REPRODUCTION AND FECUNDITY OF BLUE BREAM ABRAMIS BALLERUS L

Blue bream is one of fishes inhabiting the lower parts of some great European livers and some lakes situated in their drainage areas (see e.g. Kornpowski 1971, 1991). In Poland, the fish appears in considerable abundance only in the lower and the middle part of the river Odra and its estuary, which comprises Szczecin Lago­ on and Lake Dqbie (Kompowski 1995). The reproduction biology of blue bream has been quite thoroughly studied in the waters of Eastern Europe. The papers of many authors deal with this problem either entirely or partially (e.g. Sergeev et al. 1955; Zacharova 1955; Ginzburg 1958 a, b; Tkaceva, 1958; Balon, 1959; Rojen­ ko and Spilevskaja 1962; Aleksandrova 1966; Smirnov 1966; Spilevskaja 1967; Chaem 1969; Konstantinova and Vavilova 1969; Volodin 1980; Kuzniecov 1990).


INTRODUCTION
Blue bream is one of fishes inhabiting the lower parts of some great European livers and some lakes situated in their drainage areas (see e.g.Kornpowski 1971Kornpowski , 1991)).In Poland, the fish appears in considerable abundance only in the lower and the middle part of the river Odra and its estuary, which comprises Szczecin Lago on and Lake Dqbie (Kompowski 1995).The reproduction biology of blue bream has been quite thoroughly studied in the waters of Eastern Europe.The papers of many authors deal with this problem either entirely or partially (e.g.Sergeev et al. 1955;Zacharova 1955;Ginzburg 1958 a, b;Tkaceva, 1958;Balon, 1959;Rojen ko and Spilevskaja 1962;Aleksandrova 1966;Smirnov 1966;Spilevskaja 1967;Chaem 1969;Konstantinova and Vavilova 1969;Volodin 1980;Kuzniecov 1990).
1 Miydzyodrze is the delta area of the river Odra situated above Szczecin.It consists of two main river branches, Regalica and Odra Zachodnia, as well as a sophisticated network of canals and river arms connecting the two branches (see e.g.Kompowski 1995).The area-with its unique values of nature and scenery-is a part of the Scenic Park Lower Odra Valley, created in 1993 The only paper concerning the reproduction of blue bream in the waters of Poland (Karabanowicz and Kompowski 1994) describes the fecundity, seasonal variation of relative gonads size and the egg diameter of the blue bream caught in Szczecin Lagoon and Lake D,tbie.The paper also presents some aspects of blue bream re production in the waters of Miydzyodrze.Two spawning grounds were located: in Kurowski Canal and in Stara Regalica.The ratio of males and females among the fish spawning on the two grounds was also assessed.
The aim of this paper is to extend the knowledge about the reproduction of blue bream in the area of Miydzy odrze through examination of the age and length structure of the spawning population, the length and age of sexual maturity, gona dal development, the spawning season in each year from 1992 through 1996, as well as fecundity and relative gonads size.

MATERIAL AND METHODS
The fish samples were taken at random from the commercial catches of the Fi shery Cooperative "Regalica" in Gryfino during the autumn and spring seasons 1992-1996.Due to the selectivity of the fishing gear used (trap nets and gill nets), the samples were almost entirely composed of large, mature fish.Therefore a num ber of juvenile, immature specimens-particularly useful for the determination of the sexual maturity length-were sampled from several hauls of a summer seine.
The whole body weights (w 1 ) and the eviscerated body weights (w 2 ) of the exa mined fish were weighed to 0.1 g; the standard body lengths (SL) were measured to 1 mm; the age was read from the scales taken from the 1st or 2nd row under the lateral line, just under the first radius of the dorsal fin.The stages of gonadal de velopment were assessed according to eight-stage Maier's gonadal development scale.The weight of the gonads before preservation was estimated to 0.1 g.
In order to study the fecundity, the females with gonads in stage IV and V of Maier's scale were chosen.The ovaries taken from the females were preserved in a solution of formalin.The fecundity was assessed by weight Each pair of ovaries was thoroughly cleansed in running water and the eggs, separated from the ovary tissue, were dried on blotting paper.The dried eggs of each individual were we ighed to 0.001 g.After that three samples of about 0.05-0.1 g were weighed with the same accuracy and the number of eggs in each sample was counted under a ste reo microscope.The total number of eggs from three samples was then converted to the total number of eggs for each individual female, by proportion to its weight.
The indices of relative gonads size (RGS) were calculated according to the fol lowing formula:

RGS
g-100 RGS = relative gonads size index; g = gonads weight (before preservation); = eviscerated body weight.The list of analyses is presented in Table 1.

Len g t h a n d a g e o f t h e s t u d i e d p op u l a t i o n
The data demonstrating the length and age of the blue bream caught in the wa ters of Miydzyodrze in the years 1992-1996 are presented in Table 2.The mate rial has been divided into separate groups-for the autumn and spring seasons.Al so the two samples from the autumn of 1993 and 12th April 1995 •were taken se parately, since they were entirely or almost entirely composed of juvenile, sexual ly immature fish.
Among the mature fish prevailed large and old ones belonging to age groups VII-XII, their mean length from approx.29 cm to approx.34 cm.The oldest two blue breain specimens belonged to the group XV.The bulk of the spawning popu lation were the fish from the three age groups VII, VIII and IX. In 1993they maup 76.7%, in 1994-69.8%, and in 1995-59   The equation-based on the measurements of standard length and weight of 416 individuals with standard length range 12.1-36.9cm and whole weight (w 1 ) range 23.4-902.3g-fits the empirical data quite well, as it is demonstrated by the correla tion coefficient r = 0.9940, assessed for the logarithmic (linear) form of the equation.was assessed on the basis of the measurements of 420 individuals, with length (SL) range 12.1-36.9cm and eviscerated body weight (w2) range 21.8-829.9g.It again fits the empirical points quite well, as the correlation coefficient r-assessed f o r the logarithmic (linear) f o rm of the equation-equals 0.9901.
In Table 3 the mean values of Fulton's condition coefficient, K1 , and Clark's, K2 are demonstrated, where w 1 = whole body weight, w2 = eviscerated body we ight and l = SL in cm.
In both forms, the highest values of K were observed during the autumn se asons, whereas the lowest values occurred in the springs-after spawning.What is also clear, in the same seasons the males had generally higher condition coeffi cients than the females.

Table 3
Mean values of Fulton's condition coefficient, K 1 , and Clark's, K 2 , of mature blue breams caught in Miydzyodrze Le n g t h a n d a g e o f s e x u a l m a t u r a t i o n Fig. 2 represents the percentage of sexually immature, male and female indi viduals within each length class.All the fish smaller than 20 cm were immature, i.e. it was impossible to determine their sex with unaided eye (stage I of maturity according to Maier's scale).The smallest female, with gonads in maturity stage II, was 19.9 cm long, whereas the smallest male-20.6 cm.All the blue bream abo ve 27 cm of standard length were sexually mature.
On the assumption that the sexual maturity length is the length where 50% of fish are mature, it can be presumed, with some approximation, that the length of the maturity for blue bream in Miydzyodrze equals 20.5 cm, according to Fig.In the autumn season (the end of September till the beginning of December) the gonads of sexually mature fish were already well developed.Most of females had their gonads in gonadal development stage IV or even V (Tab.4).The relative go nads size (RGS) of the ovaries made up on the average 11-14% of eviscerated boweight (w2).The testicles were much smalkr (mainly stage II or III) and RGS measured on the average 0.8-1.7%.The above data demonstrate that the spawning concentrations of blue bream start as early as in autumn.
Table 5 contains the data concerning the gonadal development stages in the spring seasons of the years 1993-1996.In the sample caught on 30th March 1993, gonadal development stage indicated the pre-spawning season.The testicles were in stages III, IV and V, whereas the ovaries in stage IV.The relative testicles size (RGS) ranged from 0.92% to 1.88%, on the average 1.54%; the relative ova ries size varied from 16.97% to 28.68%, with the mean value 21.67%.The gona dal stages of blue bream captured on 26 April 1993 (stages V, VII and VIII) indi cated the final phase of the spawning.The relative size of testicles decreased sli ghtly (x= 1.12%), whereas in the case of the females the index apparently dropped (x= 6.5%).The above observations show that in 1993 the spawning of blue bream occurred in the second half of April.Similarly, considering the observed gonadal development stages and the level of RGS, it can be concluded that in 1994 the spawning took place in the first decade of April, in 1995-in the second decade of April and, finally, in 1996-between the end of April and the beginning of May.Table 5 a�so shows that-during the period just before spawning-the weight of ovaries in :rµature females constituted approximately 20-30% of eviscerated bo dy weight.Ttje weight of the testicles, on the other hand, was always much lower, and it has never been observed to exceed 3% of the weight of the body.The ma ximum observed RGS index value for the males reached 2.75%.What is demon strated in Tables 4 and 5, the mean length of mature males was comparable to the mean length of mature females within the same sample.The relative f1cundity was from 36.9 eggs per 1 g of the whole body weight of a female 25.0 cim.long (SL) up to 162.2 eggs (SL= 33.9 cm), with the mean re lative fecundity ok the examined sample 110.0 ± 27.8.The relative fecundity in creased apparently with the length of the blue bream females.As it is shown in Fig. 6, the relati f nship can be well expressed with the power equation: is shown by the lower correlation coefficient r = 0.6490.The relative fecundity is positively correlated with body weight (Figs. 7 and  8).This relationship is slightly curvilinear and can be quite exactly expressed in the f o rm of a power equation.Matching this formula to the empirical data, the fol lowing equation-with use of whole body weight (w 1 )-was derived: The testing of correlation significance confirmed all the equations in this chapter to be statistically significant at the confidence level ::::0.001.

DISCUSSION
The age structure comparison of the examined Mi�dzyodrze population with the populations from Szczecin Lagoon and Lake Dqbie (Karabanowicz and Ko.mpowski 1994) that larger and older fish predominated in the waters of Miiydzyodrze.N a.mely, in April 1991, directly before the spawning, in Lake Dqbie prevailed the fish belonging to age groups V-VIII, with mean Standard length SL = 27.9 cm.At the same time in Szczecin Lagoon mature females belonged to age groups IV-VIII, with mean standard length 27.9 cm, whereas mature males be longed to age groups III-VU, with mean standard length 26.0 cm.The bulk of the spawning populations in both Lake D::i:bie and Szczecin Lagoon were the fish from groups V-Vll, with standard length range 26-30 cm, whereas in the waters of Mi�dzyodrze pr�vailed older fish belonging to age groups VII-XII, and the gro ups VII-IX, wi � h mean lengths 30-34 cm, made up the bulk of the populationo The The seasonal variations of the mature blue bream condition coefficients corre with the previous observations of many authorso The highest values occur in autumn, after feeding season, the lowest after spawning ( compare Chasem 1970-Rybinsk reservoir on the Volga and Kompowski 1971-Lake Dqbie)o The phenomenon of condition coefficients being--on the average-higher f or males f or females can be explained with the higher consumption of the energy ne for the growth of female reproductive organso In the pre-spawning season the ovaries make up 5-6 times larger part of the mature fish body than the testicleso The data given by many authors indicate that the age and length of the sexual maturity differ depending on the environmental conditions in the water body, espe feeding conditionso The improvement of the feeding conditions and the in crease of growth rate have the effect in both the decrease of the maturity age and increase of the maturity length (eogo Tkaceva 1958;Spilevskaja 1967;Rojenko and Spilevskaja 1962;Smirnov 1966;Chasem 1969;Konstantinova and Vavilova 1969)0 According to these authors, this length vary from approxo 16 cm to 24-25 cm, and the males mature at the lengths slightly (approxo 1-2 cm) lower than the femaleso The age of the sexual maturity vary between 2 and 6 yearso The length age of the sexual maturity in Lake Dqbie-adjacent to Miydzyodrze-measu re 22 cm and 402 years respectively, whereas in Szczecin Lagoon-2003 cm and 3.2 years (Kompowski 1991)0 These values are comparable to the length (2005 cm) and age (307 years) for the first maturing blue bream from Miydzyodrzeo Blue bream spawning season is the earliest from among the cyprinidso The wi geographical range of blue bream causes the variation in spawning time; the spawning season falls to March -April in the drainage basin of the Danube (Balon 1959; Antipa after Banarescu 1964), whereas in Vedlozero in Karelia it occurs at end of May (Aleksandrova 1966)0 Kozin (1949) reports that in the area of the former Soviet Union the blue bream spawns in the season between the end of April and the beginning of Juneo It has been confirmed by many authors' observations (eogo Zacharova 1955;Sergeev et aL 1955;Ginzburg 1958 b;Smirnov 1966;Aleksandrova 1966;Konstantinova and Vavilova 1969)0 The spawning is single and spreads for a f�\,v days (Sergeev et al. 1955;Zacharova 1955;Karabanowicz and Kompowski 1994).Our observations show that in the waters of Miydzyodrze blue bre am spawning--depending on the year-occurs in the period from the first decade of April till the end of April -beginning of May.The previous research, carried out in 1991 (Karabanowicz and Kompowski 1994), demonstrated that the spawning season of the blue bream can exceptionally move even to the second half of May, especially when the spring is cold and comes late.
Table 6 Comparative list on blue bream fecundity in different water bodies; range in numerator, mean value in denominator Table 6 demonstrates the data concerning the absolute and relative fecundity of the blue bream from different water bodies.However, any comparisons--either be tween the extreme values or the means for the populations-are difficult, because the data depend upon the length distribution of the spawners, and this can vary from year to year because of the variations in the recruitment of the blue bream, not to mention the impact of the selectivity of various fishing gear used on the obtained results.The length,of the sexual maturity-different for various stocks and changing from year to year--can also be a very important factor in this respect.Therefore, it is the most favourable to compare the tracks of the curves which represent the relationship between the fecundity and the length or weight of fish (see e.g.Brylitiska 1971 ).Such a comparison is demonstrated in Figs 9 and 10; the lines re present the changes of fecundity with length or weight for the blue bream within the three areas of the Odra estuary: Szczecin Lagoon, Lake D<J:bie and Miydzyodrze.The tracks of the lines lie very close to each other, so it can be conc luded that the relationships between the absolute fecundity and length or weight are similar for the blue bream in the three compared water bodies.CONCLUSIONS 1.In the blue bream population from Miydzyodrze prevailed fish 29-34 cm of SL belonging to age groups VII-XU; among the juvenile fish the strong year class 1991 dominated.2. The highest values of the condition coefficients were observed in the autumns, the lowest in the springs, after spawning.The males had generally higher con dition coefficients than the females.3. On the average, the blue bream from Miydzyodrze reach their sexual maturity at the length of 20.5 cm, which reflects the age of 3-4 years.4. In the period directly preceding the spawning, the weight of the mature fema les ovaries made up 17.0-30.8% of the eviscerated body weight, whereas the weight of the testicles was much lower, comprising maximum 2.75% of the eviscerated body weight.5.The spawning time of blue bream in Miydzyodrze waters varies considerably from year to year (from the beginning of April till the second half of May) de pending on temperature conditions.6.The absolute fecundity of the examined blue bream varied from 7743 for the female of SL = 25.0 cm to 112936 eggs for the female 34.5 cm long; the me an value was 52912 ± 22331.7. The absolute fecundity rises with the increase of length or weight.This rela tionship between the fecundity and the two variables is significant when expressed either as a linear equation or in the form of a power equation.8.The relative fecundity of the blue bream changed within 36.9-162.2eggs per 1 g of the whole female body weight.9.The relative fecundity is positively correlated with the length or weight of the female blue bream.The dependence is significant when presented in either li near or power equation.10.The relationship between the absolute fecundity and length or weight in the blue bream from Miydzyodrze is very similar to the respective relationships for the blue bream from Szczecin Lagoon and Lake Dqbie.Zaleznosc masa--dlugosc wyrazic mozna w postaci r6wnati.potygowych: W 1 = 0.0113 z ' 0'1\ 2.9802 r = 0,9940 oraz W2 = 0.0141 l ; r = 0,9901, gdzie: W 1 = masa calkowita; W 2 = masa bez wnytrznosci, l = dlugosc ciala SL (Fig. 1).Najwyzsze wartosci wsp6lczynnik6w kondycji ob serwowano jesieniq, najnizsze -wiosnq po tarle, przy czym w tych samych okresach samce na og6l charakteryzowaly siy lepszc1 kondycjq niz samice (Tab.3).Dojrzalosc plciowc1 rozpi6ry z Miydzyodrza osic1gajc1 przeciytnie przy dlugosci 20,5 cm -co odpowiada wiekowi 3-4 lat (Fig. 2).

Fig. 1
also shows the relationship between eviscerated body weight and stan dard length.The power equation: �= 0.0141 l 2.9802

Fig. 2 .
Fig.2represents the percentage of sexually immature, male and female indi viduals within each length class.All the fish smaller than 20 cm were immature, i.e. it was impossible to determine their sex with unaided eye (stage I of maturity according to Maier's scale).The smallest female, with gonads in maturity stage II, was 19.9 cm long, whereas the smallest male-20.6 cm.All the blue bream abo ve 27 cm of standard length were sexually mature.On the assumption that the sexual maturity length is the length where 50% of fish are mature, it can be presumed, with some approximation, that the length of the maturity for blue bream in Miydzyodrze equals 20.5 cm, according to Fig.2.Substituting this length to von Bertalanffy's growth equation-the one expressing the growth of the blue bream caught in Miydzyodrze in the years 1990-1993 (L 00 = 40.01cm, K = 0.1817, t0 = -0.2092years;Kompowski, 1995)-the mean age of the sexual maturity derived this way equals t m = 3.74 years.The smallest female ready for spawning (stage IV in Maier's scale), among the fish captured in the spring, was 24.1 cm long and in the age 3+ (4) years.It was captured on 12th April 1995.It can also be seen from Fig.2that the sex ratio within the mature fish was li ke 1:L

FF
e c u n d i t y The absolute fecundity of blue bream in Miydzyodrze ranged from 7743 eggs (SL= 25.0 cm) up to 112936 eggs (SL= 34.5 cm).The mean fecundity of the exa mined fish was 52912 ± 22330.Fig. 3 describes how the absolute fecundity in creases with the length of fish.The relationship can be quite accurately expressed in the form of the power equation: where: F = absolute fecundity, l = SL in cm, F = 0.000823 z 5•2265 , which has a considerably high correlation coefficient r = 0.9036 for its logarith mic form.For comparison, the model based on a linear equation does not match the empirical data equally well: F = -143895.58+ 6447.97 • l; r = 0.8785.The increase of the absolute fecundity with the weight of the females is shown in Fig.4(whole body weight) and Fig.5(eviscerated body weight).In both cases the theoretical curve goes with the empirical data in the best way when use a po wer equation.The following equations were derived respectively: = 2.0351 w2; r = 0.8875; where: w 1 = whole body weight, w 2 = eviscerated body weight.The attempt to describe this relationship with a linear model resulted in the li nes fitting the empirical data slightly worse: F = -1712.86+ 152.23 w 1 ; r = 0.9153 and F = -15208.04+ 183.39 w 2 ; r = 0.8598; which is shown by the little lower correlation coefficients.The absolute fecundity is better correlated with the whole body weight than with the eviscerated body weight.

F
where: F' = relative fecundity.I The linear equation slightly worse describes the relationship: F' = -70.937+ 5.928 l;

Table 2
Mean body length (SL-<;m) and age of blue bream caught in Miydzyodrze

Table 4
Maier's degree of gonadal development of mature males and females of blue bream captured in Miydzyodrze in autumn

Table 5
Maier's degree of gonadal development of mature males and females of blue bream captured in Mitrdzyodrze in spring