The drift of ichthyoplankton in river-estuary-sea system of Cai river in Nha Trang city - Khanh Hoa province

Abstract

The object being studied in this paper is the migration of freshwater ichthyoplankton (eggs, larvae, fries) of Cai river (Nha Trang city). The purpose of the thesis is to discover the rate of daily and seasonal migration, density, components of age and size of the migratory species. The researching method is mainly based on passive and active collections of ichthyoplankton at different times of day and season at four places from upper, middle, lower parts and marine waters. The studied results have a significance since they help to describe the temporal and spatial structure of the distribution of the ichthyoplankton in the stage of catadromous migration. During the research, ten ichthyoplankton Orders were observed among twelve recorded Orders, in which Perciformes and Cypriniformes of 3-20mm in length are predominant. The migration of ichthyoplankton has been observed all year round with daily average density of 0.05-9 fish/100m³. The seasonal increase of fry’s density has been observed from March to April (upper and middle parts, and from June to July (estuary and marine waters). The greatest migration density has been reached at nights. Data have shown that after a year, there are about 4 million fries migrating through these waters. The researched results aim at expanding the theory on migration of fish and constitute methods for preserving natural resources with a view to diminishing losses of fish sources caused by irrigation works.