Mechanism study on the impacts of hydraulic alteration on fish habitat induced by spur dikes in a tidal reach

Abstract

Spur dikes have been utilized as channel regulation structures worldwide, especially in the upstream tidal reach of river estuary, where is a superexcellent habitat for resident fish or migratory fish during different life history stages. However, local hydraulic characters will be altered upon spur dikes establishment, resulting in several influence on fish habitat. The purpose of this paper was to reveal the impacts mechanism of spur dikes on fish habitat in a tidal reach through a comprehensive analysis based on some previous neglected aspects, such as fish physiological characters, hydraulic variations and essential factors alterations of the aquatic ecosystem. Hydraulic information around spur dikes was acquired through a 3D hydrodynamic model, and validations of tidal level, velocity and diversion ratio yielded a high accuracy. Turbulent rapid around spur dikes head and mild nearshore environment behind structures have some potentialities to become new spawning grounds, for fish spawning semi-pelagic eggs and fish laying adhesive eggs respectively. Benefitted from effective mixture of water during a tidal cycle, plenty of planktons and organic matters will be brought in and gradually enriched behind structures, thus a favorable feeding ground will appear there. Meanwhile, deflecting flow induced by spur dikes may cause some hazards for survival probability of migratory larval fish. It is drifted off the nearshore into the river middle, where is a higher turbulent intensity and less phytoplankton concentration. In summary, spur dikes possess tremendous ecological effects, i.e. enriching the eco-hydraulic diversity, not only as a velocity shelter. In addition, research result could afford a sufficient basis for environment conservations to restore aquatic ecosystem with spur dikes.