Inland Fisheries Institute, River Fishery Department, Żabieniec, 05-500 Piaseczno, Poland
Key words: Vistula sea trout, summer stock, winter stock, comparative analysis, fish growth, fish condition
The growth of winter and summer Vistula sea trout was compared using archival material from 1961-1968 before the construction of the dam near W�oc�awek. The average value of fish body growth during their first year in the sea was back calculated. The second growth measure was the fish size from the same year class (A.B+) during its occurrence in the river both in summer and winter. Growth variability, resulting from sex structure, smolt age and various calendar years, was eliminated by the proper choice of materials. The average body growth in the first year the fish were in the sea was greater in winter sea trout than in summer sea trout, and the differences that were evaluated with t-Student tests were statistically significant. No statistically significant differences were observed for the average lengths and masses of summer and winter sea trout or that the summer sea trout was larger than the winter one. Thus, its longer stay in feeding grounds before its spawning migration compensates for the lower rate of summer sea trout body growth.