issues in fisheries sustainability
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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
tive
catc
h
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
t
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
n of
sto
cks
not o
verf
ishe
d
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?