In the present paper the metric and meristic characters of flounder Platichthys flesus (L., 1758) from the Gulf of Gdansk are described. The examinations were performed with regard to both sexes. The results obtained were compared with those obtained from the samples of flounder collected from the Pomeranian Bay and Slupsk Furrow areas. The differences found out among the samples of flounder from three different locations in the southern Baltic may indicate for affiliation to the separate stocks (populations).

Distribution of the Siberian sculpin (Cottus poecilopus) was studied based on data collected from 138 sites in the Poprad River catchment area (the Vistula basin, southern Poland). The population density was found to range from 3 to 203 inds·100 m^-2. Large differences in fish density were found between the streams and between sites within a stream. The Siberian sculpin were significantly more abundant at shallow sites: densities of up to 50 inds·100 m^-2 were recorded at sites of mean depth <15 cm. In low gradient stream sections (<25 m·km^-1) with numerous pools (>50% bed surface area), the density exceeded 50 inds·100 m^-2. The species density was statistically significantly, negatively correlated with the distance from the stream origin (r = -0.7453), stream bed gradient (r = -0.8652), and mean depth (r = -0.7078). On the other hand, a significant negative correlation was found between the pools sections and the density (r = -0.7496). The relationships between the density and the environmental factors analysed are presented in the form of multiple regression.

Column, thin-layer, and high-performance liquid chromatographic techniques were used to study carotenoids in the Nile tilapia cultured in Polish waters. Assays were run on tissue samples from fins, skin, muscles, and liver of 3 individuals of both sexes. A total of 10 carotenoids were identified, astaxanthin being the dominant pigment. The highest total carotenoid content was typical of the liver (0.945 µg·g^-1 wet weight); the contents in the remaining parts of the body ranged from 0.216 µg·g^-1 wet weight (muscles) to 0.252 µg·g^-1 wet weight (fins).

Using column, thin layer, and high pressure chromatographies, the authors studied the occurrence of various carotenoids and their total content in the fins, skin, muscles, liver, intestine, and gonads of pre-spawning male and female smelt (Osmerus eperlanus) from Lake Wigry in the Suwałki region. A total of 15 carotenoids were identified, zeaxanthin being the dominant carotenoid. The total carotenoid content in males was higher than that in females; the carotenoid-richest male organs were the intestine and liver, while gonads (eggs) and intestine were the carotenoid-richest organs in females.

Age and growth of the perch inhabiting the Solina and Roznow dam reservoirs were studied on 124 and 206 perch individuals, respectively. Age of the fish collected in the Solina was found to range from 3 to 10 years (uncertainty coefficient of 17%), the Roznow perch age varying from 2 to 12 years (uncertainty coefficient of 10%). In the Solina dam reservoir, the perch aged 4+ and 5+ were the most numerous groups, the age groups most numerous in the Roznow encompassed the 5+ to 9+ interval. The highest annual body length gains in the Solina reservoir were observed in the perch aged 4+, 6+, 8+, and 9+, the respective gains amounting to 43.2; 44.4; 44.3, and 48.4 g. In the Roznow reservoir, the highest length gains (55 g) were typical of the perch aged 10+ and 12+. Coefficients of condition were found to range within 1.23-1.55 and 1.27-1.62 in the Solina and Roznow reservoirs, respectively.

Roach spawners 271, bream spawners 188 and 91 perch spawners were caught during fishing period carried out within 1997-2001. The aim of the work was to investigate the age, GSI and gonadal maturation in the post-spawning period. The results showed that roach was at age 3-7, bream 4-15, and perch 6-13. GSI values of particular species ranged from 1.23 to 2.85 in roach females, and from 0.8 to 1.6 in males; from 0.81 to 5.62 in bream males and from 0.276 to 0.758 in males; from 1.2 to 5.6 in perch females and from 2.4 to 4.8 in males. Histological observations of gonads showed that roach females start the first spawning at age 3. or partly at 4., while males in the second- third year of life. In case of bream, females start the first spawning in the seventh year of life, whereas males in the sixth year. The spawning time of both fish species started in the Solina reservoir about one month later than in warmer dam reservoires. Perch in the Roznow dam reservoir spawned in April, post spawning gonad regression underwent through a month, and the process of gonad recrudescence started in June. More than 60% females of perch spawned in Roznow, post spawning gonad regression underwent through a month, and the process of the protoplasmatic growth of oocytes started in June, while the trophoplasmatic growth was observed in July, August and in September.

Roach spawners 305 were collected during fishing carried out in the Solina dam reservoir and 256 roach spawners in the Tresna dam reservoir within 1997-2001.The individual body length and body weight of the fish were measured, additionally scales were collected for the age estimation. The growth of fish were determined by empirical measurements. Coefficients of condition were estimated using the Fulton formula. The results showed that roach from the Solina reservoir was at age 3-8, with the predominated age-classes of 4+ (33.3% of the whole sample) and 5+ (39%). The roach from the Tresna reservoir was at age 2-11, and the participation of fish in the particular age-classes ranged from 6% (2+) to 13.8% (4+). The highest value of the mean annual body length increment was observed at age 4+ (3.3 cm) and the lowest at age 8+ (0.6 cm) in the Solina reservoir. Similar results were obtained in case of the roach from Tresna reservoir, which showed the highest mean annual body length increment at age 4+ and the lowest at age 8+. The highest annual body weight increment of roach in Solina was recorded at age 4+ (53.9 g) and 7+ (49 g), whereas the lowest one at age 5+ (24.5 g), 6+ (26.8 g) and 8+ (8.5 g). In the case of roach from Tresna the highest annual body weight increments were noted at age 9+, 6+ and 7+ : 95.0, 57.3 g and 51.2 g, respectively. The values of condition coefficient were similar in both reservoires. No significant differences were observed between values of the annual body length and weight increments of the particular age-classes from the Solina and Tresna dam reservoires.

The ovarian structure and development in juvenile perch, Perca fluviatilis (L.) bred from the moment of hatching under 24-hour illumination and water temperature of 23°C were studied. In 4-10-month-old perch, the size of the lamellae with oocytes grew towards the opposite ovarian walls and often touched them. Oocytes in lamellae were arranged in rows along the lamellae and according to age from the oldest to the youngest towards the germinal epithelium. The oocytes could be divided into three groups in respect to colour, diameter and number of nucleolus: early oocytes, younger generation previtellogenic oocytes and older generation previtellogenic oocytes. The symptoms of cytological sexual maturation manifested by oocyte vacuolisation were observed in several 10-month females. The vacuolised oocytes atrophied. Based on the results of the study it was found that in perch bred under 24-hour illumination and stable water temperature of 23°C, the developed previtellogenic oocytes undergo vacuolisation followed by atrophy.

The aim of the study was to determine the species composition of fish passing the fish pass in the Czchow Reservoir dam in autumn. A total of 2202 individuals representing 13 species were harvested. The fish were dominated by the bleak (93.6%), roach (2.4%), spotted barbel and barbel (1.2% each), and the brown trout (1%). A comparison of the present fish fauna composition with that recorded in the 1950s showed the lack of the nase which used to be a dominant species then, and a dramatic reduction in the contribution of rheophilous species to the ichthyofauna.

Changes in exploited populations of crucian carp in 706 lakes in Northeastern Poland have been observed based on the fishing records from 1951 to 1994. The lakes, covering a total area of over 120.8 thousand hectares, were divided into five size groups. The crucian carp fishery exploitation parameters, which comprised an annual catch per area unit taking into account the number of months when the lakes were exploited and the percentage of crucian carp in the total catch, were highly varied. Trend lines in the catch of this fish species were drawn for each lake size group. The rate and tendencies in the total disappearance of crucian carp from commercial fishing catches were determined. The volume and frequency of stocking with this fish species in the lakes under study were described. High irregularity of stocking was accompanied by the decreasing number of stocked lakes and the total stocked area in each lake size category.

Total arsenic (As) contents were determined in 540 muscle tissue samples of Baltic fishes (herring, sprat, cod, and flounder) caught in 2001-2003. Effects of fish size and fishing ground location (Eastern, Central, and Southern Baltic) on As accumulation by the muscle tissue were explored, using flounder as a model. The maximum acceptable As content in fish and fish products in 2003 was 4.0 mg·kg^-1 wet weight (w.w.). In none of the samples was this level exceeded. The highest mean As contents (0.78 mg·kg^-1 w.w.) were typical of sprat, the lowest (0.26 mg·kg^-1 w.w.) being recorded in flounder. The species studied differed significantly (p≤0.05) in their muscle tissue As contents, except for cod and flounder that showed no significant interspecific differences (p≥0.05). The Eastern Baltic flounder showed significantly (p≤0.05) higher As contents than their conspecifics harvested in both the Central and the Western Baltic. The As contents in the 30-40 cm long flounder were significantly (p≤0.05) higher than those recorded in smaller fish, i.e., those measuring 20-30 cm and those smaller than 20 cm.