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

Key words: anadromous fish, Vistula sea trout, spawning migration, seasonal stocks.

Abstract.
This work presents research on the occurrence of seasonal sea trout stocks in the Vistula River which vary with respect to the beginning of the spawning migration season and the degree of spawning maturity. The observations refer to the time period before the construction of the dam in W�oc�awek and are based on catch statistics from 1953 to 1968 and data in the literature. In the summer months from June to September, so-called summer sea trout occurred immediately before spawning. In November and December the spawning migration was composed of winter sea trout which spawned in the subsequent spawning season. The numbers of fish which began the spawning migration significantly increased in these two seasons of the year. This was observed in the quantities of commercial catches, which were more abundant in periods of increased spawning runs. Catches of winter sea trout constituted, on the average, 54% of annual catches, while summer sea trout constituted 30%. These historical observations will be utilized in projects to restore sea trout populations in the Vistula River.