of trout showed a decrease of about 50 and 43%, respectively, following regulation.
Examination of length-for-age tables revealed no obvious change in growth but a significant difference in age
structure. The main consequence of the imposed fluctuating flow regime was a serious reduction in trout production
caused by a loss of suitable habitat and a loss of juveniles. Copyright © 1999 John Wiley & Sons, Ltd.
KEY WORDS:
hydropower;
Salmo trutta; Tagus basin
INTRODUCTION
Flow modification is one of the most widespread human disturbances of stream environments. Discharge
regime and the related physical effects are modified with increasing frequency by catchment management,
especially by activities such as river regulation and transfer, drainage works, afforestation and deforesta-
tion (Milner
et al.,
1981). In general, the most adverse effects of flow regulations are likely to result from
substantial, intermittent flow variations periodically exposing large arcas of the channel bed (Brooker,
1981). Bain
et al.
(1988) and Travnichek
et al.
(1995) identified these artificial flow fluctuations from
hydroelectric dams as a disturbance that can degrade fish habitat and reduce the complexity of the fish
community. It seems obvious that the effects of flow peaking on aquatic systems below dams are
i
mportant considerations in hydropower development and the management of regulated rivers (Heggenes,
1988).
Despite all major rivers in Spain being regulated by more than 1100 dams (Nicola
et al.,
1996), to date
there have been few attempts to study the consequences of river regulation upon fish communities (García
de Jalón
et al.,
1988; Casado
et al.,
1989; Camargo and García de Jalón, 1990; García de Jalón
et al.,
1994). Furthermore, Blanco and González (1992) and Elvira
(1996)
considered dams to be one of the
main negative factors affecting Spanish fishes. Specifically, Almodóvar and Burgaleta (1993) considered
water regulation as an important cause of the decline of brown trout in Spain.
The Hoz Seca is the first tributary of the River Tagus and supplies the greatest proportion of flow to
this upper part of the basin. Since autumn 1993 this stream has been regulated by a small hydropower
station
(700
kW). This paper evaluates the impacts of this recent disturbance on an indigenous brown
trout population. In order to assess changes in the trout population related to alterations in their food
* Correspondence to: Department of Ecology, Agricultural Research Institute of Madrid (IMIA), El Encín, P.O. Box 127, E-28800
Alcalá de Henares, Madrid, Spain.
CCC 0886-9375/99/050477-08$17.50
Received 14 May 1998
Copyright © 1999 John Wiley & Sons, Ltd.
Revised 19 March 1999
Accepted 23 March 1999

478
A. ALMODÓVAR AND G.G. NICOLA
supply, the benthic communities were also analysed, since macroinvertebrates are important food items
for brown trout (Elliott, 1967).
METHODS
Study area
This study was conducted on the lower reaches of a first-order tributary of the River Tagus. The River
Hoz Seca has a basin area of 175.76 km' and its altitude ranges from 1620 m at the yource to 1250 m at
its confluence with the Tagus. The mean discharge is highest in winter (2 m
3
s -') and decreases in summer
(0.4
m
3
s -'). The river flows over a limestone catchment with an average channel slope of 12.7%. The
water chemistry can be characterised as a non-polluted headstream, with low concentrations of organic
matter and high levels of dissolved salts. The ionic balance shows a dominance of HCO3 (266 mg L -',
534 tS cm' conductivity) and Ca
e
± (40 mg L -'). The water temperature ranges from a low of 8°C in
winter to a high of 14°C in summer, with a pH of 7.6. Stream-bed material was dominated by boulders
and bedrock. Substantial proportions of gravel and sand were also present. The availability of cover for
fish was high, mainly due to boulders. The bank vegetation at the site was dominated by some riparian
deciduous vegetation
(Salix
spp.,
Rosa
spp.,
Prunus spinosa
L.,
Crataegus monogyna
Jacq.,
Rubus
ulmifolius
Schott. and
Berberis hispanica
Boiss.
& Reuter) and pine
(Pinus sylvestris L.).
The aquatic
vegetation consisted primarily of
Chara vulgaris
L.,
Groenlandia densa (L.)
Fourr.,
Zannichellia contorta
(Desf.) Chamisso & Schlech,
Ranunculus peltatus
Schrank and
Cratoneurum commutatum
(Hedw.) Roth.
Brown trout is the only fish species present in this stream, which has never been stocked. Two sampling
sites
were chosen (see Figure 1), with similar sizes and habitat conditions; site 1 was located near the
mouth of the river, about 500 m downstream of the diversion dam; site 2 was a reference sector situated
approximately 3 km upstream of the power plant.
Figure 1.
Map of the study area showing sampling sites and the position of the diversion dam
Copyright © 1999 John Wiley & Sons, Ltd.
Regul. Rivers: Res.
Mgmt.
15:
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(1999)

EFFECTS OF A SMALL HYDROPOWER STATION UPON BROWN TROUT
479
Figure 2.
Mean hourly discharge per day in the downstream section of the River Hoz Seca with minimum (June and September)
and maximum flow periods (November)
The hydroelectric power station in the River Hoz Seca produces frequent and strong daily flow
fluctuations. This effect is especially notorious in autumn when the daily fluctuations range from 0.4 to
1.7
m s -' on average, in spite of being the period of the year with maximum flow (Figure 2). Thus, the
water depth in the downstream site increases 0.3 m on average in a matter of minutes whenever the
discharge from the power station arrives there, continually changing the location of the shoreline.
Benthic macroinvertebrates
Benthic macroinvertebrates were sampled in riffles every two months from January 1993 to November
1994. Three replicate samples per site, taken progressively upstream of each other, were collected on each
sampling date with a Neil cylinder with a 250 µm mesh net. Invertebrate samples were preserved in 10%
formalin for later laboratory identification, sorting and counting. Specimens were dried in an oven at
60°C for 24 h and densities and biomass (dry weight) were determined. Each taxonomic group was
assigned to one of five major functional feeding categories: predators, scrapers, shredders, filterers and
collectors (Cummins, 1973).
Brown trout
Fish were also sampled every 2 months from January 1993 to November 1994 at each site by
electrofishing using a 220
W DC generator. Fish caught were anesthetised with tricaine methane-
sulphonate (MS-222 SANDOZ) and their fork lengths (to within 1 mm) and weights (to within 1 g) were
measured. Scales were taken for age determination. Trout density was estimated by applying the three
catch removal method (Zippin, 1956). Standing crop was calculated following the formula proposed by
Mahon
et al.
(1979). Population estimates were carried out separately for each year class. The mean
instantaneous growth rate (G) was calculated as:
G=1n W
2
—1n
W
1
/t
2
-t
1
,
where
W
1
and
W
2
(in grams) were the mean weight of each year class of fish at times
t
1
and
t
2
(in days).
Production was calculated using Allen's graphic method (Allen, 1951) for each year class. A t-test was
Copyright © 1999 John Wiley & Sons, Ltd.
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480
A. ALMODÓVAR AND G.G. NICOLA
used to compare both total densities and biomass of trout before and after regulation. The same
procedure was also employed to test the mean number of trout caught of each age group.
RESULTS AND DISCUSSION
Benthic macroinvertebrates
In general, the results did not show any drastic change in benthic invertebrate communities below the
dam after the hydroelectric power station was put into operation. Furthermore, the opening of the power
station did not seem to have a negative effect on benthos with regard to total density or biomass (Table
I).
On the contrary, the benthic fauna had a slight increase in spite of the flow disturbances. Similar
results have been found by Armitage (1989) in 50 regulated sites in Great Britain and by Petts
et al.
(1993),
who noted that the regulation did not impoverish the invertebrate fauna but induced subtle
changes in faunal composition. The contribution of each feeding group to community structure was
equivalent in both sampling sites. Collectors were by far the dominant group (around 44%), followed by
predators (around 25%), shredders (15%), scrapers (9%) and filterers (7%). Moreover, frequencies of
functional feeding groups did not differ significantly through the sampling period in either site. On the
whole, the functional organisation of the macroinvertebrate community in the River Hoz Seca corre-
sponds with that predicted for headwater streams by Vannote
et al.
(1980). However, the frequencies did
not seem to follow the hypothetical co-dominance of shredders with collectors in forested headwaters
streams (Cummins, 1974; Vannote
et al.,
1980).
Maybe shredders were more abundant in other habitats
such as pools (not sampled in this study), where coarse-particulate organic matter (CPOM) could
accumulate (Bunn, 1986).
Brown trout
In relation to trout population, there was a rapid response to regulation in terms of decreasing density
and biomass. From 1993 to 1994, the estimated population density and biomass showed a significant
decrease of about 50%
(t =
6.30,
p
< 0.05) and 43%
(t =
2.69,
p
< 0.05), respectively. However, these two
parameters remained practically unchanged in the upstream section. Thus, the variations in density from
1993 to 1994 were not significant
(t =
1.57,
p
> 0.05) and the biomass did not exhibit significant
(t = 0.11,
p > 0.05) changes across the sampling period aboye the dam. Also, the recruitment of the individual
cohorts showed a progressive fall from 1993 to 1994, which was more evident in 0 + and 1 + year classes.
In Table II the mean number of trout caught by age group in both pre- and post-regulation periods are
compared. There was a significant decrease in the catches of 0 + and 1 + trout at the downstream site
following regulation and a minor but also significant decrease in 2 + and older trout. Accordingly, the
structure of the population became dominated by older fish, probably as a result of flow regulation, since
this was not observed in the non-regulated upstream site. Furthermore, the adverse flow conditions in the
Hoz Seca possibly prevented adult fish from migrating upstream to spawn. These observations agree with
those of Cowx and Gould (1989) for the River Clywedog, where recruitment of brown trout declined over
successive years after extensive regulation began. Hvidsten (1985), working in the regulated river Nidelva,
detected a similar poor recruitment and suggested that the main reason for deficiencies in the number of
0 + trout was that trout usually remain in their river bed habitats and the frequent changes in water level
generated by flow regulation led to increased mortality as a result of stranding.
Since streamflow seems to be the environmental variable likely to exert the greatest influence on
populations of young salmonids (Solomon, 1985; Elliott, 1987), it is worth noting that the loss of
recruitment in the Hoz Seca could be induced by the downstream displacement of 0 + trout as a result
of the violent fluctuations in the water level of the river. Several authors (e.g. Ottaway and Forrest, 1983;
Heggenes and Traaen, 1988; Crisp and Hurley, 1991a,b) have experimentally proved the vulnerability of
salmonid juveniles to downstream removal due to increasing water velocities. In contrast, Heggenes
(1988), testing the response of induced peaking on movement and habitat use of brown trout in a small
Copyright © 1999 John Wiley & Sons, Ltd.
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Mgmt.
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EFFECTS OF A SMALL HYDROPOWER STATION UPON BROWN TROUT
481
Norwegian stream, concluded that trout with a mean length of at least 67 mm were not washed out due
to sudden high water flows, provided that coarse substrata supplying cover and low-velocity microhabitat
were present. The area of the Hoz Seca subjected to hydropower operations suffers frequent fluctuations
of water level leading to a repeated drowning and drying up of sections near the riverbanks. This latter
Table I.
Mean density
(D,
individuals
m
-2
)and mean biomass
(B,
g dry weight m-
2
)
of each group of
macroinvertebrates for each section (upstream/downstream) within the pre- (1993) and post-regulation (1994)
periods
Upstream
Downstream
D
B
D
B
1993 1994
1993 1994
1993 1994
1993
1994
Turbellaria
1.785 1.782
0.0019 0.0030
52.595
40.115 0.3462 0.2603
Oligochaeta
5.350
112.772
0.0094 0.0960
11.145
45.465 0.1703 0.1483
Hirudinea
1.785
3.567
0.0009 0.0166 44.575
16.047
0.0339 0.0130
Gastropoda
10.700 14.707
0.0091
0.0650
Crustacea
80.230 52.150 0.1473 0.0575
Insecta
Ephemeroptera
Baetidae
310.225
193.000
0.1389
0.0835
151.100
390.460
0.0715 0.1496
Heptageniidae
67.750 30.310
0.1210
0.0358 60.175 49.475
0.1426
0.0301
Ephemerellidae
258.520
28.525
0.1281
0.0191 67.750
6.687
0.0358
0.0040
Caenidae
7.132
0.0027
58.835
0.0114
Leptophlebiidae
7.130
36.550 0.0007 0.0242 30.755
5.350
0.0313 0.0031
Ephemeridae
12.480 12.927
0.1847
0.0218
8.465 2.675
0.0107 0.0259
Odonata
Calopterigidae
1.335
4.012
0.0001
0.0044
Gomphidae
5.350
0.0030
60.175
5.347
0.0474
0.0191
Aeshnidae
1.785
0.3665
1.335
5.347
0.0001
0.3116
Cordulegasteridae
2.675
0.0001
9.360
0.2959
Plecoptera
Nemouridae
16.045
19.612
0.0051 0.0032
6.685
69.535 0.0013 0.0113
Leuctridae
131.935
168.485
0.0287 0.0563
15.155
29.417
0.0018
0.0072
Perlidae
137.285
67.307
0.5611 0.6755
12.035
0.0275
Coleoptera
73.100
48.140
0.2369
0.4930
Dytiscidae
90.930
0.892
0.0349 0.0010
Elmidae
110.540
20.950
0.0089 0.0118
58.390
42.790 0.0212 0.0123
Helodidae
41.005
7.577
0.0107 0.0009
1.785
1.337
0.0002 0.0001
Megaloptera
Sialidae
3.570
8.025
0.0085 0.0045
6.685
0.0084
Diptera
Limoniidae
1.785
1.337
0.0032
0.0011
1.335 1.337
0.0012 0.0001
Simuliidae
281.700 4.905
0.0836 0.0008
14.707
0.0043
Chironomidae
22.730 93.602 0.0013 0.0071
Ceratopogonidae
4.460 4.012
0.0003 0.0001
Stratyomyidae
46.355
8.470
0.1219
0.0151
44.575
13.372
0.1455 0.0556
Trichoptera
1.335 2.675
0.0137 0.0002
Rhyacophilidae
2.675
0.0297
1.785
4.012
0.0002 0.0009
Glossosomatidae
48.140
0.0710
1.785
10.697
0.0012 0.0151
Hydropsychidae
10.695
2.230
0.0324 0.0002
8.025 2.675
0.0291
0.0009
Polycentropodidae
6.685
0.0045
Psychomiidae
8.915
0.0030
2.675
0.0001
Limnephilidae
12.480
62.847
0.0089
0.0802
1.785
2.675
0.0002 0.0005
Sericostomatidae
133.720 109.202
0.2936
0.1952
124.805 141.742
0.1501
0.2467
Total
1790.050 1028.758
2.3167
1.5719
875.860
1131.268
1.5270
2.1410
Copyright © 1999 John Wiley & Sons, Ltd.
Regul. Rivers: Res.
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482
A. ALMODÓVAR AND G.G. NICOLA
Table II.
Mean number of trout caught for 0+, 1+ and 2+ and older age group for each
section (upstream/downstream) within the pre- and post-regulation periods
Site
Period
0+
1
+ 2+ and older
Mean
p
Mean
p
Mean
p
Pre-regulation
18 27
8
Upstream
>0.10 >0.10 >0.10
Post-regulation
20
25 9
Pre-regulation
20
33 55
Downstream <0.001 <0.001
<0.01
Post-regulation
9
9
34
The means were compared through a t-test and the results are shown in the table.
resulted in an important shift within the habitat of 0 + trout, which mostly prefer shallow waters with a
low water velocity (e.g. Heggenes
et al.,
1990; Hubert
et al.,
1994).
Growth was first examined by assessing the mean length of each age class on different sampling
occasions. Growth in length took place throughout the year but was faster between May and September.
Since growth in trout populations virtually ceases by September, the observed lengths-for-age in
November/December were considered as the mean yearly growth. No significant differences (p > 0.05)
were detected in the annual growth increments of trout between sites before and after regulation.
Nevertheless, a more precise examination of changes in the growth rate was made using the mean
instantaneous growth rate in length. A slight but not significant increase in this growth rate was observed
in the downstream site during the year following regulation for 1 + , 2 + and 3 + year classes, whereas
in the upstream site the growth rates remained mostly the same after the disturbance.
The impact of regulation in the River Hoz Seca was also evident in the annual production for trout.
Thus, considerable differences were found in the downstream total production between years, whereas in
the upstream site only a slight difference was detected (Table III). The observed decline in the downstream
site
was probably a function of recruitment loss, since no significant change was noticed in growth rate.
Crisp
et al.
(1983) and Cowx and Gould (1989) obtained comparable results in the annual production
values for salmonids as in growth rate.
In summary, the results suggest that the changes within the downstream trout population were not
induced by a scarcity of food resources. Factors closely linked to water discharge such as water velocity
and habitat modification seem to be responsible for changes in trout population. Water velocity could be
the reason for the observed loss of recruitment by removing of juveniles downstream but alteration to the
habitat involves other changes within physical features of the river like depth, cover or substratum
composition, which could alter the habitat requirements of young trout.
Table III. Percentage of total trout production (kg ha' year-') contributed by each year class and mean biomass
(kg ha-') for each section (upstream and downstream) within the pre- (1993) and post-regulation (1994) periods
Site
Year
1994
1993 1992 1991
1990
1989 1988
Total
production
(kg ha-'
year-')
Mean biomass
(kg ha-')
Upstream
1993
-
6.97
32.75
31.06
23.03
6.13
-
47.334
± 4.8937
68.992
± 6.8628
1994
11.97 19.93
44.65 23.29
-
30.907
± 3.3679
56.3850
± 5.4015
Downstream
1993
-
1.64
7.44
14.62 50.68
22.95
2.65
79.320
+ 4.4533
116.026
+ 9.2063
1994
1.35 6.98
34.23
41.66
15.77
-
-
44.402
± 15.3051
60.226
+ 12.0442
Copyright © 1999 John Wiley & Sons, Ltd.
Regul. Rivers: Res.
Mgmt.
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EFFECTS OF A SMALL HYDROPOWER STATION UPON BROWN TROUT
483
ACKNOWLEDGEMENTS
This work was funded by the project SC-95/005 of the Spanish INIA. We are grateful to J. Cubo and J.L.
Castañeras for their assistance in the field and to Dr B. Elvira, Dr M. Díaz, Dr D. García de Jalón and
Dr J. Camargo for their helpful advice and comments on the manuscript.
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