The Influence Mechanism Of D-pillar Obstacles On The Upward-tracking Behavior Of Fish In Imitating Natural Fishway

Fish passage is one of the important measures to alleviate the impact of dam construction on the connectivity of fish and rivers.It promotes the exchange of genetic information of fish upstream and downstream of the dam site,maintains the gene pool of natural fish,guarantees fish germplasm resources,and maintains The structure of fish populations plays a very important role.With the gradual increase in the number of low-head gates and dams in my country,natural fish passages as one of the important measures to assist fish to pass through low-head gates and dams have been widely concerned in recent years.The setting of obstacles in the imitating natural fishway creates a complex water flow environment,and the changes in these hydraulic characteristics will affect the swimming behavior of the target fish to a certain extent,but the changes in the hydraulic characteristics in the fishway depend on the obstacles in the fishway pond.The type and structure as well as the arrangement.To this end,this article first finds the best obstacle type for fish,analyzes the specific influence of the change of hydraulic characteristics under the best obstacle type on the swimming dynamics of the test fish,and analyzes the fish’s movement behavior in a complex water environment;Further study the behavior strategies of target fishes to hydraulic conditions under different working conditions according to the evaluation index of fish uptracking behavior in the imitation natural fishway under the optimal obstacle structure,and analyze the trial rate and success between single-tailed fish and multi-tailed fish The difference of indicators such as the uptracking rate is to explore the influence of the number of fish on the uptracking behavior of fish.In this study,the changes in fish behavior indicators under complex flow fields,the analysis of fish upward behavior characteristics,fish hydraulic factor preference curves and other research results can provide important references for fish behavior research,and can also be used for imitating natural fish in my country.The design and optimization of the flow velocity inside the channel inlet and the tank provide an important reference basis.The research work and conclusions of this article are as follows:（1）Through the study of the impact of different obstacle types on the critical swimming ability of silver carp,the results show that:in D type,the critical swimming speed of silver carp juveniles is the largest,which is 76.86±9.27cm/s,which is significantly higher than that of square pillars and cylinders.The critical swimming ability of silver carp juveniles in two types.The D-shaped column is more conducive to improving the critical swimming ability of fish,so the D-shaped column is the optimal obstacle type.（2）By studying the influence of the complex flow field under the D-shaped column on the swimming dynamics of silver carp,analyze the top flow static and head flow of silver carp at different incoming flow speeds（1 BL/s,3 BL/s and 5 BL/s）.The performance of swimming dynamics at the time,and extract its tail-wagging frequency,tail-wagging amplitude,speed,acceleration and movement step length and other indicators.At a flow rate of 3 BL/s,the silver carp’s tail swinging amplitude under the obstacle is significantly higher than the free incoming flow when the top flow is stationary（P<0.05）,and its tail swing frequency is significantly lower than that of the free incoming flow by 24%（P<0.05）;when silver carp is advancing with the top flow,the tail swing frequency,ground swimming speed,swimming acceleration and movement step length of silver carp under obstacles are significantly lower than those of free flow（P<0.05）,and The tail swing amplitude is significantly higher than the free flow.Silver carp is obviously concentrated in swimming about 5-30 cm behind the obstacle,and the residence time accounts for 92.5%.When the flow rate is 1 and 5 BL/s,the silver carp does not show a very obvious position preference;in the free flow,silver carp often swims close to the wall,and there is no obvious position preference,indicating that the combination of obstacles and flow rate can provide fish Current hiding place.（3）Through indoor simulation of natural fish channels,the target fishes under 4 different flow conditions（Q₁=11L/s,Q₂=17 L/s,Q₃=23L/s,Q₄=31L/s）were carried out.Retrospective behavior test.The results show that the attempt rate of the target fish under Q₁ flow rate is significantly lower than that of other working conditions,and the success rate of the target fish in the four working conditions are all higher than 80%;the fish presents"eight-shaped"and"sticking"in the process of upward tracing.There are three typical uptracking trajectories of wall type"and"column column type".Under low flow conditions（Q₁,Q₂）,the main choice is to use the eight-shaped trajectory for upward tracking;under high flow conditions（Q₃,Q₄）,the main choice is Tracing upwards in a cylindrical trajectory.Through the analysis of swimming speed and ascent time,it is found that the swimming speed during the upward trajectory around the cylindrical trajectory is significantly lower than the other two types of trajectories（P<0.05）;under low flow,the fish can complete the upward tracing at a faster speed and in a shorter time.The type trajectory is the best upward tracing path;under high flow,the cylindrical trajectory can maximize the fish’s movement efficiency,thereby improving the fish passing effect.（4）Coupling the distribution of the indoor flow pattern in the fishway pond with the upward trajectory of the fish found that the common preferred velocity range of silver carp under 4 flow conditions is 0.14m/s-0.28m/s,and the preferred velocity of loach is0.13m/s-0.34m/s,the preferred flow velocity of grass carp is 0.13m/s-0.36m/s.（5）The analysis of the velocity preference curve of multi-tailed fish shows that the common velocity range of three fishes under 4 flow conditions is 0.16m/s-0.37m/s;the common velocity range of five fishes is 0.18m/s-0.42m/s,a significant improvement（P<0.05）compared with single-tailed fish.The results showed that with the increase in the number of fish released,the attempt rate and the successful upward tracking rate of the fish in the fishway increased significantly at low flow rates（P<0.05）;at high flow rates,the swimming speed of the fish school increased significantly（P<0.05）,the trial time and uptrack time of the target fish were significantly shortened（P<0.05）.