Evaluation of a management measure restricting catch

below 60cm

Mark N. Maunder and Patrick Tomlinson

Resolution C-01-06

• The purse-seine fishery which takes bigeye tuna shall be closed if the Director determines, based on the best scientific and fishery data available, that the catch of bigeye tuna less than 60 centimeters has reached the level achieved in 1999, in which case he shall advise all Parties that the purse-seine fishery on floating objects shall close two weeks after such determination. However, in no event shall the fishery be closed before November 1, 2001.

The amount of bigeye tuna catch < 60cm was compared to indicators of stock status:

• 1) Catch < 60cm as a ratio of the total surface fleet catch

• 2) Catch < 60cm as a ratio of the total catch• 3) Spawning biomass ratio• 4) The scaling parameter to scale the fishing

mortality in that year to the level that would produce MSY

• 5) Average weight in the catch• 6) The fishery impact• 7) Recruitment• 8) Exploitation rate on fish < 60cm (ages 1-5

quarters)

Catch: <60cm

0

5000

10000

15000

20000

25000

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Catc

h <

60cm

Stock assessment Species composition Standard

Catch: Total

0

20000

40000

60000

80000

100000

120000

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

To

tal

Stock assessment Species composition Standard

Proportions

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

% <

60cm

Stock assessment Observer data Standard

Catch: <60cm and total

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Tota

l

1975 1980 1985 1990 1995 2000 2005

20

00

03

00

00

40

00

0

Purse seine catch: <60cm and total

Year

Pu

rse

se

ine

ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Tota

l pu

rse

se

ine

ca

tch

1975 1980 1985 1990 1995 2000 2005

05

00

01

50

00

25

00

0

Proportion <60cm

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Pu

rse

se

ine

ca

tch

< 6

0cm

/ To

tal

1975 1980 1985 1990 1995 2000 2005

0.0

0.1

00

.20

0.3

0

Proportion purse seine catch <60cm

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Pu

rse

se

ine

ca

tch

< 6

0cm

/ To

tal

1975 1980 1985 1990 1995 2000 2005

0.0

0.1

00

.20

0.3

0

SBR

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

0.0

0.1

0.2

0.3

0.4

0.5

Sp

aw

nin

g b

iom

ass

ra

tio

75 80 85 90 95 00 05

F multiplier

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Fsc

ale

1975 1980 1985 1990 1995 2000

01

23

45

Average weight

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Critical Age = 15 quartersWeight at Critical Age = 63.3 kgCritical Age = 15 quarters

75 77 79 81 83 85 87 89 91 93 95 97 99 01 03 05

0

20

40

60

Year

Ave

rag

e w

eig

ht o

f fis

h (

kg)

Recruitment

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Re

lativ

e R

ecr

uitm

en

t

75 77 79 81 83 85 87 89 91 93 95 97 99 01 03 05

0

1

2

3

4

Fishing mortality <60cm

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

Ave

rag

e T

ota

l F a

ge

s 1

-5 q

ua

rte

rs

0 20 40 60 80 100 120

0.0

0.0

50

.10

0.1

5

Recruitment

Year

Ca

tch

< 6

0cm

1975 1980 1985 1990 1995 2000 2005

01

00

03

00

05

00

0

Year

MS

Y

1975 1980 1985 1990 1995 2000

05

00

00

10

00

00

Yield per recruit analysis conducted under three scenarios:

1. No change in the relative age-specific fishing mortality;

2. 50% reduction in F for ages 5 quarters and younger; and

3. F = 0 for ages 5 quarters and younger.

Recruitment

Scenario Y/R < 60 cm

Y/R allsizes

Percentsmall

Percentincrease

1 0.7562 5.3363 14.2 0 %

2 0.4614

6.6741

6.9

25 %

3 0.0000

8.4546

0.0

58 %

YPR 1) current age-specific fishing morality and 2) age-at-entry (age 11 quarters) maximizing YPR under

current effort levels (Best-Mult).

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10 11 12 13

MULTIPLIER OF F

YIE

LD

IN

KG

PE

R R

EC

RU

IT

CURRENT

BEST-MULT=1

YPR 1) current age-specific fishing morality and 2) F for 5 quarters and less = 0 (ENTER QR 6).

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6 7 8 9 10 11 12 13

MULTIPLIER OF F

YIE

LD

IN

KG

PE

R R

EC

RU

IT

CURRENT

ENTER QR 6