Escapement of Broodstock Chinook Salmon From Whitlock Bay Spawning Station, Lake Oahe, South Dakota

Abstract

Feral spawning fall Chinook salmon (Oncorhynchus tshawytscha) in Lake Oahe, South Dakota, are captured using a fish ladder and catch raceway at Whitlock Bay Spawning Station. The number of salmon that escaped the catch raceway and descended the fish ladder prior to spawning was unknown. During October 2017, all salmon that ascended the fish ladder at the spawning station were tagged. Tagged males remained in the catch raceway. Tagged females were moved to other secure raceways and used to estimate tag retention. Of the 383 tagged males, 159 (41.5%) were initially designated as escaped from the catch raceway. Tag loss in the females was 3.9%. Thus, the estimated male salmon escapement rate from the catch raceway was 37.6%. Male salmon remained in the catch raceway for one-to-three days before escaping. The escapement rate decreased over the month-long spawn, with nearly 60% of the males going back down the fish ladder in the first week of October, compared to less than 20% escapement by the final week. Such high rates of escapement from the spawning station may be negatively impacting the spawning efficiencies. Possible solutions include re-engineering of the fish ladder or daily removal of the salmon in the catch raceway to other more secure locations at the spawning station.

Fish Ladder Weir Modifications to Minimize Escapement

“…Escapement of salmon broodstock from the station was erroneously assumed to be negligible for the first 33 years of operation. However, in 2017, a mark-recapture study revealed that nearly 40% of the salmon that ascended the fish ladder subsequently descended the ladder through the open weir and escaped from the station prior to spawning [13]. Re-engineering of the weir at the top of the fish ladder was recommended to increase the number of salmon retained at the station for spawning [13].…”