Daily and Seasonal Variation in Downstream Migration of Juvenile Pink Salmon Oncorhynchus gorbuscha (Salmonidae) in Rivers across the Sakhalin and Kuril Region

Abstract

The four-year study outcomes for downstream migration of the juvenile Pink salmon Oncorhynchus gorbuscha in the Bolshaya Khuzi, Malaya Khuzi, Lazovaya, and Voznesenka Rivers on the East Sakhalin Island and in the Rybatskaya River on the Iturup Island, located far apart from one another along a meridian line (between 45° and 51° N) in different climatic regions, are present. Almost all smolts could migrate over two months, May and June. Similar trends in dynamics of daily downstream smalt migration in all the rivers, associated with reduction in the period of mass migration recorded at dark time during each 24-hour day at approaching the date of the summer solstice, were recorded. The climate patterns of regional variations (dates and degrees of warming trends, heavy precipitation at southern latitudes) significantly modify the pattern variations in the downstream migrant number (downstream migratory phases of increased and decreased passage activities) over a season relative to the river temperature and flow regimes under the spring warming conditions. Therefore, the relationships between the smolt downstream movement rate and the river water temperature and level tend to become weaker, largely, in the southern regions under the impact of the short-term weather variations in unsettled patterns relative to the impact strength and duration.