Monitoring the Status of a Russian Red Data Book Species, Mikizha Parasalmo mykiss (Salmonidae), in Rivers of Northwestern Kamchatka. 2. Long-Term Changes in Population Structure

Long-term changes in the population structure of mikizha Parasalmo mykiss from three rivers in northwestern Kamchatka were studied over the period 1970–2024. Analysis of archival and original data revealed persistent and largely synchronous changes in biological parameters, which have become increasingly pronounced in recent years. In 1970–1971, reproduction was supported mainly by typically anadromous fish; by the late 20th and early 21st centuries, the proportion of individuals associated with freshwater and brackish estuarine habitats had increased markedly. The durations of the freshwater and marine life stages, age at sexual maturation, and life span also increased. The pattern of repeat spawning changed substantially; at present, reproduction in these populations is driven primarily by repeat-spawning males and females. Temporal changes followed a complex pattern, with several quasi-stationary periods lasting approximately 5 years or longer (1995–1998, 2000–2007, and 2015–2024). At the same time, a pronounced saltational transition occurred between 1970–1971 and 1995. The life-history strategy of Kamchatkan mikizha populations changed fundamentally over approximately 25 years. In the early 1970s, the three populations consisted of short-cycle, early-maturing individuals that spawned predominantly once and at a relatively young age. Since 1995, all three populations have been dominated by long-cycle, late-maturing, iteroparous fish. These substantial changes in population structure are most likely an adaptive response to large-scale climatic shifts in the North Pacific that occurred in the late 1970s, early 1990s, and 2014–2016.