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

Briefing Goals

1) Describe age at ocean-entry for the Snake River Basin population of full-term wild adults (i.e., II salts)

2) Describe age-at-ocean entry for transported subyearlings

3) Describe age-at-ocean entry for inriver migrants

4) Summarize the limited information on SARs

• 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

Key Points

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

Finding 1: Age at ocean-entry for random samples ofwild Snake River Basin full-term adults

1998Return Year

0

10

20

30

40

50

60

70

80

90

100

Perc

enta

ge o

f ann

ual s

ampl

e

1999 2000 2001 2002 2003 2004

(2)

(101) (55)

(6)

(65)(92)

(305)

Subyearling inter-annual mean 59%Yearling inter-annual mean 41%

2005 2006

(126)(444)

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

Method B (Continued): Use juvenile PIT-tag histories to determine migration pathway and location of first year wintering

Summer and fall transport

Subyearling entrants winterin the ocean

Yearling ocean entrants winterbelow Bonneville Dam

Inriver migration

Subyearling entrants winter in the ocean

Yearling ocean entrants winter above or below Bonneville Dam

Finding 2: Age at ocean entry for summer and fall transport groups (N = 32 adults collected in 2005; pooled 2001-2004 migration years)

Summer FallTransport season

0

20

40

60

80

100

Perc

enta

ge o

f int

er-

annu

al s

ampl

es

n = 15 n = 17

SubyearlingYearling

33% 47%

05/01/200606/16/2006

08/01/200609/16/2006

11/01/200612/17/2006

02/01/200703/19/2007

Passage date at Lower Granite Dam

0

20

40

60

80

100

120

Estim

ated

num

ber p

asse

d

Radio tagsClearwater

Dewatering

PIT-tag detectionsystem typicallydewatered

SnakeMostlysubyearling

Mostlyyearling All yearling

(reservoir types)

Never detected group(reservoir types)

Finding 3: Age-at-ocean entry for inriver migrants (e.g., wild)

Never detected

Summer 02transport

Jun-Oct 02bypassed

Spring 03(reservoir type)

Fall 02transport

0

1

2

3

4

5

72 /?????

98/16,284 21/3,990

55/1,190

80/2,500

Smol

t-to-

adul

t ret

urn

(%)

Finding 4-1: Smolt-to-adult return rates for surrogate subyearlings released into the Snake River in 2002

02 02 mortalityunknown

Jun 02 Jul 02 Aug 02 Sep 02 Oct 02 Spring 03(reservoir type)

0

10

20

30

40

50

60

0

1

2

3

4

5Juveniles

SAR

Last PIT-tag detection

Perc

ent o

f mig

rant

sSA

R (%

)Finding 4-2: Seasonal trend in SARs for bypassed surrogate subyearlings released into the Snake River in 2002

Summer transport75-100 mm FL

Fall transport 175-200 mm FL

Subyearling inriver100-125 mm FL

Yearling inriver 200-225 mm FL

Finding 4-3: Size after passing Bonneville Dam and at ocean entry likely affects SARs

Both subyearling and yearling ocean entrants made substantialcontributions to the return of full-term wild adults regardless ofof how they reached the sea because:

1. The relatively large number of early migrants destined to become subyearling ocean entrants likely compensated for the relatively low SARs for early migrants

2. 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

Conclusions

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

Jun 02 Jul 02 Aug 02 Sep 02 Oct 02 Spring 03(reservoir type)

0

10

20

30

40

50

60

0

1

2

3

4

5Juveniles

SAR

Last PIT-tag detection

Perc

ent o

f mig

rant

s

SAR

(%)