Regional and Interannual Variability of the Temporal Characteristics of the Downstream Migration of Juvenile Pink Salmon Oncorhynchus gorbuscha (Salmonidae) in the Rivers of the Sakhalin–Kuril Region

The results of a study of the downstream migration of juvenile pink salmon Oncorhynchus gorbuscha in 11 rivers of the Sakhalin and Iturup islands are presented. A large discrepancy has been established in the timing of migration, which corresponds to its known shift to earlier dates in the southern direction. The correlation has been calculated between the air temperature in months with its positive values throughout the periods of embryonic and larval development of pink salmon and the timing of migration of pink salmon juveniles in the rivers of the southeastern coast of the Sakhalin Island (r = −0.79, 14 generations) and Iturup Island (r = −0.68, 16 generations). The presence of this correlation shows that interannual changes in the timing and dynamics of the downstream migration of pink salmon juveniles within local areas are determined not only by the spring level and thermal regimes in the rivers, but also significantly by the general phenological shift in biological processes throughout the freshwater period of pink salmon development. The timing and dynamics of downstream migration within each local area are quite similar in different rivers in the same years. However, multiple differences can sometimes be observed between them in terms of the efficiency of reproduction, estimated through the downstream migration index (the ratio of the number of juveniles that migrated downstream to the number of spawners that entered the river for spawning). The reasons for such discrepancies are often obvious. They are generally determined by different densities of pink salmon aggregations on spawning grounds or by the destruction of the ground of spawning areas during autumn floods. It is very important to take into account such conditions when calculating the total number of juveniles that migrate downstream from rivers in a certain area, since this index seems to be more objective for estimating pink salmon stocks than the number of parents.