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Briefing Goals

This briefing discusses the age at which Snake River Basin fall Chinook salmon enter the ocean and its impact on adult returns. It explores the contributions of subyearlings and yearlings to the adult population and summarizes the available data on smolt-to-adult return rates. The briefing also highlights the need for further research on the impact of summer spill on the survival of early migrating subyearlings.

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Briefing Goals

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  1. Age at ocean entry of Snake River Basin fall Chinook and its significance to adult returns prior to summer spill at LGR, LGS, and LMN dams

  2. Briefing Goals • Describe age at ocean-entry for the Snake River Basin population of full-term wild adults (i.e., II salts) • Describe age-at-ocean entry for transported subyearlings • Describe age-at-ocean entry for inriver migrants • Summarize the limited information on SARs

  3. Key Points • Both subyearling and yearling ocean entrants made substantial contributions to the return of full-term wild Snake River Basin adults • Subyearling and yearling ocean entry has been evident in full-term adults from summer and fall transport groups as well as inriver migrating groups • The tendency to become a yearling ocean entrant and SARs increased as the migration season progressed

  4. Method A: Trapped random samples of adults at Lower Granite Dam and used scale pattern analyses to identify origin, age at ocean entry, and ocean age

  5. Finding 1: Age at ocean-entry for random samples of wild Snake River Basin full-term adults Subyearling inter-annual mean 59% Yearling inter-annual mean 41% 100 90 80 (65) 70 (55) (101) (92) (126) (305) (444) 60 Percentage of annual sample 50 40 30 20 10 (2) (6) 0 2001 2004 2005 1998 2000 2003 1999 2002 2006 Return Year

  6. Method B: Same as method A, but we targeted adults that were PIT-tagged as juveniles

  7. Method B (Continued): Use juvenile PIT-tag histories to determine migration pathway and location of first year wintering Summer and fall transportSubyearling entrants winterin the oceanYearling ocean entrants winterbelow Bonneville Dam Inriver migration Subyearling entrants winter in the ocean Yearling ocean entrants winter above or below Bonneville Dam

  8. Finding 2: Age at ocean entry for summer and fall transport groups (N = 32 adults collected in 2005; pooled 2001-2004 migration years) Subyearling Yearling n = 15 n = 17 100 33% 80 47% Percentage of inter-annual samples 60 40 20 0 Summer Fall Transport season

  9. Finding 3: Age-at-ocean entry for inriver migrants (e.g., wild) Clearwater Radio tags Snake Mostly subyearling 120 Mostly yearling 100 PIT-tag detection All yearling (reservoir types) system typically 80 dewatered Estimated number passed 60 Never detected group (reservoir types) Dewatering 40 20 0 05/01/2006 08/01/2006 11/01/2006 02/01/2007 06/16/2006 09/16/2006 12/17/2006 03/19/2007 Passage date at Lower Granite Dam

  10. Finding 4-1: Smolt-to-adult return rates for surrogate subyearlings released into the Snake River in 2002 02 02 mortality unknown 5 55 /1,190 4 80 /2,500 3 Smolt-to-adult return (%) 2 1 98 /16,284 21 /3,990 72 /????? 0 Summer 02 transport Fall 02 transport Jun-Oct 02 bypassed Spring 03 (reservoir type) Never detected

  11. Finding 4-2: Seasonal trend in SARs for bypassed surrogate subyearlings released into the Snake River in 2002 5 Juveniles 60 SAR 4 50 40 3 Percent of migrants SAR (%) 30 2 20 1 10 0 0 Jun 02 Jul 02 Aug 02 Sep 02 Oct 02 Spring 03 (reservoir type) Last PIT-tag detection

  12. Finding 4-3: Size after passing Bonneville Dam and at ocean entry likely affects SARs Summer transport75-100 mm FL Subyearling inriver100-125 mm FL Fall transport 175-200 mm FL Yearling inriver 200-225 mm FL

  13. Conclusions Both subyearling and yearling ocean entrants made substantial contributions to the return of full-term wild adults regardless of of how they reached the sea because: • The relatively large number of early migrants destined to become subyearling ocean entrants likely compensated for the relatively low SARs for early migrants • The relatively high SARs for late migrants destined to become yearling ocean entrants compensated for the relative small number of fish that likely survive to become yearling ocean entrants

  14. Research Needs • Summer spill (2005 to 2007) • Surrogate subyearlings were provided for research in 2005 and 2006, but not in 2007 • Support for research in 2008 is needed to evaluate spill and its influence on SARs of early inriver migrants destined to become subyearling ocean entrants

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