AGE, GROWTH RATE, AND CONDITION OF VENDACE, COREGONUS ALBULA (L.), FROM SOME POMERANIAN LAKES (NW POLAND) Przemys3aw CZERNIEJEWSKI, Mariusz RACZYÑSKI, Wawrzyniec WAWRZYNIAK

Kazimierza Królewicza 4, 71-550 Szczecin, Poland, phone: (+4891) 4231061 ext. 330, e-mail: zgl@fish.ar.szczecin.pl INTRODUCTION The vendace, Coregonus albula (L.), is one of the most valuable fish species caught in Polish lakes. The factors contributing to its high value are: high quality meat, fast growth rate, shoaling behaviour, and high market value. These qualities explain why, despite the high environmental requirements of this species, its catch had been increasing systematically in Poland, in the 1980s, reaching almost 600 t per year. However, due to the transformation of the fishing industry, the vendace fishery decreased twofold in the 1990s (Leopold and Wo3os 1999). The disappearance of Coregonus sp. in lakes has also been related to adverse changes resulting from . Vendace are found in 44 out of 1575 lakes of Polish western Pomerania (Filipiak and Raczyñski 2000), although only 23 of them are explored by professional fishermen (Czerniejewski and Filipiak 2001). Unfortunately, due to a lack of knowledge on the biological features of vendace populations inhabiting these waters it is impossible, in many cases, to implement a reasonable fisheries management. We decided to focus our attention on three, hitherto not studied, vendace populations inhabiting lakes Pe3cz Wielki, Bytyñ Wielki and We3tyñ (Table 1) and to determine the biological features of those fish, in particular their age structure, growth rate, and condition.


INTRODUCTION
10 June 2003), Bytyñ Wielki (304 specimens;19 June-25 October 2002), and We³tyñ (62 specimens; 5 June-21 September 2003). The fish were individually weighed (Axis electronic scales with 0.1-g accuracy) and measured (total length) (electronic Vernier calliper with 0.1-mm accuracy). The condition of the fish studied was determined using the Fulton's condition factor (Ricker 1975, Bolgier and Connolly 1989, Nash et al. 2006) and through analysis of "n" and "K" parameters of the (total) length and weight (L-W) relation. Fish age and growth rates were estimated from their scales, collected following the method of Bernatowicz (1952). The scales were cleaned of the mucus residues with ammonia solution and mounted on slides. Fish age determination and the measurement of scale radius were performed on the oral parts of the scales using image analysis software ("MultiScan" with 0.001mm accuracy). Due to the commonly known linear R-L relation of venadce, the back calculations were made using the Rosa-Lee procedure, assuming the length of the scale establishment as 30 mm (Grudniewski 1970). The empirical data thus obtained were used for the theoretical representation of length growth, described by the von Bertalanffy growth equation (Begenal 1978). The weight growth of fish was estimated by calculating the length of the fish in the following years on their weight according to the equation W = K · L n , where W is total individual weight [g], L is total length (TL) [mm], K and n are parameters of L-W relation.

RESULTS
The data presented in Table 2 suggest a lack of significant differences among the three populations in relation to mean length and weight. The homogeneity of values obtained can be explained by the mesh size used (24 mm) for all gillnet catches. Out of a total of 502 vendace collected, 75.7% (380) were aged 2+. In lakes Bytyñ Wielki and We³tyñ, 2+ fish constituted more than 80% of all fish caught (82.2% and 80.6%, respectively). In Pe³cz Wielki Lake, despite the domination of 2+ fish (58.8%), a rather large proportion of 1+ fish was noted (23.5%), compared to the remaining lakes. Significant disproportions between the number of males and females obtained were probably related to the period of catching (Table 2). Among 136 specimens of vendace from Pe³cz Wielki Lake, which were collected in spring, females predominated (66.9%), whilst fish obtained from autumn catches from Lakes Bytyñ Wielki and We³tyñ were characterized by greater numbers of males (82.2% and 74.2%, respectively) Fulton's condition factor of for the three populations is shown in Table 2. The difference in condition between vendace from lakes Pe³cz Wielki and We³tyñ (0.75 and 0.76, respectively) were insignificant. Slightly higher condition factor was determined for vendace from Bytyñ Wielki Lake (0.79). Fig. 1 shows the dependence between the fish length and weight for the three populations. The exponent values (n) exceeded 2.5 in all cases, with the highest value obtained for fish from We³tyñ Lake (n = 3.5748). Much higher variation was noted for the multiplier (K), which ranged from 0.0000004 to 0.00006. Note that the n values are in reverse proportion to K values.
Growth rates, determined on the basis of back-calculations, are shown in Table 3. All three populations achie-  Table 2 The average individual weights, total lengths and condition coefficients of vendace from particular lakes At the end of their 1st year, fish from Pe³cz Wielki Lake were more than 10 percentage points larger in length than fish from the remaining lakes. However, the growth rate of these fish declined in next years, probably due to an insufficient supply of plankton, which are the feeding base for vendace. Because of this, the highest values for length in the following years and the largest asymptotic length were achieved by fish from We³tyñ Lake (Table 3). The lengths determined by back-calculation with the Rosa-Lee procedure were used to determine the parameters for a mathematical length growth model, according to von Bertalanffy's equation. These models are as follows: Pe³cz Wielki Lake: L t = 213.22[1 -e -0.49576(1 + 1.055013) ] Bytyñ Wielki Lake: L t = 219.17[1 -e -0.733553(1 + 0.040147) ] We³tyñ Lake: L t = 255.08[1 -e -0.571108(1 + 0.023036) ] Where L t is total length [mm] of fish at age of t years It should be emphasized that the average absolute difference in length obtained from empirical (from back-calculations) and theoretical results (from von Bertalanffy's model) was only 2.03 mm for fish from Bytyñ Wielki Lake, 2.27 mm for We³tyñ Lake fish, and 4.85 mm for vendace of Pe³cz Wielki Lake. These small differences support the validity of the mathematical model based on the empirical data (Table 3). Table 3 also shows the results of the analysis of weight growth for the three populations. In the first year of fish life, significant differences were observed in individual weights (from 8.41 g for We³tyñ Lake fish to 18.92 g for Pe³cz Wielki Lake fish). In the second year of life these differences largely decreased, ranging from 37.15 to 39.61 g), but in the third year fish from Pe³cz Wielki Lake were definitely the smallest (53.40 g).

DISCUSSION
In the second half of the 20th century Polish inland fishermen were mainly dependent on the number of eels obtained. Nowadays, due to high stocking rates, vendace has become the most valuable species, especially in lakes with comparatively low trophic level and suitable hydrochemical parameters. The preferred targets are 2-and 3year old fish, because of their size (Bernatowicz et al. 1975). Gillnets of 24 mm mesh size are the principal gear of choice, because of their comparative selectivity. This mesh size allows the capture the fish from 17.8 to 24.2 cm long, with fish of 21.0 cm caught most effectively (Ciepielewski 1974). Consequently, the fish in Lakes Pe³cz Wielki, Bytyñ Wielki and We³tyñ have no chance to reach their maximum age and size of 9 years and 32 cm (Bauch 1966), respectively.
The most useful tools used for evaluation of fish populations, are the mathematical equations or parameters intended to estimate fish condition, using the relation between total length and weight (Le Cren 1951, Bolgier and Connolly 1989, Ritterbusch-Nauwerck 1995. The condition of vendace from the three lakes studied was evaluated using the Fulton's condition factor. The values obtained fitted the range of values most commonly reported for vendace (Szypu³a 1970, Winfield et al. 1996, Czerniejewski et al. 2002. Slight differences in the mean values between the results obtained and the published data could be the result of different year seasons at capture and the fact that the influence of fish length on the Fulton' condition factor value was not taken into consideration in the analyses. Another well-being index of the fish is the relation between length and weight of fish, which is usually an exponential function. The exponent value of this function approximates 3 for vendace (Sandlund 1992, Christianus 1995, which translates into the isometric character of vendace growth. In some lakes, rich in crustacean zooplankton, this parameter is almost 3.5 (Winfield et al. 1996), but in lakes characterized by unfavourable environmental condition for vendace, the value of the exponent is less than 2.5 (Czerniejewski et al. 2002). Among vendace from lakes Pe³cz Wielki, Bytyñ Wielki, and We³tyñ, the lowest value of "n" parameter of the relation between total length and weight was represented by fish from Pe³cz Wielki Lake. It is therefore evident that the latter body of water provides inadequate quantities of food organisms (of possible inadequate quality) for this fish species, resulting probably from water eutrophication (Bniñska andWo³os 1998, Heese andPociecha 2000). It is also possible that additional major reason contributing to the observed low values of this parameter and the condition factor of vendace were parasites, mainly tapeworms, Diphyllobothrium latum, infecting the fish (Irena Kramer, personal communication).
According to Bauch (1966), the above-mentioned characteristics, in addition to specific environmental conditions of individual lakes, play also an important role in vendace growth. It is generally thought that lakes with low trophic level index (i.e., deficient in mineral salts and fish food) are characterized by low vendace production and growth rates, in comparison with higher-trophy bodies of water. For example, in extremely oligotrophic lakes of northern Finnland, the length of vendace at the age of five does not exceed 20 cm (Viljanen 1986). In Polish lakes, which are far richer in nutrients and feed for vendace and with better conditions, fish grow much better (Table 4). In addition to the abundant zooplankton in these lakes, the fish growth is strongly influenced by the size of the reservoir (Marciak 1970), its depth, transparency, oxygenation during summer stagnation (Bernatowicz et al. 1975), and the population size. Christianus (1995) claimed that the highest growth rate of vendace is generally observed in small reservoirs (28-120 ha) with a depth of 21-42.5 m and with high water transparency. Because of high diversity of Polish lakes, growth rates of individual vandace populations are also diversified. It is evident (Table 4) that, the highest growth rates were recorded for the populations representing the Great Poland Lakeland and Pomerania (Mastyñski 1978, Marsza³ek 1961, Marciak 1970, compared to those of Masuria (Christianus 1995). Higher growth rates obviously indicate better live conditions. In order to evaluate accurately the growth rate of fish examined, taken from lakes Pe³cz Wielki, Bytyñ Wielki, and We³tyñ, the presently acquired data were confromted with individual growth classes defined by Szczerbowski (1978) (Fig. 2). Among the three populations of vendace analyzed, the fish from We³tyñ Lake were characterized by an average growth rate, whilst the populations from lakes Bytyñ Wielki and Pe³cz Wielki were characterized by slow growth. It is interesting that the initial rapid growth of vendace from

Total length [cm]
Pe³cz Lake Pe³cz Wielki Lake declined in older fish. This may be due to unfavourable hydrochemical conditions during intensive growth period in the lake, along with the parasite infection (mainly Diphyllobothrium latum). As shown in the work of Waldemar Piasecki (unpublished) in Pe³cz Wielki Lake, the lack of oxygen restricts vendace movement and feeding to the upper 7-m layer of water, limiting the feeding area of the fish and contributing to easier transmission of certain pathogens.

RECAPITULATION
• The growth of vendace from the three lakes was typical for the species, i.e., very fast growth in the first year of life and significant (almost 2-3-fold) decrease in the following years. • The growth-rate analysis determined through back-calculation showed differences between the populations of vendace from lakes Pe³cz Wielki, Bytyñ Wielki, and We³tyñ in sequential years of fish life. The fish from We³tyñ Lake were characterized by an average growth rate, whilst the fish from the two other lakes grew slowly or very slowly. • The lengths calculated by back-calculation, using the Rosa-Lee procedure, were used to determine a mathematical length growth model according to the von Bertalanffy's equation. These models are shown as follows: • Pe³cz Wielki Lake: L t = 213.22[1 -e -0.49576(1 + 1.055013) ] • Bytyñ Wielki Lake: L t = 219.17[1 -e -0.733553(1 + 0.040147) ] • We³tyñ Lake: L t = 255.08[1 -e -0.571108(1 + 0.023036) ] • Mean values of Fulton's condition factor were 0.75 (range 0.66-0.90) for Pe³cz Wielki Lake, 0.79 (range 0.65 to 1.01) for Bytyñ Wielki Lake, and 0.75 (range 0.59 to 1.09) for We³tyñ Lake. Estimated condition indexes for particular lake are close to those presented in the literature for other vendace reservoirs.