Download Issues in fisheries sustainability

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
Catch
Fishing “fleet”
Fish stock(s)
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
From Mullon et al. 2005.
The dynamics of collapse
in world fisheries. Fish
and Fisheries 6: 111-120.
(an examination of 1500
catch time series)
20
Relative catch
1
0.5
0
10
8
6
4
2
0
2
0
2
0
Years before collapse
Smooth
Relative catch
31
12
16
38
1
0.5
0
10
8
6
4
Years before collapse
Erratic
5
17
8
16
1
Relative catch
Many (25-30%)
of the world’s
fisheries have
“collapsed”
to catches less
than 10% of
historical peak
Plateau
0.5
0
10
8
6
4
Years before collapse
Where are fisheries collapsing?
Newfoundland
India
North sea
China
California current
Indonesia
Australia
Benguela
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”
30000
Biomass
25000
Production
Catch x 10
Food consumption
20000
Biomass
• 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
15000
10000
5000
0
0
0.2
0.4
0.6
Fishing m ortality rate F
0.8
1
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 decline
Total Fraser River sockeye run size, Pacific Salmon Commission estimates
50,000,000
Total Run
Spawners
Total run size
40,000,000
30,000,000
20,000,000
10,000,000
0
1890
1910
1930
1950
1970
1990
2010
Productive fisheries often depend on diverse
mixtures of individual spawning stocks, most
obvious with Pacific salmon
Fraser sockeye abundance by stock
latemisc
12,000,000
harrison
cultus
portage
10,000,000
weaver
early shuswap
adams
8,000,000
birkenhead
stellako
late stuart
6,000,000
quesnel
chilko
upi
4,000,000
gates
nadina
estuart
2,000,000
seymour
scotch
raft
2008
2004
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
0
fennell
bowron
Hilborn showed a similar
pattern of shifting
contributions for major
Bristol Bay stocks
There is a severe tradeoff between
harvesting and maintenance of
stock structure (biodiversity)
1
0.9
0.8
0.7
15000000
0.6
0.5
10000000
0.4
0.3
5000000
0.2
0.1
0
Total Harvest
Total Escapement
Pextinct
Poverfished
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?
Tradeoff between catch and stock "health"
Fraser River sockeye
1
0
0
0.2
0.4
0.6
0.8
0.9
1
Overall exploitation rate
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
Proportion of stocks not overfished
Catch, Escapement
20000000
Proportion of stocks overfished or extinct
25000000
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
2000000 4000000 6000000 8000000 10000000 12000000 14000000 16000000
Average long term catch