Witkowski A., Kuszewski J. 1995. Characteristics of the population of Lampetra fluviatilis (L.) entering the Drwęca and Grabowa rivers (North Poland). Acta Ichthyol. Piscat. 25 (1): 49-56.
The size, length/weight relationship, sex ration and gonadosomatic index of the autumn and spring upstream run populations of the river lamprey have been studied. The size attained by the lampreys from the lower Vistula basin (Drwęca R.) is among the largest recorded for this species. In autumn males in the studied rivers outnumber females. In this period lampreys are bigger than during the spring run. The mass of gonads and the GSI in early spring are higher than in autumn.