Issues in fisheries sustainability

• What is a “fishery”?• The global status of fisheries: will we soon

be eating only jellyfish?• What does “sustainability” mean, and what

is the ecological basis for it?• What does it mean to “manage” a fishery?

What is a “fishery”

• A linked dynamic relationship between a set of valued fish and a set of fishermen who pursue those fish

Fish stock(s) Fishing “fleet”

Catch

Mortality

The global status of fisheries

The global status of fisheries, revisited

From Branch et al. 2010. The trophic fingerprint of marine fisheries. Nature, doi:10.1038/nature09528

Many (25-30%) of the world’s fisheries have

“collapsed”to catches less

than 10% of historical peak

Plateau

0

0.5

1

0246810

Years before collapse

Rela

tive

catc

h

20

Smooth

0

0.5

1

0246810

Years before collapse

Rel

ativ

e ca

tch

31 12 16 38

Erratic

0

0.5

1

0246810

Years before collapse

Rela

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catc

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5 17 8 16From Mullon et al. 2005. The dynamics of collapse in world fisheries. Fish and Fisheries 6: 111-120.(an examination of 1500 catch time series)

Where are fisheries collapsing?Newfoundland

North sea

California current

India

China

Indonesia

AustraliaBenguela

What does “sustainability” mean?

• Lack of collapse?• Capable of recovery after collapse,

especially for collapses not caused by fishing?

• Harvested at near maximum sustainable yield?

• Harvested at near maximum sustainable harvest rate?

Sustainable fisheries depend on creation of “surplus production”

• Surplus production is biological production (growth) that can be translated either into catch or into population growth.

• On average, surplus production is zero in unharvested natural populations

• High fishing mortality rate can result in sustainability, but at low biomass and catch

0

5000

10000

15000

20000

25000

30000

0 0.2 0.4 0.6 0.8 1Fishing mortality rate F

Bio

mas

s

Biomass

Production

Catch x 10

Food consumption

What causes surplus production to occur when fishing reduces stock size?

• “Compensatory” improvement in juvenile survival rates and/or growth rates

• These compensatory improvements result from– Reduction in predator abundances (uncommon)– Increase in food abundance (more common)– Increase in available food abundance leading to

better growth and/or reduced predation risk (very common)

– Reduction in juvenile mortality due to cannibalism (common)

What does it mean to “manage” a fishery?

• Protect the ecological basis for production (biophysical habitat, forage base)

• Control the quality (size, age) of fish harvested• Regulate the fishing mortality rate F

– Input control: control fishing activity, area swept by fishing

– Output control: control the catch, given estimate of biomass (since F=catch/biomass)

• Seek balance in situations where fishing impacts multiple stocks so as to create tradeoffs

Most fisheries impact multiple stocks, create tradeoffs where not all stocks

can be harvested at best rates• Fishing may “target” particular

stocks/species, but fishing activity typically causes catch of other species

• Discarding non-target stocks is typically wasteful

• “collateral damage” reduces biological diversity and threatens ecological basis for sustainability

Fraser sockeye salmon have returned to near historical peak levels, but there

has been a worrisome declineTotal Fraser River sockeye run size, Pacific Salmon Commission estimates

0

10,000,000

20,000,000

30,000,000

40,000,000

50,000,000

1890 1910 1930 1950 1970 1990 2010

Tota

l run

siz

e

Total Run

Spawners

Productive fisheries often depend on diverse mixtures of individual spawning stocks, most

obvious with Pacific salmon

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

1952

1956

1960

1964

1968

1972

1976

1980

1984

1988

1992

1996

2000

2004

2008

latemisc

harrison

cultus

portage

weaver

early shuswapadams

birkenhead

stellako

late stuart

quesnel

chilkoupi

gates

nadina

estuart

seymour

scotch

raft

fennellbowron

Hilborn showed a similar pattern of shifting contributions for major Bristol Bay stocks

Fraser sockeye abundance by stock

There is a severe tradeoff between harvesting and maintenance of

stock structure (biodiversity)

0

5000000

10000000

15000000

20000000

25000000

0 0.2 0.4 0.6 0.8 1Overall exploitation rate

Catc

h, E

scap

emen

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Prop

ortio

n of

sto

cks

over

fishe

d or

ext

inct

Total HarvestTotal EscapementPextinctPoverfished

Tradeoff between catch and stock "health"Fraser River sockeye

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2000000 4000000 6000000 8000000 10000000 12000000 14000000 16000000

Average long term catch

Prop

ortio

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sto

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not o

verf

ishe

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At the harvest rate expected to produce maximum average yield, about 50% of the (mostly small) stocks would be overharvested, and about 10% would be threatened with extinction. The tradeoff will be even worse if diverge in productivity continues

Is it wise or just for people who will not pay the bill to demand that fishers give up 50% of their income as an insurance policy for biodiversity?