We PIT tagged wild spring/summer Chinook salmon parr in the Snake River Basin in 1995 and subsequently monitored these fish during their smolt migration through Lower Granite, Little Goose, Lower Monumental, McNary, John Day, and Bonneville Dams during spring and summer 1996. This report details our research and findings, which are summarized below.
- In August 1995, we PIT tagged and released 1,407 wild chinook salmon parr to the South Fork Salmon River and two of its tributaries.
- Average overall observed mortality from collection, handling, tagging, and after a 24-h holding period to assess delayed mortality was 1.0%. No PIT tags were lost during this 24-h holding period.
- In 1996, the overall adjusted average proportion of PIT-tagged fish detected at dams was 13.1% (range 9.4-17.0%).
- Fish larger at release were detected at a significantly higher rate the following spring and summer than their smaller cohorts (P < 0.0001).
- Wild fish that began migration in April 1996 were significantly larger at release than fish that began migration after April (P < 0.05).
- In 1996, as observed in all years from 1989 to 1995, peak detections of wild spring/summer chinook salmon smolts at Lower Granite Dam were highly variable and generally independent of river flows before about 9 May. However, peak detections of wild fish coincided with periods of peak flow at the dam from 9 May to the end of May. In both 1995 and 1996, well over 90% of wild fish had passed Lower Granite Dam by the time peak flows occurred in June. In 1989, peak detections of wild fish coincided with peak flows at the dam in June.
- At Lower Granite Dam, the 50th and 90th percentile passage dates of wild PIT-tagged fish from combined Idaho and Oregon streams occurred on 3 and 22 May 1996, respectively. However, unlike previous years, few wild fish from Idaho streams had been marked as parr in 1995. Therefore, detections at Lower Granite Dam in 1996 were composed of 91% fish from Oregon streams. We caution against comparing migration timing in 1996 to that in previous years, since in all previous years less than 50% of wild fish detected at the dam were from Oregon streams.
- Before 1995, we observed a 2-week shift in timing of wild stocks passing Lower Granite Dam between relatively warm and relatively cold years. In cold years (1989, 1991, and 1993) 50% of all wild fish passed the dam by mid-May and 90% passed by mid-June. In warm years (1990, 1992, and 1994) 50% of all wild fish passed Lower Granite from 29 April to 4 May, and 90% passed by the end of May. In 1995, we experienced intermediate weather conditions in late winter and early spring compared to the previous 6 years. Similarly, we observed intermediate passage timing at the dam, with the 50th and 90th passage percentiles occurring on 9 May and 5 June 1996 respectively.