CHARACTERISTICS OF SEA TROUT (SALMO TRUTTA M. TRUTTA) FROM THE DRWĘCA RIVER BASED ON SCALE SAMPLES COLLECTED BETWEEN 1988-1992
The Stanisław Sakowicz Inland Fisheries Institute in Olsztyn, Poland
ABSTRACT. The spawning stock of migratory sea trout in the Drwęca River was investigated 20 years after the Vistula River was dammed near Włocławek (1968). Fish scales and data were obtained from fish traps and anglers fishing in the lower Drwęca River. The age structure of sea trout which enter the Drwęca River to spawn and the percentage of them which are repeat spawners are presented taking into consideration the stocking coefficient. The age distribution was as follows: specimens returning to the river after one year in the sea - age A.0+ - 1.3%; age group A.1+ - 36.6%; age group A.2+ - 52.9%. The latter had spent three growth seasons in the sea and was the most numerous. Fish from age group A.3+ comprised an average of 8.5% of the samples. Repeat spawners comprised 4.3%. In addition to the more numerous summer sea trout, winter sea trout still occur in the Drwęca River, although they currently constitute only about 10% of all sea trout.
Key words: SEA TROUT (SALMO TRUTTA M. TRUTTA), AGE STRUCTURE, SUMMER STOCK, WINTER STOCK, SPAWNING MARKS
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