Scholarly article on topic 'Long-term changes in commercial fish catches in Lake Mamry Północne (northeastern Poland) on the background of physical, chemical, and biological data'

Long-term changes in commercial fish catches in Lake Mamry Północne (northeastern Poland) on the background of physical, chemical, and biological data Academic research paper on "Earth and related environmental sciences"

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Academic research paper on topic "Long-term changes in commercial fish catches in Lake Mamry Północne (northeastern Poland) on the background of physical, chemical, and biological data"

Arch. Pol. Fish. (2009) 17: 195-210 DOI 10.2478/v10086-009-0011-2

RESEARCH ARTICLE

Long-term changes in commercial fish catches in Lake Mamry Polnocne (northeastern Poland) on the background of physical, chemical, and biological data

Arkadiusz Wolos, Boguslaw Zdanowski, Malgorzata Wierzchowska

Received - 20 July 2009/Accepted -10 October 2009. Published online: 16 November 2009; ©Inland Fisheries Institute in Olsztyn, Poland

Abstract. Lake Mamry Polnocne, a part of the Great Masurian Lake System in northeastern Poland, is a valuable body of water from environmental, recreational, and economic points of view. The paper evaluates long-term physical, chemical, and biological parameters as well as commercial fisheries data. Nitrogen and phosphorus influx data as well as physical and chemical data indicate that the trophic state of Lake Mamry Polnocne can be described as moderate from 1967 to 2006, making the lake borderline j5-mesotrophic and eutrophic. The lake's trophic state increased with time, manifesting itself in an increasing concentration of biogenic substances, decreasing water transparency, and worsening oxygen conditions in the hypolimnion. Changes in physical, chemical, and biological parameters generally corresponded with changes in the commercial fish catches of selected species and groups of species indicative of the process of eutrophication.

Keywords: eutrophication, commercial fisheries, physical, chemical and biological data

A. Wolos [H]

Department of Fisheries Bioeconomics

The Stanislaw Sakowicz Inland Fisheries Institute in Olsztyn

Oczapowskiego 10,10-719 Olsztyn, Poland

Tel. +48 89 5241045; e-mail a.wolos@infish.com.pl

B. Zdanowski, M. Wierzchowska Departament of Hydrobiology

The Stanislaw Sakowicz Inland Fisheries Institute in Olsztyn, Poland

Introduction

The eutrophication of lakes is caused by increasing amounts of allochthonous matter transported in from their direct as well as total basins. The element fundamentally responsible for eutrophication is phosphorus, which regulates biological production, limits the development of algae, and determines the degree of eutrophication (Vollenweider 1968, Schindler 1977, Kajak 1979). Phosphorus generally stimulates the eutrophication of lakes with the results ranging from oligotrophy to eutrophy (Vollenweider 1968, 1976, Carlson 1977, Hickman 1980).

Eutrophication is a process that causes far reaching qualitative and quantitative changes at all levels of the trophic state (Kajak 1979). As fish represent one of the higher trophic levels, changes in their number and type are not short lived or accidental but are signs of clearly emerging patterns. This fact can be used to assess the degree of eutrophication in lakes and identify the impact of other factors associated with human pressure. The theoretical framework for this type of an assessment can be found in Colby et al. (1972), Hartmann (1977, 1979), Leach etal. (1977), Persson (1991), Zdanowski (1995), and Bninska (1996). The principles that govern the succession of particular species and groups of species of fish from Salmonidae to Cyprynidae were used to help assess the degree of eutrophication in lakes in Poland. The assessment

was performed based on commercial fisheries data for selected lakes (Bninska 1991, Wolos and Falkowski 2007), selected fish farms (Bninska 1985, Mickiewicz et al. 2003), and all lakes used for commercial fishing purposes with a total area of 280,000 hectares (Leopold etal. 1986).

The aim of the paper is to present long-term changes in commercial fish catches in Lake Mamry Polnocne with special attention being paid to species and groups of species indicative of eutrophication. The hypothesis that eutrophication impacts the size of fish populations in lakes was tested by comparing changes in fish catches with long-term variability in the physical, chemical, and biological parameters of water.

Materials and methods Study area

Lake Mamry Polnocne is located in the Great Masurian Lake District. It is a western mesoregion of the Eastern Baltic Lake District located across the Eastern Baltic-Belarussian Lowland (Kondracki 2000). The Great Masurian Lakes are the largest lake district in Poland - interconnected either naturally or via a system of manmade canals. The lakes' water level is normally at about 116 m above sea level and is maintained via a system of dams on the W^gorapa River and Jeglinski Canal. The Great Masurian Lakes occupy an area of about 307 km2 and constitute about 10% of Poland's total lake surface area (Mikulski 1966, Choinski 1991).

The Great Masurian Lakes are located in a drainage divide zone between the drainage basins of the Wisla and the Pregola rivers. The main tributaries are: the Krutynia, the Jorka, the Orzysza, and the Sapina. Water exits the lake system to the north via the W^gorapa River and to the south via the Pisa and Narew rivers that flow into the much larger Vistula River. The drainage divide in the Great Masurian Lake District runs along the southern edge of Lake Kisajno at the mouth of the Gizycki Canal (Mikulski

1966). The location of the divide can change from Lake Dargin in the north to Lake Mikolajskie in the south, while during periods of drought, from Przystan to Lake Sniardwy (Bajkiewicz-Grabowska 2008). Pollutants in the waters of the central part of the Lake District are usually carried south but in some cases, they can be carried north to Lake Mamry Polnocne.

Lake Mamry Polnocne is the northernmost of the Great Masurian Lakes (Table 1). It occupies an area of 2,504 hectares with a mean depth of 11.7 m and a maximum depth of 43.8 m. The lake formed inside a basal moraine. Today, it is supplied with water by Lake Dargin via Lake Kirsajty as well as by Lake Swi^cajty whose main tributary is the Sapina River. Water exits the lake via the W^gorapa River. Lake Mamry Polnocne possesses two basins: Przystan and Bodma. A slightly separate portion of the lake is called Lake Mamry Male. The bottom of Lake Mamry Polnocne features a diverse topography with numerous depressions and shallows. Table 1

Limnological characteristics of Lake Mamry Polnocne (unpublished Inland Fisheries Institute (IFI) data)

Parameters

Surface area (ha) 2,504.0

Maximum depth (m) 43.8

Mean depth (m) 11.7

Meane depth x maximum depth-1 0.27

Volume (thousands m3) 298,300.0

Maximum length (km) 7.6

Maximum width (km) 5.2

Area of direct basin (km2) 32.0

Area of total basin (km2) 562.3

Mean annual runoff rate (mln m3 year-1) 90.2

Water exchange percentage per year (%) 30

Retention time ( years) 3.3

Hydraulic load (m year-1) 3.6

The lake is surrounded by agricultural land and some forests. Numerous wetlands and thickets line its fringes. The small town of Przystan and a part of the town of Trygort are located next to the lake with the city of W^gorzewo also nearby. There are two recreational centers, a number of summer homes, and five

camp sites in close proximity to the lake (Rozanski 2001). According to the GUS Almanac (2001), the average population density in the lake's direct basin is 15 persons km-2. The Almanac also reports 911 tourists day-1 based on hotel beds. The lake does not have any point sources of pollution while wastewater from the recreational centers is shipped to the municipal treatment plant in W^gorzewo. An inland waterway runs across the lake connecting the port of W^gorzewo in the north to ports located in the southern part of the lake district.

Data collection and treatment

The lake's biogenic substance load was calculated using the Giercuszkiewicz-Bajtlik (1990) estimation method. Data on phosphorus and nitrogen sources were obtained from the following institutions: the Regional Inspectorate for Environmental Protection in Olsztyn and its branch in Gizycko, the county governments of Gizycko and W^gorzewo, the National Highway Authority, as well as the County Road Authority in Gizycko. Population and tourist data for the basin were obtained from the 2001 GUS Almanac. Calculations were performed using biogenic compound runoff coefficients produced by Giercuszkiewicz-Bajtlik (1990). The calculated load data was compared to results produced by a static model and a hydraulic model (Vollenweider 1968, 1976).

Data acquired during the 2000 growing season (spring - autumn) at the deepest sampling sites on Lake Mamry Polnocne were used to test for changes in physical and chemical parameters. Two more sampling sites were added in 2001. Oxygen content was measured using an oxygen probe recording data every one meter from the bottom to the surface of the lake. A Secchi disk was used to measure the transparency of water. Chlorophyll and seston content was determined using water samples collected from the 0-10 m layer. Samples were filtered through glass filters (45 p.m), dried in order to obtain a constant mass at 105°C, and then weighed. Information on chlorophyll content in the epilimnion was obtained from a paper by Napiorkowska-Krzebietke and Hutorowicz (2005). A Ruttner sampler was used to obtain water for physical and chemical testing purposes

from the following three layers: subsurface (0.5 m), metalimnion (15 m), and bottom (0.5 m above the bottom). Standard analytical methods were then employed (Standard Methods 1992, Hermanowicz et al. 1999). The concentration of phosphate and total phosphorus as well as ammonia nitrogen was determined colorimetrically using a Schimadzu UV-1601 spectrophotometer. Total nitrogen and nitrite were determined using an Epoll ECO 20 spectrophotometer. Total nitrogen content, that is the sum of the concentrations of organic and inorganic compounds, was determined for summer periods from 1991 to 1994 and from 2000 to 2001. The trophic state of the lake was determined for summer seasons based on measurements of Secchi disk visibility (SD), total phosphorus concentration (Ptot), and chlorophyll content (Chl). Carlson's Formula (Carlson 1977) was used to transform the results into the trophic state index (TSI). The oligo-mesotrophic state was characterized by TSIsd, TSIPtot, TSIChl values below 40, while values between 40 and 60 were considered moderately trophic (mesotrophic), and eutrophic exceeding 60. The paper includes archival materials from the Inland Fisheries Institute concerning research on Lake Mamry Polnocne from 1986-89 and 1990-99 (Zdanowski and Hutorowicz 1994, unpublished IFI data). Change patterns in the physical and chemical parameters of lake water were tested using the nonparametric statistical Mann-Whitney and Kruskal-Wallis tests. A significance level of P < 0.05 was used for each test.

Commercial fisheries data obtained from Lake Mamry Polnocne fisheries logs for the 1967-2006 time period were used to analyze changes in the fish population. The Lake Mamry Polnocne commercial fishing was supervised by the Gizycko State Fisheries Cooperative until 1993. In 1994, the lake's fishing came under the jurisdiction of the Polish Anglers' Association (PAA) in the provinces of Suwalki.

The paper identifies three groups of species that are considered to be indicative of eutrophication processes:

A. Species and their sizes indicative of poorly eutrophic lakes:

- coregonids (vendace, Coregonus albula (L.) and common whitefish, Coregonus lavaretus (L.))

- littoral species (pike, Esox lucius (L.), tench, Tinca tinca (L.), and crucian carp, Carassius carassius (L.))

- perch, Perca fluviatilis (L.)

- "large" cyprinids (large and midsize stocks of bream, Abramis brama (L.) and large roach, Rutilus rutilus (L.))

B. Species and their sizes indicative of increasing eutrophication:

- "small" cyprinids (small varieties of bream -under 0.5 kg, roach - under 0.2 kg, and white bream, Abramis bjoerkna (L.))

- pikeperch, Sander lucioperca (L.)

C. Species that may be considered "indifferent" given their level of tolerance for environmental changes. On the other hand, eel, Anguilla anguilla (L.) comes exclusively from fish farms:

- Other (smelt, Osmerus eperlanus (L.), bleak, Alburnus alburnus (L.), as well as small trash fish)

Two methods were used in the analysis of commercial fisheries data. The first method was used to analyze the catches of selected species. In the case of bream and roach, stock size was also taken into consideration via the use of time series (trends) including

polynomials up to the fourth degree and exponential curves with a threshold significance level of P < 0.05. Statistically significant polynomials were graphed. In the absence of such polynomials, raw data was graphed instead. Commercial fisheries data (in %) were also analyzed for four consecutive decades (1967-1976, 1977-1986, 1987-1996, 1997-2006) based on the following classification of species: coregonids, littoral species, perch, "large" cyprinids, "small" cyprinids, pikeperch, eel, and others.

Results

Basin influence on the lake

In 2000-2001, the direct basin of Lake Mamry

Polnocne was capable of delivering a phosphorus

load no larger than 0.13 g P m year and a nitrogen

load no larger than 1.61 g N m year , a load value close to the critical value (Table 2). Its total basin, on the other hand, was capable of delivering a phosphorus load of 0.62 g P m 2 year 1 and a nitrogen load of -2 -1

9.83 g N m year , values that are three times critical levels. Over 75% of the loads came from spatial

Table 2

Estimated potential phosphorus and nitrogen influx to Lake Mamry Polnocne from its direct basin and total basin in 2001-2002

Annual phosphorus Annual phosphorus Annual nitrogen Annual nitrogen

load (kg P year-1) load (kg P year-1) load (kg N year-1) load (kg N year-1)

Source

direct basin

total basin

direct basin

total basin

Point1

Spatial 1,502 7,679

Dispersed (animal husbandry) 931 4,007

Atmospheric precipitation 887 3,905

Swimming areas 5 24

Linear 1 2 Lake's actual phosphorus

and nitrogen loads (g m-2 year-1) 0.133 0.624

Lake's critical nitrogen and 0.109-0.219

phosphorus loads (g m-2 year" )_0.111-0.2223_

1 Point sources of pollution were not identified

2 Vollenweider (1968) based on average depth criteria

3 Vollenweider (1976) based on hydraulic criteria

32,562 3,725 3,860 100 2

1.607 1.75-3.50

164,075 16,029 65,620 376

and dispersed sources such as arable land, animal husbandry, recreation, as well as the influx of more fertile waters from Lake Swi^cajty and the southern part of the Mamry Lake System.

Oxygen conditions

Lake Mamry Polnocne is a dimictic lake where waters mix twice during the year - in the spring and autumn. It is a third degree static-type lake, highly susceptible to the mixing of epilimnion waters during the summer stagnation period. Oxygen content

ranged from 8.0 to 15 mg dm in the surface layer

and from 0 to 13.8 mg dm in the bottom layer. Trace amounts of oxygen were detected near the bottom of the lake from 1995 to 2001. Significantly reduced oxygen content in the metalimnion of Lake Mamry Polnocne was observed in the thermocline layer. Oxygen content variability along the vertical cross section of the lake was described by a negative clinograde curve, typical for /3-mesotrophic lakes

(Fig. 1). Oxygen content in the metalimnion was as

low as 1.2 mg dm . Oxygen content in the

hypolimnion fluctuated from 1.3 to 5.1 mg dm (10.7% to 42.4% saturation).

Supply of biogenic substances

Average total phosphorus concentration in the

epilimnion was 0.060 mg dm 3. The concentration of

phosphorus in the bottom layer of the lake was, on

average, 0.090 mg dm (Table 3). Only trace amounts of phosphate were normally detected in the epilimnion. Phosphate content did not exceed 20% of total phosphorus content during the summer season. In the bottom layer, phosphate made up about 70% of total phosphorus content. No significant change patterns in the phosphorus supply of surface waters of Lake Mamry Polnocne were detected.

The concentration of nitrogen in the lake increased in 2000-2001. Organic compounds made up the major portion of total nitrogen in the epilimnion. Inorganic nitrogen (nitrate, nitrite, ammonia) accounted for about 5% during the summer. Ammonia nitrogen was the key determinant of the quantity of total nitrogen in the bottom layer (about 80%). Its

Oxygen (mg O2 dm3) Oxygen (mg O2 dm3)

Figure 1. Oxygen conditions during the summer stagnation period from 1986 to 1994 (A) and from 1995 to 2001 (B).

Table 3

Changes in the trophic parameters of water in the surface layer (a) and the bottom layer (b) of Lake Mamry Polnocne (sampling site #1)

Time Period

1986-19891 1991-19942 1995-19992 2000-2001

Parameter (n=22) (n=12) (n=13) (n=13)

Tot-P (mg dm-3) a 0.032-0.104 0.034-0.039 0.009-0.096 0.032-0.072

b 0.044-0.152 0.072-0.110 0.076-0.200 0.055-0.188

PO4-P (mg dm-3) a 0.001-0.052 0.0-0.006 0.0-0.023 0.007-0.055

b 0.001-0.100 0.010-0.082 0.042-0.095 0.006-0.147

Tot-N (mg dm-3) a - 0.63-1.75 - 1.12-2.02

b - 0.86-2.31 - 0.89-2.56

NO3-N (mg dm-3) a 0.03-0.17 0.05-0.06 0.0-0.06 0.0-1.42

b 0.02-0.25 0.14-0.50 0.0-0.09 0.0-0.29

NO2-N (mg dm-3) a 0.0-0.010 0.0-0.004 0.0-0.008 0.0-0.015

b 0.0-0.019 0.0-0.005 0.0-0.020 0.003-0.035

NH4-N (mg dm-3) a 0.02-0.15 0.03-0.15 0.06-0.21 0.04-0.09

b 0.010-0.460 0.100-0.180 0.060-0.390 0.038-0.405

Secchi disk visibility (m) 2.5-9.5 2.4-4.6 2.0-4.5 2.5-6.5

Seston content (mg d.m. dm-3) 0.3-2.5 - 0.4-3.8 0.4-2.1

Chlorophyll content (mg m-3) 3 0.7-8.9 5.8-5.8 1.7-5.6 2.7-14.9

CODMn(mg O2 dm-3) 8.7-20.5 8.3-14.4 3.2-13.8 5.4-12.8

1 Zdanowski and Hutorowicz (1994)

2archival material from the IFI Department of Hydrobiology in Olsztyn 3 Napiorkowska-Krzebietke (2004)

concentration did not exceed 0.46 mg dm during the summer (Table 3).

No appreciable differences in the supply of nitrogen and phosphorus were detected in the open waters of the lake. Higher concentrations of total

phosphorus (up to 0.250 mg dm ), mainly phos-

phate and ammonia nitrogen (under 1.2 mg dm ), were detected in 2000-2001 only at an isolated sampling site on Upalty Island. Here, a total lack of oxygen was recorded in the bottom layer.

Water transparency

Decreased water transparency was noted in the summer period with an average of about 4.0 m. In most

cases, suspended material (about 2.0 mg d.m. dm ) and chemical oxygen demand (CODMn) were the reasons for this. Long-term data indicates that water transparency decreased temporarily (as low as 2.0 m)

during the last year analyzed. The decrease can be attributed to extensive phytoplankton development in Lake Mamry Polnocne as measured by increased

chlorophyll content (14.9 mg m ). Changes in water transparency in the lake tended to be more related to seston content than chlorophyll content. Changes in seston content in Lake Mamry Polnocne explained 47% of the changes in Secchi disk visibility (r = 0.4735, P = 0.007) while changes in chlorophyll content explained only 28% of changes in visibility (r = 0.277, P=0.036). Only changes in chlorophyll content could have been connected with changes in nitrogen content (r2= 0.580, P = 0.028).

Trophic state

The epilimnion of Lake Mamry Polnocne was classified, in most cases, as mesoeutrophic based on its water transparency index (TSI sd - 37-50) and

chlorophyll content (TSIchl - 45-59). The phosphorus concentration index (TSItp - 36-70) varied within a range characteristic of waters experiencing elevated eutrophication. A rise in the lake's fertility was noted for the latter part of the research period.

Trends of commercial fish catches

Total commercial fish catches clearly decreased from 1967 to 2006 (Fig. 2). Maximum catches - over 80 tons - were recorded from 1967 to 1977. The smallest catches - about 20 tons - were recorded in 1981-1982 and 1991. Fish catches consisted of twelve species of fish - some were labeled as "other" in commercial fishing statistics. Vendace catches fluctuated

120000

100000 80000

60000 40000 20000 0

significantly (Fig. 3) but did not tend to decrease. Common whitefish (Fig. 4) catches have been on the decline since 1976. Pike catches have been rather constant over the last 20 years (Fig. 5), however, they were much lower than in the 1960s and 1970s. Commercial perch catch (Fig. 6) resembles that of pike catch. Despite some small fluctuations, pike and perch catch has stabilized at about 4,000 kg (each) in recent years. Maximum tench catch was recorded from 1968 to 1976, however, beginning in 1977, catch began to drop rapidly with minimum catch being recorded in the 1990s (Fig. 7). Tench catch has seen a small rebound in the last few years. Bream catch was characterized by substantial fluctuations, which made it impossible to construct a statistically significant trend line (Fig. 8). Lake Mamry Poinocne is

y = -1303.6x + 96057 r = 0.5947, P < 0.01

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 2. Total commercial fish catches from 1967 to 2006 in Lake Mamry Poinocne.

S 15000

y = -0.2209x4 + 18.107x3 - 464.91x2 + 3987.5x + 867.17 r = 0.5246, P< 0.01

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 7. Commercial tench catches from 1967 to 2006.

n,n n nn n

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 8. Commercial pikeperch catches from 1967 to 2006.

25000 -20000 -

jr 15000 -■g

10000 -

ll.l ■-

n - n n n n n n n n „ n

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 9. Commercial large bream catches from 1967 to 2006 (large bream - fish with body weight above 1.0 kg).

9000 -

8000 -

7000 -

6000 -

"f 5000 -

" 4000 -

3000 -

2000 -1000

y = -0.4347x3 + 19.908x - 114.68x + 1085.5 r = 0.5325, P < 0.01

rr^TïïTTrrïï

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 10. Commercial medium-size bream catches from 1967 to 2006 (medium-size bream - fish with body weight 0.5 -1.0 kg).

4000 -3500 -3000

Ik 2500 -

3 2000

1500 -1000 500 -0

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 11. Commercial small bream catches from 1967 to 2006.

y = 0.036x - 12.513x - 212.97x + 35670 r = 0.6674, P < 0.01

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

30000 -

25000 -

20000 -

15000 -

10000 -

5000 -

y = -0.286x + 15.223x - 512.01x + 19412 r = 0.6089, P < 0.01

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 13. Commercial large roach catches from 1967 to 2006.

35000 -, 30000 -25000

' 20000 -I 15000 10000 -5000 -

y = 0.322x - 27.735x + 299.04x + 16258 r = 0.6427, P < 0.01

ÈOttL

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

Figure 14. Commercial small roach catches from 1967 to 2006.

-a 6000

nlIn.íIníL

I . ■ I ■ ■ ■ ■ - I ■

1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

dominated by larger bream, however, it is worth noting that the catches of smaller bream have been clearly on the rise since 1997 (Figs. 9,10,11). Roach catch is characterized by a clearly decreasing trend (Fig. 12). The size of the different sizes of roach in the lake are quite similar and the calculated trends point to a clear decrease towards the end of the period of interest (Figs. 13, 14). White bream catch in Lake Mamry Polnocne has been rising since the middle 1990s compared with earlier periods. Given the huge differences in white bream catch during the period of interest, the calculated trend line was statistically insignificant (Fig. 15).

Changes in composition of commercial fish catches

Commercial fish catches were dominated by large sizes of cyprinids from 1967 to 1976. Cyprinids made up 21.3% of total catch (Table 4). The second most common type of fish caught were "small" cyprinids at 20.6%. Large cyprinids continued to dominate commercial fish catches over smaller sizes in the years that followed. Cyprinids made up most of the lake's commercial fish catches during the time period of interest, however, coregonid catches (as a percentage of total catch) continued to rise from year to year, with the exception of the period from 1977 to 1986. Coregonids became the dominant group of fish (23.8% of total catch) during the last decade studied

(1997-2006). Perch catches remained steady as a percentage of total catches during all four analyzed decades, ranging from 4.9% in 1987-1996 to 7.1% in 1967-1976. The highest percentage of littoral species noted in commercial fishing records was 15.5% during the first of the four analyzed decades. The catches of littoral species continued to decline in the years that followed, with the lowest years of catch (5.9%) on record from 1977 to 1986.

Discussion

In light of its environmental and economic value, Lake Mamry Polnocne has been the subject of numerous hydrobiological research studies. Zdanowski (1983) as well as Zdanowski and Hutorowicz (1994) studied the lake's thermal and oxygen conditions, its water chemistry, water transparency (Secchi disk), and chlorophyll content. Research on the relationship between chlorophyll, nutrient concentration, and visibility of Secchi's disk was performed by Kufel (1998). The lake's biogenic substance loads were estimated by Giercuszkiewicz-Bajtlik and Gl^bski (1981) as well as Giercuszkiewicz-Bajtlik (1990). Changes in the types of species present as well as changes in phytoplankton, chlorophyll, and phaeophyta content were also studied (Napiorkowska-Krzebietke and Hutorowicz 2005).

The data used in the paper indicate that the load of biogenic substances from the lake's drainage basin

Table 4

Catch composition (%) of selected fish species and groups of species from 1967 to 2006 in Lake Mamry Polnocne

Species/groups of species 1967-1976 (%) 1977-1986 (%) 1987-1996 (%) 1997-2006 (%) 1967-2006 (%)

Coregonids 16.2 7.3 19.8 23.8 16.3

Littoral species 15.5 5.9 6.2 7.5 9.7

Perch 7.1 6.1 4.9 6.5 6.3

"Large" cyprinids 21.3 30.4 30.4 21.0 25.2

"Small"cyprinids 20.6 25.6 21.9 19.5 21.8

Eel 3.6 9.1 7.4 3.1 5.6

Other 15.7 15.6 9.4 18.6 15.1

was largely responsible for the trophic conditions in Lake Mamry Polnocne. Potential phosphorus and nitrogen loads coming from the lake's total basin were three times greater than critical levels in 2000-2001 and ten times greater than load values forecasted by Giercuszkiewicz-Bajtlik and Gl^bski (1981). The direct basin of the lake supplied only 20% of its biogenic substance load and did not exceed critical levels. Elevated concentrations of phosphorus in the epilimnion indicate a growing degree of eutrophication in Lake Mamry Polnocne in 2000-2001 (< 0.100 mg dm-3). Other signs of encroaching eutrophication include reduced water transparency (as low as 2.0 m) and larger losses of oxygen content from the hypolimnion during summer periods with saturation at 11% (Gieysztor and Odechowska 1958, Zachwieja 1975, Gliwicz et al. 1980, Soszka et al. 1979, Zdanowski et al. 1984, Zdanowski and Hutorowicz 1994, Wroblewska 2002). Oxygen content losses taking place in 2000-2001 and increasing concentrations of phosphate and ammonia nitrogen near the bottom may have been signs of the release of biogenic substances from lake bottom deposits, which would make internal ion delivery more significant as means of lake eutrophication. Physical and chemical parameters were used to classify the lake in 2001 as Class II Water Purity (Wroblewska 2002) while Carlson's trophic index (1977) was used to classify the lake as mesoeutrophic for the purpose of this paper.

Phytoplankton research studies also identified an increase in the fertility of the waters of Lake Mamry Polnocne (Napiorkowska-Krzebietke and Hutorowicz 2005). Common cyanophytes such as Microcystis aeruginosa and Leptolingbya thermalis were determined to be responsible for the elevated concentration of chlorophyll and greater abundance of plankton. The two types of cyanophytes are characteristic of eutrophic waters (Eloranta 1998, Bucka and Wilk-Wozniak 2002).

In addition to purely environmental considerations, Lake Mamry Polnocne and all of the Great Masurian Lakes are also quite important in terms of their recreational and fishing value (Nowicki and Glinska 2000). The lakes' fishing, in conjunction

with increasing level of lake eutrophication, has been the subject of many research papers (Zachwieja 1966, Wolos 1988, Grabowska and Wolos 1989, Pyka et al. 1989, Zakçs and Pyka 1990, Krzywosz 1999, Czerwinski and Wolos 2001, 2004).

The 40-year commercial fisheries data set for Lake Mamry Polnocne presented in the paper illustrates the changes that have taken place in its fish populations. It is important to note that the lake has been classified as a vendace-type lake. According to Szczerbowski (1981), lakes of this type are generally over 20 m deep, feature a hard bottom, sandy shores, little lake bottom vegetation, where the main "indicator" species are vendace, smelt, bleak, common whitefish, and burbot (Lota lota). Other species in the lake include common predators and cyprinids. Mehner et al. (2005) identified two types of fish populations in German lakes: 1) cold water-type with vendace and perch in deeper lakes, and 2) warm water-type with cyprinids in shallower lakes. At the same time, the researchers argued that population types are determined primarily by maximum depth, average depth, chlorophyll, and lake volume. In Poland, other factors affecting fish populations include commercial fishing and fish farming.

Analysis of commercial fisheries data has shown unequivocally that changes taking place in Lake Mamry Polnocne are generally consistent with the model based on the influence of eutrophication developed by the pioneers in the field: Colby et al. (1972) and Hartmann (1977, 1979). However, some departures have been identified, which may be the result of changes in the intensity of fishing efforts.

Clear cut differences in the types of species caught by commercial fishermen are obvious from the 40-year data set (1967-2006). This is especially true of selected species and groups of species indicative of the process of eutrophication. "Large" and "small" cyprinids made up almost 50% of total commercial fish catches. Coregonids, on average, were at 16.3%. Littoral species made up 10% and perch 6.3% of total catch. It is important to note that species labeled "other" made up as much as 15% of total commercial fish catch. Such "other" species include smelt and bleak - species found along with vendace

in lakes with low or moderate trophic states (Szczerbowski 1981). These results unequivocally show that the trophic state of the lake as well as the rate of change of the trophic state were moderate from 1967 to 2006. One only needs to note that the percentage ofcoregonids remained quite high, reaching as much as 24% during the last years of the study period. The percentage of littoral species stood at 7.5% during the same period of time, while perch was at 6.5%, and species labeled "other" made up as much as 19% of total catch.

All species exhibited a declining catch trend, which is consistent with national catch trends (Wolos et al. 2007). Vendace catch fluctuated significantly with no clear trend in sight. Vendace catch reached a 40-year maximum of over 27 tons in 2006. Strong fluctuations in vendace catch are characteristic of this species. In the case of pike, catch fell abruptly towards the end of the 1970s and then stabilized at a rather low level. Pike catch experienced a small rebound after 2000, which may have been associated with increased lake-based fish stocking efforts throughout Poland (Mickiewicz 2008). The trend in the case of the catch of the other common predator, perch, is almost identical to that of pike. Tench catch declines rapidly down to zero in the 1990s and rebounds somewhat after 2000. Total bream catch fluctuates a lot and a statistically significant tendency cannot be established. Large bream dominates commercial fish catch throughout the time period of interest, however, small bream catch increases rapidly starting in 1997, which may be a sign of a moderate increase in the trophic state of the lake. Roach catch continues to decline. This is true of both large and small sizes.

In summary, it can be argued that commercial fishing trends for most species were in agreement with the eutrophication-based fish population model. However, some of the trends presented herein are substantially different from those produced by the model. Examples include the decrease in roach catch and the absence of an increase in the case of bream. These exceptions are associated with reduced fishing efforts caused by low market prices as well as increased cormorant activity (Krzywosz 2003). Such

factors are used by Wolos et al. (2007) to explain the

decrease in the yield of less valuable cyprinid species

throughout Poland.

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Wieloletnie zmiany w odlowach gospodarczych ryb z jeziora Mamry Pólnocne na tle wskazników fizyko-chemicznych i biologicznych

Jezioro Mamry Pólnocne usytuowane w kompleksie Wielkich Jezior Mazurskich nalezy do zbiorników szczególnie cennych z przyrodniczego, rekreacyjnego i gospodarczego punktu widzenia, a jednoczesnie przez wielolecia charakteryzowalo siç bardzo niskim poziomem troficznym i korzystnymi warunkami srodowiskowymi. W pracy zalozono, ze zmiany troficzne wód w jeziorze implikujq. zmiany odlowów gospodarczych ryb. Jej zasadniczym celem bylo zatem zweryfikowanie tezy, ze zmiany trofii mogly wpfynqœ w znaczqcym stopniu na wielkosc i struktur^ gatunków odlowów ryb wskaznikowych dla róznych poziomów troficznych. Weryfikacjç tej hipotezy oparto na podstawie oceny wieloletnich parametrów fizyko-chemicznych i biologicznych oraz statystyk polowów gospodarczych ryb. Przeprowadzone analizy doplywu azotu i fosforu do jezior oraz zmian parametrów fizyko-chemicznych (tlen, przezroczystosc wody, chlorofil, seston, utlenialnosc (ChZT^, fosforany i fosfor calkowity, azot amonowy, azot calkowity i azotyny) wykazaly, ze w calym badanym okresie trofia jeziora Mamry

Pólnocne byla stosunkowo niska, kwalifikujqc jezioro na pograniczu beta-mezotrofii i eutrofii. Wykazano jednak, ze w miarç uplywu czasu w jeziorze Mamry Pólnocne nastçpowal staly umiarkowany wzrost trofii, przejawiajqcy siç m.in. wzrostem koncentracji biogenów, spadkiem przezroczystosci i pogorszeniem warunków tlenowych w hipolimnionie. Zmiany parametrów fizyko-chemicznych i biologicznych na ogól korespondowaly ze zmianami w odlowach gospodarczych gatunków, a takze wyodrçbnionych grup gatunków wskaznikowych dla procesu eutrofizacji. Symptomatyczne byly zmiany odlowów koregonidów, które przy pewnych fluktuacjach utrzymujq. siç stale na wysokim poziomie. Modelowy przebieg mialy tendencje odlowów gatunków litoralowych i okonia, natomiast odbiegaly od modelu trendy odlowów mniej cennych gatunków karpiowatych (ploci i leszcza), co mozna m.in. tlumaczyc zmniejszon^ intensywnosciq. eksploatacji rybackiej, oraz zwiçkszonq. presj^ kormoranów.