Volume 64, Nº 5 (2024)

Growth, peculiarities and anomalies of the ossification and development of skeletal elements, as well as body proportions and meristic characters of laboratory-reared larvae and fry of two lacustrine dwarf forms of Arctic charr Salvelinus alpinus complex from lakes Tokko and Bol’shoe Leprindo (Transbaikalia) were studied. Retardations of the formation of blood cells, anomalies of morpho- and osteogenesis, and increased mortality of unhatched embryos, free embryos and prelarvae of reciprocal hybrids were revealed. Initial characteristics of prelarvae, linear growth, rate of osteogenesis, body proportions, and colouration of hybrid individuals matched matroclinal developmental pattern. Heterochrony of the ossification of vertebral centra was revealed in prelarvae of the hybrid between females from Lake Tokko and males of the deep-water form from Lake Bol’shoe Leprindo. Their ossification was displaced to an earlier stage in the typical ossification and differentiation sequence of skull bones and fin rays, which indicates the presence of respective regulator in parental genome. Such effect was not observed in reciprocal hybrid; however, it demonstrated earlier ossification of predorsalia than the pure form from Lake Bol’shoe Leprindo. The formation of morphological features in the ontogeny of hybrid larvae and juveniles as compared with parental forms was traced. All lethal anomalies of reciprocal hybrids and non-lethal anomalies influencing their viability as well as accelerated use of yolk by hybrids between the females from Lake Bol’shoe Leprindo and males from Lake Tokko, which resulted in mass mortality of prelarvae and weak development of the skeleton of early larvae are considered as the elements of post-zygotic reproductive isolation between pure forms of gen. Salvelinus from the two lakes. This isolation is obviously incomplete since part of hybrid individuals developed normally, though their fertility remains unknown.

The results of a study of the formation of chum salmon Oncorhynchus keta stocks at the Kalininsky, Sokol’nikovsky, and Yasnomorsky salmon hatcheries, located in the southwestern part of Sakhalin Island, using the method of otolith marking of juveniles are presented. According to long-term data, the commercial stocks of all hatcheries are formed by both hatchery production and natural spawning; they are significantly dominated by individuals of older age groups, and the numbers of fish of the respective generations at all hatcheries are closely correlated. The stock of the Kalininsky hatchery is mostly formed due to the work of the farm itself. The Sokol’nikovsky hatchery stock is greatly influenced by the work of neighboring farms: the number of brood fish with tags of Kalininsky and Yasnomorsky hatcheries among the fishes that approached this hatchery becomes clearly pronounced in years with low catches in the region. In stocks of the Yasnomorsky hatchery, brood fish with marks of neighboring farms were also revealed. However, the formation of the Yasnomorsky hatchery stock is influenced more, compared to the stocks of other farms, by natural reproduction and primarily by spawning in the Yasnomorka River, which is the most productive chum salmon riverin the southwestern part of Sakhalin.

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.

Migration of Sakhalin sculpin Cottus amblystomopsis larvae from the river to the sea for early rearing – the primary dispersion, occurs in the form of passive downstream migration in the first 1.0–1.5 hours after dark. Reaction of larvae to decrease of illumination ensures their involvement into the flow and the implementation of downstream migration. Total duration of primary dispersion period does not exceed 3 days. Body length of the larvae is on average 7.7 mm, their vertebrae and spinous processes are formed, the anlages of unpaired fins are noticeable, and pectoral fins are well developed.