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ISSN 19950829, Inland Water Biology, 2011, Vol. 4, No. 4, pp. 461–467. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © Yu.I. Solomatin, M.I. Bazarov, 2011, published in Biologiya Vnutrennikh Vod, No. 4, 2011, pp. 64–71.
461
INTRODUCTION
An important characteristic of the state of the eco
system of a water body is the stability of its species
composition, abundance, and biomass [13]. The
abundance and biomass of individual species in water
bodies commonly vary from year to year, in particular
because of variations in the yield of generations, deter
mined by some ecological, biological, and anthropo
genic factors, as well as by catch volumes [10].
The pelagic zone of the Rybinsk Reservoir is a feed
ing area of young fish of commercial species, such as
perch and, partially, carp, as well as young and full
grown zopes. The pelagic fish complex also includes
species of low commercial value—smelt, kilka, bleak,
sabrefish—which are important food components for
predatory fishes.
The data on fish density in pelagic areas are of
importance for monitoring the abundance of young
fish of valuable species and the abundance of low
value species, which are main food components for
predators. Moreover, very low or rapidly falling abun
dance and biomass of young commercial fishes in res
ervoir pelagic area can give grounds for stricter control
of catches of those fishes.
The objective of this work is to study the density of
fishes of pelagic complex in the Rybinsk Reservoir.
MATERIALS AND METHODS
The materials of this study were the results of trawl
catches taken in individual parts of the Rybinsk Reser
voir (inundated channels of former rivers) in August–
September 1991, 1994, 2006, and 2007. The trawling
was carried out in the daytime over a predetermined
station network (Fig. 1).
The instrument for fish catching was a midwater
pelagic trawl with horizontal opening of 12 m, vertical
opening of 1.5 m, and mesh in the cod end of 6 mm.
The trawling time varied from 5 to 12 min. The trawl
ing was carried out one or two times at each station,
depending on the number of horizons in which pelagic
fishes were concentrated. The trawling horizon was cho
sen in accordance with the readings of Simrad EYM
echo sounder (beam angle of
22
°
) [15–17].
The catches were sorted by species and counted,
the fishes were measured (body length to the end of the
scale cover) and weighed [12]. The total of 55 trawling
catches where taken at 9 stations (Fig. 1).
The abundance and biomass of each species, the
total abundance and biomass of fishes in catches, and
the density of fishes were calculated at each station.
The collected data were processed in Microsoft Excel
spreadsheets.
Fish density was calculated based on trawling catch
data by using the formulas
P
1
=
P
2 =
where
Р
1
is fish density, ind/m
2
;
Р
2
is fish density, g/m
2
;
N
is fish count in the catch, individuals;
L
is trawling
distance, m;
В
is fish mass in the catch, g; 0.22 is the
catching power of the trawl; 10 is the horizontal open
ing of the trawl in operation, m.
Since the obtained density values do not follow
normal distribution, nonparametric methods of statis
tical analysis [1, 7] available in “Statistica 6” software
package were applied. In particular, the reliability of
the differences in the abundance and biomass of
pelagic fishes was evaluated by Mann–Whitney
U
test.
N
10L0.02×
,
B
10L0.22×
,
Fish Population Density in Rybinsk Reservoir Pelagic Area
Yu. I. Solomatin and M. I. Bazarov
Papanin Institute for Biology of Inland Waters, Russian Academy of Sciences,
Borok, Nekouzskii raion, Yaroslavl oblast, 152742 Russia
email: salamatin08@mail.ru
Received November 17, 2010
Abstract
—The density of individual species and the total density of pelagic fishes at stations in the Rybinsk
Reservoir was evaluated in 1991, 1994, 2006, and 2007. A reliable difference was found to exist between the
total fish density in pelagic areas in the 1990s and the 2000s. The density of many species (smelt, zander,
perch, and zope) showed considerable variations in both abundance and biomass.
Keywords
: trawl catches, pelagic area, fish density (abundance and biomass), Rybinsk Reservoir.
DOI:
10.1134/S1995082911040195
ICHTHYOLOGY
462
INLAND WATER BIOLOGY
Vol. 4 No. 4 2011
SOLOMATIN, BAZAROV
RESULTS OF STUDIES
Fish population in the pelagic area of the Rybinsk
Reservoir is represented by 12 fish species, whose size
characteristics are given in Table 1.
In 1991, the mean fish density in the pelagic area
was 3.2 ind/m
2
in terms of abundance (Fig. 2a) and
15.6 g/m
2
in terms of biomass (Fig. 2b). These charac
teristics show wide variations in the examined reser
voir areas. The minimal values of the total density dif
fered 36 times in terms of numbers of individuals and
18 times in terms of biomass (Table 2).
Smelt dominated at six out of nine stations in terms
of both abundance and biomass (stations 2–5, 7, 8),
and the highest values of those characteristics were
recorded at stations 2 and 7 (Table 2). Its share in the
catches was mostly 49–91%. At stations 6 and 9,
despite its greater abundance, smelt biomass was much
less than that of zope, which was represented by fishes
of older age groups. Zander young of the year domi
1
2
3
4
6
7
8
9
5
Fig. 1.
Trawl survey station network in the Rybinsk Reservoir: (1–9) station nos.
INLAND WATER BIOLOGY
Vol. 4 No. 4 2011
FISH POPULATION DENSITY IN RYBINSK RESERVOIR PELAGIC AREA 463
nated in terms of abundance (94%) and biomass
(74%) only at station 1.
In 1994, the mean fish density dropped to 2 ind/m
2
(Fig. 2a) and 4.5 g/m
2
(Fig. 2b), while the minimal
and maximal values differed thousands of times, rather
than tens of times as it was the case in 1991. Smelt
dominated at all stations in terms of both abundance
and biomass (Table 3), and it was only at station 4 that,
still dominating in terms of abundance, it ranked sec
ond after vendace in terms of biomass. At station 1,
where zander young dominated in 1991, a singlespe
cies accumulation of smelt was recorded, though its
density was minimal here.
Analysis of catches of 2006 showed considerable
changes in fish population in the examined areas. The
mean fish abundance and biomass in the pelagic area
decreased to 0.12 ind/m
2
(Fig. 2a) and 0.45 g/m
2
(Fig. 2b), respectively, i.e., since 1994, the abundance
Table 1.
Pelagic fish length and species distribution in the Rybinsk Reservoir
Species Length, mm
minimal maximal mean
Kilka (
Clupeonella cultriventris
(Nordmann)) 23 98 57
Smelt (
Osmerus eperlanus
(L.)) 38 118 65
Roach (
Rutilus rutilus
(L.)) 30 156 87
Zope (
Abramis ballerus
(L.)) 42 264 133
Bream (
Abramis brama
(L.)) 40 355 141
Zander (
Stizostedion lucioperca
(L.)) 51 100 72
Pike (
Esox lucius
(L.)) 458 458 458
Perch (
Perca fluviatilis
(L.)) 46 104 61
Ruft (
Gymnocephalus cernuus
(L.)) 42 81 59
Bleak (
Alburnus alburnus
(L.)) 20 159 91
Vendace (
Coregonus albula
(L.)) 76 182 114
Sabrefish (
Pelecus cultratus
(L.)) 88 242 136
Table 2.
Pelagic fish density in the Rybinsk Reservoir in 1991
Station
no. Ruft Perch Roach Vendace Zope Smelt Zander Bream Bleak Sabrefish
Σ
10 0
20 0 0 0 0
300
4000 00
50 0 0 00 0
60 0 0 0
70 00 00 0
80
90 0
Note: Here and in Tables 3–5, above the line is the density, ind/m
2
, below the line is the density, g/m
2
.
0.023
0.115
0.023
0.137
0.022
0.338
0.023
0.206
1.671
5.498
0.021
0.698
0.003
0.082
0.003
0.229
1.79
7.30
0.292
1.304
0.001
0.065
7.676
34.898
0.109
3.862
0.007
0.109
8.09
40.24
0.001
0.005
0.005
0.033
0.003
0.240
1.871
15.371
0.383
1.249
0.002
0.120
0.014
0.207
0.015
0.185
2.30
17.41
0.003
0.071
0.001
0.131
0.615
3.153
0.089
0.360
0.010
0.098
0.72
3.81
0.001
0.005
0.007
0.147
2.856
12.905
0.305
1.091
3.17
14.15
0.032
0.153
0.016
3.175
0.569
2.625
0.028
0.235
0.002
0.480
0.014
0.569
0.66
7.24
2.025
5.749
0.017
0.017
8.730
37.887
0.001
0.011
10.78
43.66
0.007
0.022
0.048
0.191
0.008
0.229
0.008
0.502
0.422
2.040
0.065
0.322
0.005
0.513
0.014
0.115
0.002
0.115
0.58
4.05
0.043
0.172
0.002
0.005
0.001
0.016
0.007
0.971
0.145
0.916
0.086
0.260
0.001
0.007
0.020
0.098
0.30
2.45
464
INLAND WATER BIOLOGY
Vol. 4 No. 4 2011
SOLOMATIN, BAZAROV
and biomass decreased 17 times and 10 times, respec
tively. The smelt, which has dominated in catches
before, was detected in small amounts only at one out
of the nine stations. A new species, kilka, appeared in
catches, where it was recorded in seven out of the nine
examined areas. It became the dominant species in
pelagic area in terms of the number of species (Table 4).
Kilka accounted for 34 to 100% of the total catch. In
terms of biomass, kilka dominated (66–100%) only at
stations 6–9. At stations 3 and 4, where kilka domi
nated in terms of abundance, the major portions of
catches consisted of vendace (50% at station 3) and
zope (84% at station 4). In 1991 and 1994, fishes in
some combinations occurred in the catches at all sta
tions, while in 2006, catches at stations 1 and 2 were
zero.
The changes in the fish population of pelagic areas
that were recorded in 2006 persisted in 2007 (Table 5).
Fish density was still low in the examined areas of the
reservoir, the mean density being 0.12 ind/m
2
in terms
of abundance (Fig. 2a) and 0.79 g/m
2
in terms of bio
mass (Fig. 2b). Kilka was met in catches at all stations,
but it dominated in terms of abundance only at sta
tions 1, 2, 5, 6, and 9. Its share at those stations varied
from 43 to 100%. Young perches—zander (stations 4
and 8) and perch (station 3)—or bleak (station 7)
dominated at other stations in terms of abundance.
Kilka dominated in terms of biomass only at stations 6
and 9. At other stations, the majority of fishes were a
few older zope (station 1), roach (station 2), bream
(station 8), and sabrefish (station 5).
Comparative analysis of fish density characteristics
in the pelagic zone of the Rybinsk Reservoir with the
use of nonparametric statistical methods confirmed
the abovementioned quantitative changes in fish den
sity in the study period. Fish density at most stations in
that zone in 1991 was higher than in other years. The
widest difference was found to exist between the 1990s
and the 2000s. (Mann–Whitney test value in those
cases varies between 0 and 11, i.e., below
U
crit
—the
critical value of twotailed Mann–Whitney test with
the significance level of 0.05; in all cases considered by
the authors,
U
crit
= 17.) The mean density of smelt in
the 1990s was greater than that of kilka in the 2000s
(Fig. 3a). It is important to note that no reliable differ
ence was found to exist in smelt density between 1991
and 1994 (
U
= 40,
U
>
U
crit
) and kilka density between
2006 and 2007 (
U
= 35).
3
2
1
ind/m
2
(а)
(b)
12
8
4
2007200619941991
g/m
2
16
Years
Fig. 2.
(a) Mean abundance and (b) biomass of pelagic
fishes in different years.
2
1
ind/m
2
(а)
(b)
0.2
0.1
2007200619941991
0.3
Years
I
II
I
II
Fig. 3.
Mean density of (a, I) smelt, (a, II) kilka, and
(b) zander.
INLAND WATER BIOLOGY
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FISH POPULATION DENSITY IN RYBINSK RESERVOIR PELAGIC AREA 465
Table 3.
Pelagic fish density in the Rybinsk Reservoir in 1994
Station
no. Pike Perch Roach Vendace Zope Smelt Zander Bream Bleak Sabrefish
Σ
100000 0000
2000 0 000
3000
4000 000
5000 0 00
6000 0 000
70 000
80 0 0
90 0
0.002
0.002
0.002
0.002
0.011
0.207
6.157
5.564
0.001
0.006
6.17
5.78
0.016
0.458
0.001
0.022
1.437
5.270
0.014
0.044
0.001
0.011
0.001
0.022
0.002
0.016
1.47
5.84
0.093
2.804
0.002
0.545
0.240
1.418
0.009
0.305
0.34
5.07
0.019
0.458
0.456
2.520
0.009
0.044
0.001
0.011
0.48
3.03
0.019
0.327
1.571
1.309
0.001
0.001
1.59
1.64
0.002
0.016
0.013
0.087
0.008
0.175
0.001
0.011
1.183
1.500
0.002
0.011
1.21
1.80
0.001
0.011
0.019
0.153
0.003
0.022
0.016
4.364
5.077
7.091
0.024
0.055
0.003
0.049
5.14
11.74
0.001
0.862
0.002
0.016
0.009
0.027
0.002
0.224
1.897
3.851
0.045
0.155
0.002
0.038
0.002
0.065
1.96
5.24
Table 4.
Pelagic fish density in the Rybinsk Reservoir in 2006
Station
no. Ruft Perch Roach Vendace Zope Smelt Zander Kilka Bleak Sabrefish
Σ
1000 00 000000
2000 00 000000
30 000
400 0 0 0
50 0 0 0 0
60 0 0 0 0
700 0 00 00
8000 00 00 00
900 0 0
0.001
0.006
0.001
0.005
0.031
0.744
0.155
0.367
0.006
0.211
0.003
0.173
0.20
1.51
0.001
0.003
0.001
0.222
0.001
0.003
0.002
0.008
0.001
0.030
0.01
0.27
0.002
0.012
0.001
0.023
0.002
0.005
0.004
0.016
0.001
0.00023
0.01
0.06
0.004
0.009
0.005
0.213
0.0004
0.001
0.111
0.465
0.0004
0.022
0.12
0.71
0.001
0.001
0.001
0.004
0.490
0.929
0.49
0.93
0.134
0.267
0.13
0.27
0.001
0.004
0.001
0.004
0.001
0.005
0.006
0.019
0.133
0.285
0.001
0.003
0.14
0.32
466
INLAND WATER BIOLOGY
Vol. 4 No. 4 2011
SOLOMATIN, BAZAROV
Appreciable changes have taken place in perch
young density. The abundance of zander young
decreased in period 1991 to 2006 (Fig. 3b). In 1994,
this characteristic was reliably higher than in 2006
(
U
= 13). No reliable difference was found to exist
between zander abundance in 2006 and 2007 (
U
= 20).
The situation with perch and zope was similar. No reli
able difference was recorded between the 1990s and
the 2000s in the densities of roach, bleak, sabrefish,
and vendace. At the same time, their abundance did
not reach considerable values (
≥
0.1
ind/m
2
) at any sta
tion. Pike, ruft, and bream (the latter two belong to
benthic ichthyofauna) appeared occasionally in the
examined areas, because of which, the assessment of
their density would be incorrect.
DISCUSSION OF RESULTS
The obtained results are in agreement with the data
of previous studies, which have shown that the fish
population of the pelagic zone of the Rybinsk Reser
voir is distributed nonuniformly with the largest
amount concentrated in biomass accumulation zones.
Such zones are generally situated at the interface
between water masses or at the margins of water eddies
[2, 11, 14]. The total fish density in the pelagic zone in
terms of abundance and biomass was reliably higher in
the 1990s than in the 2000s, with smelt dominating in
accumulations in the 1990s and kilka dominating in the
2000s. In most cases, the maximal values of fish density
were recorded at stations 2 and 7 (or at stations 2 and 8
in 1994). In the study period, these areas were situated
at the interface between water masses: station 2 lied at
the interface between the water mass from the Mologa
River and the central water mass; and station 8, at the
interface between the water mass from the Sheksna
River and the central water mass; or, as for example, at
station 7, on the margin of water eddies. High values of
fish absolute abundance (5–11 ind/m
2
) throughout the
study period were recorded only at two stations,
medium values (1–3.5 ind/m
2
) were recorded at six
stations, and low values (<1 ind/m
2
) were recorded at
all stations. Note that in the 2000s, the fish density at
all stations was low. No regular occurrence of either
high or low fish density at certain stations was detected
in the study period.
The studies confirmed the published data suggest
ing considerable qualitative changes in the species
composition of pelagic fish complex, i.e., the change
from smelt to kilka in catches [3, 5, 6]. The density of
major pelagic fishes (smelt, young perch, and young
zope) abruptly dropped in the mid1990s; kilka was
the dominating species in catches in the 2000s. The
high spring water temperature in 1995 (the mean water
Table 5.
Pelagic fish density in the Rybinsk Reservoir in 2007
Station
no. Ruft Bream Perch Roach Vendace Zope Zander Kilka Bleak Sabrefish
Σ
10 0 00
20 00 0
30000 00
400 0 00 00
5000 00 0
60000 0 00
7000 00 0
80 0 00
90000 000 00
0.001
0.038
0.008
0.031
0.001
0.041
0.021
0.829
0.004
0.017
0.082
0.215
0.12
0.17
0.001
0.005
0.042
0.154
0.067
1.443
0.001
0.055
0.385
0.744
0.045
0.534
0.54
2.94
0.002
0.002
0.238
0.923
0.001
0.003
0.002
0.008
0.24
0.94
0.001
0.004
0.006
0.028
0.003
0.003
0.01
0.03
0.001
0.003
0.001
0.002
0.002
0.008
0.001
0.180
0.005
0.19
0.002
0.071
0.016
0.050
0.103
0.578
0.12
0.70
0.003
0.014
0.006
0.020
0.003
0.022
0.012
0.003
0.02
0.06
0.001
0.913
0.001
0.003
0.002
0.015
0.001
0.002
0.016
0.062
0.010
0.022
0.03
1.02
0.033
0.069
0.03
0.07
INLAND WATER BIOLOGY
Vol. 4 No. 4 2011
FISH POPULATION DENSITY IN RYBINSK RESERVOIR PELAGIC AREA 467
temperature in May at station 8 was
12.7
°
С
) could
have a strong adverse effect on smelt spawning, result
ing in that some individuals failed to spawn well [8, 9].
The high summer temperatures (the mean water tem
perature in June at station 8 was
22.7
°
С
) could kill part
of smelt population in the Rybinsk Reservoir [8, 9].
Such conditions disturbed the life cycle of this species.
Kilka (a thermophile species, which can tolerate high
temperature over a long time), occupied the niche that
had become free.
Smelt (as well as kilka) is an immigrant species.
However, unlike kilka, smelt entered the reservoir
from the north, rather than the south, and this took
place long before (in the 1940s [4]); therefore, the
smelt density in the 1990s being much higher than that
of kilka in the 2000s seems to be due to the smelt hav
ing been originally better adapted to the temperature
regime of the Rybinsk Reservoir.
The decrease in the abundance of perch and zope
in the reservoir’s pelagic zone seems to have resulted
from the abrupt increase in the catch load on those
species in the 1990s [3] with the result of a drop in the
number of breeders and a decrease in their replenish
ment rate.
CONCLUSIONS
The overall density of fish in the pelagic zone in the
1990s was reliably greater than that in the 2000s. Smelt
was the dominating species in catches in the 1990s,
and kilka was such species in the 2000s; smelt density
in the 1990s was appreciably greater than that of kilka
in the 2000s. An abrupt drop in the density of young
zander, zope, and perch was recorded in the 2000s. No
reliable changes in either abundance or biomass were
recorded in sabrefish, bleak, and roach in the study
period.
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