Comparative analysis of fish aggregation in channel depressions of the Irtysh

919

ISSN 0032-9452, Journal of Ichthyology, 2008, Vol. 48, No. 11, pp. 919–936. © Pleiades Publishing, Ltd., 2008.

The distribution of fish is an integral function oftheir behavior, reflecting ecological conditions in awater body. It is obvious that the main differences offish distribution take place in contrasting aquatic envi-ronments—lotic and lenitic. However, even undermainly similar conditions—in strong water courses—the specific traits of the distribution of fish formed as aresult of the complex action of external and internalfactors are characterized by diversity and the essentialdynamics (Pavlov, 1979).

In life of most fish species, continental watercourses play the role of transport mainlines. Alongthese courses, fish migrations, drifting of eggs and lar-vae, and transfer of trophic resources occur(Welcomme, 1985). Fish migrations along the riverchannel are not permanent but alternate with their resi-dence and concentration in areas with minimum cur-rent—an abrupt bend in the channel, trenches, ordepressions (Martinsen, 1937, 1940).

A biological explanation of the causes and regulari-ties of fish aggregations in a river channel is importantboth theoretically and in application. Until recently,widely known facts of the concentration of fish in chan-nel depressions were traditionally interpreted as part ofthe wintering function of these formations (Nikolsky,1963; Ioganzen, 1972). The results of our investigationson the Gornoslinskinskaya depression of the Irtysh(Pavlov and Mochek, 2005; Pavlov et al., 2006) suggesta wider aspect of understanding the biological signifi-cance of channel depressions. With reference to theGornoslinkinskaya depression, it is shown that not onlydo adult fish winter in such river stretches, but theirjuveniles also aggregate in enormous quantities.

Obviously, a general understanding of the biologicalrole of channel depressions should be based not only onone, though demonstrative, example. Such investiga-tions should consist of a series of similar stretchesalong a considerable length of the river. Such anapproach is necessary for a general understanding ofthe biological significance of channel depressions.

The present study investigates, on a general method-ical basis, large channel depressions of the lower Irtysh:from Tobolsk (southern Tymen oblast) to the mouth ofthe Konda (Khanty-Mansi Autonomous Region).

The goal was to elucidate special traits of fish aggre-gations in principal channel depressions of the lowerIrtysh. For this purpose, traits of fish distribution in thearea of channel depressions were investigated, as wellas the size composition, density, and dynamics of fishaggregations in channel depressions in different sea-sons.

MATERIAL AND METHODS

The study is based on the method of quantitativeestimation of the biomass of hydrobionts by verticalsounding from surface to bottom. This was performedusing an Askor hydroacoustic complex consisting ofthe source of an echo signal (an adapted Furuno serialfish scouting echo sounder with a working frequency of200 kHz and an electric power of the generated pulse atthe antenna of 300 W), an IBM-compatible notebookcomputer, an external appliance of input and output ofdata through a USB port, an external receiver for a GPSsatellite navigation system with data input via a consec-utive port using NMEA protocol, and specialized math-

Comparative Analysis of Fish Aggregation in Channel Depressions of the Irtysh

D. S. Pavlov

a

, A. D. Mochek

a

, E. S. Borisenko

a

, A. I. Degtev

b

, and E. A. Degtev

c

a

Institute of Ecology and Evolution, Russian Academy of Sciences, Leninsky prosp. 33, Moscow, 119071 Russia

b

Petrozavodsk State University, Petrozavodsk, Russia

c

Tobolsk Biological Station, Russian Academy of Sciences, Tobolsk, Russiae-mail: [email protected]

Received January 11, 2008

Abstract

—We present the results of ichthyological studies in principal channel depressions of the Irtysh devel-oping the notions on polyfunctional biological role of these formations. The study is made by means of a com-puterized hydroacoustic complex “Askor”. Quantitative estimation of the size composition, abundance, density,and spatial distribution of fish on water area of channel depressions in different seasons confirms the hypothesisof multifaceted biological significance of these formations. Universal traits in the organization of fish aggrega-tions, specificity of distribution, composition, and seasonal dynamics of the fish population on the largest chan-nel depressions of the Irtysh are revealed.

DOI:

10.1134/S0032945208110015

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ematical software. Askor software was used both forcollection of the material and in processing of theobtained data.

The field material was collected during researchvoyages downstream the Irtysh on KS water-jet vessels(Radioekolog of the Tobolsk Biological Station, Rus-sian Academy of Science (RAS); and the Neptun of theBasin Administration Nizhneobrybvod). Taking intoconsideration the obvious seasonal dynamics of fishoccupying channel depressions, surveys were per-formed in different periods of open water—in the peakof the flood season (July 1–10, 2006) and before ice for-

mation (October 20–26, 2006). In these periods of phe-nological contrast, the most demonstrative differencesin the composition and the type of fish distribution indepressions can be expected. The study covered a chan-nel of the Irtysh for over more than 460 km to the north,from 54.7

°

N to 66.8

°

N (Fig. 1).

Computer analysis of the material of hydroacousticsurveys was made under laboratory conditions andincluded calculation of the following characteristics:abundance, density, and size composition of aggrega-tions; and distribution of fish in different parts of waterarea. The results of processing of hydroacoustic sur-

70

67

63

59

55

51969289848076726864

60

0 500 1000

70.1

69.9

69.7

69.5

69.3

69.1

68.9

68.7

68.5

E, deg

60.8

58.8

60.7

60.6

60.5

60.4

60.3

60.2

60.1

60.0

59.9

59.8

59.7

59.6

59.5

59.4

59.3

59.2

59.1

59.0

58.9

58.7

N, d

egre

es

0 km 10 km20 km30 km40 km

Urals

Ob R

.

Irtysh R.

Kondinskaya

Kirovskaya

Sotnikovskaya

Slishkinskaya

Tugalovskaya

Kokuyskaya

Varlamovskaya

Burenskaya

Esaulskaya

Karbinskaya

Gornoslinkinskaya

Fig. 1.

Localities of the investigated channel depressions in the Irtysh.


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COMPARATIVE ANALYSIS OF FISH AGGREGATION 921

veys have been presented as tables, histograms, andthree-dimensional images.

Application of hydroacoustic devices does not per-mit automatic determination of the taxonomic status ofhydrobionts. Therefore, the notion of the species com-position of fish was based on our seinings, on the dataof commercial seining in the Irtysh in the Uvat regionof Tymen oblast and the Khanty-Mansi AutonomousRegion, as well as on literature data. According toregional information (Pavlov and Mochek, 2006), thebackground fish species are the following: sterlet

Aci-penser

ruthenus

, muksun

Coregonus

muksun

, roach

Rutilus

rutilus

, id

Leuciscus

idus

, dace

L

.

leuciscus

,bream

Abramis

brama

, perch

Perca

fluviatilis

, ruffe

Gymnocephalus

cernuus

, sander

Stizostedion

lucio-perca

, pike

Esox

lucius

, and burbot

Lota

lota

.Altogether, 11 channel depressions were surveyed,

as well as adjacent parts of the river area. The followingchannel depressions were explored: Karbinskaya, Gor-noslinkinskaya, Esaulskaya, Burenskaya, Varlam-ovskaya, Kokuyskaya, Tugalovskaya, Slushkinskaya,Sotnikovskaya, Kirovskaya, and Kondinskaya.

RESULTS

The results of processing of the data of hydroacous-tic surveys are shown in the summary table and inFigs. 2–12, illustrating the density of aggregations andsize composition of fish in the investigated channeldepressions in July and October 2006.

According to the data shown in the table, each chan-nel depression of the Irtysh has a certain hydrologicalspecificity. Thus, the greatest depth (44 m) wasrecorded in the Gornoslinkinskaya depression. Signifi-cant depths (30–35 m) have been found in Komdin-skaya, Kirovskaya, and Kokuyskaya depressions.Depths exceeding 20 m have been recorded in theEsaulskaya, Varlamovskaya, Tugalovskaya, Slushkin-

skaya, and Sotnikovskaya depressions. Burenskaya andKarbinskaya are comparatively shallow depressions(about 10 m).

The specificity of hydrological conditions and fishaggregations have made it necessary to consider theresults of different channel depressions individually.

The Karbnskaya channel depression

(Fig. 2) hassmall depths. The contours of the depression copy theform of the Irtysh channel and only in some of its partsare there comparatively deep stretches of limitedlength.

In summer, the total abundance of fish in the Karbin-skaya depression was about 37400 at a density of294 ind./ha. In autumn, these parameters decreased to22800 and 245 ind/ha, respectively. At the beginning ofsummer, the bulk of fish aggregations consisted ofsmall-sized fish. In autumn, the values of abundance,density of aggregations, and fish size in the Karbin-skaya depression changed significantly—the totalabundance and density of the fish population noticeablydecreased against an increase in the relative abundanceof large fish.

In the Karbisnkaya depression, the spatial distribu-tion of fish aggregations changed significantly by sea-son. During spring–summer flooding, fish were distrib-uted over the water area relatively homogeneously. Ashort time prior to ice formation, four areas of theirincreased concentration were recorded, assigned tofunnel-like depressions of the bottom.

The Gornoslinkinskaya channel depression

(Fig. 3) is unique in its hydrological traits in the lowerreaches of the Irtysh. This is the sole channel depres-sion and has two centers of considerable depths: thedepth in the lower part of the depression attains themaximum value for this river, 44 m.

At the beginning of summer, the abundance of fishin the Gornoslinkinskaya depression was 91 800; in lateautumn, 27 400. The density of fish aggregations here,

General results of surveys of channel depressions in the Irtysh in different seasons

Channel depression Max depth, mAbundance, thousands Mean density, individuals/ha

summer autumn summer autumn

Karbinskaya 14 37.4 22.8 294 245

Gornoslinkinskaya 44 91.8 27.4 905 305

Esaulskaya 21 16.9 8.5 169 80

Burenskaya 18 7.6 5.4 109 23

Varlamovskaya 23 88.9 12.1 692 94

Kokuyskaya 30 35.8 3.6 722 53

Tugalovskaya 23 61.1 7.5 610 51

Slushkinskaya 24 35.2 38.3 824 565

Sotnikovskaya 27 44.2 16.1 629 117

Kirovskaya 32 10.5 31.2 240 190

Kondinskaya 35 34.5 56.3 444 584

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PAVLOV et al.

100

8–12

%

30.9

58.770

68.55 68.56 68.57 68.58 68.59 68.60

58.775

N

N

E

(a)

(b) (c)

Area, ha — 127.6Abundance, thous. individuals — 37.4Mean density, individuals/ha — 294Maximum density, individuals/ha — 7114

individuals/ha


Size distribution by hydroacoustic survey data

5000

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Length, cm

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–52

4–6

3–4

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8–12

%

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80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

24.9 24.113.5

5.1 0.8 0.2 0.5 0 0 0 0

57.3

23.214.6

0.9 1.8 1.4 0.7 0.1 0.1

Fig. 2.

Morphology of the Karbinskaya channel depression (a) and the distribution tablet and histogram of the size composition offish in the depression in July (b) and October (c) 2006.


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100

8–12

%

23.4

68.708

58.740

N

N

E

(a)

(b) (c)


individuals/ha



9000

500

200

100

10

0

1000

500

200

100

10

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individuals/ha

Length, cm

0

80604020

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>74

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12–1

7

25–3

636

–52

4–6

3–4

2–3

100

8–12

%

0


Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

68.704

68.700

68.696

68.692

58.736

58.732

58.728

19.9 20.2 16.3 10.6 5.9 2.4 0.8 0.3 0.1 0 00 0

46.131.5

13.85.3 0.9 1.3 1.1

Fig. 3.

Morphology of the Gornoslinkinskaya channel depression (a) and the distribution table and histogram of the size composi-tion of fish in the depression in July (b) and October (c) 2006.

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100

8–12

%

20.4

58.932

68.808 68.814 68.820 68.826 68.832

58.952

N

N

E

(a)

(b) (c)


individuals/ha



1500

500

200

100

10

0

1000

500

200

100

10

0

individuals/ha

Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

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636

–52

4–6

3–4

2–3

100

8–12

%

0


Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

58.947

58.942

58.937

000

36.625.5

11.95.2 0.2 0.20 0

50.1

31.2

9.0 6.5 0.9 1.4 0.5 0.3 0.1

Fig. 4.

Morphology of the Esaulskaya channel depression (a) and the distribution tablet and histogram of the size composition offish in the depression in July (b) and October (c) 2006.


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100

8–12

%

7.6

59.295

68.850 68.855 68.860 68.865 68.870 68.875

59.315

N

N

E

(a)

(b) (c)


individuals/ha

Area, ha — 237Abundance, thous. individuals — 5.4Mean density, individuals/ha — 23Maximum density, individuals/ha — 948


1000

500

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100

10

0

900

500

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100

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–52

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0

80604020

17–2

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636

–52

4–6

3–4

2–3

59.305

59.325

59.335

00000

34.022.5 20.7 14.5

0.7 0.2

40.8

19.7 21.612.0

1.4 3.2 0 0.9 0.1

Fig. 5.

Morphology of the Burenskaya channel depression (a) and the distribution tablet and histogram of the size composition offish in the depression in July (b) and October (c) 2006.

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PAVLOV et al.

both in summer and in late autumn was very high,905 and 305 ind./ha, respectively.

In summer, the bulk of fish aggregations in the Gor-noslinkinskaya channel depression consisted of earlyjuveniles of dace, id, roach, and small-sized fish: ruffeand perch. Prior to winter, the composition of the fishpopulation of this depression significantly changed dueto an obvious decrease in abundance of small-sized fishand an increase in large sized specimens: id, sander,pike, and nelma.

The Esaulskaya channel depression

(Fig. 4) cop-ies the river channel in shape, and its depth is compar-atively small in the upper part of the meander. Thedeep-water part is situated in the lower part of thedepression, where depths of over 20 m have beenrecorded.

The abundance of fish recorded in the Esaulskayadepression was low in early summer and autumn:16900 and 8500, respectively. The density of aggrega-tions in the Esaulskaya channel depression differed sig-nificantly by season. In early summer, small-sizedspecimens prevailed here; in late October, the numberof specimens whose body size did not exceed 5 cmdecreased with a simultaneous increase in the share oflarge fish.

Distribution of fish in the Esaulskaya depressionwas characterized by fish concentration at large depths,i.e., in the lower part of the depression. In early sum-mer, two large aggregations were clearly identified,obviously attributed to maximum depths. In lateautumn, the fish were mostly dispersed, though fourcenters of increased concentration of fish could be dis-cerned, situated not only in deep parts of the depressionbut also in its comparatively shallow part.

The Burenskaya channel depression

(Fig. 5) is theshallowest of the investigated depressions, and its max-imum depth does not exceed 18 m. The deepest part ofthe depression is situated directly at the bend of theriver, and downstream the channel, depth rapidlyincreases. In the deep-water part of the depression,there are several funnel-like pits. Thus, this depressionis just a somewhat deeper stretch of the mainstreamformed at the bend.

In this depression, comparatively few fish wererecorded—7600 in early summer and only 5400 priorto winter. The density of fish aggregations also waslow—109 and 23 ind./ha in July and October, respec-tively. In summer and autumn, the fish population of theBurenskaya depression significantly differed in size. InJuly, the bulk of fish aggregations in this depressionconsisted of small-sized fish; in October, the share oflarge fish increased against a decrease in the relativeabundance of small-sized fish.

In summer and autumn, the distribution of fish in thewater area of the Burenskaya depression was different.In July, the aggregations of fish repeated the outline ofthe depression with the sole center of maximum con-centration in the deepest part of the water area. In Octo-

ber, the fish were dispersed all over the depression,including its shallow part. At the same time, severalcenters of increased concentration were observed, bothin relatively shallow places and the above-listed depths.

Varlamovskaya channel depression

(Fig. 6). Dur-ing our observations, the maximum depth of the depres-sion was 23 m. The deepest part of the depression isconfined to the straight part of the channel, while down-stream, at a small bend, depths were minimal.

The highest abundance of fish in this depression wasrecorded in July, 88900, and towards the end ofautumn, this value decreased many times, to 12100.The mean density of fish was as follows: in early sum-mer, 692 ind./ha; in late autumn 94 ind./ha. Seasonaldifferences in fish size of the aggregations in thisdepression were very large. In summer, the maximumabundance of small-sized specimens was recordedhere; in late autumn, the abundance of small-sizedspecimens decreased against an increase in the relativenumber of average-sized and large-sized fish.

The distribution of fish in the Varlamovskayadepression differed noticeably in different seasons. Inearly summer, dense fish aggregations were recordedall over the water area, with four centers of increasedconcentration. In late autumn, according to the univer-sal tendency toward a decrease in the general abun-dance of fish, the majority of the water area was notpopulated and small aggregations were recordedmostly in the middle part of the depression.

Kokuyskaya channel depression

(Fig. 7). Themaximum recorded depth of the depression was 30 m,and the deepest areas are situated in the upper part,above the bend.

In this depression, small fish aggregations formed,whose abundance in early summer was 35800; in lateautumn, only 3600. The density of aggregations waslow both in summer and in autumn: 722 and 53 individ-uals/ha, respectively. In summer, in the water area ofthis depression, small-sized specimens definitely pre-dominated; before winter, numerous average-sized andlarge-sized fish appeared here against an abruptdecrease in the abundance of small-sized fish.

In early summer, fish aggregations occupied theentire water area of this depression, producing fourlarge centers of increased concentration. In lateautumn, fish aggregations occurred in the investigatedwater area only in individual areas, forming patches ofaggregations. In this season, only one center of rela-tively large aggregations in the lower party of thedepression was recorded.

The Tugalovskaya channel depression

(Fig. 8) ischaracterized by comparatively shallow depths, to23 m.

In summer, the abundance of aggregations was61100 and in late autumn, 7500. The density of fishaggregations was 610 and 51 individuals/ha, respec-tively.


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In the size composition of fish aggregations, the sea-sonal dynamics were characteristic of all depressions.In early summer, small-sized specimens dominated inthe water area, and of their abundance in late autumnnoticeably decreased. In autumn, the relative abun-dance of average-sized and large-sized fish signifi-cantly increased.

In summer, the fish were distributed all over thewater area, producing in the lower part of this depres-sion, over large depths, two centers of high concentra-tion. In late autumn, the fish preferred the middle andupper parts of this depression.

The Slushkinskaya channel depression (Fig. 9) issituated at a steep bend in the Irtysh. Its maximumdepth is 24 m, and the deepest places are confined

directly to the steep turn of the river flow at the mean-der.

The total abundance of fish in the water area of thischannel depression was 35200 in June and 38300 inOctober, remaining invariable during the period of openwater. The mean values of density of fish aggregationswere as follows: in early summer, 824 individuals/ha, inlate autumn, 565 individuals/ha.

The size composition of fish recorded in the Slush-kinskaya depression in early summer indicated obviousdomination of small-sized fish. However, even in thisperiod in the water area of this depression, quite a fewaverage-sized fish (4–12 cm) were recorded. In autumn,when the abundance of small-sized fish decreased4 natural reasons, the relative part of average-sized and

100

8–12

%

24.2

59.45468.875 68.890 68.905 68.920 68.935

59.458

N

N

E

(a)

(b) (c)individuals/ha


5000

500

200

100

10

0

3000

500

200

100

10

0

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4

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12–1

7

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–52

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3–4

2–3

1008–

12

%

0


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0

8060402017

–25

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

59.462



00

14.9 21.3 20.312.1

4.9 0.6 1.6 0.1 0

55.2

16.6 13.8 8.8 3.0 0.8 1.0 0.4 0.2

Fig. 6. Morphology of the Varlamovskaya channel depression (a) and the distribution tablet and histogram of the size compositionof fish in the depression in July (b) and October (c) 2006.

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PAVLOV et al.

large-sized fish in the total abundance of aggregationsgrew still higher.

It is remarkable that in the seasonal aspect in thewater area of the Slushkinskaya depression, thebiotopes preferred by fish change. Whereas in summerthe main fish aggregations are recorded in the upper

part of the depression, in shallow areas, in late autumn,they shift downstream to the deepest zone.

The Sotnikovskaya channel depression (Fig. 10)is situated in an extended and comparatively gentlysloping meander of the Urtysh, and the maximum depthreaches 27 m.

100

8–12

%

22.6

59.555

69.055 69.061 69.067 69.073 69.079 69.085

59.565

N

N

E

(a)

(b) (c)


individuals/ha



2000

500

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500

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17–2

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–52

4–6

3–4

2–3

59.560

59.550

000

19.6 24.014.8 10.4 4.9 2.9 0.7 0.1

63.6

18.33.1 6.0 3.8 4.4 2.4 0.6 1.0

Fig. 7. Morphology of the Kokuyskaya channel depression (a) and the distribution tablet and histogram of the size composition offish in the depression in July (b) and October (c) 2006.



100

8–12

%

30.8

59.740

69.775 69.782 69.789 69.796 69.803 69.810

59.764

N

N

E

(a)

(b) (c)



2000

500

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80604020

17–2

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–52

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Length, cm

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80604020

17–2

5

52–7

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6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

59.746

59.752

59.758

59.770

individuals/ha

8000

500

200

100

10

0


00013.4

26.814.6

8.92.2 1.5 1.8 0 0 0

63.3

14.2 11.3 6.5 4.1 0.2 0.4

Fig. 8. Morphology of the Tugalovskaya channel depression (a) and the distribution tablet and histogram of the size composition offish in the depression in July (b) and October (c) 2006.

930


PAVLOV et al.

The total abundance of fish in this water area in sum-mer was 44200, and in autumn 16100. The dynamics ofthe mean values of the density of fish aggregations byseason is as follows: 629 individuals/ha in summer and117 individuals/ha in autumn. The spring–summerperiod is characterized by a prevalence in the depres-sion of small-sized fish, and in late autumn it is partlydissipated due to the appearance of a noticeable numberof average-sized and large-sized fish.

In the water area in early summer and in lateautumn, the distribution of fish over the water area wasdifferent. In July, the fish were distributed homoge-

neously all over the water area, producing three centersof increased concentration in the deepest parts. In Octo-ber, the fish were densely concentrated in four parts ofthe depression at different depths.

The Kirovskaya channel depression (Fig. 11) issituated at a steep bend of the river. Its deepest parts areformed at the area of an abrupt change in direction ofthe river flow. The maximum depth of the depression is32 m.

The total abundance of fish recorded in theKirovskaya depression in July was 10500, and the den-

Fig. 9. Morphology of the Slushkinskaya channel depression (a) and the distribution tablet and histogram of the size compositionof fish in the depression in July (b) and October (c) 2006.

100

8–12

%

35.2

60.215

69.795 69.802 69.809 69.816 69.823 69.830

60.220

N

N

E

(a)

(b) (c)


individuals/ha



11000

500

200

100

10

0

4000

500

200

100

10

0

individuals/ha

Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

100

8–12

%

0


Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

60.210

00

22.9 16.9 15.65.4 2.3 0.7 1.0 0.1 0

60.7

21.98.5 3.3 1.6 2.9 0.7 0.2 0.2



sity of aggregations was 240 individuals/ha. A consid-erable part of fish (over 30%) in the summer aggrega-tion consisted of early juveniles up to 2 cm in length.Simultaneously, larger specimens were recorded here,from 3 to 12 cm. In October, the abundance of fish inthe water area was 31200 and the mean density of theaggregation was 190 individuals/ha. In October, thesize composition of fish in the Kirovskaya depressionchanged significantly. The quantity of small-sized fish

noticeably decreased, but numerous average-sized andlarge-sized specimens accumulated whose body sizemay have surpassed 50 cm.

The distribution of fish in the Kirovskaya depressionto a considerable extent reflects the biological featuresof this object. The boundaries of fish aggregations areattributed to deep parts of the depression, and the cen-ters of their increased density are attributed to drop-offsin depth. In autumn, the fish in this depression were

100

8–12

%

55.8

60.350

69.785 69.805 69.815 69.825 69.835 69.845

60.365

N

N

E

(a)

(b) (c)


individuals/ha



4000

500

200

100

10

0

3000

500

200

100

10

0

individuals/ha

Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

100

8–12

%

0


Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

69.795

60.360

60.355

000

18.913.0 9.5 2.2 0.2 0.2 0.2 0.10

58.1

19.212.1

4.3 1.8 3.0 0.8 0.4

Fig. 10. Morphology of the Sotnikovskaya channel depression (a) and the distribution tablet and histogram of the size compositionof fish in the depression in July (b) and October (c) 2006.

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PAVLOV et al.

100

8–12

%

32.3

60.655

69.880 69.886 69.892 69.898 69.904 69.910

60.660

N

N

E

(a)

(b) (c)


individuals/ha



2000

500

200

100

10

0

6000

500

200

100

10

0

individuals/ha

Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

100

8–12

%

0


Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

60.665

60.670

60.675

00

25.4 19.5 19.2

2.9 0.2 0.4 0.2 0

51.2

0.3

25.6

8.1 6.1 3.9 2.6 1.3 0.7 0.2

Fig. 11. Morphology of the Kirovskaya channel depression (a) and the distribution tablet and histogram of the size composition offish in the depression in July (b) and October (c) 2006.



rather dispersed, but the maximum amount of fish werestill over deep spots. In deep-water areas, four centersof large aggregations of fish were recorded, while inshallow channel stretches, only one site with increaseddensity was recorded.

The Kondinskaya depression (Fig. 12) is one ofthe deepest depressions in the Irtysh: the maximumdepth attains 35 m. In early summer, the total abun-dance of fish in this depression was 34500, and in lateautumn, 56300. A seasonal increase in the density of

100

8–12

%

20.3

60.710

69.650 69.657 69.664 69.671 69.678 69.685

60.725

N

N

E

(a)

(b) (c)



6000

500

200

100

10

0

individuals/ha

Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

100

8–12

%

0


Length, cm

0

80604020

17–2

5

52–7

4

>74

6–8

12–1

7

25–3

636

–52

4–6

3–4

2–3

60.715

60.720

60.730


individuals/ha

7000

500

200

100

10

0

000

21.2 17.5 19.2 12.7 7.41.5 0.1 0

74.5

17.34.3 1.6 1.0 0.6 0.3 0.2 0.2

Fig. 12. Morphology of the Kondinskaya channel depression (a) and the distribution tablet and histogram of the size compositionof fish in the depression in July (b) and October (c) 2006.

934


PAVLOV et al.

fish aggregations was also observed: from 444 individ-uals/ha in July to 584 individuals/ha in October.

The series of sizes of fish living in the depression insummer included different groups: small-sized speci-mens of 2–4 cm (over 40%), fish 4–12 cm in length(about 50%), and relatively large fish (about 10%). Inlate autumn, the bulk of the fish population (about 75%)consisted of grown underyearlings. In this period, therelative abundance of average-sized and large-sized fishnoticeably increased.

Distribution of fish in the Kondinskaya depressionsignificantly differed by season. In summer, the fishaggregation was situated in the deep-water part of thedepression, at the Konda inflow. In late autumn, twocenters of aggregations were formed. One, the largest,was situated in the upper part of the depression; the sec-ond, in its lower part, below the river inflow. It isremarkable that both aggregations that formed beforewinter were found at low depths and beyond the directhydraulic influence of the strong flow from the KondaRiver.

DISCUSSION

The results of our study encompassing all largechannel depressions of the Irtysh River from Tobolsk tothe confluence with the Ob suggest that in these objectsabundant fish of various sizes, ages, and species areconcentrated. The phenomenon of the increase in con-centration of fish in a water area is designated by vari-ous terms taking into consideration factors that attractfish and interactions of members of the formed associ-ations. Common definitions are school, group, commu-nity, and aggregation. In the present case, the latterseems to be the most adequate.

A fish aggregation is understood as the associationformed by the predominant action of external factors.Within such aggregations, certain biotic relationshipsare formed between the fish, indicating the formation oftemporary schools (Mochek, 1987) and probably inter-species schools (Radakov, 1972). We believe that eco-logical interactions between fish are, in the presentcase, derivatives and the initial cause of concentrationof fish in depressions stems from environmental fac-tors, primarily irregular bottom relief and flow hydro-dynamics.

When discussing the causes of formation of fishaggregations in channel depressions, one has to keep inmind that the main water courses perform, in relation tofish, a predominantly transit function, while feeding,reproduction, and survival of unfavorable seasonalinfluences are achieved in inhomogeneous biotopes ofshallow water or abrupt drop-offs in depth (Welcomme,1985; Pavlov et al., 2006). The factors of heterogeneityof aquatic biotopes are, primarily, an irregular bottom;the presence of reefs, vegetation, or submersed items;strong turbulence of the flow; gradients of temperature,hydrochemical parameters, and turbidity; and contrast-

ing optical effects. Heterogeneity of the environmentcontrols to a considerable extent the character oftrophic interrelations of fish and the aggregation oftheir distribution (Mikheev, 2006). We consider the fishconcentration in heterogeneous biotopes irrespective ofthe physical nature of their heterogeneity as a universalbiological phenomenon, the manifestation of the “mar-ginal effect” known in ecology (Odum, 1975).

The hydrographic features of different depressionscontrol to a considerable extent the spatial distributionof fish, as well as the abundance and density of theiraggregations. It is obvious that the major factor influ-encing fish distribution is the morphology of a depres-sion, primarily, sudden declines of depth. At the sametime, a great depth is not a sufficient condition for for-mation of an aggregation. The largest fish aggregationsare formed in depressions where the flow abruptlychanges direction; there is also an abrupt drop-off inbottom and circulation currents, and strong turbulentflows are formed due to the form of the sline and bot-tom relief. A similar phenomenon takes place at mean-ders all over the river flow, and early juveniles in theflow are redistributed towards the concave shore, wherethe turbulence level is especially high (Pavlov andPakhorukov, 1973).

In this aspect, the Gornoslinkinskaya channeldepression is a demonstrative example, situated at asteep meander of the Urtysh with stretches of an almostvertical increase of depth. During the entire year, thecenters of maximum fish concentration are formedhere, confined principally to abruptly decreasing bot-tom structures and to open water above a drop-off.Probably, it is here, above great depths that strongeddies are formed that may suck small fish into thedepression. In addition, beyond spurs of the underwatercanyon there are sites of so-called “hydraulic shade,” apermanent zone where fish of different size always stay(Pavlov, 1979). Undoubtedly, these factors cause accu-mulation of numerous fish in deep places.

Comparison of the abundance of fish aggregationsin different depressions does not reveal a specific rela-tionship of this parameter with depth. Abundant fishaggregations are present in relatively shallow-waterdepressions—Tugalosvskaya, Slushkinskaya, Sotnik-ovskaya, Varlamovskaya, and Karbinskaya. On thecontrary, among deepwater depressions, only the Gor-noslinkinskaya and Kondinskaya are characterized by apermanent presence of abundant fish aggregation; inthe Kirovskaya and Kukuyskaya depressions, the abun-dance of fish aggregations in some seasons is minimal.

With reference to the Irtysh, it was shown that in thewater areas of all, without exception, channel depres-sions aggregations were continuously present. The den-sity of such aggregations considerably exceeded thecorresponding parameters for the main channel of theriver. For example, the maximum density of aggrega-tions in the Gornoslinkinslaya depression attained10000 individuals/ha, while in adjacent stretches of the



channel, this parameter did not exceed 2000 individu-als/ha (Pavlov et al., 2006). At the same time, the sizecomposition of fish present in the depression and in thechannel hardly changed at all. In early summer, small-sized fish prevailed everywhere, predominantly, earlyjuveniles. In late autumn and winter, the peak of thesize curve of fish population in all biotopes shifted tothe right due to growth and natural loss of underyear-lings and to translocation of average-sized and large-sized fish (Pavlov and Mochek, 2005; Pavlov et al.,2006) from faraway and adjacent stretches of the river.

Accordingly, in late autumn in most channel depres-sions, in comparison with the spring-summer period,the total abundance and density of fish aggregationsdecreased. This is understandable: it was caused bymass occupation of channel depressions by juveniles inearly summer and their later decrease towards winter.Only in the Kondinskaya and partly in the Kirovskayachannel depressions situated in the lower reaches of theIrtysh was there a reverse process—a considerableincrease of the relative abundance and density of fishaggregations in late autumn. It is characteristic that thesize composition of fish populating the two latterdepressions was very heterogeneous in all seasons:along with abundant juveniles and small-sized fish, theaverage-sized and large-sized fish continuouslyoccurred here. In late autumn, the relative abundance oflarge-sized fish in the water area of these channeldepressions noticeably increased. This confirms thewintering function of these formations.

The results of the present study indicate that in allchannel depressions during the year, juveniles andsmall-sized fish constantly accumulate and in lateautumn the abundance of large-sized fish significantlyincreases and wintering concentrations are formed.Thus, the assumption, based on the results of study ofthe Gornoslinkinskaya depression (Pavlov andMochek, 2005), of the polyfunctional biological role ofchannel depressions as rearing and wintering stationsof fish is generally confirmed for channel depressionsof the Irtysh.

CONCLUSIONS

(1) Channel depressions of the Irtysh are sites ofconstant concentration of fish. In different seasons,aggregations of juveniles and adults are situated here.For this reason, these objects may be considered notonly as wintering stations but also as feeding and rear-ing water areas.

(2) Small-sized fish are the most abundant in chan-nel depressions, and the relative abundance of large-sized fish increases in such water areas only in lateautumn.

(3) Formation of fish aggregations in channeldepressions depends, in the biological aspect, on fishpreferring an inhomogeneous environment.

(4) Distribution of fish in channel depressions isaggregated, with the formation of centers of increasedconcentration predominantly in depth drop-off zones,at a steep turn in the channel, and in eddies.

(5) Changes in the size composition, type of distri-bution, parameters of abundance and density of fishaggregations are observed in all channel depressions.However, the dynamics of this process is specific foreach of these objects.

ACKNOWLEDGMENTS

The authors are deeply grateful to the Tobolsk Bio-logical Station, RAS (Tobolsk), and to the BasinAdministration “Nizhneobrybvod” (Tyumen) for allo-cation of research vessels under conditions of technicalcooperation.

The study has been supported by the West-SiberianExpedition of the Institute of Ecology and Evolution,RAS; the Program of the Department of General Biol-ogy of the RAS, “Biological Resources of Russia: Fun-damental Bases of Their Rational Exploitation” (grantno. II.2.14.); and the Russian Foundation for BasicResearch (project no. 07-04-00639).

REFERENCES

1. Ecology of Fish of the Ob-Irtysh Basin, Ed. by D. S. Pav-lov and A. D. Mochek, (KMK, Moscow, 2006) [in Rus-sian].

2. B. G. Ioganzen, “Zonal and Biotopic Distribution of Fishin the Ob Valley,” in Biological Resources of the ObFloodplain (Nauka, Novosibirsk, 1972), pp. 270–291.

3. Yu. V. Martinsen, “Fish Movement in the River,” Rybn.Khoz., No. 2, 27–30 (1937).

4. Yu. V. Martinsen, “The Impact of Currents on FishBehavior,” Rybn. Khoz., No. 12, 23–27 (1940).

5. V. N. Mikheev, Environmental Nonuniformity andTrophic Relations in Fish (Nauka, Moscow, 2006) [inRussian].

6. A. D. Mochek, Etological Organization of Coastal Com-munities of Marine Fish (Nauka, Moscow, 1987) [inRussian].

7. A. D. Mochek, “Fish Concentration in DissectedBiotopes,” in Proceedings of the Fourth Regional Scien-tific and Practical Conference, Scientific Tobolsk (Tobo-lsk, 2007), pp. 35–36.

8. G. V. Nikolsky, Ecology of Fish (Vysshaya Shkola, Mos-cow, 1963) [in Russian].

9. E. P. Odum, Fundamentals of Ecology (Saunders, Phyl-adelphia, 1971).

10. D. S. Pavlov, Biological Basis of Control of Bioresources(Nauka, Moscow, 1979) [in Russian].

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11. D. S. Pavlov and A. D. Mochek, “Fish Resources of theOb-Irtysh Basin and the Role of Wintering Depressionsin Their Preservation,” in Fundamental Bases of Controlof Bioresources (KMK, Moscow, 2005).

12. D. S. Pavlov and A. M. Pakhorukov, Biological Basis ofProtecting Fish from Falling into Water-Intake Struc-tures (Pishchepromizdat, Moscow, 1973) [in Russian].

13. D. V. Radakov, Schooling of Fish as an Ecological Phe-nomenon (Nauka, Moscow, 1972) [in Russian].

14. D. S. Pavlov and A. D. Mochek, “The Seasonal Distribu-tion of Fishes in the Gornoslinkin’s Wintering Depres-

sion (the Irtysh River),” J. Ichthyol. 45 (Suppl. 2), S206–S213 (2005).

15. D. S. Pavlov, A. D. Mochek, E. S. Borisenko, et al., “Bio-logical Significance of the Gornoslinkinskaya RiverbedDepression in the Irtysh,” J. Ichthyol.; 46 (Suppl. 2),S125–S133 (2006).

16. R. L. Welcomme, River Fisheries. FAO Fish Tech. Pap.(Rome, 1985).

Translated by N.N. Smirnov

Documents

Comparative analysis of fish aggregation in channel depressions of the Irtysh