Abstract: To examine the fish resources in rivers and its protection, this paper develops a fluvial egg drift model in the Eulerian-Langrangian approach coupling a traditional 2D shallow water model to a 3D particle tracking model. This egg drift model is capable of simulating river hydrodynamics, 3D movement of fish eggs and their dispersive patterns in natural rivers under advection and turbulent diffusion. Its performance is verified by comparing our simulations with the previously published measurements on laboratory flumes using particle tracking velocimetry (PTV), and a good agreement is achieved. Then, the model is used to simulate a field experiment of egg release and capture in the Ouchi tributary of the middle Yangtze, focusing on analysis of egg transport dynamics, vertical egg mixing, transverse distribution of egg concentration, and the influence of river geography and hydrodynamics. The inaccuracy of traditional methods using cross-sectional averages in estimating the drifting egg resources due to the non-uniformity of transverse distribution is addressed and improved through certain correction in numerical simulations. The improvement is verified against field egg capture data, showing that after correction the simulation errors are reduced significantly. Moreover, the simulations reveal that average egg drift velocities are considerably lower than those of the mean flows, which must be considered in predicting the locations of fish spawning grounds.

Key words: fish egg movement, hydrodynamics, Eulerian-Lagrangian method, drifting eggs, dispersive patterns, fish spawning scale estimation