CONFERENCEPROCEED]NGS 
HUNGAR~JOURNAL 

OF INDUSTRIAL CHEMISTRY 
VESZPREM 

Vol. 2. pp. 1 - 6 (2000) 

ASSESSING IMPACTS OF A HYDROPOWER PLANT: EBRO RIVER, SPAIN 

F.M. CAPEL, F.H. RODRIGUEZ1, D.G. DE JAL6N LASTRA and M.S. DE LOS TERREROS 

(Lab oratorio de Zoologia, Escuela Tecnica Superior de Ingenieros de Montes, Universidad Politecnica de Madrid, 
Ciudad Universitaria, Madrid, 28040, ESPANA 

1Centro de Investigaciones Forestales de Louriziin, Aptdo. de Correos 127, Pontevedra, 36.080, ESPANA) 

This paper was presented at the Second International Conference on Environmental Engineering, 
University ofVeszprem, Veszprem, Hungary, May 29- June 5, 1999 

Ebro River is one of the largest river in the Iberian Peninsula. A small hydropower plant, Viana III, was built in Navarra 
province (North Spain), so a small dam was constructed to divert flow from the mainstream to the plant channel. 
In the reach (''by-pass") affected by the flow reduction (around 263.550 :d of aquatic habitat) we studied the impact on 
the benthic and fish assemblages. Before (autumn 1995) and after (autumn 1996) the plant started to operate, different 
habitat types (fast and slow waters) were sampled by electrofishing and trammel nets to estimate fish populations. 
Invertebrates were sampled by Neil cylinder in lotic habitats. Densities and biomasses were estimated for invertebrates 
and fish populations. Besides, growth, condition coefficients and mortalities were estimated for fish. 
Having compared the flow regime before and after the station began to work, we estimated there was a considerable loss 
of aquatic habitat (40%). In consequence, in fast and shallow waters we noticed a dramatic decrease for the biomass 
(61 %) of the fish assemblage, although we observed an important increase for density (28%). In different types of habitat 
(fast and slow waters) where we sampled by nets, we also detected notable decreases in the catch ratio (40%) and average 
weight (59%). We also estimated a remarkable reduction in the density (56%) and biomass (35%) of the benthic fauna. 
Results suggest the need of developing latest methods for the impact assessment, studying the natural regime of flow by a 
set of parameters based on historical data [1] and estimating habitat loss for fish and benthonic assemblages by the 
InstreamFlow Incremental Methodology [2]. 

Introduction 

Ebro River has the highest mean annual flow among the 
rivers flowing in the Iberian Peninsula. In the 
mainstream a small dam was constructed in order to 
divert flow to a channel from the mainstream to the new 
hydropower plant "Viana liT'. This channel connects to 
the Ebro River downstream the study reach again. 

We studied the by-pass reach of the river affected 
by the flow reduction. The study site (around 3360 m 
long) is located 10 km downstream Logroiio city. We 
surveyed benthos and fish fauna before the plant started 
to operate, and after 1 year of work. The questions we 
were wondering were: 

- What changes occurred in the flow regime? How 
could they affect the available habitat for benthic 
and fish fauna? 

- How were the benthic and fish populations going 
to change in terms of biomass, density and 
structure? 

Study Reach 

The by-pass reach affected is part of the mainstream of 
the Ebro River, 10 km downstream Logrofio city. It 
represents the boundaries between Navarra and La Rioja 
provinces, entering the last one in the lowest part of the 
reach. Since the reach is 3360 m long and the mean 
width is 78.4 m, we estimated the aquatic habitat to be 
263,550 m2 after the dam, 1.2 m height, was constructed 
and before the plant started to operate. 

The thalweg gradient is estimated to be 0.126% and 
the mean altitude 341 m asl. We found the main part of 
the reach was slow waters (56.2%) but not much more 
than the fast water areas (43.8%), the ratio fast/slow 
lengths was 0.93. 

The flow regime was studied in Mendavia gauge 
station, based on the data from October 1966 to 
September 1995. The results are included in Fig.!. The 
mean annual flow is 123.4 m3 s·1 and the river has a 
pluvial discharge regime. We found a low flow period 



2 

400.00 

~ 350.00 

~ 300.00 
g 250.00 
~ 200.00 
tQ 150.00 

.<:: 
u 
.!! 

100.00 

0 50.00 

0.00 

~ 
1\ 
I \ 
I \ 
I \ _. ........._ 
I ~ ~ 

.-/" ...... ....,_ .-........... 

~ 4 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ 
d>.?CJ((f~'<'<i~'r./f~"o/ 'S ~tft 

/--Natural regime -----1996 Regime / 

Fig.] Historical data: Mendavia (Navarra) 

! 
J 
J 
I 
I 

! 
I 
! 

Fig.2 Map of the study 

between July and October, the minimum flow occurs in 
September and the maximum in February. 

The riparian habitat was strongly affected by 
human activities (mainly agriculture). However, we 

' found little areas wher~ the shrubs and trees provided a 
well structured riparian habitat: Basically Populus, 
Salix, Tamarix, Ulmus and Alnus. A map of the study 
reach and the sampling sites can be seen in Fig.2. 

Method 

Based on the gaugings controlled by the Confederaci6n 
Hidrografica del Ebro the flow regime was studied. 
Mendavia was the gauge station selected because only 
Leza Creek {with very little flow compared with the 
mainstream) joins Ebro River between the study reach 
and the gauge station. 

We calculated the mean annual flow for every 
month using the historical data from 1966 to 1995. After 
the plant started to operate, a new study was done. 
Based on the turbinated flow by the plant {data given by 
the company in charge}. the new gauge in the Ebro 
River was estimated, obtaining the mean flow for every 
month in the period January-September 1996. 

Both methods, electrofishing and nets, were used to 
survey the fish assemblage in different sites of the 
reach. Every fish was weighed (1-2 gr. error) and its 
fork length measured. Hydroacoustic techniques were 
not suitab~e in the study reach, where the depths (less 
than 2-3m) do not get to the necessary values to apply 
this technique successfully. 

Thanks to the low flow in September-October, a 
wadeable unit (1491 m2 in 1995 and 1161 ~in 1996) 
was found. We netted off the unit and applied 
electrofishing in order to obtain quantitative results. The 
unit was a glide in a side channel with an estimated 
mean depth of 50 em. 

Electrofishing technique was applied with removal 
method (we did 3 passes). The rectificator device was 
custom-made in our laboratory and the power was 
supplied by a 1200 W generator. With 220 V, we 
obtained 2.5 A, avoiding danger for the fish [3] and the 
sampling team. 

The first step to study the fish populations was the 
calculation of length frequency [4], assimilating normal 
distribution to every age class. Mean length and weight 
was calculated for every age class with a 95% 
confidence interval. Weight/length relationship was 
calculated according to RICKER {5,6] and condition 
coefficients were calculated for every age class. 



3 

Table 1 Historical Data (Q natural regime avg.: 129.9; Q 1996 regime avg.: 44.1) 

HxctrolO!;Iical xear OCT NOV DEC JAN 
1966 44.0 76.8 251.2 167.3 
66-67 105.2 357.2 210.2 122.7 
67-68 38.1 120.2 257.7 327.7 
68-69 48.3 25.8 38.6 66.4 
69-70 33.3 28.1 272.0 319.7 
70-71 39.7 37.7 43.7 47.1 
71-72 33.8 150.3 191.4 180.8 
72-73 78.2 57.4 7524.0 110.8 
73-74 62.4 54.1 96.9 103.1 
74-76 116.4 187.7 101.0 100.3 
76-76 55.0 148.0 145.1 74.3 
76-77 51.7 74.9 78.3 105.5 
77-78 58.3 58.3 103.9 287.8 
78-79 77.2 69.9 122.1 307.7 
79-80 103.5 276.7 114.1 225.1 
80-81 72.8 67.2 255.3 345.7 
81-82 49.0 33.2 95.5 115.2 
82-83 51.8 101.3 328.9 108.5 
83-84 50.8 41.4 75.6 137.5 
84-85 70.9 145.4 88.8 166.3 
85-86 28.4 41.4 36.2 101.8 
86-87 33.2 31.8 61.4 116.1 
87-88 41.9 83.8 116.2 122.3 
88-89 34.8 26.3 38.3 32.6 
89-90 19.9 28.8 43.2 45.0 
90-91 12.5 22.3 82.3 66.2 
91-92 29.8 119.1 57.1 32.3 
92-93 185.7 159.3 188.3 54.9 
93-94 60.1 41.6 170.8 209.4 
94-95 20.6 37.8 35.4 252.1 

Natural regime (m3 s'1) --t I 56.9 90.1 374.1 148.4 
1996regime 69.2 

Length/age relationship was calculated adjusting a Von-
Bertalanffy curve [7]. Using the same asymptotic value 
pennitted the comparison between 95 and 96 population 
growth. 

In the unit surveyed, the mathematical method to 
estimate the total number of fish was the maximum 
weighted likelihood [8], the most robust method 
recommended by Cowx [9]. In order to minimize the 
error due to the different catch probability in every 
species and age class, we calculated the 95% confidence 
interval independently for every species and, when 
necessary, for every age class. Biomass was calculated 
by assigning a mean weight to the fish in every species 
and age class. Finally, relative density (number of fish 
per m2) and biomass (g m·2) was calculated for the unit 
surveyed. 

We studied the changes in the fish assemblage 
comparing the total number of fish, total biomass and 
composition between 1995 and 1996. Indexes of 
diversity and evenness were also calculated. 

Besides, we made electrofishing from a raft in 
deeper sites. As we could not net off a piece of water, 
no quantitative results could been obtained. These data 
completed our data base to study the fish assemblage 
composition and the age, length and weight 
relationships for every species. 

We got more data about the :fish assemblage by 
sampling different habitats with trammel nets. These 
nets consist of three parallel panels of netting, the two 
outer panels are of large-mesh netting and the inner 
panel is of a small mesh. The three panels are suspended 
from a float line and attached to a lead line. Our 

FEB MAR APR MAY JUN JUL AUG SEP 
241.7 232.4 133.0 105.3 158.3 77.4 62.8 64.1 

95.2 98.1 101.0 104.0 57.0 43.4 50.7 43.2 
220.0 200.3 217.9 112.6 68.6 44.2 48.1 83.9 
75.7 128.4 154.7 119.1 76.7 41.8 42.1 74.9 

231.6 251.2 88.7 84.8 43.1 21.1 23.6 26.9 
54.7 116.0 128.7 248.8 164.0 63.8 42.9 40.4 

380.1 217.0 169.0 258.6 126.6 70.8 58.2 71.9 
312.9 125.0 147.0 76.0 183.2 61.9 58.2 53.1 
199.9 282.7 184.3 74.8 58.6 63.6 55.6 54.6 
90.7 145.8 350.1 152.7 110.3 64.0 59.6 60.9 

235.4 99.6 201.9 92.9 57.3 57.3 52.3 54.2 
132.1 78.4 134.8 199.8 277.9 104.4 85.1 61.2 
515.4 283.3 322.5 307.0 131.2 94.3 85.3 77.2 
430.9 222.6 347.0 157.8 92.1 85.9 78.2 84.3 

92.4 199.9 210.5 230.4 126.5 78.4 67.6 63.6 
132.4 114.7 183.7 130.1 62.9 50.7 51.8 54.7 
103.5 137.5 57.3 39.7 52.6 65.4 58.3 43.9 
185.0 161.4 25o.4 116.3 56.6 64.2 139.0 75.6 
182.6 197.3 157.4 202.5 132.6 61.4 64.4 66.3 
128.7 126.5 124.9 142.9 65.9 65.4 51.0 47.4 
201.1 119.4 116.2 91.5 52.8 58.9 58.6 42.2 
174.8 109.2 160.0 52.9 52.0 60.2 58.9 53.2 
156.7 208.4 396.9 156.2 107.4 86.6 57.5 44.9 
26.0 45.8 120.6 53.4 48.0 56.6 45.0 32.6 
43.6 26.4 148.4 35.7 22.7 13.3 
51.0 143.1 215.6 229.5 49.2 42.3 40.0 26.9 
26.4 67.3 275.7 77.7 116.6 74.3 52.7 33.0 
33.0 154.4 93.6 112.5 65.2 65.2 50.9 25.8 

102.0 84.4 103.5 56.9 50.8 55.7 45.9 27.1 
102.4 218.8 55.1 44.4 46.1 62.4 52.0 28.5 

165.3 153.2 178.3 128.9 90.4 63.5 58.5 51.~ 
116.1 84.7 36.3 21.5 23.2 17.6 14.9 13.7 

trammel nets were 1.5 m tall and 12 m long. They were 
sited in different habitats and stayed floating one night. 

Based on all the data obtained by trammel nets, we 
calculated catch per unit effort (CPUE) and the mean 
weight for every species. We grouped the results 
obtained from nets sited in fast water and slow water 
habitats. Then we compared the situation in every group 
before and after the plant was in operation. This way of 
sampling gave us more data about the assemblage 
composition and its variation in different types of 
habitats in the study reach. 

The benthonic fauna plays a crucial role as a bio-
indicator for the quality of the aquatic habitat. To study 
the benthos assemblage we used the Neil cylinder, 
taking 4 replicates of 0.1 m2 each. The sample was done 
in lotic habitats, close to the same glide where we did 
electrofishing. In every replicate, we removed the 
cobbles to sample the complete assemblage living upon 
the river bed. We estimated the density and biomass for 
every population and the assemblage in 1995 and 1996. 
Diversity and water quality indexes were calculated as 
well. 

Results 

The natural flow regime based on the monthly data from 
1966 to 1995 was compared with the flow regime 
registered after the plant started to operate. The natural 
flow had a winter peak in December (374,12 m3 s·1) and 
a low period between July and October, with the 
minimum in September (51 m3 s"1}. 



4 

Table 2 Data obtained by sampling with trammel nets in 1995 and 1996 

TRAMMEL NETS Year 1995 Year 1996 
FASTWATER SLOWWATER FASTWATER SLOWWATER 

HABITATS HABITATS HABITATS HABITATS 
CPUE Mean weight CPUE Mean weight CPUE Mean weight CPUE Mean weight 

5.75 307.9 7.00 559.2 4.25 31.7 5.25 64.7 Barbus graellsii 
Carassius auratus 
Cyprinus carpio 

Chondrostoma toxostoma 
Gobiogobio 

TOTAL (MEAN) 

0.75 278.3 3.00 746.7 
2.75 283.8 1.00 410.0 2.25 

2.25 
340.1 
42.33 

3.25 
4.50 

312.3 
29.8 9.50 34.5 6.25 33.3 

0.25 10.0 
19.00 (162.6) 17.25 

The flow regime based on the data from 1996 
showed strong changes in the drought period. From 
April to September we observed a mean flow of 21,18 
m3 s·1 (17% of the natural mean annual flow -MAF-) 
and a reduction of 27% in the minimum (13,66 m3/s) 
that still occurs in September. 

Using the month averages, the maximum/MAF 
ratio was studied. It reveals strong changes in the 
torrential regime: the ratio was 3 for the natural regime 
and 0.94 for the available data of 1996. As we expected, 
the flow regime line started to become flatter in 1996. 

The mean reduction in flow was calculated and a 
66% of the MAF was obtained. The available series of 
data is considered time enough to evaluate the changes 
in the habitat available for benthos and fish fauna. The 
lFIM, Iqstream Flow Incremental Methodology (2), 
permitted us to estimate the potential usable habitat (m

2 

m-1 of available habitat) for fish based on a very simple 
survey. At least ten cross-sections (measuring mean 
velocity, depth, substrate and cover for fish) enable us 
to run a hydraulic model and predict changes in the 
habitat caused by changes in flow. Therefore, a 66% 
reduction in the MAF can be evaluated in terms of 
suitable habitat for fish, based on the suitability curves 
that were calculated for cyprinids in Spain [10]. 
Processing data by the RHYHABSIM program [11], the 
weighted usable area (WUA) was related to the 
discharge and a 40% of potential loss of habitat was 
obtained for the cyprinids. 

The structure of the fish assemblage suffered patent 
changes after the plant started io operate. The data 
obtained by electrofishing and trammel nets are showed 
in Tables 2 and 3. In the unit where we made 
electro fishing, a notable increase of relative density was 
detected (+28%), mainly due to the increase of the 
young of the year. Relative density was 1.14 ind. m·2 in 
1995 and 1.46 ind. m·2 in 1996. Dominant species in 
number were Barbus graellsii and Chondrostoma. 
toxostoma. The frrst increased its importance from 5% 
to 17%. whereas the second decreased from 89% to 
17% in the assemblage. The changes in the importance 
for the rest ofspecies were not representative, except for 
Rutilus arcasii and Phoxinus pho;-.;inus that were not 
found in 1996. 

In terms of biomass. the relative values of the unit 
surveyed feil a 61% related to 1995, due to the 
reduction in the number of fish belonging to the older 
classes. Biomass changed from 41.4 g m"2 (1995) to 
15.9 g m·2 (1996). Relative importance of th"e principal 

(392.6) 8.75 (113.75) 13 (114.54) 

Table 3 Results obtained for the benthic assemblage 

Year 1995 Year 1996 
BENTHIC 

ASSEMBLAGE Average Standard Average Standard 
deviation deviation 

Mean density of 
samples (ind m·2) 
Mean biomass of 
samples (g m"2) 

1856 

1.15 

765.9 

0.50 

808 452 

0.74 0.62 

species stayed the same in 1995 and 1996. Barbus 
graellsii (70%), Cyprinus carpio (20%) and 
Chondrostoma toxostoma (9%) were the most 
important, and the rest of species maintained each one 
under 1% of the assemblage biomass. 

The results obtained by trammel nets also reveal 
great differences between the 1995 and 1996 situation. 
In the slow water areas, the mean decrease of the 
capture per unit effort (CPUE) for all the species was 
25% related to 1995 and the decrease of the mean 
weight was 71%. In fast water areas, the results were a 
54% of reduction in the average of CPUE and a 30% of 
the mean weight. Carassius auratus was captured only 
in the first survey, in 1995. 

Based on the two methods, every fish population 
was studied. In the electrofishing area. Barbus graellsii 
increased its stock of youngest classes (0-2+), which 
number were multiplied by 10. Oldest classes (5-7+) 
suffered an important reduction: 50% for relative 
density and a 65% in terms of relative biomass. Results 
by trammel nets were in concordance with the results 
obtained by electrofishing. No data could be obtained 
for the young classes by nets, because the smallest fish 
captured was 115 mm long (age 1+) for this species. 
Both types of results showed that the population was 
getting younger; results represent a change in the mean 
weight from the typical barbel 6-8 years old to a 2 years 
old barbel. 

Chondrostoma. toxostoma. suffered an increase in 
the stock of young of the year -YOY- (10%) but a 
decrease in the rest of the classes. In general, it 
supposed a 62% reduction in the total biomass in the 
electrofishing area. The results obtained by nets also 
revealed big reductions in the stocks and a movement of 
the adults to the faster waters from the lentic areas. The 
minimum size of the fish captured was 120 mm {age 
l+}. 

ln the area shocked, Cyprinus carpio suffered a 
general reduction in the stocks (47%), which supposed a 
6.3% reduction in the biomass. By trammel nets we 



captured 3 times more carps in the slow water habitats 
and the number decreased in the nets sited in the fast 
water habitats. The minimum size of the fish captured 
was 78 mm (age 0+). Gobio gobio was the only species 
that clearly increased its stocks. In the electrofishing 
area, its number was multiplied by 4.5 and the biomass 
by 2.5. We only captured a fish in 1995 by nets, due to 
their small size. Noemacheilus barbatulus stayed in the 
same patterns in both surveys. We only fished 2 
individuals of Rutilus arcasii and none of Phoxinus 
phoxinus by electrofishing in 1996. Carassius auratus 
was only fished in 1995 by nets and none was observed 
in the 1996 survey. 

Length/weight and length/age relationships were 
calculated only for Barbus g., Cyprinus c. and 
Chondrostoma t. Lengtblweight relationship didn't 
show relevant changes between 1995 and 1996. The 
Von-Bertalanffy curves showed the same pattern as 
well. Condition coefficients K (100 g cm.3) did not 
reveal clear changes in any species. Mortalities could 
not be calculated due to the lack of fish in different age 
classes of the principal species. 

The benthic fauna is crucial to study how the 
physical changes affect the aquatic habitat. Most of the 
fish species living in the study reach feed on the benthic 
fauna, therefore its density and biomass are considered 
fundamental elements to explain the changes that we. 
observe in the fish assemblage. 

The index of TUFFERY-VERNEAUX [12], Extended 
Biotic Index [13] and BMWP' [14] were calculated in 
1995 and 96, showing no representative changes in the 
water quality. In both_years, the water was polluted and 
diversity was very low. The survey team found that silt 
particles were abundant in the site where the samples 
were taken. The flow was between 20 and 25 d s·1 in 
both surveys. 

Strong reduction in terms of density and biomass 
was observed. Density fell 56% and biomass 36% 
related to the 1995 survey, so we noticed an important 
loss in the available quantity of food for cyprinids in the 
lotic areas. 

Discussion 

The instream flow·regime after the plant started to 
operate was characterized by a notable reduction in the 
minimum month average. Besides, the drought period 
became longer and more intense (the flow was lower 
than before in the summer). Based on the available data 
from 1996, we calculated an important decrease in the 
mean annual flow. Applying the Instream Flow 
Incremental Methodology to the cyprinid populations, 
we calculated that a reduction of 66% in the mean 
annual flow potentially represented a 40% of loss for 
the aquatic habitat. The changes in the aquatic habitat 
affected the fish assemblage in different manners: the 
assemblage composition, the populations' structure and 
the relative density and biomass of each species. 

5 

The percentages of relative importance for the 
principal species mantained the same in the assemblage, 
in terms of biomass. However, in terms of density, we 
observed higher importance of barbels and lower of 
Chondrostoma toxostoma related to 1995. We think that 
these changes are due to the physiological conditions of 
each species, as barbel is considered a genus that 
tolerates a very wide range of conditions while 
Chondrostoma is a genus very adapted to the fast water 
habitats that decreased in the study reach. 

The population structure also changed for the 
, principal species. In the area where we could sample the 
young classes, a general increment of the recruitment 
was noticed for Chondrostoma toxostoma and specially 
for Barb us graellsii. We think that the reason was the 
reduction in the mean velocity and depth in the area, 
because this reduction created better conditions for the 
survival ofthe young of the year. 

The relative biomass obtained by electrofishing 
survey for Barbus graellsii, Chondrostoma toxostoma 
and Cyprinus carpio suffered great reductions (the 
average loss was about 60%). In general, the results 
obtained by tranunel nets were in accordance with these 
results, showing great reductions in the CPUE and the 
mean weights of the fishes captured. Only the carp was 
a different case, because it multiplied its number by 3 in 
the slow water'habitats; the explanation is that the carp 
is considered as a species completely adapted to the 
slow waters. 

Gobio gobio, species that was introduced in Spain, 
was the only one that increased its stocks greatly in the 
area of electrofishing. The wide range of conditions that 
it tolerates in terms of velocity and depth permits it to 
spread widely in the Spanish rivers today. The same as 
the carp, Carassius auratus prefers slow water habitats, 
and it was captured by nets only in 1995. We think that 
they have not disappeared of the study reach. The rest of 
species: Rutilus arcasii, Phoxinus phoxinus and 
Noemacheilus barbatulus, are benthic and ephibenthic 
species. They vary in their capture efficiency because 
they are very small, mobile and cryptic, and they hide 
on the river bed. The population density and structure of 
the environment strongly modify the size of error, so it 
is difficult to get reliable results by electro fishing [ 15]. 

We did not notice changes in the water quality 
based on the study of macroinvertebrates. However, 
their relative density suffered a 56% reduction and a 
36% in their relative biomass after 1995. Another study 
in Austrian rivers also found important reductions in the 
benthic biomass with no changes in the diversity {Hi]. 
The declination of the benthos assemblage was due to 
the fine particles deposition because of the flow 
reduction that caused the increasing filling of the 
interstitial habitat. The great reduction of fish biomass is 
explained by the loss of the basic food for the principal 
species. Chondrostoma toxostoma and Barbus graellsii 
feed on the benthos fauna in different stages of growth. 
However, the carp feeds on the abundant silt of the river 
bed and does not depend on the benthos populations. 



6 

Therefore, the increase of lentic habitats largely benefits 
the carp populations. 

Conclusion 

The impacts of Viana III hydropower plant in River 
Ebro ecosystems can be synthesised as follows. An 
increasing bed-filling in the study reach was due to the 
abundance of fine particles and a notable decrease of the 
mean flow. This loss of suitable habitat for 
macroinvertebrates caused a great reduction of their 
biomass. In different manners, every fish population 
was affected by the changes in the available habitat and 
by the food reduction in the reach. While a slow waters 
species (the carp) found greater areas of available 
habitat, the rest of the species suffered important 
decrement in the stock of adults. We remark that a 
better comprehension of the habitat selection by fish, 
and the application of new methods to study the aquatic 
habit~t [1,2,10,11,17] are hot points to take into account 
for this kind of studies. 

Acknowledgments 

We thank for the help of ALICIA LIZARRAGA 
(Hidroelectrica de Navarra) and the friends of the Lab. 
of Hydrobiology who participated in the surveys. 
Special thanks to ESTER, she was crucial for the final 
redaction of this text. 

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