Temporal differentiation and conditions of reproduction of chum salmon Oncorhynchus keta (Salmoniformes, Salmonidae) from the Tugur River basin (Khabarovsk Krai)

Chum salmon Oncorhynchus keta from the Tugur River of Khabarovsk krai was studied. It was found that, in the river basin, chum salmon of two ecological groups reproduces: the first spawns at river sections with a pronounced underflow and the second spawns at river sections with a groundwater outlet. In the first anadromous migration, usually three maxima of the number of approaches of migrating fish are observed. The first in time maximum is represented by early (or summer) chum salmon, the second includes migrants of early and late (or autumn) form, and the third is represented by chum salmon of only late form. Forms of chum salmon from the Tugur River basin different in dates of the run and the spawning sites differ not only in the ecology of reproduction but also in biological indices. The later chum salmon is larger; its gonads at entry into the river for spawning are less mature as compared with early chum salmon. The autumn form of chum salmon reproducing at key spawning grounds is distributed more widely than the summer form. It is found along the Asian coast from Chukotka in the north to Japan and Korea in the south. Summer chum salmon occupies only part of the range of the autumn form.