fishery selection on alaskan sockeye salmon and potential changes in size at maturity
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Fishery selection on Alaskan sockeye salmon and potential changes in size at maturity
Neala KendallTom Quinn
School of Aquatic and Fishery SciencesUniversity of Washington
Need for quantifying harvest selection Life history traits are changing over time (Darimont et al.
2009 PNAS)
Need for quantifying harvest selection Life history traits are changing over time (Darimont et al.
2009 PNAS)
Importance of quantifying harvest selection (Carlson et al. 2007 Ecology Letters, Edeline et al. 2007 PNAS)
Need for quantifying harvest selection Life history traits are changing over time (Darimont et al.
2009 PNAS)
Importance of quantifying harvest selection (Carlson et al. 2007 Ecology Letters, Edeline et al. 2007 PNAS)
Evaluating implications of selection (Olsen et al. 2004 Nature, Heino and GodØ 2002 Bulletin of Marine Science)
Difficult to quantify fishery selection
Hutchings, Nature 2005
Fish caught
Length
Fishery selectivity curve?
Alaska salmon: good model to study fishery selection
Semelparous and anadromous
Length and age at maturity easy to measure
Know population size and structure
Long-term gillnet fisheries Large, long term data set
J. Carter
Research questions
Does fishery selection vary by fish length and sex?
J. Carter
Research questions
Does fishery selection vary by fish length and sex?
Are fish length at age changes over time correlated with fishery selection?
J. Carter
Alaskan gillnet sockeye salmon fisheries
Bristol Bay
Upper Cook Inlet
Methods
Yearly historical fishery reconstruction
Characterize annual length distributions in catch, escapement, and total run
Estimate:1) Length-specific vulnerability2) Selection differentials
Length frequency histogram example
Females
Males
-
100,000
200,000
300,000
450 500 550 600 650
Length (mm)
Num
ber o
f fis
h
Total run
CatchEscapement
-
100,000
200,000
300,000
400,000
400 450 500 550 600 650
Length (mm)
Num
ber o
f fis
h
Total runCatchEscapement
Vulnerability profiles by length
Females
Males
199000.
20.
40.
60.8
1
400 450 500 550 600 650
Upper Cook Inlet males
Body length (mm)
Vul
nera
bilit
y
1994
Body length (mm)
Vul
nera
bilit
y
1999
400 450 500 550 600 650
Body length (mm)
Vul
nera
bilit
y
2002
Body length (mm)
Vul
nera
bilit
y
2003 00.
20.
40.
60.8
1
400 450 500 550 600 650
Body length (mm)
Vul
nera
bilit
y
1990
00.2
0.4
0.6
0.8
1
400 450 500 550 600 650
Upper Cook Inlet males
Body length (mm)
Vulnerability
1994
Body length (mm)
Vulnerability
1999
400 450 500 550 600 650
Body length (mm)
Vulnerability
2002
Body length (mm)
Vulnerability
2003
00.2
0.4
0.6
0.8
1
400 450 500 550 600 650
Body length (mm)
Vulnerability
1990 1994 1999 2002 2003
400 650 400 650 400 6500
10
1
Selection on length: SSDs
SSD
Larger fish are escaping to
spawn than are getting caught
+-
Smaller fish are escaping to
spawn than are getting caught
Standardized selection differential =
lengthescapement – lengthtotal run
std. deviation of lengthtotal run
0
0.1
0.2
0.3
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6
Standardized selection differential
Prop
ortio
n fre
quen
cy
MaleFemale
J. Carter
Female SSDs more consistently negative than male SSDs
Female average
Male average
Fishery selection on Bristol Bay sockeye salmon and length at age changes over time
Bristol Bay, Alaska fishing districts
Differential fishery selection on ocean age 2 fish
Prop
ortio
n fr
eque
ncy
Prop
ortio
n fr
eque
ncy
Standardized selection differential
Standardized selection differential
Nushagak
0.0
0.1
0.2
0.3
0.4Male
Fem ale
Naknek-Kvichak
0.0
0.1
0.2
0.3
0.4
Egegik
0.0
0.1
0.2
0.3
0.4
-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
Ugashik
0.0
0.1
0.2
0.3
0.4
Togiak
0.0
0.1
0.2
0.3
0.4
-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
P < 0.001P = 0.143
0.0
0.1
0.2
0.3
0.4Male
Fem ale
0.0
0.1
0.2
0.3
0.4Male
Fem ale
0.0
0.1
0.2
0.3
0.4Male
Fem ale
0.0
0.1
0.2
0.3
0.4Male
Fem ale
P < 0.001P < 0.001
P < 0.001P = 0.002
P < 0.001P = 0.051
P = 0.010P = 0.756
Nushagak
Naknek-Kvichak
Egegik
Ugashik
Togiak
Nushagak
Naknek-Kvichak
Ugashik
Togiak
1960 1970 1980 1990 2000 2010Year
NushagakMale
Female
Naknek-Kvichak
Egegik
1960 1970 1980 1990 2000 2010Year
Average length of ocean age 2 fish has decreased over time
Aver
age
leng
th (m
m)
slope=-0.60
slope=-0.64
slope=-0.26
slope=-0.31
slope=-0.16
slope=-0.05slope=-0.19
slope=-0.21
slope=-0.16
slope=-0.21
450
550
450
550 Aver
age
leng
th (m
m)
450
550
450
550
Egegik
Ugashik
Togiak
Nushagak
Naknek-Kvichak
slope: P = 0.003
slope: P = 0.033
slope: P = 0.005
slope: P = 0.218
slope: P = 0.869
SSDs somewhat correlated with decreasing length at age over time
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0-0.20 -0.15 -0.10 -0.05 0.00
Standardized selection differential
Slop
e of
oce
an a
ge 2
le
ngth
ove
r tim
e
Conclusions Run sizes/harvest rates vary Average fish sizes vary Fishing regulations (mesh size, timing, breaks
between fishing periods) vary
ConclusionsWhy different selection among fishing districts and over time?
Conclusions
Are larger than average fish more vulnerable to being caught?
YES Fish caught
ConclusionsConclusions
Males
Does the fishery harvest different lengths of males than females?
YES
Length
Females
Conclusions
Are fish length at age changes over time correlated with fishery selection?
YES…Fishing districts that
harvest larger fish show a greater decline in fish length at age over time
Conclusions
Other factors, such as ocean temperature and competition, also affect
growth
But…
What can fishery managers do?
Time
Exploitation rateFishery selectivity
Escapement counts
Create quantitative genetics models to understand impacts of fishery selection and environmental factors on length at age at maturation
Calculate maturation reaction norms for spawning populations to understand potential genetic changes associated with fishery selection
Using estimated selectivities, model ideal length and age at maturity under different harvest scenarios
J. Bennis J. Carter
Future work
J. BennisJ. Bennis
Tim Baker and Fred West, ADFG Mark Willette and Terri Tobias, ADFG Jeff Hard, NOAA Fisheries Alaska Salmon Program, UW Funding:
School of Aquatic and Fishery Sciences Alaska Sustainable Salmon Fund National Science Foundation Gordon and Betty Moore Foundation
Acknowledgements
Ugashik
0.0
0.1
0.2
0.3
0.4
Togiak
0.0
0.1
0.2
0.3
0.4
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
Nushagak
0.0
0.1
0.2
0.3
0.4Male
Female
Naknek-Kvichak
0.0
0.1
0.2
0.3
0.4
Egegik
0.0
0.1
0.2
0.3
0.4
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
Differential fishery selection on ocean age 3 fish
Prop
ortio
n fr
eque
ncy
Prop
ortio
n fr
eque
ncy
Standardized selection differential
Standardized selection differential
P = 0.01P = 0.057
0.0
0.1
0.2
0.3
0.4Male
Fem ale
0.0
0.1
0.2
0.3
0.4Male
Fem ale
0.0
0.1
0.2
0.3
0.4Male
Fem ale
0.0
0.1
0.2
0.3
0.4Male
Fem ale
P = 0.083P = 0.009
P = 0.52P = 0.005
P = 0.47P = 0.001
P = 0.273P < 0.001
Nushagak
Naknek-Kvichak
Egegik
Ugashik
Togiak
Nushagak
Naknek-Kvichak
• In most districts, smaller than average male fish caught, average female
• Differences in selection among districts
*
**
*
**
**
*
*
Ugashik
Togiak
1960 1970 1980 1990 2000 2010Year
Nushagak MaleFemale
Naknek-Kvichak
Egegik
1960 1970 1980 1990 2000 2010Year
Average length of ocean age 3 fish has decreased over time
Aver
age
leng
th (m
m)
slope=-0.60
slope=-0.64
slope=-0.26
slope=-0.31
slope=-0.16
slope=-0.05slope=-0.19
slope=-0.21
slope=-0.16
slope=-0.21
520
620
520
620 Aver
age
leng
th (m
m)
520
620
520
620
Egegik
Ugashik
Togiak
Nushagak
Naknek-Kvichak
slope: P < 0.001
slope: P = 0.172
slope: P = 0.009
slope: P = 0.041
slope: P = 0.018
• In most districts, fish getting smaller over time
• Differences in size decline among districts
Vulnerability differs by length & sex example
Females
J. Carter
Males
0
0.2
0.4
0.6
0.8
1
450 500 550 600 650
Length (mm)Vu
lner
abili
ty
0
0.2
0.4
0.6
0.8
1
430 480 530 580 630
Length (mm)
Vuln
erab
ility
0
0.2
0.4
0.6
0.8
1
Vuln
erab
ility
Females
Vulnerability curves differ among years
J. Bennis
Length (mm)
1980 1991 2002
420 620420 620 420 620
J. Carter
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