Acta Sci. Pol., Piscaria 5 (2) 2006, 17 36

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1 Acta Sci. Pol., Piscaria 5 (2) 26, SIZE AND FECUNDITY OF PERCH FEMALES AT AGE OF MATURITY Władysław Ciepielewski, Anna Hornatkiewicz-Żbik University of Warmia and Mazury in Olsztyn Abstract. The perch, a species present in most Polish freshwater reservoirs, is not protected. A continuous decline of the perch population abundance in Poland, reflected in catches of fishermen and anglers over the second half of the last century and in the first years of the 21st century, is mainly a result of increased intensity of fishing. At present, the perch abundance is much lower than that required by sustainable functioning of aquatic communities in which the species diversity and abundance of fish is kept at a stable level corresponding to the trophic status of an area. The aim of the analysis presented was to asses the size and fecundity of perch females at age of maturity and to use the results to propose a protective size which, when deemed legally binding, would contribute to a faster recovery of overfished perch populations without drastic restrictions to fisheries. Materials for the study were collected from the Polish coastal lakes on the southern Baltic coast, from the Vistula Lagoon, from Mazurian lakes in the vicinity of Olsztyn, and from the power station-heated lakes near Konin. Samples were collected in 2 25 from catches effected with trap and drag nets; sampling was conducted in late autumn, when the perch growth is arrested, as well as in winter and spring, prior to and during spawning. The smallest females which were ready to spawn had survived three growth seasons (age group 2+) and measured cm total length (L.t.). They contributed 25% to all the females of that age group caught by fishermen. The largest 3-year-old females ( cm L.t.) occurred sporadically in the catches. The most frequent females at age of first maturity measured about 17.5 cm L.t. Absolute fecundity of the smallest females that spawned for the first time was 5 1 thou. eggs per fish. A female measuring 17.5 cm L.t. at age of first maturity carried about 13 thou. eggs. A protective size of 18. cm L.t. is sufficient for spawning of 7% of the perch females that have reached maturity for the first time. Key words: age, fecundity, length distribution, perch, protective size, sex INTRODUCTION The Polish perch population has been observed to decline since the 197s, the decline being reflected in catches [Szczerbowski 1993]. The decline gained momentum within the last fifteen years not only because of increased fishing pressure in lakes but mainly, as it seems, because of the constantly increasing pressure of angling [Ciepielewski and Dominiak 24, Oglęcki 26]. Corresponding author Adres do korespondencji: Dr hab. Władysław Ciepielewski, Ichthyology Department, University of Warmia and Mazury, M. Oczapowskiego1, Olsztyn, Poland, wciepielewski@infish.com.pl

2 18 W. Ciepielewski, A. Hornatkiewicz-Żbik The perch is not protected in Poland, as the species used to be, in the past, considered a pest, a predator feeding on eggs and juveniles of commercially valuable fishes, and as a species penetrating all parts of a water body. Therefore it was recommended to destroy the perch eggs and to catch the fish regardless of their size [Żuromska 1961]. Studies on feeding of perch, carried out in [Antosiak 1963a] in some Mazurian lakes showed the species to consume, in addition to invertebrates, mainly its smaller conspecifics (almost 7% of the fish consumed), and to be the second largest component, after roach, of the pike food to which it contributed more than 2% [Antosiak 1963b, Załachowski 1965]. When controlling the number of predators in a water body, excessive removal of pike enhances the abundance of small perch that feed on juveniles of other species. Therefore protecting the perch of older age classes, which feed mainly on the conspecific fry and juveniles, together with simultaneous removal of perch eggs, may prove an efficient way to depress the perch population size, as suggested by Antosiak [Antosiak 1963b]. However, increased demand for fish, growing numbers of anglers, and increasingly more effective angling methods have led to a situation whereby the perch, the most common predator which, until recently, occurred in mass abundances in almost all the Polish water bodies, occurs at present at abundances much lower than those necessary for an aquatic community to function in a balanced way, in which the fish species diversity and abundance is maintained at a stable level, appropriate to the trophic status of a water body. Even partial protection measures (commercial sizes, fishing limits) observed by water body leasees might increase the species abundance to the desired level in those water bodies in which the too-abundant populations of some cyprinids (roach, bream, bleak, and Prussian carp) indicate the absence of perch, a predator feeding voraciously on all developmental stages of various fish species. This study was aimed at analysing the size structure of small perch (size category M, consisting of individuals weighing less than 1 g) caught by fishermen, and at assessing the age, size, and fecundity of females present in catches of that perch category. It was also deemed necessary to propose a protective size, such that would facilitate a faster recovery of the too-intensely exploited population, without any drastic restrictions in fisheries of the species. MATERIAL AND METHODS The materials for the study were collected from coastal lakes in the Polish coastal zone of the southern Baltic as well as from the Vistula Lagoon, Mazurian lakes in the vicinity of Olsztyn (2º 3 E, 53º 47 N), and power station-heated lakes near Konin (18º 16 E, 52º 13 N). 1. The coastal lakes studied are large and shallow flow-through basins: Lake Bukowo (1898 ha; average and maximum depth 1.8 and 3 m, respectively); Lake Gardno (2468 ha; average and maximum depth 1.3 and 2.5 m, respectively), and Lake Łebsko (714 ha, Acta Sci. Pol.

3 Size and fecundity of perch females at age of maturity 19 average and maximum depth 1.6 and 5. m, respectively). Each lake is permanently connected with the Baltic via a discharge channel, and separated by a narrow spit from the sea. Due to their exposed and flat shores, the water in the lakes is well-mixed by the prevailing winds, most frequently the northerlies and north-westerlies. The northerlies cause storm surges along the southern Baltic shore, as a result of which the water level in the sea is higher than that in the coastal lakes. Consequently, the seawater, usually colder by a few degrees Celsius than the water in the lake, flows into the lake. Such influxes, varying in size and depending on wind force and duration, occur several times a year. Effects of the Baltic climate (summer air temperatures usually lower by a few degrees Celsius than those inland), influxes of cool seawater, and intensive wind-driven mixing reduce the water temperature in the coastal lakes during the growth season, compared to the situation in lakes located further inland. 2. The Polish part of the Vistula Lagoon is a ca ha basin exposed to wind; its average depth is 2.7 m, and the bottom is level. The Lagoon is subjected to climatic and hydrological effects identical to those operating in the coastal lakes. 3. The lakes in the vicinity of Olsztyn (Mazurian Lake District) selected for the study are deep, vendace-type lakes with a narrow littoral and an extensive pelagic zone. They are flow-through lakes located in the River Łyna catchment: Lake Pluszne (92.4 ha; average and maximum depth 15.1 and 52 m, respectively); Lake Mielno (362 ha; average and maximum depth 12 and 39 m, respectively); Lake Maróz (345.5 ha; average and maximum depth 11.9 and 41 m, respectively); and Lake Łańskie (1133 ha; average and maximum depth 16 and 54 m, respectively). 4. Heated lakes near Konin (central part of Poland) are medium-size basins. Three of them: Lake Licheńskie (152 ha; average and maximum depth 4.9 and 13.3 m, respectively); Lake Wąsosko-Mikorzyńskie (244 ha; average and maximum depth 11.9 and 38 m, respectively); and Lake Ślesińskie (162 ha; average and maximum depth 7.5 and 25.7 m, respectively) are typical trough lakes with a narrow littoral. The remaining two lakes of the complex are shallow, oval in shape, twice as large as the others, and exposed to wind. The Konin lakes form an open complex receiving, and cooling down, the water discharged by two power stations located on their shores. Throughout a year, the lakes receive discharges of heated water, varying in quantity and temperature [Zdanowski and Korycka 1976]. The fish fauna of the lakes (both the non-predatory species and predators) has adapted to novel thermal regime involving higher water temperature throughout a year, compared to that in non-heated lakes. As a result, the fish spawn earlier and their growth rate is faster as of their first year of life, an effect induced not only by a raised water temperature, but also by a prolonged growth season [Zamojska 1971, Ciepielewski 1977, Marciak 1977, Wilkońska 1977, Żuromska 1977]. Materials for the analyses were collected in 2 25 in late autumn, when the perch growth is arrested [Le Cren 1951] as well as in winter and spring, prior to and during spawning. Samples were collected at random from fishermen s catches effected with different gear types (traps, vendace seines, drag nets) equipped with the smallest mesh sizes (18 2 mm) applicable to a gear type in question. Piscaria 5 (2) 26

4 2 W. Ciepielewski, A. Hornatkiewicz-Żbik The fish were weighed (to 1. g) and measured (total length, L.t., to 1 mm). It should be mentioned that Polish fishermen sort the perch caught into the following size categories: M (small fish) comprising individuals weighing less than 1 g; S (medium-sized fish) covering larger individuals weighing more than 1 g (usually about 2 g); and D (large fish) with individuals weighing more than 4 g. The size classification of perch, and other fish species as well, is dictated by the market, with larger fish attaining higher price. Samples for this study consisted of the fishes assigned to category M, on the assumption that this is the category that groups the youngest females, those at age of first maturity. Some samples were collected from size category S and from catches not sorted into size categories, to check if the catches would contain the youngest mature females of an exceptionally high growth rate. Such an exceptionally high growth rate is typical of those perch individuals that feed primarily on fish [Le Cren 1992]. Some of the fish sampled were incised to determine their sex. Maturity stage of ovaries was determined following the classification of Sakun and Buckaja [1968]. The absolute fecundity was assessed by wet weight, assuming after Hornatkiewicz-Żbik [23] that an overestimation by a few per cent, inherent in the method (when estimating the fecundity, the egg weight contains also the weight of the connective tissue in the ovary), does not affect the order of magnitude of the result in any substantial manner. Age of the individuals examined was determined both from scales and from the opercular bone to increase the accuracy of reading [Le Cren 1947]. The skewness of distributions was calculated from the formula: mean (x) modal (Mo) Skewness (Sk) = standard deviation (SD) where the modal value is the mid-point of the length class most abundant in the materials on hand. The significance of differences between the distributions was tested using the Kolmogorov-Smirnov test. A total of 127 perch individuals representing size category M, caught in different lakes, were examined. The heated lakes yielded as few as 63 individuals; individual lakes from that group produced from a few to several individuals each, as the species is only sporadically caught by the fishermen operating there [Ciepielewski and Dominiak 24]. Length distributions of females and males were determined from samples collected on a single occasion from the Vistula Lagoon (n = 148) and from two Mazurian lakes: the Mielno (n = 146) and the Łańskie (n = 15). A single sample of small perch (n = 25) was obtained from a winter (January 25) catch of smelt; such catches are sporadically only (once in a few years) made in Lake Łańskie. The fishermen use then a small mesh size trawl (6 mm in codend). The sample obtained from that trawl was additionally used to assess the size structure of category M perch females and males in Lake Łańskie. Length distributions of the smallest mature perch females representing age group 2+, found in the catches, were plotted based on samples containing slightly more than 1 individuals, on the assumption that the plots would fairly accurately reflect length variability of the females belonging to that age group. A sample of the youngest mature females collected from the heated lakes and containing as few as 4 individuals was Acta Sci. Pol.

5 Size and fecundity of perch females at age of maturity 21 included in the analysis as well, since the heated lake fish are the fastest growing among all the Polish populations. The samples analysed for their age structure contained females a year older (age group 3+), representing the smallest individuals of their age group. To obtain a length distribution representative of individuals of that age group, data on about 1 females from different samples of non-sorted catches were used. RESULTS Length distributions The size structure of the small (size category M) perch caught by fishermen is illustrated with length (L.t.) distributions plotted with 2-cm wide class intervals. a. Coastal lakes and Vistula Lagoon Length distributions of the small perch caught by fishermen (Fig. 1) are positively skewed, with a distinct mode in the cm class. The mean length of the fish caught with traps was similar (about 18 cm L.t.) It was only in Lake Bukowo that the mean was slightly lower and amounted to 17 cm (the sample was collected from a drag net catch). 7 Vistula Lagoon Zalew WiÊlany Number of individuals Liczba osobników N = 148 O 17.6 cm Lake Gardno Jezioro Gardno N = 88 O 18.3 cm Length classes L.t., cm Klasy długości L.t., cm Piscaria 5 (2) 26

6 22 W. Ciepielewski, A. Hornatkiewicz-Żbik Number of individuals Liczba osobników Lake ebsko Jezioro ebsko N = 67 O 18. cm N = 261 O 17. cm Lake Bukowo Jezioro ebsko Length classes L.t., cm Klasy długości L.t., cm Fig. 1. Length distributions of small perch (size category M) in fishermen s catches from coastal lakes and the Polish zone of the Vistula Lagoon during late fall, winter and early spring. Except for Lake Bukowo where drag nets were sampled, all other samples were collected from trap net catches Rys. 1. Rozkład długości okonia (M) w połowach rybackich z jezior przymorskich i z polskiej strefy Zalewu Wiślanego. Próby zbierano z połowów dokonywanych żakami. Dane z jeziora Bukowo zebrano z połowów niewodowych The smallest fish, sporadically caught by traps, belonged to the cm class. The Bukowo drag net catches contained individuals representing the length class below that. The largest individuals, caught occasionally by traps, represented the cm length class (lakes: Gardno and Łebsko). The largest fish caught in the Bukowo belonged to a smaller length class ( cm), while still smaller fish represented the largest length class in the Vistula Lagoon ( cm). b. Lakes in the vicinity of Olsztyn The size range of the fish examined (Fig. 2) was widest in the largest samples (lakes: Mielno and Maróz) ( cm). Smaller samples obtained from the Łańskie and Pluszne showed distributions spanning the range of cm. Acta Sci. Pol.

7 Size and fecundity of perch females at age of maturity 23 Number of individuals Liczba osobników Number of individuals Liczba osobników Lake Mielno Jezioro Mielno N = 183 O 17.1 cm Lake Maróz Jezioro Maróz N = 317 O 18.1 cm Lake aƒskie Jezioro aƒskie N = 15 O 18.2 cm Lake Pluszne Jezioro Pluszne N = 38 O 18.2 cm Length classes L.t., cm Klasyd ugoêci długości L.t., L.t., cm cm Fig. 2. Length distribution of small perch (size category M) in fishermen s catches from lakes: Mielno, Maróz, and Łańskie obtained in winter (24/25), and from Lake Pluszne in April 24. Winter and spring samples were collected from drag net and trap net catches, respectively Rys. 2. Rozkład długości małego okonia (M) w połowach rybackich dokonywanych w okresie zimowym (24/25) na jeziorach: Mielno, Maróz, Łańskie i na Jeziorze Pluszne (kwiecień 24). Próby zimowe pozyskano z połowów niewodowych, wiosenną porą z żaków Piscaria 5 (2) 26

8 24 W. Ciepielewski, A. Hornatkiewicz-Żbik As shown by the respective length distribution plots, the dominant individuals measured cm L.t. (lakes: Mielno and Pluszne) and cm (the Maróz and Łańskie). The mean length of the size category M perch caught in the Maróz, Łańskie, and Pluszne was, in spite of various sample sizes, virtually identical and amounted to about 18.2 L.t. It was only in Lake Mielno, the perch population of which was clearly dominated by the fish measuring cm, that the mean length was lower by about 1 cm and amounted to 17.1 cm L.t. c. Heated Konin lakes Due to the small size of the samples (a few to several individuals per sample), length distributions of the size category M perch from all the lakes of the complex were pooled and plotted in a single graph (Fig. 3). Number of individuals Number of individuals Liczba osobników Liczba osobników N = 62 O 18.8 cm Length Length classes L.t., cm Klasy długości d ugoêci L.t., cm cm Fig. 3. Length distribution of small perch (size category M) in the Konin Lake Complex. Samples were collected during winter seasons of from drag net catches Rys. 3. Rozkład długości małego okonia (M) w jeziorach kompleksu konińskiego. Próby zebrano zimą w latach z połowów niewodowych The smallest individuals assigned to the size category M measured cm, the cm class grouping the largest fish. Thus, the size range was similar to that found in fishermen s catches in other regions. The population was dominated by the individuals representing the cm length class. The mean length amounted to 18.8 cm. d. Size structure of the size category M perch across the regions A total of 56 individuals from the coastal region (the coastal lakes and the Vistula Lagoon) and 645 individuals from lakes in the vicinity of Olsztyn were measured in this study (Fig. 4). The mean length of the category M perch caught in those regions was about 17.8 cm, the modal length being situated in the cm length class. The distributions were moderately positively skewed (Sk about +.3). The cm length class was the second most abundant class. Those individuals smaller than 14 cm L.t. and larger than 24 cm did not contribute more than 2% of all the fish caught. The length distributions obtained Acta Sci. Pol.

9 Size and fecundity of perch females at age of maturity 25 for those regions were identical (λ =.79 lower than the critical λ = at the significance level α =.1) A N = 56 O 17.6 cm Number of individuals Liczba osobnikûw Number of individuals Liczba osobników Number of individuals Liczba osobników B N = 645 O 17.8 cm C N = 62 O 18.8 cm Length Length classes classes L.t., L.t., cm Klasy d ugoêci długościl.t., L.t., cm cm Fig. 4. Perch (size category M) length distribution in fishermen s catches from different regions. A, coastal lakes and Vistula Lagoon; B, Mazurian Lake District (vicinity of Olsztyn); C, heated lakes of the Konin Lakes Complex Rys. 4. Rozkład długości okonia (M) w połowach rybackich z różnych regionów. A jeziora przymorskie i Zalew Wiślany, B jeziora z okolic Olsztyna, C podgrzewane jeziora kompleksu konińskiego Piscaria 5 (2) 26

10 26 W. Ciepielewski, A. Hornatkiewicz-Żbik Distribution C (repeated in Fig. 3) is strongly positively skewed (Sk = +.72), the mean of 18.8 cm being by almost 2 cm higher than the mode (17.). The comparison of the distributions A and B with the less abundant group of fish producing distribution C showed them to be identical as well, as the test values of λ A,C = and λ B,C = were lower than the critical λ = at α =.1. e. Length distribution (L.t.) of the size M perch in fishermen s catches across all the water bodies studied The size structure of the size category M perch caught by fishermen in the water bodies studied is shown in Fig. 5. The graph was plotted by pooling the data from all the water bodies because, as shown by the statistical tests, the length distributions were not significantly different from one another. % catch Udział procentowy, % N = 1267 O 17.7 cm Length classes L.t., cm Klasy długości L.t., cm Fig. 5. Perch (size category M) length distribution in lakes studied. All individuals obtained from a lake were pooled Rys. 5. Rozkład długości okonia (M) w badanych zbiornikach wodnych. Wszystkie osobniki zebrane z badanych jezior są przedstawione razem The mean length of the size M perch caught by fishermen in the regions studied was 17.1 cm L.t., the mode being situated within the cm class which grouped about 45% of all the size M perch caught. The distribution is moderately positively skewed (Sk = +.34). Those individuals smaller than 14 cm L.t. contributed about 3% to the catches, the fish larger than 24 cm contributing.5%. About 6% of all the size M perch caught by the fishermen were thus contributed by individuals measuring less than 18 cm in total length. Size of females at age of first maturity In late autumn and in winter, the ovaries of maturing females were at maturity stage IV, i.e., at an early developmental stage. During spawning, some females were found to contain hard ovaries (advanced stage IV), others were running (stage V), and still others spent (stage VI); stage classification followed that of Sakun and Buckaja [1968]. The spent females were not numerous in the catches as the samples were usually collected Acta Sci. Pol.

11 Size and fecundity of perch females at age of maturity 27 at the beginning of the spawning season, in April, when the mass perch spawning in Polish waters usually takes place [Terlecki 2, Epler et al. 25]. Those individuals larger than 14 cm L.t. found in the fishermen s catches of the size M perch were dominated by females. No females mature to spawn were found among the fish smaller than 14 cm in the materials analysed. The lower size class ( cm) was found to contain only mature males and immature individuals (Fig. 6). 5 4 Vistula Lagoon Zalew WiÊlany N = Number of individuals Liczba osobników Lake Mielno Jezioro Mielno N = Lake aƒskie Jezioro aƒskie N = immature niedojrza e males samce females samice Length classes L.t., cm Klasy długości L.t., cm Fig. 6. Length distribution of males and females of small perch (size category M) caugth by fishermen with traps in the Vistula Lagoon and with drag net in lakes Mielno and Łańskie Rys. 6. Rozkład długości samców i samic okonia sortymentu (M) złowionego w żaki przez rybaków w Zalewie Wiślanym i niewodem w jeziorach Mielno i Łańskie Piscaria 5 (2) 26

12 28 W. Ciepielewski, A. Hornatkiewicz-Żbik The smallest females found in the samples, both maturing and mature, had already survived three growth seasons (age group 2+) and represented the length class cm L.t. ( cm L.c.). Length distributions of the females belonging to the age group 2+ in different regions are illustrated in Fig. 7. The regions are denoted as in Fig. 4. The coastal region (A), the vicinity of Olsztyn (B), and the Konin lakes (C) yielded 19, 112, and as few as 4 females for the analysis, respectively. Despite varying sample sizes, the distributions show identical modal values if, as before, the mode is regarded for simplicity as the mid-point of the size class most abundant in the distributions. The means of distributions A and B are similar (17.4 and 17. cm, respectively), the distribution C mean being higher (18.9 cm) by 1.5 and 1.9 cm, respectively. Distributions A and C are positively skewed (Sk values of +.22 and +.68, respectively). The distribution C skewness is three times that of distribution A. The mean and mode of distribution B are identical (17 cm), in which case the distribution should be assumed symmetrical. The data presented suggest that the length growth of the perch females belonging to age group 2+ in natural, non-heated water bodies is similar, and that the females that have survived three growth seasons attain an average length slightly exceeding 17 cm L.t. The positive skewness of length distributions is small (slightly over +.1). The means and modes are located in the same length class ( cm). The left-hand side of the distributions contain a single length class, the second most abundant one. To the right of the distribution there are three classes, their pooled abundance being similar to that of the class to the left of the distribution. In the heated lakes (distribution C), at a mode similar to that in distributions A and B, the length range is wider. The mean is by almost 2 cm higher than the mode and amounts to about 19 cm L.t. The females to the right of the mode are distributed between four length classes. The left-hand side of the distribution is similar to that observed in distributions A and B discussed above. The high skewness of the distribution (Sk = +.68) and the magnitude of the mean suggest that growth of the perch females that have survived three growth seasons in heated waters is more variable (coefficient of variation V C = 14.8%, as compared to V A = 1.7% and V B = 1.8%), and that the fish are, on the average, larger than those in the non-heated water bodies. The small sample size (few to several individuals from a single lake) precludes any more detailed analysis of the growth rate of females living in lakes that are differently heated during the growth season. Acta Sci. Pol.

13 Size and fecundity of perch females at age of maturity A N = 19 O 14.4 cm 3 2 Number of individuals Liczba osobników B N = 112 O 17. cm C N = 4 O 18.9 cm Length classes L.t., cm Klasy długości L.t., cm Fig. 7. Length distribution of perch females aged 2+ (after three growth seasons) in fishermen s catches in different regions: A, coastal lakes and Vistula Lagoon; B, vicinity of Olsztyn (Mazurian Lake District); C, heated lakes of the Konin Lake Complex Rys. 7. Rozkład długości samic okonia z grupy wieku 2+ (po trzech sezonach wzrostu) w połowach rybackich z różnych regionów: A jeziora przymorskie i Zalew Wiślany; B jeziora położone w okolicy Olsztyna, C podgrzewane jeziora konińskie Piscaria 5 (2) 26

14 3 W. Ciepielewski, A. Hornatkiewicz-Żbik The length distribution of the age group 2+ females plotted for all the individuals in the regions of study is slightly positively skewed (Sk =+.24; Fig. 8). The low skewness was produced in this case by a small difference (.5 cm) between the mean (17.5) and the mode (17. cm), for which reason the skewness, as calculated by the formula: Skewness (Sk) = X Mo SD assumes a low value at an actual standard deviation (SD = 2.12). Those females representing the cm L.t. length class contributed about 7% to all the females aged 2+ which spawned for the first time. Those individuals larger that 18 cm L.t. belonged to a few (4) length classes. The most abundant class (18%) was that grouping females cm long. The largest females aged 2+ in the materials examined were found to measure cm and occurred sporadically in the samples, only in those from the Konin Lake Complex. The largest females from the non-heated waters represented the cm L.t. length class. 5 % catch Udział procentowy, % N = 261 O 17.5 cm Length classes L.t., cm Klasy długości L.t., cm Fig. 8. Length distribution of perch females aged 2+ (after three growth seasons) in lakes studied Rys. 8. Rozkład długości samic okonia z grupy wieku 2+ (po trzecim sezonie wzrostu) w badanych akwenach The females one year older (age group 3+) measured cm L.t., the mode and the mean of the distribution amounting to 21 and 23.2 cm, respectively. The lowest length class in the distribution ( cm) grouped about 2% of the females. The distribution is positively skewed (Sk = +.6) and spans 3 classes more than the distribution of the age group 2+ (Fig. 9). Acta Sci. Pol.

15 Size and fecundity of perch females at age of maturity 31 Number of individuals Liczba osobników N = 99 O 23.2 cm Fig. 9. Length distribution of perch females aged 3+ (after four growth seasons); all individuals from the regions of study were pooled Rys. 9. Rozkład długości samic okonia z grupy wieku 3+ (po czwartym sezonie wzrostu). Wszystkie samice z badanych regionów przedstawiono razem Absolute fecundity of females at age of first maturity Absolute fecundity was assessed for 74 females aged 2+ collected from different water bodies. The gonads for analyses were collected only during the spawning season from the hard females (advanced stage IV). The smallest females ready to spawn measured cm L.t. and showed fecundity of about 5 1 thou. eggs per female. Fecundity of the largest females ( cm L.t.) was about 3 45 thou. eggs. The mean length (L.t.) and the mean absolute fecundity of a female aged 2+, as estimated from data shown in Fig. 1, were 18.2 cm and 16.5 eggs, respectively. The fecundity of a female of that length, as calculated from the equation, was lower(14.9 thou. eggs). Number of eggs in thousands Liczba jaj w tysiącach Total length L.t., cm Długość całkowita L.t., cm Fig. 1. Absolute fecundity of perch females aged 2+ Rys. 1. Płodność absolutna samic okonia z grupy wieku 2+ Piscaria 5 (2) Length classes L.t., cm Klasy długości L.t., cm y =.7x R 2 =.7669

16 32 W. Ciepielewski, A. Hornatkiewicz-Żbik DISCUSSION As mentioned earlier, perch is not protected in Poland. The size of the smallest individuals caught by fishermen depends on the net mesh size applied. Those individuals smaller than 14 cm L.t. contribute about 3% to the size M perch catch. On the other hand, the right-hand size of the length distributions analysed in that size category is limited primarily by the cm L.t. class (about 2.5% of the fish caught). Larger individuals (24 26 cm L.t.) are found in the category M perch catches only sporadically (.5%). It should be added that the right-hand size of the category M perch length distributions analysed depend on the size criteria adopted in the inland fisheries, which have to be respected during sorting of catches. The size M perch catches in Poland are dominated by females, particularly in length classes higher than that containing the modal value (Fig. 6). The smallest females ready to spawn after three growth seasons are about cm L.t. long, their fecundity ranging within 5 1 thou. eggs per female, which has been confirmed also by other authors [Treasurer 1981, Wandzel and Neja 1998, Terlecki 2], although Holmgren [23], who studied perch in the eutrophic Swedish lake Stensjon, recorded smaller females (about 13.5 cm L.t.) at age of maturity, with a similar fecundity. This suggests that the oocytes of the females examined by Holmgren [23] were smaller. As shown by this study, the mean size of the females at age of first maturity (age group 2+) in the Polish water bodies covered by the present research was about 18 cm L.t. A single female of this size produced about 14.5 thou. eggs. All the females aged 2+ and smaller than that total length, contributed about 7% of all the females caught by the fishermen s nets. On the other hand, as few as about 2% of the females aged 3+ were smaller than 18 cm L.t. Thus, setting the protective size at 18 cm will mean that virtually about 7% of the females at age of first maturity and about 2% of the older females will be given a chance to spawn. A ban on catching perch smaller than 15 cm L.t., in force in some areas of Poland, does not protect the females, because both fishermen and anglers remove even the smallest females (aged 2+) at age of first maturity, as indicated by the data obtained in this study. As a result, intensive exploitation (effected by fishermen and anglers alike), at an absence of a protective size and catch quota, leads to a fast reduction of the perch population size in the areas operated in. Both the protective size suggested (18. cm L.t.) and catch quota determined separately for each water body, will prevent a further reduction in the perch population size in the Polish inland waters. The catch quota should be determined in fisheries documentation submitted by water body leasees who have to detail their plans for fisheries management in basins covered by individual fishing areas. CONCLUSIONS The smallest mature perch females were found to have survived three growth seasons (age group 2+) and represented the cm L.t. length class. They were fairly abun- Acta Sci. Pol.

17 Size and fecundity of perch females at age of maturity 33 dant in the fishermen s catches (about 25% of the females of that age group). The largest females in the age group in question ( cm L.t.) were rare in the catches. The most frequent females at age of first maturity measured about 17.5 cm L.t. Absolute fecundity of the smallest females that were ready to spawn for the first time was 5 1 thou. eggs per fish, the largest females producing 3 4 thou. eggs. Absolute fecundity of a female (17.5 cm L.t.) at age of first maturity averaged 13 thou. eggs. The protective size of 18 cm L.t. will make spawning possible for about 7% of perch females reaching age of first maturity each year. REFERENCES Antosiak B., 1963a. Udział ryb w pokarmie starszych roczników okonia (Perca fluviatilis L.) w niektórych jeziorach okolic Węgorzewa [Fishes in the food of older perch (Perca fluviatilis L.) in lakes of Węgorzewo District]. Rocz. Nauk Rol. 82B, 2, [in Polish]. Antosiak B., 1963b. Pokarm szczupaka (Esox Lucius L.) w niektórych jeziorach okolic Węgorzewa [Pike food in some lakes of Węgorzewo District]. Rocz. Nauk Rol. 82B, 2, [in Polish]. Ciepielewski W., Wzrost sandacza (Stizostedion lucioperca L.) w podgrzanym jeziorze Licheńskim [Growth of pike-perch (Stizostedion lucioperca L.) in artificially heated Licheńskie Lake]. Rocz. Nauk Rol. 97H, 4, 7 15 [in Polish]. Ciepielewski W., Dominiak J., 24. Ryby drapieżne kompleksu jezior konińskich odłowy, zarybienia, perspektywy [Predatory fish in the Konin Lakes Complex catches, stockings, prognoses]. Kom. Ryb. 2, [in Polish]. Epler P., Łuszczek-Trojnar E., Szymocha J., Drąg-Kozak E., Socha M., 25. Maturity stage of roach (Rutilus rutilus L.) and bream (Abramis brama L.) gonads from the Solina dam reservoir, and perch (Perca fluviatilis L.) gonads from the Roznow dam reservoir. Acta Sci. Pol., Piscaria 4 (1 2), Holmgren K., 23. Omitted spawning in compensatory-growing perch. J. Fish. Biol. 62 (4), Hornatkiewicz-Żbik A., 23. Fecundity of roach Rutilus rutilus (L.) from the coastal lakes Gardno and Łebsko. Acta Sci. Pol., Piscaria 2 (1), Le Cren E.D., The determination of the age and growth of the perch (Perca fluviatilis) from the opercular bone. J. Anim. Ecol. 16, Le Cren E.D., The length-weight relationship and seasonal cycle in gonad weight and condition in the perch (Perca fluviatilis). J. Anim. Ecol. 2, Le Cren E.D., Exceptionally big individual perch (Perca fluviatilis L. ) and their growth. J. Fish. Biol. 4, Marciak Z., Wpływ podgrzania wody przez elektrownię cieplną na wzrost leszcza w jeziorach konińskich [Influence of thermal effluents from an electric power plant on the growth of bream in the Konin Lakes Complex]. Rocz. Nauk Rol. 97H, 4, [in Polish]. Oglęcki P., 26. Co wiemy o okoniu? [What do we know about perch?]. Wiad. Wędkarskie 2, 14 [in Polish]. Sakun O.F., Buckaja N.A., Opredielenie stadii zrelosti i izucenie polovych ciklov ryb. Izd. Nauc. Isl. Inst. Morsk. Ryb. Hoz. Murmańsk, 5 46 [in Russian]. Piscaria 5 (2) 26

18 34 W. Ciepielewski, A. Hornatkiewicz-Żbik Szczerbowski J.A., Inland Fisheries in Poland. IRŚ, Olsztyn. Terlecki J., 2. Okoń (w: Ryby słodkowodne Polski) [Perch (in: Freshwater fishes of Poland)]. M. Brylińska (Ed.). PWN, Warszawa [in Polish]. Treasurer J.W., Some aspects of the reproductive biology of perch Perca fluviatilis L. Fecundity, maturation and spawning behaviour. J. Fish. Biol. 18, Wandzel T., Neja Z., The fecundity of perch from Międzyodrze waters. Bull. Sea Fish. Inst. 3 (145), Wilkońska H., Żuromska H., Wzrost narybku w podgrzewanych jeziorach konińskich [Growth of the fry in the heated Konin Lakes Complex]. Rocz. Nauk Rol. 97H, 4, [in Polish]. Wilkońska W., Wzrost płoci (Rutilus rutilus L.) w podgrzanym jeziorze Licheńskim [Growth of roach (Rutilus rutilus L.) in heated Licheńskie Lake]. Rocz. Nauk Rol. 97H, 4, [in Polish]. Załachowski W., Odżywianie się szczupaka z jezior Legińskich [The food of the pike from Legińskie Lakes]. Zesz. Nauk. WSR Olszt. 2, [in Polish]. Zamojska B., Współczynnik pokarmowy młodego sandacza z jeziora Licheń [The food coefficient of young pike-perch from Lake Licheń]. Rocz. Nauk Rol. 97H, 4, [in Polish]. Zdanowski B., Korycka A., Wpływ zrzutu wód podgrzanych na stosunki termiczno-tlenowe i przezroczystość wody jezior konińskich [The influence of heated effluent waters on thermaloxygen relations and waters transparency in the Konin Lakes Complex]. Rocz. Nauk Rol. 3H, [in Polish]. Żuromska H., Wzrost okonia (Perca fluviatilis L.) w jeziorach okolic Węgorzewa [The growth of perch (Perca fluviatilis L.) in lakes of Węgorzewo District]. Rocz. Nauk Rol. 77B, 2, [in Polish]. Żuromska H., Wzrost wzdręgi (Scardinius erthrophthalmus L.) w jeziorze sztucznie podgrzewanym [Growth of rudd (Scardinius erthrophthalmus L.) in an articficially heated lake]. Rocz. Nauk Rol. 97H, 4, [in Polish]. WIELKOŚĆ I PŁODNOŚĆ SAMIC OKONIA W WIEKU OSIĄGANIA DOJRZAŁOŚCI PŁCIOWEJ Streszczenie. Okoń zamieszkujący prawie wszystkie akweny w Polsce nie podlega ochronie. Stały spadek liczebności populacji okonia w wodach śródlądowych Polski mający odzwierciedlenie w połowach rybackich i wędkarskich w ostatnim półwieczu i pierwszych latach bieżącego stulecia jest głównie efektem zwiększonej intensywności połowów. Okoń występuje obecnie w ilościach dużo mniejszych niż wymagane do zrównoważonego funkcjonowania biocenoz wodnych w których różnorodność gatunkowa i ilościowa ryb jest utrzymywana na ustabilizowanym poziomie odpowiednim dla trofii danego akwenu. Celem przedstawionej w opracowaniu analizy jest określenie wielkości i płodności samic w wieku osiągania pierwszej dojrzałości płciowej i wykorzystanie tych wyników do zaproponowania wymiaru ochronnego, którego legalizacja umożliwiłaby szybszą odbudowę zbyt intensywnie eksploatowanych populacji okonia, bez drastycznych ograniczeń w eksploatacji gatunku. Materiał do analiz zebrano z jezior przymorskich polskiej strefy brzegowej południowego Bałtyku, Zalewu Wiślanego, jezior mazurskich z okolicy Olsztyna oraz z podgrzewanych przez elektrociepłownie jezior położonych w okolicy Konina. Próby pozyskano z połowów rybackich dokonywanych za pomocą niewodów, żaków i wontonów w latach 2 25 w okresie późnej jesieni, kiedy wzrost Acta Sci. Pol.

19 Size and fecundity of perch females at age of maturity 35 okonia już ustaje oraz w zimie i w okresie wiosennym, przed tarłem i w czasie tarła. Najmniejsze, dojrzałe do tarła samice okonia były po trzech sezonach wzrostu (grupa wieku 2+) i mieściły się w klasie wielkości 14,1 16 cm L.t. Stanowiły one w połowach rybackich około 25% samic odławianych z tej grupy wieku. Największe trzyletnie samice (klasa 22,1 24 cm L.t.) występowały w połowach sporadycznie. Najczęściej spotykane samice w wieku pierwszej dojrzałości płciowej, to osobniki około 17,5 cm L.t. Płodność absolutna najmniejszych samic, pierwszy raz przystępujących do tarła, kształtowała się na poziomie 5 1 tys. ziaren przypadających na jednego osobnika. Na jedną samicę o długości 17,5 cm L.t. w wieku pierwszej dojrzałości płciowej przypada około 13, tys. ziaren ikry. Wymiar ochronny: 18 cm L.t., umożliwia odbycie tarła około 7% samic okonia osiągających pierwszą dojrzałość płciową. Słowa kluczowe: okoń, płeć, płodność, rozkłady długości, wiek, wymiar ochronny Accepted for print Zaakceptowano do druku: Piscaria 5 (2) 26