Deepwater Spawning of Fall Chinook Salmon (Oncorhynchus tshawytscha) Near Ives and Pierce Island of the Columbia River, 2000.

Fall chinook salmon (Oncorhynchus tshawytscha), thought to originate from Bonneville Hatchery, were first noted to be spawning downstream of Bonneville Dam by Washington Department of Fisheries and Wildlife (WDFW) biologists in 1993 (Hymer 1997). Known spawning areas included gravel beds on the Washington side of the river near Hamilton Creek and Ives Island. The size of this population from 1994 to 1996 was estimated at 1,800 to 5,200 fish (Hymer 1997), and 554 fish in 1998 (Van der Naald et al. 1999). These estimates were based on carcass surveys and visual observation of redds by boat near the shoreline. Pacific Northwest National Laboratory (PNNL) conducted underwater video surveys in the fall of 1999 and 2000 to determine the extent of the fall chinook salmon spawning and to estimate the number of redds occurring in deeper water. Estimates of redds occurring in water depths exceeding 2.2 m at 143,000 cubic feet per second (kcfs) were 499 in 1999 (Mueller and Dauble 1999) and 567 redds >2.2 m at 127 kcfs in 2000 (this study). The majority of the redds found were confined near the main river channel adjacent to Pierce Island. Chum salmon (O. keta) also have been documented using the mouth of Hamilton Creek and portions of Hamilton Slough for spawning. The majority of chum salmon were found to spawn in shallow water at the mouth of Hamilton Creek adjacent to Ives Island. Estimates of the natural chum salmon spawning population for 1998 were 226 (Van der Naald et al. 1999). Chum salmon spawning near Ives Island are part of the Columbia River evolutionary significant unit (ESU), and are included in the Endangered Species Act of 1973 (ESA) listing in March 1999. Our main objective of this study was to locate deep water spawning locations of fall chinook salmon in the main Columbia River channel and to collect additional data on physical habitat parameters at spawning sites. The secondary objective was to map any chum salmon redds located in the deep sections of Hamilton Slough. There are several ongoing investigations to define the physical habitat characteristics associated with fall chinook and chum salmon spawning areas downstream of Bonneville Dam. A major concern is to determine what flows (i.e., surface elevations) are necessary to ensure their long-term survival. This objective is consistent with the high priority placed by the Northwest Power Planning Council's Independent Advisory Board and the salmon managers on determining the importance of mainstem habitats to the production of salmon in the Columbia River Basin