Irena BORZĘCKA
Inland Fisheries Institute, River Fishery Department in Żabieniec 05-500 Piaseczno, Poland

Key words: original Vistula sea trout, summer stock, winter stock, age in river, age in sea, spawning marks, smolt recruitment variability.

Abstract.
The age structure of the two seasonal stocks of Vistula sea trout is described during the 1953-1968 period for both the river and sea periods of their life cycle. In both the winter (89%) and summer stocks (75%) it usually took sea trout two years to reach the smolt stage. However, the percentage of summer sea trout which reach the sea after spending one year in the river was significantly higher � on average 22%. Winter sea trout stayed in the sea for two (45%) or three (51%) trophic seasons until beginning their spawning migration. Summer sea trout usually started their spawning migration after three trophic seasons in the sea (60%). Fish younger than A.1⁺ and older than A.3⁺ were rare in both stocks. The percentage of fish which repeated spawning was about 1% for both winter and summer fish. The recruitment strength of winter sea trout smolt to the sea was more variable than in the case of summer sea trout in subsequent years. The recruitment variability coefficient for winter and summer sea trout was 88% and 67%, respectively.