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 vulnerability

2) 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

Catch

Escapement

-

100,000

200,000

300,000

400,000

400 450 500 550 600 650

Length (mm)

Nu

mb

er

of f

ish

Total runCatchEscapement

Vulnerability profiles by length

Females

Males

199000

.20

.40

.60

.81

400 450 500 550 600 650

Upper Cook Inlet males

Body length (mm)

Vu

lne

rab

ility

1994

Body length (mm)

Vu

lne

rab

ility

1999

400 450 500 550 600 650

Body length (mm)

Vu

lne

rab

ility

2002

Body length (mm)

Vu

lne

rab

ility

2003 00

.20

.40

.60

.81

400 450 500 550 600 650

Body length (mm)

Vu

lne

rab

ility

199000.2

0.40.6

0.81

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

0.40.6

0.81

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

Pro

po

rtio

n f

req

ue

nc

y

Male

Female

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

Pro

po

rtio

n f

req

ue

nc

y

Pro

po

rtio

n f

req

ue

nc

y

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 < 0.001

P = 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

Ave

rag

e le

ng

th (

mm

)

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

Ave

rag

e le

ng

th (

mm

)

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

Slo

pe

of

oc

ea

n a

ge

2

len

gth

ov

er

tim

e

Conclusions

Run sizes/harvest rates vary

Average fish sizes vary

Fishing regulations (mesh size, timing, breaks

between fishing periods) vary

Conclusions

Why different selection among fishing districts and over time?

Conclusions

Are larger than average fish more vulnerable to being caught?

YESFish 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 rate

Fishery 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

Pro

po

rtio

n f

req

ue

nc

y

Pro

po

rtio

n f

req

ue

nc

y

Standardized selection differential

Standardized selection differential

P = 0.01

P = 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 = 0.083

P = 0.009

P = 0.52P = 0.005

P = 0.47

P = 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 Male

Female

Naknek-Kvichak

Egegik

1960 1970 1980 1990 2000 2010Year

Average length of ocean age 3 fish has decreased over time

Ave

rag

e le

ng

th (

mm

)

slope=-0.60

slope=-0.64

slope=-0.26

slope=-0.31

slope=-0.16

slope=-0.05

slope=-0.19

slope=-0.21

slope=-0.16

slope=-0.21

520

620

520

620

Ave

rag

e le

ng

th (

mm

)

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)V

uln

era

bilit

y

0

0.2

0.4

0.6

0.8

1

430 480 530 580 630

Length (mm)

Vu

lne

rab

ilit

y

0

0.2

0.4

0.6

0.8

1

Vu

lne

rab

ility

Females

Vulnerability curves differ among years

J. Bennis

Length (mm)

1980 1991 2002

420 620420 620 420 620

J. Carter