- Results of a study of spatial and temporal utilization of a tidal river estuary by hatchery and wild juvenile salmonids
(Oncorhynchus spp. and Salmo spp,), of overlap in food habits of hatchery and wild juvenile salmonids, and of size and relative
abundance of associated fish species are reported in this thesis. The investigation was conducted in Yaquina Bay, Oregon in 1977
and 1978 to provide information for evaluation of concerns over the biological impact of large releases of hatchery salmon on wild fish in the estuary. A 100- X 3-m beach seine was used to sample four beach
study areas from July 1977 through December 1978, and a 222-m lampara net was used to sample two channel study areas from March 1978 through October 1978. Approximately 2. 2 million hatchery salmon were released into Yaquina Bay in 1977, and 9.6 million were released in 1978. Tags, fin clips, dye marks,
scales, species, release date, external parasites, visceral fat, size, and fin erosion were used to determine hatchery or wild origin of individual salmonids in the catch. Chinook (Oncorhynchus tshawytscha), chum (0. keta), and coho (0. kisutch) salmon, in decreasing order, were the most abundant wild salmonid species, and coho salmon were the most abundant hatchery species. In 1978 wild populations of chum and coho salmon were present in the estuary for 2-3 mo (March-June), and wild Chinook were present during 9 mo (January, April-November). Increase in mean length of wild chum and Chinook, and decrease in mean length of wild coho, indicated that wild chum and Chinook
utilized the estuary as a rearing area, and wild coho did not. Lack of overlap in peak migration periods of wild chum (early April),
coho (mid May), and chinook (late July-early August) suggests the need to minimize overlap in utilization of the estuary by hatchery and wild juvenile salmonids. The length of residence of hatchery coho in Yaquina Bay was described by the equation: N=N 0 e-kt. The
"residency half-life" (E®2) ranged from 1. 7 to 9. 0 days for different No release groups of hatchery coho in 1977 and 1978. Juvenile hatchery coho that remained for an extended period (1-3 mo) in Yaquina Bay during 1977 increased in mean length from 11.5 cm FL in mid July to 21. 0 cm FL in October. Some individuals within summer release groups of hatchery chinook also remained in the estuary for extended periods (> 2 mo)a Groups of juvenile hatchery coho and chinook released
into Yaquina Bay earlier in the year (June-August) remained in the estuary for longer periods than groups released later in the year (September-October), Overlap in food habits of hatchery and wild juvenile salmonids in the estuary was C ften high, although overlap was found to vary with species, time, habitat, space, length of estuarine residence, and prey abundance. In terms of biomass, larval and juvenile fish
(Clupeidae, Engraulidae, and Osmeridae) were the most important prey organisms of hatchery and wild coho and chinook salmon in Yaquina Bay. Approximately 58 fish species were captured at the study areas in 1977 and 1978, and 17 were identified in the stomach contents of hatchery and wild salmonids.
Overlap in spatial and temporal utilization and in food habits of hatchery and wild juvenile salmon in the estuary indicates that the potential for competition between these groups does exist, should space or food resources become a limiting factor. To reduce overlap in spatial and temporal utilization, consideration should be given to not releasing hatchery salmon during peak migration periods of wild chum, coho, and chinook salmon. To reduce length of residence
of hatchery coho and chinook released after May, mid to late summer releases should be considered.