The ovarian condition was studied in juveniles of six species of the Pacific salmon of different ages, taken for research at four hatcheries, as well as captured in lakes and rivers in the Sakhalin Province and Kamchatka Territory. The formation of the older generation of germ cells, consisting of previellogenic oocytes, in females of the Pacific salmon ends at the age of 0+, in pink salmon, with a mass of about 0.2–0.3 g, in other species, with a mass of about 1–2 g. In all species, the replenishment of this generation ceases during the habitat of juvenile fish in fresh water. After the formation of the older generation of germ cells is completed and its number reaches a certain level characteristic of each fish species, two oogenesis processes, that are not externally related to each other, continue to be carried out in the ovaries. The first process is the growth of the older generation oocytes, which develop relatively synchronously, varying 1.5–2 times in diameter. The second process is the mitotic reproduction of the gonies, their entry into meiosis, and subsequent resorption at the stage of pachytene and early diplotene. The mitotic activity of the gonies is minimal in females of the pink salmon, and, in fact, it is not detected in the fish caught in the coast. In females of other species, a decrease in both mitotic activity and initiation of new meiotic cycles does not occur during the entire period of their habitat in fresh water.
Comparative Analysis of Japanese Three-Spined Stickleback Clades Reveals the Pacific Ocean Lineage Has Adapted to Freshwater Environments while the Japan Sea Has Not
