Fish Ladder Weir Modifications to Minimize Escapement

Abstract

A fish ladder, weir, and raceway at Whitlock Spawning Station are used to capture and retain feral fall Chinook salmon (Oncorhynchus tshawytscha) for spawning from Lake Oahe, South Dakota. Nearly 40% of the salmon ascending the fish ladder have been documented to escape from the holding raceway by descending the ladder. This manuscript describes modifications to the weir that eliminated nearly all escapements, except for an inconsequential number of small non-spawning fish. The modified design using the existing structure incorporated free-swinging, spaced steel bars with a back-stop to create a one-way gate. The modified gate does not require springs, hydraulics, or electrical power. It is relatively simple to construct and requires minimal maintenance.