GROWTH PATTERN OF FLOUNDER, PLATICHTHYS FLESUS (L.), FROM THE GULF OF GDA¡SK (SOUTHERN BALTIC SEA)

The landings of flatfishes (Pleuronectiformes) in the FAO area of north-eastern Atlantic, within 1996–2002, ranged from 9765 to 23 380 t. They constituted up to 0.15% of the total catch from this area. The highest flatfish landings were recorded from 1997 through 1998 (Anonymous 2004). The contribution of flatfishes to Polish fish landings in the Baltic Sea tends to increase. Within 2000–2002 it increased from 4.0% to 6.3% of the total weight of all fishes captured there (Mi∏osz et al. 2003). The flounder, Platichthys flesus (L.), is one of the most commercially important flatfishes that occur in the Baltic Sea. Flounder used to be the most important component of flatfish catches in the Baltic, in both Polishand neighbouring zones. Within 1989–1994, flounder constituted from 85% to 99% (depending on the area of capture) of all Polish flatfish landings (by weight) (Kuczyƒski 1996). There has been a number of papers focused on the biology of the Baltic flounder, although many of them were published in the 1930s and 1940s (e.g. Ci ́glewicz 1935, 1947, Ci ́glewicz et al. 1969). The literature, appearing within the last decade, was based predominately on the flounder landings (e.g. Kuczyƒski 1992, 1995, 1996). A revived interest in the Baltic flounder, however, could be observed in recent years (Antoszek and Krzykawski 2003, 2004). The above-mentioned papers dealt with morphometry and growth patterns of flounder from the southern Baltic (Pomeranian Bay and S∏upsk Furrow). The above-mentioned characteristics, significant from the taxonomic point of view, are also essential for fisheries management and sustainable exploitation of this species. ACTA ICHTHYOLOGICA ET PISCATORIA (2005) 35 (1): 51–60


INTRODUCTION
The landings of flatfishes (Pleuronectiformes) in the FAO area of north-eastern Atlantic, within 1996-2002, ranged from 9765 to 23 380 t. They constituted up to 0.15% of the total catch from this area. The highest flatfish landings were recorded from 1997 through 1998 (Anonymous 2004).
The contribution of flatfishes to Polish fish landings in the Baltic Sea tends to increase. Within 2000-2002 it increased from 4.0% to 6.3% of the total weight of all fishes captured there (Mi∏osz et al. 2003).
The flounder, Platichthys flesus (L.), is one of the most commercially important flatfishes that occur in the Baltic Sea. Flounder used to be the most important component of flatfish catches in the Baltic, in both Polish-and neighbouring zones. Within 1989Within -1994, flounder constituted from 85% to 99% (depending on the area of capture) of all Polish flatfish landings (by weight) (Kuczyƒski 1996).
There has been a number of papers focused on the biology of the Baltic flounder, although many of them were published in the 1930s and 1940s (e.g. Ci´glewicz 1935, 1947, Ci´glewicz et al. 1969. The literature, appearing within the last decade, was based predominately on the flounder landings (e.g. Kuczyƒski 1992Kuczyƒski , 1995Kuczyƒski , 1996. A revived interest in the Baltic flounder, however, could be observed in recent years Krzykawski 2003, 2004). The above-mentioned papers dealt with morphometry and growth patterns of flounder from the southern Baltic (Pomeranian Bay and S∏upsk Furrow). The above-mentioned characteristics, significant from the taxonomic point of view, are also essential for fisheries management and sustainable exploitation of this species.
The objective of the presently reported study was to assess the growth rates of length and weight of flounder collected from the Gulf of Gdaƒsk. Also, distributions of length and age of flounder were presented, as well as the length-weight relationship was determined. The results obtained were compared with the available data (following the same standard procedures) on flounder from the Pomeranian Bay (Antoszek and Krzykawski 2003) and from the S∏upsk Furrow (Antoszek and Krzykawski 2004).

MATERIAL AND METHODS
A total of 200 flounder was collected in 1997, in the coastal waters off the village of Hel (Gulf of Gdaƒsk) by small fishing vessels, using flatfish gill-nets (Fig. 1). The detailed description of fish studied is given in Table 1. The fish caught were frozen and sent to Szczecin to the laboratory of the Division of the Fish Systematics. The biometric analysis was made on the thawed material, performed for the whole sample and for both sexes. All measurements were made with a slide caliper, to the nearest 0.1 cm. The weight (of ungutted fish only) was determined with the accuracy of 0.1 g.
The age of individuals examined in the present study was derived from otolith analysis, following Draganik and Kuczyƒski (1993). Whole sagittae were examined in water, using a dissecting microscope with reflected light over a dark background (Ci´glewicz et al. 1969, Draganik andKuczyƒski 1993). Measurements of annual growth increments were made to the nearest 0.01 mm, with the reference to nucleus. The dissecting microscope used, was linked to a computer with a measuring-and image-processing software "Multiscan".
As the total length (TL)-otolith radius (OR) relationship was curvilinear, the method of Vovk (1956) was used to determine the growth rate of flounder's length by back-calculated reading. Length values calculated by this method, as empirical values, were employed to show a growth rate of flounder in von Bertalanffy model (Ricker 1975). All measurements were performed using MS Excel macro platform. The alternative Fi SAT software is also known by the present authors, although it was not used because it is restricted to users of WINDOWS operating system.
The length-weight relationship was determined based on the mean weights calculated in the particular length classes (2-cm interval). Weights estimated by this relationship allowed to show a growth pattern of weight of flounder with the modified von Bertalanffy model; however instead of exponent equalling "3", the value of coefficient "n" was introduced, estimated from the length-weight relationship what allowed to obtain results approximate the empirical data. Minet (1973) showed the growth rate of weight of American plaice with the von Bertalanffy model using similar procedures like in the present paper.

RESULTS
Distributions of total length (TL) and age of the flounder studied (200 fish), with respect to sexes, are presented in Fig. 2.
The total length of flounder males (113 individuals) was 14.9-35.6 cm, with the mean length of 22.2 cm. The most numerous was the 20-21.9 cm length class, with 35.4% of males.
Females (86 specimens) were 12.3-46.0 cm long (TL); the mean length (25.9 cm) was higher than in males. The length class of 20-21.9 cm was the most numerous (18% of females).
The total length (TL) of all fish examined ranged from 4.3 to 46.0 cm, with the mean value of 23.7 cm. The most numerous was the 20-21.9 cm length class (TL) (28.0% of whole sample).
Antoszek and Krzykawski 52  Table 1 Characteristics of flounder sample collected * one specimen unsexed Males predominated in the sample studied. The age of the fish studied ranged from one to ten. Four-year-old specimens were the most abundant in the whole sample. Also four-yearold males where the most numerous, with age groups eight and nine absent altogether. In females, the range of age was narrower (three-ten) with III and IV age-classes prevailing. The mean age of females was higher by one year when compared to males. The smallest fish in the sample (4.3 cm TL) was immature and unsexed, included to I age-class.
Because Antoszek and Krzykawski (2003) showed that the total length (TL)-otolith radius (OR) relationship in the flounder was curvilinear, the method of Vovk (1956) was used to determine the growth rate of flounder's length by back-calculated reading.
Back-calculated growth rates of length (TL) in males and females from the area studied are presented in Tables 2 and 3. As can be seen from Table 2, greatest increments of length (¢ L) in males were observed in the first-and sec-Growth of flounder from Gdaƒsk Bay 53   Table 3 Growth rate of flounder females based on the back calculations (TL) [cm] ond years of life, then in consecutive years it tended to decrease, to slightly increase in the last year of life. The highest fluctuations of individual length were noted in the first year of life, what is indicated by the highest coefficient of variation (CV).
In females (Table 3) the greatest increments of length were also seen in the first-and second years of life, then in the consecutive years it decreased gradually, except for the sixth-and ninth years, when the slight rises were noted. The highest fluctuations of length were noted in the first year of life. As can be seen from Tables 2 and 3, males to the third year of life were characterized by a little higher growth rate, whereas from the fourth year females grew faster.
The growth rate of length of fish in the whole material studied is shown in Table 4. As can be seen, the greatest increments of length of flounder were noted in the firstand second years of life, then in consecutive years (with some fluctuations) it tended to decrease. Coefficients of variability indicated the highest fluctuations of length in the first year of life. Those data were taken from the sections from l 1 to l 8 of Table 4. The mean lengths for l 9 and l 10 were omitted due to low number of fish in these age-classes. Fig. 4 demonstrates the total length (TL)-weight (W) relationship of the flounder studied, based on the averaged values of total length of fish, derived from the 2 cm length classes. The relationship between the total length (TL) and weight (W) in the whole sample can be expressed as follows: The individual mean weight in age-classes was calculated, introducing a mean length to the formula of total length (TL)-weight (W) relationship. In Fig. 5  Growth of flounder from Gdaƒsk Bay Table 4 Growth rate of sex combined of flounder based on the back calculations (TL) [cm]

DISCUSSION
The flounder sampled from the Gulf of Gdaƒsk and characterised in the present paper, shows greatest variability in total length when compared to the samples of flounder captured in the S∏upsk Furrow (Antoszek and Krzykawski 2004) and in the Pomeranian Bay (Antoszek and Krzykawski 2003).
The lowest variability in length was observed in the fish sample from the S∏upsk Furrow, where flounder from older age-classes dominated, as components of spawning stock (Antoszek and Krzykawski 2004). This sample was also characterized by the greatest mean length of 31.4 cm (29.8 cm in males, 32.1 cm in females).
The mean length of the fish from the Pomeranian Bay was 23.4 cm (23.1 cm in males, 23.6 cm in females).
Individuals collected in the Gulf of Gdaƒsk represented age-classes of I-X, with only a few fish from age-classes I and II. Age-classes III, IV, and V were the most abundant. The age of flounder from the S∏upsk Furrow (Antoszek and Krzykawski 2004) ranged from four to ten, while in the sample from Pomeranian Bay (Antoszek and Krzykawski 2003)-from two to seven, with age-class II represented by a single fish.
The age of flounder, studied by the present authors at three earlier-mentioned areas of the southern Baltic Sea ranged from one to ten. Similar age structure was observed by other researchers (Feso∏owicz and Wiktor Antoszek and Krzykawski 56  1959, Ci´glewicz 1947, 1972, 1974, Ci´glewicz and Netzel 1978, Kuczyƒski 1992, 1995, 1996, Draganik and Kuczyƒski 1993, 1996, Kosior and Kuczyƒski 1995. The absence of younger fish in the samples studied was obviously the affect of the protective regulations imposed on the flatfish catches in the Polish Exclusive Economic Zone, translating into bigger mesh-size in the coastal flounder gill-nets used by local fishermen (Kuczyƒski 1995). An additional factor affecting the presence of young fish in the samples studied is the zonal distribution of this species. The fish for the presently reported study were captured at the depth below 10 m, whereas the youngest flounder occur in the shallow coastal waters. To collect young specimens, Ci´glewicz (1935) sampled areas not deeper than 1.5 m, with a Peterson's fishing net. Malorny (1990) caught juveniles in the coastal zone, at the depth range of 0.5-1 m.
In the presently reported study, from the Gulf of Gdaƒsk, males were more common than females (113 males, 86 females). The predominance of males in this area, although not so distinct, was also observed by Ci´glewicz (1972Ci´glewicz ( , 1973. It could be affected by the sampling period (May-August). At that time the fish were on their way back from the spawning grounds in the Gdaƒsk Deep, where males remain longer than females (Reimann 1959). The above observations are consistent with Feso∏owicz and Wiktor's (1959) conclusions on the predominance of males on the fishing grounds in the summer season. It was also reflected in the sample from the S∏upsk Furrow (Antoszek and Krzykawski 2004), where females dominated in older year-classes.
It is evident from the comparison of the mean lengths c back-calculated from the otolith reading, in the sample examined, that the mean annual increments of length tend to decrease with the increasing age of fish. Krzykawski (2003, 2004) found out similar phenomenon in flounder captured in the Pomeranian Bay and S∏upsk Furrow. In each of the areas compared, the intensive length increments are observed, particularly in the first 3 years of life, and then, with some fluctuations, a progressive decline with the increasing age of fish is recorded. These fluctuations could be affected by a low number of fish from older age-classes. The analysis of growth rates based on length of fish from the Gulf of Gdaƒsk, determined for both sexes, showed that in the first 3 years of life females grew slower, then in contrast, males had lower rate of growth in the consecutive years (Tables 2 and 3). Similar observations were presented by Ci´glewicz (1947).
The fish from the Pomeranian Bay showed a faster growth rate in the first four years in comparison to flounder from Gulf of Gdaƒsk. It should be emphasized that, in spite of different procedures used, the similar tendency was observed by Ci´glewicz (1935Ci´glewicz ( , 1947. In the following years (i.e. from the fifth year of life) a faster growth rate was observed in flounder from the Gulf of Gdaƒsk.
A faster growth rate of length in the first-and last years of life was recorded in flounder from the Gulf of Gdaƒsk than in S∏upsk Furrow, whereas in the fourth-, fifth-, and sixth years of life flounder from the S∏upsk Furrow demonstrated a faster rate of growth. The differences between the growth rates of flounder from individual areas may result from the different thermal conditions which strongly affect the growth rate of this species (Ci´glewicz 1962, Ci´glewicz andHoppe 1970). In summer season, the highest average temperatures are recorded in the surface waters of the Pomeranian Bay, slightly lower ones-in the Gulf of Gdaƒsk, and the lowest-in the S∏upsk Furrow area. It is evident, particularly in the case of young flounder (which prefer warm coastal waters) that temperature affects their rate of growth. That is probably why the fastest growth rate of flounder in the first years of life is observed in fish sampled from the Pomeranian Bay, slower in fish from the Gulf of Gdaƒsk, and the slowest in flounder from the S∏upsk Furrow. In their more-advanced age the thermal factor is undoubtedly less significant, because adult fish clearly prefer deeper waters, much colder, where the temperature changes are not so distinct.
The growth rate based on length is demonstrated, fitted with the von Bertalanffy growth model for compared samples from three locations in the southern Baltic (Fig. 6).
Based on analysis of the results obtained from these locations, it can be concluded that the growth rate of length established with the von Bertalanffy equation, in the first years of life is the fastest in the western part of Polish Baltic waters, then it gradually decreases towards the east. Hence in the first years a higher growth rate of length is noted in flounder from the Pomeranian Bay, slower in fish from the S∏upsk Furrow, and the slowest in fish from the Gulf of Gdaƒsk. As the fish get older, however, the opposite tendency is observed. Fig. 7 shows the growth rates based on weight calculated with the modified von Bertalanffy model, in flounder from the three locations compared. Flounder from the Gulf of Gdaƒsk showed a slower growth rate of weight in the first years of life when compared to fish from the Pomeranian Bay; then this tendency (similarly like in the case of length growth) was inverted.
The greatest weight growth rate was noted in the sample of flounder from the S∏upsk Furrow. It should be emphasized, however, that sampling in this location was performed on the spawning ground. Therefore the individuals examined were older and bigger than in the remaining locations, moreover, the weight of matured gonads was considerably high. These factors, as well as the predominance of females in the sample, usually bigger than males (Antoszek and Krzykawski 2004), undoubtedly affected the growth rate of weight in fish from this location.
The differences stated both in growth rates based on length and weight, as well as in the biometric characters (Antoszek and Krzykawski 2005) amongst flounder from the Pomeranian Bay, the S∏upsk Furrow, and the Gulf of Gdaƒsk areas, may indicate that they represent separate stocks (populations).