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Characterisation of salmon (Salmo salar) aged 0+, 1+, 2+, stocked in Pomeranian rivers, involved 25 plastic and 13 meristic characters, the opercular bone arrangement, and coloration. Coloration of body and fins, appearing at the age of 0+ at the fork length of 6-10 cm belonged to the earliest species-specific traits. Typical values of certain species-specific characters, such as tail base width, caudal fin incision (as per cent of l. caudalis), and the length of the upper jaw (as per cent of l. capitis) were recorded as early as in the fish aged 0+. Other characters, such as the opercular bone arrangement, were observed in those individuals aged 1+ and only in some fish aged 0+. The typical salmon anal fin shape occurred in some of the salmon aged 1+ and 2+ only. The study showed a high variability and significant differences in meristic and plastic characters between individuals grown in various rivers.
Four cranial bones (supraethmoid, glossohyal, premaxilla, vomer) of nine-month-old reciprocal hybrids of Salmo salar and Salmo trutta and the arrangement of the opercular bones in reciprocal hybrids aged from 4 to 24 months were analyzed. The supraethmoid bone in the majority of the hybrids studied was similar to that of salmon. In some hybrids, an atypical additional process was found on the lower part of the supraethmoid bone. The large process of the premaxilla bone had a short base in the two groups of hybrids as in salmon, and it was low as in trout. The shape of the vomer plate was intermediate between a triangle (as in trout) and a pentagon (as in salmon) in the majority of salmon x trout hybrids and almost half of the trout x salmon hybrids. The glossohyal in some hybrids had an uneven number of teeth in each row, and these rows of teeth were uneven.
Furunculosis appears from time to time in salmonids in different rivers. It was noted in sea tout from the Dunajec River in 1923 and 1924. In 2007 and 2008, it was noted in sea trout and salmon from Pomeranian rivers, where from 80 to 2,400 sea trout spawners were caught. The degree of infection ranged widely from 0.00% in the Łeba to 74.7% in the Słupia and Rega rivers. In 2007, of the 1,381 sea trout weighing 1949.5 kg caught in the Słupia River, 1031 fish weighing 1,664 kg were cleared for utilization. The most diseased fish were observed in the Słupia and Rega rivers. The number of salmon caught in 2007 and 2008 was lower than that of sea trout and ranged from 2 to 65 individuals from the various rivers. The percentage of infected fish ranged from 8.3% (Słupia) to 58.3% (Rega). Funuculosis was noted more frequently in males than females, as well as in more salmon than in sea trout. Bacteriological assays indicated the presence of Aeromonas hydrophila Complex, Aeromonas sorbia, Pseudomonas flurescens, Streptococus mutans, Streptococus sp. Internal organ bleeding was also noted. In the Vistula, Drwęca, Łeba, and Łupawa rivers and in hatcheries rearing spawners of both salmon and sea trout, no symptoms of furunculosis were noted.
The last remaining salmon population in Poland went extinct in the mid 1980s. The salmon restoration program in Poland in based on Daugava salmon, and the first eyed Daugava salmon eggs were imported to Poland in 1985, and they continued to be imported in subsequent years. Spawners were reared in net cages in the Gulf of Gdańsk. Salmon smolts have been released into Pomeranian rivers and the Vistula River since 1994. Between 1994 and 1999, 1, 644, 936 salmon smolts were released, of which 63, 602 were tagged. Overall, there was a low recovery rate of tagged specimens, and from many experiments there were no recoveries at all. The highest recovery rate was 4.5%. Tagged salmon migrated throughout the Baltic Sea, but the highest concentration of catches were in the Gulf of Gdańsk, near Bornholm and in the Gulf of Finland. In the first winter, the tagged salmon attained an average length of 62.9 cm and an average weight of 2.9 kg, in the second winter theses figures were 76.1 cm and 5.3 kg, and in the third they were 90.7 cm and 7.6 kg.
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