Study On Drifting Fish Egg Movement Patterns In Non-uniform Water Flows

In recent years,the species and quantity of fish resources were dramatically decreased due to the variation of hydrological conditions,change of river geomorphology,and overfishing in the upper reaches of the Yangtze River.China has implemented a ten-year fishing ban since 2020 for fish protection in the Yangtze River.Chinese Four Carps are the typical fishes that lay drifting eggs,these eggs need to be floated or suspended in the water to keep alive from spawning to hatching.The incubation process of drifting fish egg would affected by hydrodynamic characteristics,river topography,and water temperature.To strengthen the protection of early-stage fish resources,it is necessary to find more effective conservation methods based on fish egg movement patterns.In this thesis,the particle size and density range of drifting fish eggs in the upper reaches of the Yangtze River were measured by using high magnification electron microscopy and salt selection method.And the hydrostatic settling velocity of fish eggs was measured by Particle Track Velocimetry（PTV）method.The transport velocity of the fish egg in longitudinal and vertical direction were studied in non-uniform deceleration flow and non-uniformly accelerated flow conditions by using high-precision variable-slope flume and PTV.Then the movement patterns of drifting fish eggs were obtained and the main results are as follows.（1）Physical properties and hydrostatic settling velocity of drifting fish eggs:the particle size of drifting fish eggs captured in the upper reaches of the Yangtze River ranged from 4.0 mm to 8.0 mm,and 63.9%eggs has the diameter between 5.5 mm and6.5 mm.The density of those fish eggs was 1.0012 g/cm³ and 1.0034 g/cm³,and 78.9%of them were ranged in 1.0022 g/cm³-1.0032 g/cm³.The hydrostatic settling velocity was ranged from 3 mm/s to 13 mm/s with average value of 7.16 mm/s.（2）The main factors affecting the hydrostatic settling velocity were:1)the effect of water turbulence,2)the error caused by the deformation of the egg membrane,and 3)the error caused by the movement of the embryo.The settling equation of fish eggs could be written as=0.609*（（ρ_g-ρ_w）g D/ρ_w）^0.5,whereis the settling velocity,D represents the size of the eggs,ρ_g represents the density of fish eggs,ρ_w stands for water density.（3）In non-uniform decelerating flow,the longitudinal velocity of fish eggs was 0.83times of the current flow velocity,and the mean value of the vertical velocity of fish eggs was-0.00515m/s.By statistical analysis of the trajectory of fish eggs under all conditions,the slope of the trajectory of fish eggs was fitted as-0.13.The lateral distribution of fish eggs decreases with the increase of flow velocity;the recovery rate of fish eggs increases with the increase of flow velocity.（4）In non-uniform accelerated flow,the longitudinal velocity of fish eggs was 0.84times of the current flow velocity,the vertical velocity of fish eggs was-0.0031m/s;the increase of water flow velocity reduced the trend of lateral spread of fish eggs.The experimental results systematically counted the physical properties and hydrostatic settling characteristics of drifting fish eggs and introduced the egg movement patterns in detail,which could provide reliable theoretical and data support for the conservation of early-stage fish resources in the future.