Larvae of parasitic European river lamprey (Lampetra fluviatilis) were studied in the Kalajoki, Pyhäjoki, and Siikajoki rivers which flow into Bothnian Bay in Finland. In the Kalajoki river, ammocoetes occur up to 40 km upstream from the mouth, but in the Pyhäjoki rapids block most of the adult migration about 20 km from the river mouth. The Siikajoki river is blocked by a hydroelectric dam 16 km from the estuary. The mean abundance of larvae in 1977 was 12/m2 in the Pyhäjoki and 21/m2 in the Siikajoki river. In 1978 the mean abundance was 6/m2 in the best area (an 8-km stretch) of the Kalajoki, 15/m2 in the Pyhajoki, and 21/m2 in the lower 7 km of the Siikajoki river. The mean length of larvae was smallest in the Siikajoki (45 mm in 1977 and 55 mm in 1978) and largest in the Kalajoki river (77 mm in 1978). Reasons for such size differences and the variation in year-class strengths may be attributable to oxygen deficiencies and other problems caused by dams and other construction work on the rivers. Both the Siikajoki and Kalajoki rivers appear to be affected in this way. The Pyhäjoki appears to be the least disturbed of the three rivers. Ammocoetes favor mean grain sizes from 0.05 to 0.10 mm. Myomere studies on larvae from the Siikajoki river gave a mean count of 62.3 (range, 60–64) and verified that the larvae involved were those of the river lamprey.Key words: lamprey, Finland, larvae, ammocoete, Lampetra fluviatilis, European river lamprey
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