Ichthyoplankton species composition and characteristics of its seasonal and interannual dynamics in the lagoon Lake Izmenchivoye (southeastern Sakhalin) in the iceless period
First obtained data are presented on species composition, abundance, spatial distribution and seasonal variability of ichthyoplankton in the lagoon Lake Izmenchivoye (southeastern Sakhalin) during summer 2004-2005. Lake salinity varies between 25.8 and 31.7%o due to a season. Water column is being warmed faster than in the coastal sea zone. A unified hydrologic-hydrochemical and thermal regime is being formed over the lake area. It greatly depends on the intensity of water exchange with a sea. The eggs and larvae of 16 marine euryhaline and stenohaline fish species belonging to 8 families were found in ichthyoplankton of the lagoon Lake Izmenchivoye. 94.3% of ichthyoplankton complex was formed by the flounder eggs and larvae. The maximum ichthyoplankton abundance (52,8 million individuals) occurred in July. This time period was characterized by the optimal salinity and temperature for the longsnout flounder spawning. Its relative eggs abundance reached 83% and density 19.2 ind./m<sup>3</sup>. In June-July, the main concentrations of eggs accumulated on the well-warmed sites with a weak water dynamics. Larvae concentrated in the southern part. In May and September, when the eggs and larvae habitat conditions were going down, ichthyoplankton concentrated near the channel. In the second half of July, the water warming exceeded the upper temperature threshold that was favorable for flounder spawning. There happened an abrupt decline in ichthyoplankton abundance. In autumn, we observed only the residual spawning of yellowfin sole and Japanese anchovy. The lowered temperature in June 2005 had caused a decline in abundance of pelagic stages of species with spring and summer spawning that resulted in lowering of total ichthyoplankton abundance more than twice and half. The ichthyoplankton complex of the Lake Izmenchivoye is not stable and completely depends on the intensity of water exchange with the sea.