195 / 2024-02-27 12:23:46

A Computational Model for Upstream Migration of Ayu (Plecoglossus altivelis) in an Agricultural Canal Considering Tilting Weir Conditions

Fish behavior,PIT tag,Numerical simulation,Shichika irrigation canal,diadromous fish

Agricultural canals are generally equipped with tilting weirs, gates and hydraulic drops to control flow conditions, which potentially hinder the migration of fish. The Shichika irrigation canal constructed in the alluvial fan of the Tedori river, Ishikawa Prefecture, Japan, has some tilting weirs and drops in it and discharges directly into the sea, and then 11 species of diadromous fish have been found in the canal. The purpose of this study is to estimate the effect of the tilting weir conditions on fish migration in the canal.

A computational model was developed to represent the migration of Ayu (Plecoglossus altivelis), a representative species of migratory fishes, based on the biotelemetry observation using PIT (Passive Integrated Transponder) tags in the Yamashima irrigation canal, which is a part of the Shichika irrigation canal. There are 19 drops in the study area of Yamashima irrigation canal, of which six are equipped with tilting weirs.

The computational model consists of sub-models of channel and drop sections. In the channel section sub-model, Ayu is modeled to swim along the channel based on a swimming duration curve, which swims upstream for the swimmable time at a swimming speed corresponding to the channel flow velocity. When the swimmable time is exceeded, the Ayu is modelled to drift for a certain period. In the drop section sub-model, Ayu is modeled to decide whether to swim or jump upstream based on the comparison of the velocity of falling water from the top of a drop and Ayu’s burst speed which is assumed to be normally distributed with an expected value of 20 times of the body length. If the velocity is larger than the burst speed, Ayu is modelled to jump up. If Ayu could jump over the height of the drop, the Ayu is considered to have successfully ascended the drop. If it fails, it tries again after some time.

　The computed results were validated by the observations of the time taken to pass two PIT tag antennas placed 870 m apart in the channel. And using identified parameters of Ayu’s swimming ability, upstream migrations of Ayu through the Yamashima canal of 3.5 km simulated under various conditions of tilting weirs which demonstrated how significant impact of tilting weir conditions affect upstream migration of Ayu.