HYDROBIOLOGICAL NOTES IN SHIMOKITA PENINSULA, WITH SPECIAL REGARD TO THE PLANKTON AND FISH DISTRIBUTION

Abstract

A hydrobiological survey was carried out in two lakelets in the Shimokita lake group, Lake Usori-ko and River Ohata in the Shimokita Peninsula at the northernmost part of Honshu, Japan. Lakelet Akagawa-numa, Tate-numa and Caldera Usori-ko are respectively eutrophic, dystrophic and acidotrophic. Their planktological characteristics correspond to their limnological designations. The phytoplankton are rich in Lakelet Akagawa-numa, on the other hand in Lakelet Tate-numa the phyto-and zooplankton are poor not only quantitatively, but qualitatively, and some species of Chlorophyceae are characteristic to the moor, Pediastrum or Hyalotheca are dominant. Lake Usori-ko contains strong acid water due to sulphuric acid ion and the phytoplankton is scanty but zooplankton is rather rich. The lake bottom is covered almost by the vegetation of the water moss Leptodictyum riparium, which plays an important role in basic biotic production. Only one fish species, Tribolodon hakonensis, inhabits the caldera lake ; it takes Asellus, insect larvae and Macrocyclops as food. River Ohata is one of the so-called natural rivers, it shows no pollution or human impact. Twenty-four species and 2 ecotypes of fish inhabit the river, especially"suginoko"an ecotype of the land-locked cherry salmon Onhorhyncus masou, is found throughout the year in the upper water area more than the habitat of Salvelinus pulvius.