Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.
2003
Volume 6
Issue 2
Topic:
Fisheries
ELECTRONIC
JOURNAL OF
POLISH
AGRICULTURAL
UNIVERSITIES
Hornatkiewicz-Żbik A. , Ciepielewski W. 2003. SELECTED BIOLOGICAL PARAMETERS AND THE ABUNDANCE OF THE SPAWNING POPULATION OF ROACH RUTILUS RUTILUS (L.) FROM LAKES GARDNO AND ŁEBSKO , EJPAU 6(2), #01.
Available Online: http://www.ejpau.media.pl/volume6/issue2/fisheries/art-01.html

SELECTED BIOLOGICAL PARAMETERS AND THE ABUNDANCE OF THE SPAWNING POPULATION OF ROACH RUTILUS RUTILUS (L.) FROM LAKES GARDNO AND ŁEBSKO

Anna Hornatkiewicz-Żbik, Władysław Ciepielewski

 

ABSTRACT

The abundance of the spawning population of roach females from the coastal lakes Gardno (2 468 ha) and Łebsko (7 140 ha) located in the Słowiński National Park was estimated for the 1987-1992 period. Following bream, roach is the second most common cyprinid species in regulated fisheries catches. The size of the spawning population was estimated using Virtual Population Analysis and long-term fisheries statistics. Female roach dominated the exploited population at 62.4% in Lake Gardno and 64.3% in Lake Łebsko. During the analyzed period, more roach females were caught in Lake Gardno, which is more than twice as small as Lake Łebsko. Of the generations analyzed (1985, 1986, 1987, 1988, 1989 and 1990), more abundant years occurred in Lake Gardno. The most abundant generation in the two lakes was that of 1985, and following it, each subsequent generation was “weaker” than the preceding had been. The size of the spawning stock was shaped the youngest age groups (2+, 3+, 4+), with the 4+ and 3+

Key words: roach Rutilus rutilus (L.), age composition, generations, exploited population, spawning population..

INTRODUCTION

The progressive eutrophication of water causes changes in the biomass and species composition of fish. The effect of this phenomenon in the waters of northern Europe is seen in the increased abundance of cyprinid fishes, especially bream and roach. Roach dominates in more trophic waters because its food spectrum is wide and it replaces species which prefer clearer waters [10]. This species is able to feed on zooplankton and benthos and, in hypereutrophic water, even on macrophytes and detritus [12, 18, 20, 23, 24, 26, 30, 33].

Other factors which stimulate the dynamic development of roach populations is that it is a highly fecund species and its main predators, pikeperch and pike, are overfished.

As it feeds on zooplankton, roach removes the organisms which filter phytoplankton and through its excreted feces it quickly reintroduces mineral substances into circulation; this induces worsening water clarity and can prompt algal blooms [1, 32]. Attempts at biomanipu-lation are thus being made to prevent the further degradation of waters by removing species, including roach, which are the most impervious to the effects of progressing eutrophication [11, 22]. Species such as these which dominate the environment pose a threat to the balance of faunal biodiversity [13, 25].

The aim of the observations presented in this paper was to analyze the dynamics of changes in the abundance of virtual and spawning populations of roach in the largest lakes in the Słowiński National Park (SNP) - Gardno (2 468 ha) and Łebsko (7 140 ha) (Fig.1).

Fig 1. Area and maksymal depth of the Gardno and Łebsko Lakes

Fig 2. Percentage of different fish species in catches made in lakes Gardno and Łebsko from 1991 to 2001

Of the many lakes in the SNP, fishing is conducted only in these two. The fisheries statistics in the logs of the two studied lakes indicate that bream and roach dominate the catches. Since the 1970s, catches of predatory pikeperch and pike have remained on a low level with the former constituting only several percent and the latter not exceeding 1% of total catches. In the last decade, roach was second only to bream Abramis brama (L.) in Gardno and third following bream and perch Perca fluviatilis (L.) in Łebsko in terms of the biomass of fish caught (Fig.2). Pike and pikeperch catches remained on a low level from 1991 to 2001 at an average of 0.4 % in Gardno and approximately 7% in Łebsko. These species could not have had a significant impact on the size of the stock of planktivorous roach in these lakes.

MATERIALS AND METHODS

The structures of the exploited roach populations in lakes Gardno and Łebsko was analyzed based on material caught by fishermen from 1983 to 2001. These samples provided information regarding the sex, body length (Lc), total weight and age of the roach caught by fishermen. Data concerning the magnitude of fisheries catches in the 1987-2001 period was drawn from the management logs of the individual lakes.

The period from June 1 of a given year (T) to May 31 of a given year (T+1) was used to determine number of females caught by fishermen. This meant that the beginning of the year was during the period when new generations are born and the first annual ring is laid down.

Samples for analyses were chosen randomly from catches which had been made with seines (22 mm mesh size in the bag) and fyke-nets (20 mm mesh size) [4, 5]. The fish were measured to the nearest mm and weighed to the nearest g. Age was determined from scales taken from the first row above the lateral line under the dorsal fin.

The method used in this paper to estimate the abundance of the virtual population allows calculating the number of fish from various years which are alive in a given year. Fisheries statistics can be used for this providing that the exploitation intensity of the studied population is similar [21]. The virtual population describes the minimum abundance of the exploited population, since the calculations do not take into consideration those fish which died of natural causes.

Weight data was recorded for 1 801 grade M and 243 grade S specimens from Lake Gardno and 835 grade M and 561 grade S specimens from Lake Łebsko. This data was used to determine the average weight of roach from these grades in both lakes (Tables 1, 2), which was 76.3 g and 236.8 g in Gardno and 88.5 g and 265.3 g w Łebsko for grades M and S, respectively. According to Polish inland fisheries standards, grade M refers to roach up to 150 g while grade S refers to those above 150 g.

Table 1. Mean weight of roach from Gardno Lake samples

Sampling date

Sortyment (Individual)

M, g

n

S, g

n

13.04.1983

65.365

159

210.100

10

2.10.1984

69.467

75

185.000

2

25.09.1985

76.442

95

222.963

27

26.08.1986

94.158

101

200.000

2

7.10.1987

75.080

250

187.647

17

21.09.1988

90.875

80

196.667

27

7.11.1989

66.129

62

196.250

4

18.11.1992

45.377

106

190.000

1

26.07.1995

49.500

55

174.500

2

14.11.1996

85.856

59

209.800

8

30.09.1997

62.293

41

-

0

22.04.1998

106.056

36

489.857

7

12.05.1998

69.705

15

-

0

9.09.1998

127.056

18

182.273

22

7.10.1998

60.265

49

-

0

21.04.1999

71.382

54

277.369

11

12.05.1999

75.663

12

-

0

12.10.1999

67.560

52

-

0

13.10.1999

73.833

12

280.000

2

9.11.1999

86.961

82

250.333

9

4.04.2000

51.822

20

303.862

38

12.10.2000

91.341

126

199.389

18

15.11.2000

120.375

24

176.500

8

19.04.2001

80.604

87

286.749

10

15.05.2001

72.533

45

272.833

6

7.08.2001

91.500

42

184.143

7

18.09.2001

88.273

44

301.200

5

76.34

1 801

236.80

243

Table 2. Mean weight of roach from Łebsko Lake samples

Sampling date

Sortyment (Individual)

M, g

n

S, g

n

24.09.1985

116.667

3

270.213

47

8.10.1987

83.077

13

325.000

14

20.11.1987

133.333

12

310.331

121

27.03.1995

125.000

2

247.478

23

29.03.1995

71.766

30

154.000

1

26.07.1995

135.929

14

204.414

29

21.05.1996

75.256

39

219.833

6

1.10.1997

89.375

16

198.750

4

28.10.1997

119.895

19

221.200

10

17.03.1998

99.083

12

168.333

6

21-22.04.1998

78.800

60

380.552

29

13.05.1998

76.200

31

-

0

8.10.1998

99.371

35

285.604

48

22-25.03.1999

93.096

88

223.919

32

20-22.04.1999

64.305

93

292.375

14

9.11.1999

79.333

18

279.000

4

7.12.1999

133.000

5

212.880

25

4.04.2000

39.419

10

335.408

14

27.04.2000

84.723

101

158.750

4

19.05.2000

109.083

24

185.000

4

7.06.2000

93.780

50

179.417

12

16.11.2000

107.068

59

205.125

16

13.12.2000

98.792

24

238.656

32

19.04.2001

104.726

10

437.281

7

16.05.2001

87.797

36

211.500

46

8.08.2001

87.613

31

195.000

13

88.7

835

265.27

561

The number of specimens of a given grade caught in a given year was determined by dividing the weight of the fish of the grade by the average specimen weight. The statisitcs regarding catches in Lake Gardno in 1999 and 2000 indicate that from this period the roach were no longer sorted. This is why calculations of the number of specimens caught were based on the average figures of both grades combined.

The sex of 1 252 Gardno and 920 Łebsko specimens from various periods was determined and provided a basis for establishing the male to female ratio and the percentage of sexually immature specimens from the exploited population. Evaluating the percentage of the sexes allowed the number of females caught by fishermen from 1987-1988 to 2000-2001 to be determined.

The female age distribution which was determined from the samples allowed for the abundance of particular age groups in both exploited populations to be estimated for the years analyzed.

In order to simplify calculations, it was assumed that the percentage of females in the fished population remained constant and that the mortality of roach females in particular age groups did not vary significantly during the analyzed period.

The material collected in the fall-winter period over a dozen years was used to determine the body length growth rate of roach. The values used were obtained from direct measurements of 2 002 Gardno and 1 318 Łebsko specimens, and the average length for each age group of fish was calculated. The dependence was expressed as a second degree polynomial.

The virtual abundance of subsequent female generations V(x,T) in the catches was determined by summing the number of fish of a given year which were caught in subsequent years of the analyzed period beginning with age group x (age at recruitment) in year T and ending with the oldest female of that age group noted in the catches. The formula by Ricker [21] was used in the calculations:

V(x,T) = N(x,T) + N(x+1, T+1) + N(x+2, T+2) + ...... ,

where:

N(x,T) is the number of females caught from age group x in year T.

The life span of the female generation from recruitment (age group 2+) to the upper age limit of 13 (no older specimens were noted in the material collected) was taken into consideration in the calculations, and this is how the virtual abundance of generations of females which spawned in the 1987-2000 period was determined.

The sum of the virtual abundance of all the generations of females in a given year is the estimated virtual population of female roach in the analyzed year VT:

VT = V(x,T) + V(x+1,T) + V(x+2, T) + ......

This is considered by many researchers to be the most accurate method for estimating the abundance of fish generations in a natural basin in which the fish are caught with the same gear and at a constant intensity throughout the years [2, 3, 16, 17, 21].

RESULTS

The body length of the roach caught in the two lakes ranged from 7.1 to 33.0 cm (Figs 3, 4). A greater quantity of larger specimens was caught in Lake Łebsko. The plots illustrating the body length distribution of the fish are positively skewed with the modal value shifted to the left. The most numerous group in Lake Łebsko, at approximately 23% of the caught fish, were specimens from the 16.1-18.0 cm length class, while in Lake Gardno the share of roach in the most numerous length class (14.1-16 cm) was 35%. Fish from this class constituted 20% of the material collected from Lake Łebsko.

Fig 3. Percentage of roach specimens in body length classes in the samples from Lake Gardno according to sex

Fig 4. Percentage of roach specimens in body length classes in the samples from Lake Łebsko according to sex

Juvenile specimens occurred in the catches at a body length (Lc) above 6 cm. In the 10.1-12.0 cm length class they comprised 25% of the fish caught in Lake Gardno and 100% in Lake Łebsko. Males occurred at a body length of 9.1 cm in Gardno and 11.1 cm in Łebsko, and dominated the population at body lengths of approximately 13 and 15 cm, respectively. Mature females were noted in the two lakes in classes above 10 cm and were most numerous in the 14.1-16.0 cm length class in Gardno, comprising 40% of the specimens caught. Larger females dominated in Lake Łebsko with 30% of the specimens in the 16.1-18.0 cm length class.

Specimens which weighed less dominated in the catches from Lake Gardno, and roach from the 100 g weight class constituted 70% of the fish caught, while specimens in the next weight class (to 200 g) constituted 25%. Fish in the remaining weight classes totaled 5% of the specimens caught (Fig. 5). Most of the roach from Lake Łebsko also fell into the first two weight classes with 40% in the first (to 100 g) and 37% in the second (to 200 g). Larger roach in this lake constituted 23%, which is 18% more than in Lake Gardno.

Fig 5. Percentage of different weight classes in catches

Roach from age groups 3+ (37%) and 4+ (34%) dominated the catches from Lake Gardno (Fig. 6). Specimens from the 2+ and 3+ groups comprised about 10% of the fish caught, while specimens from the remaining age groups of 1+, 6+ to 10+ and 12+ occurred rarely and comprised less than 2% of the catches. Specimens from the 4+ and 5+ age groups dominated in the catches from Lake Łebsko at 26% and 24%, respectively (Fig. 7). There were fewer fish caught in the 2+ (about 2.5%), 3+ and 6+ (15%) and 7+(above 5%) age groups. Roach from the youngest (1+) and oldest (8+ - 12+) age groups were noted rarely, and the share of none of the groups exceeded 3%.

Fig 6. Sex ratio of roach in Lake Gardno by age groups

Fig 7. Sex ratio of roach in Lake Łebsko by age groups

Roach females appeared in the catches generally in the third year of life (2+ age group), and fish from this age group and older dominated the catches of this species. No male specimens older than nine or eleven years old were noted in lakes Gardno or Łebsko, respectively.

The growth rate of the roach from the analyzed Słowiński National Park lakes was rapid, and the fish attained 19 cm in body length by the sixth year of life (Fig. 8). According to Wilkońska [29], this is very rapid. Younger specimens (three and four year olds) as well as eight, nine and ten year olds in Lake Gardno grew faster than their counterparts did in Lake Łebsko. Similar growth rates were confirmed for specimens from the population aged five, six and seven. The oldest fish, past the tenth year of life, grew faster in Lake Łebsko. Thirteen-year-old females in both lakes attained body lengths exceeding 30 cm.

Fig 8. Growth rate of roach body length

The total catch magnitude of the fish in Lake Gardno during the 1987-2001 period was an average of 24.79 kg ha–1year–1, including 7.84 kg ha–1year–1 of roach (which constituted 31.7% of the catch). Roach catches during this same period in Lake Łebsko comprised only 8.61% of the total catch of 23.45 kg ha–1year–1. At the beginning of the 1990s, catches in Lake Gardno fell from the level of more than 14 kg ha–1 in the 1980s to less than 5 kg ha–1 in the 1990s. During this period there was a decrease noted in the catches of all fish species. By the end of the 1990s, there was an increase in total catches, with the exception of those of roach which remained below the levels of the 1980s (Fig. 9).

Fig 9. Catches in Lake Gardno

Catches in Lake Łebsko did not fluctuate as much as those in Lake Gardno during the analyzed period, with catches of both roach and other species remaining on a similar level. The largest catch was in 1987 (more than 35 kg ha–1 of mainly bream) and the smallest in 1998 (12 kg fish ha–1Fig. 10).

The roach from Lake Łebsko had a higher average specimen weight at 88.5 g for grade M and 265.3 g for grade S than that of the roach caught in Lake Gardno which weighed approximately 12 g and 28 g less, respectively.

Fig 10. Catches in Lake Łebsko

Grade M roach dominated in Lake Gardno comprising from 98-100% of the weight of the catch of this species. The share of these smaller specimens in Lake Łebsko was lower and ranged from approximately 73 to 8% of the catch weight of this species; only in 1994 was the catch of this specimen size exceptionally large at 91%. Grade S roach were caught by fishermen in Lake Łebsko in quantities not exceeding 27% of the total weight of the annual catch of this species. An even smaller quantity of this grade was caught in Lake Gardno (2%), and the management logs indicate that from 1999 this grade was not caught. This was because fishermen stopped segregating the catch in 1999 and entered the total catch figure under the grade M heading. From 1987/88 to 1989/90 roach catches exceeding 300 000 specimens were made in Lake Gardno. Catches exceeding 100 000 fish were noted in Lake Gardno in 1990/91, 1994/95 and 1996/97 and in Lake Łebsko in 1991/92, 1992/93 and from 1996/97 to 2000/2001. There was a sign ificant fall in the abundance of roach catches in Lake Łebsko in 1998/99 when only 41 500 specimens were caught (Table 3).

Table 3. Number of roach caught by fishermen between 1987 and 2000

Year

Gardno

Łebsko

Sortyment (Individual)

S

M

S+M

S

M

S+M

1987/88

2 830

509 899

512 729

23 412

77 985

101 397

1988/89

6 918

607180

614 097

44 650

164 711

209 361

1989/90

7 751

436 932

444 683

36 616

126 974

163 590

1990/91

1 534

80 733

82 267

35 590

139 385

174 976

1991/92

1 517

188 243

189 760

19 365

54 897

74 261

1992/93

1 755

217 318

219 072

20 712

77 180

97 892

1993/94

1 984

103 675

105 659

19 467

124 464

143 931

1994/95

646

78 046

78 692

22 488

134 094

156 582

1995/96

1 011

271 676

272 687

13 192

139 009

152 201

1996/97

13

87 057

87 070

12 007

63 565

75 572

1997/98

8

114 868

114 877

18 388

61 077

79 464

1998/99

161

116 629

116 791

7 471

33 996

41 468

1999/00

122 249

122 249

10 118

72 453

82 571

2000/01

148 313

148 313

9 947

54 395

64 342

S – specimens of more than 150 g.
M – specimens of less than 150 g.

Table 4. Percentage of females, males and juveniles in the samples

Lake

Sex

n

juv

Gardno

1 252

63.10

36.34

0.56

Łebsko

920

64.35

35.54

0.11

Females were the dominant sex in the catches both in terms of abundance and biomass, and they comprised 62.4% of all the specimens from Lake Gardno and 64.35% of those from Lake Łebsko (Table 4).

The fewest females (under 50 000 specimens) were caught in Gardno in 1994/95 and in Łebsko in 1991/92, 1996/97, 1998/98 and 2000/01. Exceptionally high numbers of female roach (exceeding 100 000 specimens) were caught in 1987/88-1989/90, 1991/92, 1992/93, 1995/96 in Lake Gardno and in 1988/89-1990/91 and 1994/95 in Lake Łebsko. Over the course of the 14 years of the analyzed period, more females were caught in Lake Gardno; this was the result mainly of abundant roach catches (exceeding 250 000 females annually) in the first three years of the analyzed period. During the periods mentioned above, among the highest catches of roach females in Lake Gardno were made in 1987/88, 1988/89 and 1989/90 and the maximum was in the 1988/90 season when 387 495 female specimens were caught. More than six-fold fewer specimens (49.6-54.9 thousand) were caught in Lake Gardno in 1990/91, 1994/95 and 1996/97. In Lake Łebsko the highest roach catches were noted in 1988/89 at 134 724 females, and in the 1 989/90 and 1990/91 seasons when 105 271 and 112 597 specimens were caught, respectively. The smallest catch was recorded for the 1998/99 season when only 26 685 specimens were caught. In the remaining periods, the fluctuation of this figure was higher and fell within the range of 47 800 to 100 800 roach females caught annually. The average catch of females in Lake Gardno in 1987-2001 was 140 125 specimens per computational year; this figure for Lake Łebsko was almost two-fold lower at 74 352 specimens. In Lake Gardno, in 1987/88 and 1996/97 higher catch figures were recorded and both of these seasons were preceded by years with lower catches. From the 1997/98 season, catches in this lake have remained stable at a level of 70 000 specimens. Meanwhile, fluctuations in the abundance of catches of female roach in Lake Łebsko follows an annual cycle (Table 5).

Table 5. Number of females caught in different period

Year

Lake

Gardno

Łebsko

1987/88

323 534

65 248

1988/89

387 495

134 724

1989/90

280 595

105 271

1990/91

51 911

112 597

1991/92

119 739

47 788

1992/93

138 236

62 992

1993/94

66 671

92 620

1994/95

49 653

100 760

1995/96

172 066

97 942

1996/97

54 939

48 630

1997/98

72 487

51 135

1998/99

73 694

26 685

1999/00

77 139

53 136

2000/01

93 585

41 406

Table 6. Age structure of females in the samples

Age groups

Lake

Gardno

Łebsko

n

%

n

%

2+

30

5.78

17

3.19

3+

175

33.72

94

17.64

4+

210

40.46

166

31.14

5+

72

13.87

135

25.33

6+

12

2.31

62

11.63

7+

12

2.31

23

4.31

8+

3

0.585

20

3.75

9+

3

0.585

10

1.88

10+

1

0.19

4

0.75

11+

0

0

1

0.19

12+

1

0.19

1

0.19

519

100

533

100

Females from the 4+ age group dominated in both lakes (Gardno – 40.46% and Łebsko – 31.14%), while those from the 3+ (33.72%) and 5+ (13.87%) age groups in Gardno and the 3+ (17.4%) and 5+ (25.33%) age groups in Łebsko were less abundant. Older females from the 6+ to the 9+ age groups were noted rarely in Lake Gardno (less than 3%). Females which were even older were observed sporadically and some age groups, i.e. 11+, were not noted at all in the catches (Table 6).

The material collected permitted estimating the number of females caught from six generations confirmed in the two lakes from recruitment to the upper age limit in the analyzed period. It should be added that there is a lack of data regarding the abundance of females from the 11+ and 12+ age groups in the last two generations (1989 and 1990); however, in light of the probable low numbers of specimens this age, something which can be inferred from the data presented, the years 1989 and 1990 were also used in comparisons.

Of the analyzed roach generations in 1985, 1986, 1987, 1988, 1989, the most abundant were in Lake Gardno (Tables 7, 8). Only the 1990 generation was more abundant in Lake Łebsko. The most abundant generation of female roach was in 1985 with 277 000 specimens in Gardno and 102 000 in Łebsko (Table 9). After this year, each subsequent generation, with the exception of that of 1990 in Łebsko, was “weaker” than the previous one had been. The least abundant virtual female generation in Lake Gardno was that of 1990 (78 000 specimens) and in Lake Łebsko that of 1989 (74 000 females). More females spawned at the age of recruitment (2+ age group) in Lake Gardno (78 000-277 000) than in Lake Łebsko (73 000-101 000) in the years analyzed. The ten-year-old generation in Lake Gardno and the eleven-year-old generation in Lake Łebsko probably did not number more than a thousand females, and there were about 100 of the oldest specimens (13+) noted in the years analyzed.

Table 7. Virtual abundance of roach females V(x,T) in different generations in Gardno Lake

Generations

Years of catches

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

V (x,T)

1975

615

 

 

 

 

 

 

 

 

 

 

 

 

 

615

1976

0

736

 

 

 

 

 

 

 

 

 

 

 

 

736

1977

615

0

533

 

 

 

 

 

 

 

 

 

 

 

1 148

1978

1 893

736

0

99

 

 

 

 

 

 

 

 

 

 

2 728

1979

1 893

2 267

533

0

228

 

 

 

 

 

 

 

 

 

4 921

1980

7 474

2 267

1 641

99

0

263

 

 

 

 

 

 

 

 

11 744

1981

7 474

8 951

1 641

304

228

0

127

 

 

 

 

 

 

 

18 725

1982

44 874

8 951

6 482

304

700

263

0

94

 

 

 

 

 

 

61 668

1983

1E+05

53 746

6 482

1 199

700

809

127

0

327

 

 

 

 

 

194 291

1984

1E+05

2E+05

38 919

1 199

2 766

809

390

94

0

104

 

 

 

 

310 157

1985

18 700

1E+05

1E+05

7 200

2 766

3 193

390

290

327

0

138

 

 

 

277 196

1986

22 397

94 617

21 003

16 608

3 193

1 540

290

1 007

104

0

140

 

 

160 899

1987

 

 

16 218

17 504

48 446

19 173

1 540

1 147

1 007

321

138

0

147

 

105 641

1988

 

 

 

3 000

40 376

55 930

9 247

1 147

3 975

321

424

140

0

178

114 738

1989

 

 

 

 

6 921

46 613

26 975

6 887

3 975

1 269

424

431

147

0

93 642

1990

 

 

 

 

 

7 990

22 481

20 090

23 865

1 269

1 674

431

451

178

78 429

1991

 

 

 

 

 

 

3 854

16 744

69 618

7 620

1 674

1 702

451

547

102 210

1992

 

 

 

 

 

 

 

2 870

58 020

22 229

10 054

1 702

1 782

547

97 204

1993

 

 

 

 

 

 

 

 

9 945

18 526

29 328

10 221

1 782

2 162

71 964

1994

 

 

 

 

 

 

 

 

 

3 176

24 443

29 817

10 699

2 162

70 297

1995

 

 

 

 

 

 

 

 

 

 

4 190

24 850

31 210

12 980

73 230

1996

 

 

 

 

 

 

 

 

 

 

 

4 260

26 011

37 865

68 136

1997

 

 

 

 

 

 

 

 

 

 

 

 

4 459

31 557

36 016

1998

 

 

 

 

 

 

 

 

 

 

 

 

 

5 409

5 409

Table 8. Virtual abundance of roach females V(x,T) in different generations in Łebsko Lake

Generations

Years of catches

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

V (x,T)

1975

 

 

 

 

 

 

 

 

 

 

 

 

 

 

124

1976

124

256

 

 

 

 

 

 

 

 

 

 

 

 

380

1977

489

256

200

 

 

 

 

 

 

 

 

 

 

 

945

1978

1 227

1 010

200

214

 

 

 

 

 

 

 

 

 

 

2 651

1979

2 447

2 533

790

214

91

 

 

 

 

 

 

 

 

 

6 075

1980

2 812

5 052

1 979

844

91

120

 

 

 

 

 

 

 

 

10 898

1981

7 588

5 807

3 948

2 117

358

120

176

 

 

 

 

 

 

 

20 114

1982

16 528

15 668

4 537

4 222

898

472

176

191

 

 

 

 

 

 

42 692

1983

20 318

34 126

12 243

4 853

1 792

1 184

695

191

186

 

 

 

 

 

75 588

1984

11 510

41 953

26 665

13 095

2 060

2 362

1 741

756

186

92

 

 

 

 

100 420

1985

2 081

23 765

32 781

28 521

5 558

2 715

3 473

1 894

735

92

97

 

 

 

101 712

1986

 

4 298

18 570

35 063

12 105

7 326

3 992

3 779

1 841

365

97

51

 

 

87 487

1987

 

 

3 358

19 862

14 881

15 956

10 772

4 343

3 673

914

384

51

101

74 295

1988

 

 

 

3 592

8 430

19 616

23 460

11 718

4 221

1 824

961

200

101

79

74 202

1989

 

 

 

 

1 524

11 112

18 842

25 523

11 391

2 096

1 918

502

399

79

73 386

1990

 

 

 

 

 

2 009

16 338

31 377

24 809

5 656

2 204

1 001

999

311

84 704

1991

 

 

 

 

 

 

2 955

17 774

30 499

12 318

5 947

1 150

1 993

778

73 414

1992

 

 

 

 

 

 

 

3 214

17 277

15 144

12 953

3 103

2 290

1 553

55 534

1993

 

 

 

 

 

 

 

 

3 124

8 578

15 923

6 759

6 180

1 785

42 349

1994

 

 

 

 

 

 

 

 

 

1 551

9 020

8 310

13 459

4 815

37 155

1995

 

 

 

 

 

 

 

 

 

 

1 631

4 707

16 546

10 488

33 372

1996

 

 

 

 

 

 

 

 

 

 

 

851

9 373

12 893

23 117

1997

 

 

 

 

 

 

 

 

 

 

 

 

1 695

7 304

8 999

1998

 

 

 

 

 

 

 

 

 

 

 

 

 

1 321

1 321

Table 9. Females spawning populations from 1987 to 1992

Lake

Age groups

Spawning year

1987

1988

1989

1990

1991

1992

Gardno

2+

277 196

160 899

105 641

114 738

93 642

78 429

3+

310 157

258 496

138 502

89 423

111 738

86 721

4+

194 291

201 062

127 833

43 885

71 919

71 362

5+

61 668

63 390

44 281

14 304

22 882

23 473

6+

18 725

16 794

9 644

5 362

7 104

6 274

7+

11 744

11 251

7 843

3 162

4 163

4 338

8+

4 921

4 270

2 300

1 361

1 963

1 397

9+

2 728

3 028

2 003

659

1 057

1 263

10+

1 148

835

761

362

355

357

11+

736

533

99

228

263

127

12+

615

736

533

99

228

263

VT

883 929

721 294

439 440

273 583

315 314

274 004

Łebsko

2+

101 712

87 487

74 295

74 202

73 386

84 704

3+

100 420

99 631

83 189

70 937

70 610

71 862

4+

75 588

88 910

75 866

64 619

51 075

62 180

5+

42 692

55 270

46 957

43 085

29 556

36 194

6+

20 114

26 164

21 144

20 292

24 564

17 451

7+

10 898

12 526

10 496

8 901

7 197

9 006

8+

6 075

8 086

6 719

5 959

4 048

5 137

9+

2 651

3 628

3 034

2 771

1 737

2 256

10+

945

1 424

1 095

1 055

654

839

11+

380

456

414

305

211

296

12+

124

256

200

214

91

120

VT

361 599

383 838

323 409

292 340

263 129

290 045

The abundance of the spawning population in Lake Gardno decreased in the 1987-1992 period. In both 1990 and 1992 as much as 69% fewer females spawned in comparison with 1987, the year in which the highest female abundance of 884 000 specimens was recorded. Meanwhile, the abundance of the spawning stock in Lake Łebsko did not fluctuate to the same degree; the difference between the most and least abundant years was 32% with values ranging from 263 000 females in 1991 to 384 000 females in 1988 (Table 9).

The size of the spawning stock was shaped principally by the three youngest age groups of 2+, 3+ and 4+, with the 4+ and 3+ groups dominating in lakes Gardno and Łebsko, respectively. Beginning in 1988, the abundance of the 2+ and 3+ age groups fell, and in 1992 the most significant fall in the abundance of females (72%) in these age groups in comparison to 1987 was recorded. This was responsible for the limited abundance of the spawning stock. In Lake Łebsko the abundance of the roach spawning stock in these two age groups did not exhibit such a pronounced decline. In 1991 when the abundance of the spawning stock was at its lowest, females from the 3+ and 4+ age groups comprised 71% of the number of specimens from 1987.

DISCUSSION AND CONCLUSIONS

The age composition of fish from different basins varies depending on factors such as the intensity of fishing and predation. When exploitation is heavy, the stock is composed of younger and smaller specimens, and predation by cormorants and predatory fish is limited, above all else, to young fish. The effects of these factors can lead to a rapid decline in population size in a short period. The intensity of fishing in the studied lakes has not varied much over the last two decades, and seine catches and the setting of fyke-nets was similar in both lakes at the beginning of the 1990s [4]. The abundance of the cormorant population feeding in these lakes in the 1980s and early 1990s was large (approximately 5 000 specimens – information from park officials). A decline in the numbers of these birds has been noted since the late 1990s, and currently the size of the cormorant population feeding in the SNP does not exceed 1 500, according to park officials. Populations of predatory pike and perch were not numerous as is indicated by low catches of these species. It is plausible that predation by the cormorants had an impact on the abundance of roach in these lakes. Nor can it be ruled out that the significant fall in the numbers of roach caught in the early 1990s in Lake Gardno was caused by climate change. Analyses of the average monthly and annual temperature in this region indicates that there was a long-term increase from 7.2°C in the 1881-1930 period to 8.4°C in 1991-2000. Air temperature impacts the temperature of aquatic basins, especially in the case of the studied lakes which are shallow, devoid of thermal stratification and well-mixed to the bottom. This means that the water temperature in these lakes adapts more rapidly to the air temperature. Thus, air temperature can be useful in describing changes in the thermal state of lakes Gardno and Łebsko in the analyzed period.

The annual temperature variations in 1984-2000 ranged between 6.4-9.7°C, with the average at 8.2°C. The coldest periods, which could have decelerated roach gonad development, were noted in 1987 (6.4°C), 1996 (6.6°C) and 1985 (6.8°C). During the pre-spawning months (January to May) in these years the average temperature fell below 0°C.

Statistical analyses of the correlation of the annual air temperature averages (TY) and the average from five months (TS – from January to May) in the analyzed years (for example in 1985) with the abundance of females caught two years later (in 1987/88) indicated that there was a negative correlation for the population from Lake Gardno (r = - 0.573) at a level of a = 0.05, as is illustrated by the following:

 

TY-N(2+,T) TS-N(2+,T) r (a=0.05)
Lake Gardno -0.573 -0.573 0.532
Lake Łebsko -0.153 -0.169 0.532

This means that a rise in temperature in the lake during the year as well as in the period preceding the spawning season causes higher mortality rates in the new generation. Such a correlation was not confirmed for the roach from Lake Łebsko (r = - 0.153, r = - 0.169).

There is a negative correlation between air temperature in the year in which spawning occurred and catches of females in Lake Gardno two years later when the new generation was at recruitment age. This is evidence of the negative impact higher temperatures have on the mortality rates of young roach stages in Lake Gardno.

This process can be explained as follows: the smaller surface area and volume of Lake Gardno in comparison with Lake Łebsko and its less intense water exchange render it more susceptible to eutrophication and degradation. It follows that in warm years the air-water heat exchange is more intense and the water in Lake Gardno heats faster. This lake is heavily loaded with nitrogen and phosphorous in its waters, organisms and bottom sediments. The most threatening source of nutrients for roach are decomposing sediments (especially when the ratio of active bottom area to lake volume is high – according to Cydzik et al. [8]). As the water in the shallow littoral becomes warmer during warm springs and summers, hydrogen sulfide displaces oxygen. Oxygen deficiencies and toxic hydrogen sulfide probably caused high mortality among the larvae and fry inhabiting this environment. The warmest years, during which this process would have been most intense, were 2000 (9.7°C), 1990 (9.5°C), 1999 (9.3 °C), 1989 (9.3°C), 1992 (8.9°C), 1994 (8.8°C). It should also be added that the average temperature in the five months preceding spawning in 1989, 1990, 1998 and 2000 was exceptionally high at more than 6°C, with an average for the entire period of 4.52°C. Thus, environmental conditions during these years were not conducive to the development of larvae which is evident in the weak recruitment of the roach population in Lake Gardno.

No such dependence was observed in Lake Łebsko. This is most certainly due to this lake’s lower susceptibility to degradation thanks to its more advantageous morphometry, larger volume and stronger inflows of marine waters [28].

The recruitment of fish to the exploited population is shaped by the fish’s behavior and habitat and its sensitivity to hydrological and climatic factors, such as temperature of water level variations which can occur in warm years [15]. Mingelbier et al. [15] studied two smelt populations in the St. Lawrence estuary and confirmed that there is a positive correlation between the aforementioned factors and the recruitment of smelt which inhabited the deep canals in the north of the basin. The second population, which inhabited the shallow, southern coastal area of the estuary, reacted differently; there was a fall in the number of recruits when temperatures or water levels rose.

Many authors have reported that climatic factors have an impact on the recruitment and growth of various fish species. Mann and Mills [14] and Cowx [6, 7] link an exceptionally hot summer in the year preceding dace and roach spawning with accelerated somatic growth in that year, which, as a consequence, reduced fecundity in the following spawning season. Ponton and Gardeaux [19] maintained, that in Lake Geneva, climatic factors, such as strong wind, increase the mortality of roach larvae and limit the abundance of some generations.

Dippner [9] reported a negative correlation between the influence of global warming (temperature anomalies in surface waters) on the recruitment of marine species such as mackerel, cod, saithe and whiting.

The cyclic occurrence of strong and weak generations in roach populations inhabiting eutrophic lakes in England is explained by Townsend and Perrow [27] as the result of the absence of any specimens older than 4+ and the related feeding competition for zooplankton. When a strong generation occurs and food availability is low, the older specimens grow more slowly and fecundity is limited. This results in the next generation being weaker. Townsend and Perrow maintained that cyclicality does not occur when the roach population is composed of many older age groups because competition for food does not occur. They also suggested that the occurrence of abundant and less abundant generations might be related to climatic factors which initiate this cyclicality.

Wysokiński et al. [31] linked the abundance of pikeperch generations with years when water temperatures during the spawning period were lower in comparison with periods when weaker years were born.

Similar conclusions were drawn from the analyses of changes in the abundance of the roach population in Lake Gardno. Cyclicality in recruitment was not confirmed. Advanced eutrophication and a rise in the average air temperature probably contributed to worsened spawning conditions, which led to a decrease in the abundance of the roach population in Lake Gardno at the end of the 1980s. The direct consequence of this was that fewer eggs were released by the population, which meant that the generations were less and less abundant.

The population composed of larger roach specimens in Lake Łebsko, in which degradation was less pronounced, remained balanced, and during the six-year study period the abundance of the spawning stock did not vary significantly.

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Anna Hornatkiewicz-Żbik, Władysław Ciepielewski
Department of Ichthyobiology
Inland Fisheries Institute
M. Oczapowskiego 10, 10-719 Olsztyn, Poland
e-mail: zbiczek@infish.com.pl
wciepielewski@infish.com.pl

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed ‘Discussions’ in each series and hyperlinked to the article.


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