GROWTH OF VENDACE LENGTH IN TWO LAKES IN THE VICINITY OF OLSZTYN (MASURIAN LAKE DISTRICT)

Acta Sci. Pol., Piscaria 5(1) 2006, 29 44 GROWTH OF VENDACE LENGTH IN TWO LAKES IN THE VICINITY OF OLSZTYN (MASURIAN LAKE DISTRICT) W adys aw Ciepielewski, Anna Hornatkiewicz- bik Inland Fisheries Institute, Olsztyn, Poland Abstract. The length growth was estimated, using the Dahl-Lea back-calculation method, in the vendace inhabiting two vendace-type lakes ( a skie and Pluszne), similar in surface area (about 1000 hectares) and depth (more than 50 m), which, for many years, have been showing extensive fluctuations in vendace catches. The total length (Lt.) of the vendace caught in the Pluszne was lower by more than 2 cm, on the average, compared to the total length of the vendace caught in Lake a skie, although gill nets of identical mesh sizes were used in both lakes. Analysis of length increments showed the a skie vendace to have grown at a faster rate. As from the second year of life on, the differences between mean lengths were very highly significant (Student s t test, <0.001). As demonstrated by comparing vendace growth between the early 21st century (samples collected during the 2003 spawning season and March 2004; in the case of Lake Pluszne, a sample obtained in July 2005 was included into the analysis) and the 1960s and 1980s, the present (early 21st century) growth rate in the Pluszne is slower than it used to be. In contrast, the vendace growth rate in the anskie has not changed. The Pluszne vendace grow rate shows a pattern similar to that reported from the vendace of Lake Narie, known in past decades for its slow growth. Most probably, the unfavourable conditions of life the Pluszne offers the older vendace at present (the fish stay mainly in the epilimnion during the summer), manifested as intensified competition for food, result in periodic food deficiencies and retard the individual growth rate. Key words: back calculations, growth rate, vendace INTRODUCTION Polish lakes lie at the southern boundary of the vendace, a typical short-lived species; the vendace growth rate in those lakes is highly variable [Bernatowicz 1953, 1961]. A similarly variable growth rate of the vendace was reported from Finnish lakes [Viljanen 1986] which lie at the northern boundary of the species range in Europe; those lakes are mostly oligotrophic; the process of eutrophication is less advanced than it is in lakes of Poland. Corresponding author Adres do korespondencji: Dr hab. W adys aw Ciepielewski, Ichthyology Department, Inland Fisheries Institute, M. Oczapowskiego 10, 10-719 Olsztyn, Poland, e-mail: wciepielewski@infish.com.pl

30 W. Ciepielewski, A. Hornatkiewicz- bik Differences in growth rate between age classes of vendace generations in various water bodies are usually explained by invoking food deficiency. Insufficient food resources may be a result of competition for food between the vendace and the smelt (both species occupy identical ecological niches) or the juvenile roach and perch and adults of other species (e.g., bleak) that feed in the epilimnion. The competition arises when the vendace is forced, by oxygen deficiency in the hypolimnion in summer, to move to better oxygenated upper layers of the water column. A lower growth rate may be also a result of the presence of a strong generation of the vendace under favourable conditions for reproduction and fry growth. In effect, elimination of zooplankton by the vendace is then faster than the recovery of zooplankton biomass, which may lead to the vendace starving as early as during the initial months of their life. Toxic phytoplankton (cyanobacteria) blooms may also reduce vendace food resources by restricting zooplankton abundance hot and windless days. In addition, water temperature exceeding 20 o C may diminish feeding intensity of the vendace, which is reflected in a slower growth rate. The variety of phenomena affecting vendace growth, which may often appear concurrently in a water body, concerns not only the typical vendace lakes [Szczerbowski 1993], but primarily those lakes which, in the past decades, were classified as the vendace fishery lakes and, at present, should be more appropriately termed bream lakes. Those lakes are usually large (several hundred hectares in surface area) and deep (mean depth of several metres, maximum depth exceeding 40 m); they show oxygen deficiency in the lower layers of the thermocline and in the hypolimnion, a hydrogen sulphide-containing layer extending high above the bottom in summer. As a result, pelagic species (vendace, smelt) are forced to move to the epilimnion and, during their intensive individual growth, are affected by particularly adverse physical processes (high temperature of the water, its low transparency during phytoplankton blooms) and densitydependent ones (competition for food with other species feeding in the epilimnion). No study has dealt with variations in the vendance growth rate in a lake, over a few, several, and more years, under conditions of progressing eutrophication of lakes in Poland. Therefore, the rate of length increase of the vendace was estimated in two vendacetype lakes located close to each other (the smallest distance did not exceed 5 km) in the vicinity of Olsztyn: Lake a skie (53 o 35.9N; 20 o 29 E) and Lake Pluszne (53 o 35.9N; 20 o 24.5 E) (Fig. 1). The two lakes are placed within the same category (vendace fishery lakes) of fisheries-based lake classification. They are similar in their surface area (1128 and 903 ha of the a skie and Pluszne); the maximum and mean length amounting in both to about 55 and 15 m, respectively. Both are flow-through lakes. The a skie lies in the course of River yna (mean flow rate of about 2 m 3 s -1 ), while the Pluszne receives discharges of some small streams (a total flow rate of about 1 m 3 s -1 ; Figs. 2 and 3). The small outflow of the Pluszne is discharged into Lake Poplusz (14.7 ha surface area), connected via a small stream with River Marózka emptying into the yna. The vendace catches in those lakes are highly variable, from a few to several kilograms per hectare. The populations are regularly supplemented by stocking vendace Acta Sci. Pol.

Growth of vendace length... 31 hatchlings at densities which vary depending on the amount of stocking material obtained in different years. The vendace catches in the Pluszne within 1990 2004, despite large fluctuations, were maintained at a level of about 10 kg ha -1, while a distinct hyperbolic decreasing trend has been observed in the a skie since the early 1990s, from 26.6 kg ha -1 in 1991 to 0.4 kg ha -1 in 2001 (an increasing trend was recorded in 2002, 2003, and 2004: 1; 2.7; and 3.3 kg ha -1, respectively; Fig. 4). Fig. 1. Situation of the lakes studied Rys. 1. Po o enie badanych jezior Piscaria 5(1) 2006

32 W. Ciepielewski, A. Hornatkiewicz- bik Lake Pluszne Jezioro Pluszne Fig. 2. Bathymetric chart of Lake Pluszne Rys. 2. Jezioro Pluszne plan batymetryczny Acta Sci. Pol.

Growth of vendace length... 33 River yna rzeka yna River Orzechówka rzeka Orzechówka 222 River yna rzeka yna Piscaria 5(1) 2006

34 W. Ciepielewski, A. Hornatkiewicz- bik wyl gu ha -1 Fig. 4. Vendace catches and stocking densities in the lakes studied Rys. 4. Od owy sielawy i zarybienia wyl giem w badanych jeziorach Commercial fishing operations, carried out in both lakes with gill nets of identical mesh size as well as with a vendace trawl, showed in recent years the Pluszne catches to consist of smaller individuals (Fig. 5), which suggests that the vendace growth in the Pluszne is slower than that in the a skie. To check this suggestion, age structure of the commercial catches was determined and length growth rates were compared between the vendace years of life. In addition, the present length growth rates of the vendace in the two lakes were compared with unpublished data collected in the 1960s and 1980s. The research was supported by the Polish Scientific Research Committee grant No. P04G 034 25 The quality status of the aquatic environment in the catchment of the upper reaches of River yna and its effects on the fish fauna. Acta Sci. Pol.

Growth of vendace length... 35 25 20 % Lake Pluszne n 129, x 20.1 cm Jezioro Pluszne n 129, x 20,1 cm Lake Pluszne 129, X 20.1 cm Lake a skie n 86, x 22.7 cm Jezioro Lake a skie a skie n 86, 8 6, x X 22,7 22.7 cm cm 15 10 5 0 15.1 16 16.1 17 17.1 18 18.1 19 19.1 20 20.1 21 21.1 22 22.1 23 23.1 24 24.1 25 25.1 26 26.1 27 27.1 28 28.1 29 29.1 30 30.1 31 31.1 32 32.1 33 33.1 34 Percentage Udzia procentowy Length classes (Lt.), cm Klasy d ugo ci (Lt.), cm Fig. 5. Vendace length (Lt.) distribution in commercial catches in the spawning season 2003 Rys. 5. Procentowy udzia osobników sielawy w poszczególnych klasach d ugo ci (Lt.) w po owach rybackich w okresie tar a w 2003 roku Piscaria 5(1) 2006

36 W. Ciepielewski, A. Hornatkiewicz- bik MATERIAL AND METHODS Samples for the analysis of the vendace growth rate were picked out at random from commercial catches effected, in autumn 2003 during the spawning season, with sets of gill nets of different mesh sizes and, in March 2004, with a vendace trawl. The fish were measured to 1 mm and weighed to 0.2 g. Scales for age and annual increment determinations were collected from a spot located above the lateral line, between the dorsal and adipose fins [Bernatowicz 1952]. The individuals examined were sexed. Growth rate was assessed by back calculations. The ring radii were measured on the oral part of a scale under a measuring microscope (21x magnification). The analyses were performed with the Dahl-Lea method, selected for the simplicity of the calculations involved and also for practical reasons: the authors who, in past decades, studied vendace growth in the lakes in question, assumed a proportional relationship between the scale oral radius and the fish length. The data obtained in this study are thus comparable, without any additional computations, with data collected over previous years. The Lake a skie vendace growth rate was calculated for 161 individuals of both sexes; 168 individuals were collected for the purpose from Lake Pluszne catches. The samples were dominated by individuals aged 2+; the sex ratio amounted to 40 50%. The difference in body length growth between vendace males and females was significant as of the third year of life on and amounted to about 1 cm, a difference of 2 cm being observed in the older vendace. Males grew at a slower rate. As of the age group 3+, males in vendace populations are scarce (a few to several per cent of the number of vendace caught by fishermen) [Ciepielewski 1974a, b, 1987]. For this reason, the vendace growth rate in this study was analysed for both sexes together, assuming that an error made with respect to the first three years of life would be small and would not significantly affect the mean values obtained. The lengths (Lt.) calculated for consecutive years of life were compared with the values obtained with the Rosa-Lee method to find out if the slightly lower values arrived at by using the Dahl-Lea method would alter the assignation of the vendace to growth rate categories in Polish lakes, proposed by Bernatowicz [1953] and Szczerbowski [1981]. The 1957 data on vendace growth rate in Lake a skie were drawn from a study on fisheries management of the lake, conducted in 1963 (manuscript, Institute of Inland Fisheries library). The analysis includes also the unpublished data collected from samples obtained from Lake Narie whose vendace catches in the past decades were dominated by small (mean total length below 20 cm) vendace individuals. The 1968 data on the vendace growth in the lake were drawn from Radziej [1973]. Logarithmic curves were used to illustrate mean growth rates of the vendace in their consecutive years of life in the a skie and the Pluszne. Differences in length increments were evaluated by applying the parallelism test on direction coefficients of the regression lines obtained by transformation of the logarithmic curves. Significance of differences between the pairs of means in the consecutive years of life was tested with Student s t test. Acta Sci. Pol.

Growth of vendace length... 37 RESULTS Over the period of study (spawning season 2003 and winter 2004), the Pluszne commercial vendace catches did not contain individuals older than 5 years, while the a skie catches contained even 11-year-old fishes (Fig. 6). The Lake a skie vendace growth rate, illustrated by Fig. 6, was faster than that in the Pluszne from the second year of life on. Total length (Lt.), cm D ugo ca kowita (Lt.), cm 40 35 30 25 20 15 10 5 0 161 161 n -168 y = 10.192ln(x) + 10.996 R 2 R2 = 0.9955 = 0.9955 50 52 87 165 27 6 47 37 20 11 3 y = 8.1124ln(x) + 10.68 RR2 2 = = 0.9987 1 2 3 4 5 6 7 8 9 Age Wiek 10 1 2 Fig. 6. Rys. 6. Logarithmic trend lines of vendace growth rate (1 Lake a skie; 2 Lake Pluszne). Tempo wzrostu sielawy przedstawione za pomoc krzywych logarytmicznych (1 Jezioro a skie; 2 Jezioro Pluszne) The parallelism test showed the direction coefficients of the regression lines examined (Fig. 7) to be very highly significantly different ( = 0.001); the calculated value of t (11.36) was higher than the critical value (t = 4.587) for 10 degrees of freedom. Numerical values which served to plot the curves shown in Fig. 6 are given in Table 1. Table 1. Length (Lt.) of the vendace in consecutive years of life, as determined by back calculations (Dahl-Lea method) Tabela 1. D ugo (Lt.) sielawy w kolejnych latach ycia okre lona metod odczytów wstecznych Dahl-Lea Age Wiek I II III IV V VI VII VIII IX X Lake a skie Jezioro a skie Mean length rednia d ugo 10.3 18.2 22.4 25.8 28 29.8 30.7 31.8 32.9 34 n 161.0 161.0 87.0 52.0 50 47.0 37.0 20.0 11.0 3 Lake Pluszne Jezioro Pluszne Mean length rednia d ugo 10.8 16.1 19.5 22.1 n 165.0 162.0 27.0 6.0 n number of individuals used for calculating the means. n liczba osobników, z których liczono rednie. Piscaria 5(1) 2006

38 W. Ciepielewski, A. Hornatkiewicz- bik Total lenght (Lt.), cm D ugo ca kowita (Lt.), cm R 2 = 0.9955 R 2 = 0.9987 2.5 Fig. 7. Rys. 7. Vendace growth rate as plotted using logarithmic (ln) scale (1 Lake a skie; 2 Lake Pluszne) Tempo wzrostu sielawy po przedstawieniu wieku w skali logarytmicznej (ln). 1 Jezioro a skie, 2 Jezioro Pluszne The mean length (Lt.) attained by the Pluszne vendace older than 1 year was by 0.5 cm higher than that in the a skie, the difference being significant (t = 2.35; = 0.02). In the second year, the a skie vendace grew faster, its mean total length exceeding 18 cm as opposed to the mean Lt. of about 15 cm in Lake Pluszne; that difference was highly significant (t = 9.58; < 0.01). In the third year, the a skie vendace grew still faster: the difference between the mean lengths was about 3 cm and was highly significant (t = 8.43; < 0.01). The sporadic occurrence of the vendace older than 3 years in Lake Pluszne catches made it impossible to compare mean lengths of the older fish. How ever, as suggested by the shape of the curves pl otted, the difference would increase with time (Fig. 6). The between-decade comparisons of growth rates in various water bodies showed that the rate of grow th in the first three years of life in Lake a skie was not subject to any major changes, while the Pluszne vendace grows at a lower rate at present, as was the case in previous years in Lake Narie (Fig. 8). Acta Sci. Pol.

Growth of vendace length... 39 Lake Pluszne Jezioro Pluszne Total length (Lt.), cm D ugo ca kowita (Lt.), cm 30 25 20 15 10 5 0 2005 2004 1985 1961 1 2 3 4 5 Age Wiek Total length (Lt.), cm D ugo ca kowita (Lt.), cm 35 30 25 20 15 10 5 0 Lake a skie Jezioro a skie 2004 1985 1970 1961 1957 1 2 3 4 5 6 7 8 9 10 Age Wiek Lake Narie Jezioro Narie Total length (Lt.), cm D ugo ca kowita (Lt.), cm 25 20 15 10 5 0 1 2 3 4 Age Wiek 1985 1967 1961 Fig. 8. Vendace growth rate in the lakes studied in different years Rys. 8. Tempo wzrostu sielawy w ró nych latach w badanych zbiornikach Piscaria 5(1) 2006

40 W. Ciepielewski, A. Hornatkiewicz- bik DISCUSSION The vendace in Polish lakes was observed to grow at a varying rate throughout its distribution range [Bernatowicz 1953, Walczak 1953]. Bernatowicz [1953] distinguished between three growth rate categories (poor, not exceeding 18 cm; moderate, 18 20 cm; and good, exceeding 20 cm) based on the total length (Lt.) attained in the third year of life. According to this criterion, the a skie and Pluszne vendace growth rates, as evaluated from samples collected in 2003 and 2004, should be considered good and moderate, respectively. According to growth rate criteria developed, for several fish species, by Szczerbowski [1981] who differentiated between 5 growth categories, the Pluszne vendace grew slowly, while the a skie vendace was a fast-growing fish. To find out if the Dahl-Lea method-based growth rate calculation affected the vendace growth classification, the calculations were repeated using the Rosa-Lee method recommended for the species by Szypu a [1970]. The latter method focuses on the fish length at which the first scales appear; that length (Lt.) in the vendace is 3 cm. Table 2 shows length growth of the vendace as calculated by the two methods. In Lake a skie, the mean Lt. values in the 3-year-old vendace are 23.1 and 22.4 cm, as produced by the Rosa-Lee and Dahl-Lea methods-based calculations, respectively, the difference amounting to 0.7 cm. The respective values in Lake Pluszne are 19.7 and 19.5 cm (the difference of 0.2 cm). Thus, the lengths estimated using the Rosa-Lee method do not alter the gist of the growth rate classification of the vendace in the lakes studied, confirming the conclusions drawn on the basis of the growth classifications developed by both Bernatowicz [1953] and Szczerbowski [1981]. Table 2. Length (Lt.) growth rate of the vendace in consecutive years of life as estimated by back calculations (Rosa-Lee and Dahl-Lea methods) Tabela 2. Tempo wzrostu d ugo ci (Lt.) sielawy w kolejnych latach ycia ocenione metodami odczytów wstecznych: Rosa-Lee i Dahl-Lea Age Wiek I II III IV V VI VII VIII IX X Lake a skie Jezioro a skie Rosa-Lee 12.1 19.0 23.1 26.5 28.5 30.1 31.0 332.0 33.0 34 Dahl-Lea 10.3 18.2 22.4 25.8 28.0 29.8 30.7 31.8 32.9 34 n 161.0 161.0 87.0 52.0 50.0 47.0 37.0 20.0 11.0 3 Rosa-Lee 12.1 16.5 19.7 22.3 Dahl-Lea 10.8 16.1 19.5 22.1 n 168.0 165.0 27.0 6.0 n number of individuals. n liczba osobników. Lake Pluszne Jezioro Pluszne The different length increments in the consecutive years of life in the lakes studied and a higher contribution of individuals older than 3 years in commercial catches in Acta Sci. Pol.

Growth of vendace length... 41 Lake a skie suggest that the conditions for growth of the species in the a skie differ from those offered by Lake Pluszne. Echo-sounding, temperature measurements, and dissolved oxygen assays carried out in those lakes by wierzowski and Doroszczyk [2004, 2005] showed the summer thermocline to be thin and the epilimnion thickness not to exceed 5 6 m. The rapid reduction in the dissolved oxygen content starts already in the thermocline, the minimum (reduction to 0 mg O 2 dm -3 ) being recorded in the upper hypolimnion. At that time, the fishes of various species stay at high densities within the depth layer of 0 10 m; low abundances of the fish, mostly the vendace, were recorded deeper in the hypolimnion in both lakes. The a skie hypolimnion dissolved oxygen content at 15 45 m amounted to 4 5 mg O 2 dm -3, the corresponding values in the Pluszne being close to 0. Thus, in summer, the vendace can feed in the a skie hypolimnion, while in Lake Pluszne, during the intensive growth of body length (the first half of summer) [Marciak 1962, Ciepielewski 1971] the vendace feeds in the epilimnion, competing heavily with other species staying there and feeding on the plankton (juvenile cyprinids and bream, smelt, and bleak). Food deficiency, most probably a result of a heavy competition produced by environmental effects mentioned in the introduction (too high epilimnion temperature and oxygen deficiency in the hypolimnion) as well as phytoplankton blooms reducing water transparency and adversely affecting the feeding process, act in concert to the effect that the Lake Pluszne vendace should be regarded as growing poorly. CONCLUSION Length growth rate of the vendace inhabiting Lake Pluszne has changed. At present, the fish grows at a rate slower than that in the 1960s and 1980s when the growth was fast; in the early 21st century, the Pluszne vendace is classified as growing slowly. In the nearby Lake a skie, the length growth rate of the vendace has not changed over the last 50 years; there, the species is classified as growing fast according to growth categories developed by Szczerbowski [1981]. The reduction of the Pluszne vendace growth rate has been most probably a result of unfavourable living conditions in the lake, manifested as intensified competition for food which causes periodic (summer) food deficiency in the epilimnion where the vendace is forced to stay in summer. REFERENCES Bernatowicz S., 1952. Zagadnienie trafno ci oznaczania wieku i przyrostu sielawy na podstawie usek z ró nych okolic cia a [Determining age and rate growth of Coregonus albula by reading scales from different parts of the body]. Rocz. Nauk Rol. 65, 311 335 [in Polish]. Bernatowicz S., 1953. Wyst powanie i przyrost sielawy jezior Mazurskich na tle warunków rodowiska [Distribution and growth of the small whitefish in Mazurian Lakes on the background of the environmental conditions]. Rocz. Nauk Rol. 67-B (1), 1 20 [in Polish]. Piscaria 5(1) 2006

42 W. Ciepielewski, A. Hornatkiewicz- bik Bernatowicz S., 1961. Wp yw rodowiska na populacj sielawy [The influence of environment conditions on Coregonus albula L. population]. Rocz. Nauk Rol. 77-B (2), 641 679 [in Polish]. Ciepielewski W., 1971. Tworzenie si pier cienia rocznego na uskach sielawy (Coregonus albula L.) [Annual ring formation on the scale of Coregonus albula L.]. Rocz. Nauk Rol. 93-H (3), 25 34 [in Polish]. Ciepielewski W., 1974a. Wzrost, miertelno i stosunek produkcji do biomasy populacji sielawy w jeziorze Maróz [Growth, mortality and the relation of production to biomass of Coregonus albula population in the Lake Maróz]. Rocz. Nauk Rol. 96-H (2), 7 22 [in Polish]. Ciepielewski W., 1974b. Obfito sk adanych jaj i ocena prze ywalno ci narybku sielawy w jeziorze Maróz [The estimation of eggs quantities and of hatch survival rate of Coregonus albula in the Lake Maróz]. Rocz. Nauk Rol. 96-H (2), 23 36 [in Polish]. Ciepielewski W., 1987. Fluktuacje liczebno ci roczników sielawy w dwóch jeziorach mazurskich [Fluctuations in numbers of vendace year classes in two Masurian Lakes]. Rocz. Nauk Rol. 101-H (2), 27 41 [in Polish]. Marciak Z., 1962. Sezonowe zmiany w od ywianiu si i wzro cie sielawy (Coregonus albula L.) z jeziora Pluszne [Seasonal variations in feeding habits and growth of Coregonus albula L. in Pluszne Lake]. Rocz. Nauk Rol. 81-B (2), 335 357 [in Polish]. Radziej J., 1973. Wp yw rodowiska na wolnorosn c sielaw [Coregonus albula (Linnaeus 1758)] wsiedlon z jeziora Narie do jeziora Wierzbiczany [The effect of site upon slowly growing Coregonus albula (Linnaeus 1758) transferred from the Lake Narie to the Lake Wierzbiczany]. Rocz. Nauk Rol. 95-H (1), 129 146 [in Polish]. Szczerbowski J. A., 1981. Criteria for estimating the rate of growth in fish. Rocz. Nauk Rol. 99-H (4), 123 136. Szczerbowski J. A., 1993. Inland Fisheries in Poland. IRS, Olsztyn. Szypu a J., 1970. Wzrost i rozmieszczenie narybku sielawy w jeziorze Legi skim i Wydry skim [Growth and distribution of Coregonus albula L. in Legi skie i Wydry skie Lakes]. Rocz. Nauk Rol. 92-H (3), 45 60 [in Polish]. wierzowski A., Doroszczyk L., 2004. Seasonal differences in situ measurements of the target strength of vendace (Coregonus albula L.) in Lake Pluszne. Hydroacoustics 7, 217 226. wierzowski A., Doroszczyk L., 2005. Spatial variation in the occurrence and density of pelagic fish in Lake Lansk: monitoring with hydroacoustic and catch methods. Hydroacoustics 8, 209 216. Viljanen M., 1986. Biology, propagation, exploitation and management of vendace (Coregonus albula L.) in Finland. Arch. Hydrobiol. Beih. 22, 75 97. Walczak J., 1953. Sielawa (Coregonus albula L.) kilku jezior Pomorza Zachodniego [The small whitefish (Coregonus albula L.) of several Western Pomeran ia Lakes]. Rocz. Nauk Rol. 67-B (1), 21 37 [in Polish]. WZROST D UGO CI SIELAWY W DWÓCH JEZIORACH Z OKOLICY OLSZTYNA (MASURIEN LAKELAND) Streszczenie. W dwóch blisko siebie po o onych jeziorach sielawowych (Jezioro a skie i jezioro Pluszne) o podobnej powierzchni (ok. 1000 ha) i g boko ci (ponad 50 m), charakteryzuj cych si od wieloleci du ymi wahaniami od owów sielawy, oceniono metod odczytów wstecznych Dahl-Lea, tempo wzrostu sielawy, gdy d ugo (Lt.) osobników od awianych z jeziora Pluszne wontonami (gill nets) o takich samych rozmiarach oczek jak stosowane w drugim zbiorniku jest mniejsza rednio o ponad 2 cm od sielawy owionej w Jeziorze a skim. Analiza tempa przyrastania d ugo ci sielawy w tych zbiornikach wykaza a szybsze tempo wzrostu w Jeziorze a skim. Ocenione testem Studenta ró nice Acta Sci. Pol.

Growth of vendace length... 43 pomi dzy rednimi pocz wszy od drugiego roku ycia s bardzo istotne statystycznie ( <0,001). Porównanie tempa wzrostu ocenionego z prób zebranych w okresie tar a 2003 roku i marca 2004 (dla sielawy z jeziora Pluszne oceniono równie tempo wzrostu z próby zebranej w lipcu 2005 r.) z wzrostem oszacowanym z prób pozyskanych w latach 60. i 80. wykaza o, e sielawa w jeziorze Pluszne obecnie (pierwsze lata XXI wieku) ro nie wolniej. W Jeziorze a skim tempo wzrostu nie uleg o zmianie. Sielawa w jeziorze Pluszne przyrasta, podobnie jak (sielawa) z jeziora Narie, znanego w minionych dekadach z wolno rosn cej sielawy. Prawdopodobnie niekorzystne obecnie, warunki bytowania starszej sielawy w jeziorze Pluszne (w sezonie letnim przebywa g ównie w epilimnionie) przejawiaj ce si nasilon konkurencj pokarmow, skutkuj okresowym niedostatkiem pokarmu i powoduj w efekcie spowolnienie tempa wzrostu osobniczego. S owa kluczowe: odczyty wsteczne, sielawa, tempo wzrostu Accepted for print Zaakceptowano do druku: 16.09.2005 Piscaria 5(1) 2006