Abstract
A comparative study of the size distribution of river lamprey, Lampetra fluviatilis (L.), was conducted in several rivers in the Baltic Sea basin. Total length and weight data were collected from 7503 lamprey individuals. Sex ratio distributions were also examined in some of the river lamprey populations. The length of lamprey varied from 20.0 to 51.0 cm. The smallest lamprey were in Finland, where mean length in different rivers ranged from 27.9 to 30.8 cm. Larger individuals were noted in Latvia, with mean lengths from 34.0 to 39.5 cm, and in Lithuania from 33.5 to 36.4 cm. The largest lamprey were in Poland, where mean length was from 38.3 to 42.9 cm. The heaviest lamprey were noted in Poland with mean body weights from 45 to 230 g, while slighter individuals were noted in Finland with mean body weights from 12 to 90 g. The body length and weight ranges indicate regional variation. The smallest lamprey individuals were noted in the populations in the rivers of the northern region of the Baltic Sea basin. The analysis of the sex ratio distribution indicated that in most rivers populations of river lamprey males slightly outnumbered females.